Differences in Trunk and Hip Flexion/Extension Strength

Georgia Southern University Digital Commons@Georgia Southern

Electronic Theses and Dissertations Graduate Studies, Jack N. Averitt College of

Spring 2017

Jasmin Brown

Follow this and additional works at: https://digitalcommons.georgiasouthern.edu/etd

Part of the Physical Therapy Commons

Recommended Citation Brown, Jasmin, "Differences in Trunk and Hip Flexion/Extension Strength" (2017). Electronic Theses and Dissertations. 1575. https://digitalcommons.georgiasouthern.edu/etd/1575

This thesis (open access) is brought to you for free and open access by the Graduate Studies, Jack N. Averitt College of at Digital Commons@Georgia Southern. It has been accepted for inclusion in Electronic Theses and Dissertations by an authorized administrator of Digital Commons@Georgia Southern. For more information, please contact [email protected]. DIFFERENCES IN TRUNK AND HIP FLEXION/EXTENSION STRENGTH by JASMIN BROWN (Under the Direction of Li Li)

ABSTRACT Context: The definition of the “core” within the literature is misconstrued: some researchers believe the core only involves muscles of the trunk while others believe it also includes muscles of the hip. Core strength tests typically include exercises that activate hip flexors and extensors without a firm definition of the “core” including the muscles of the hip. Purpose: The purpose of this study was to differentiate between the strength of the trunk and hip during flexion and extension.

Methods: Participants included 28 Division I collegiate athletes from a single university (12 males, 16 females, height (in.) = 69.14 ± 4.81, weight (lb.) = 171.57 ± 45.54, age = 20.82 ± 1.31).

Trunk and hip joint strength was tested on the Biodex Isokinetic Dynamometer using the hip and the back attachments. Measurements were taken of peak torque isometrically and both peak and average torque isokinetically at contraction speeds 60 deg/s, 120 deg/s, and 180 deg/s. The independent variables are joint, contraction speed, and flexion/extension. The dependent variables are peak and average torque.

Results: One-factor ANOVAs with repeated measures were ran to compare between peak and average torques for both joints at the different contraction speeds. A Tukey’s post hoc analysis was ran in order to control the amount of error within the data. There was a significant interaction between joint and speed for peak isokinetic hip flexion torque (F(1,28)=22.75, p< 0.05), average isokinetic hip flexion torque (F(1,28)=13.93, p< 0.05), peak isokinetic hip extension torque

(F(1,28)=32.72, p< 0.05), and average isokinetic hip extension torque (F(1,28)=37.90, p< 0.05). For the isometric tests, there was significance between joints for both flexion (F(1,28)=86.15, p< 0.05) and extension (F(1,28)=66.58, p< 0.05). For all post hoc comparisons of isokinetic tests, trunk strength was significantly different between the different test speeds. For post hoc comparisons of peak and average isokinetic extension torque, hip strength was significantly different from trunk strength at 60 and 120 deg/s. For post hoc comparisons of peak and average isokinetic flexion torque, hip strength was significantly different when compared to the trunk at all testing speeds.

Hip strength was significantly different when compared to trunk strength at all testing speeds during peak and average flexion torque. When looking at the post hoc comparison for peak isometric flexion and extension torque in, trunk strength is significantly different when compared to hip strength

Conclusion: Because the trunk and hip joints are different from each other when comparing movement of the two joints at different contraction speeds, researchers must be careful when defining and testing the “core”.

INDEX WORDS: Trunk, Core, Hip, Strength, Biodex, Torque DIFFERENCES IN TRUNK AND HIP FLEXION/EXTENSION STRENGTH

JASMIN BROWN

B.S., Western Carolina University, 2015

M.S., Georgia Southern University, 2017

A Thesis Submitted to the Graduate Facility of Georgia Southern University in Partial

Fulfillment of the Requirements for the Degree

MASTER OF SCIENCE

STATESBORO, GEORGIA

DIFFERENCES IN TRUNK AND HIP FLEXION/EXTENSION STRENGTH by JASMIN BROWN

Major Professor: Li Li Committee: Barry Munkasy Tamerah Hunt

Electronic Version Approved: May 2017 2

ACKNOWLEDGEMENTS I would like to thank my chair, Dr. Li, and my committee members Dr. Munkasy and Dr. Hunt for helping me throughout this entire process and making it as enjoyable as it can be. I would also like to thank Peter Chrysosferidis and Brian Szekely for going out of their way to help me analyze my data. I also thank any and all other helped that I received during this entire project.

TABLE OF CONTENTS Page ACKOWLEDGEMENTS…………………………………………………………………………2 LIST OF TABLES………………………………………………………………………………...4 LIST OF FIGURES……………………………………………………………………………….5 CHAPTER 1 INTRODUCTION……………………………………………………………………....6 2 METHODS……………………………………………………………………………...9 3 RESULTS……………………………………………………………………………...12 4 DISCUSSION………………………………………………………………………….19 5 CONCLUSION………………………………………………………………………...22 APPENDICES A RESEARCH OUTLINE…………...………………………………………….23 B REVIEW OF LITERATURE………………………………………………….24 C IRB APPROVAL/INFORMED CONSENT………………………………..…40 D STATISTICS…………………………………………………………………..43 REFERENCES…………………………………………………………………………...……...53

LIST OF TABLES Table 1: Subject Demographics…………………………………………………………………...9

LIST OF FIGURES Page Figure 1: Peak Trunk vs. Hip Isokinetic Flexion Torque………………………………………..13 Figure 2: Average Trunk vs. Hip Isokinetic Flexion Torque…………………………………….14 Figure 3: Peak Trunk vs. Hip Isokinetic Extension Torque…………………………………...…15 Figure 4: Average Trunk vs. Hip Isokinetic Extension Torque………………………………….15 Figure 5: Peak Trunk vs. Hip Isometric Flexion Torque………………………………………...17 Figure 6: Peak Trunk vs. Hip Isometric Extension Torque…………………………………...…18

Introduction

The core is the center of the kinetic chain and allows the generation and transmission of forces for optimal movement and function.1,2 The core is seen as a muscular corset that works as a unit to stabilize the body, particularly the spine, both with and without limb movement.3 Some researchers believe the core involves musculature of the lumbopelvic-hip complex, which involves deeper muscles such as the internal oblique, transversus abdominis, transversospinalis

(multifidus, rotatores, semispinalis), quadratus lumborum, and psoas major and minor, and superficial muscles, such as the rectus abdominis, external oblique, erector spinae (iliocostalis, spinalis, longissimus), latissimus dorsi, gluteus maximus and medius, hamstrings, and rectus femoris.4–6 Others define core musculature as “a matrix of bones, ligaments, muscles, and nerves mostly comprised of muscles from the trunk”.2 For the purpose of this study, the trunk joint is defined as the angle of movement between the pelvis and rib cage and the hip joint is defined as the angle of movement between the pelvis and the femur.

Core stability is typically defined as the ability to control the position and motion of the trunk over the pelvis and leg to allow optimum production, transfer, and control of force and motion to the terminal segment in integrated kinetic chain activities.7 The muscles of the trunk and pelvis are responsible for the maintenance of stability of the spine and pelvis and help in the generation and transfer of energy from large to small body parts during many sports activities.8,9

For optimal core stability, both the smaller, deeper core muscles and the larger, superficial core muscles must contract in sequence with appropriate timing and tension.10,11 Stabilization of the trunk is crucial for maintaining static or dynamic balance, especially to provide a solid base when attempting to exert forces upon external objects, which forms the base of success in the majority of sports and is a necessity in the activities of daily living.8 7

Core training programs include processes that target muscular strengthening and motor control of the core musculature. Core strengthening exercises are very popular in rehabilitation and strength training programs despite little scientific evidence existing as to their efficacy on improving subsequent performance.12 A common method for testing core strength in research is the protocol established by McGill. The protocol consists of four different isometric holds to test trunk flexors, trunk extensors, and lateral musculature. Trunk flexor endurance is tested with the person in an upright seated position with the back resting against a prop angled at 60 degrees from the floor, arms crossed against the chest, and feet secured on the floor. The prop is pulled back 10 cm and the person has to try and hold this position as long as possible until any part of the back touches the prop. Trunk extensors are tested with the upper body cantilevered out over the end of a test bench with the pelvis, knees, and hips secured and the arms held across the chest. Failure occurs when the upper body drops below the horizontal position. Lateral musculature is tested with the person lying in a side-bridge position (testing is done on both sides) with the legs extended and the top foot placed in front of the lower foot for support.

Testing begins when the subject supports themselves on one elbow and lifts the hips off of the ground, forming a straight line from head to toe. Failure occurs when the person can no longer keep the body in a straight line or the hips return to the ground.13,14

Isokinetic testing of trunk muscle performance has also gained popularity in research within the past 30 years. This sort of testing is unique because it measures muscle torque at constantly changing angles and associated muscle moment arms, which is presumed to more closely represent a dynamic spinal loading event.15 Research has also found that, as the speed of contraction increases for different joints and muscle groups, peak and average torque decreases.16,17 This is most likely due to the fact that the dynamometer can only register external 8 torque when the machine’s lever arm can be made to exceed the preselected machine speed. If the subject is unable to accelerate his or her limb to a speed greater than the machine speed, the machine’s internal load cell records zero torque being produced. This recording does not mean that the contracting muscle is generating zero force, but rather that all of the resultant force is being transformed into limb movement. As the machine’s speed setting is progressively increased, more of the muscle’s force production goes into generating limb speed and less is responsible for producing externally measured torque.16 Very little research reports on whether or not this same pattern of decreased torque with faster contraction speeds exists at the hip or trunk.

Due to much confusion on which muscles of the lumbo-pelvic-hip complex are actually defined as the “core”, it is hard to determine whether or not the core strength tests used in the literature are actually testing muscles of the trunk joint or the hip joint and whether or not these tests should be used to assess strength of the “core”. Anatomically and physiologically, it is well understood that the trunk and hip joints and the surrounding musculature work differently, but these muscles are grouped together within some research and called the “core”. For the purposes of this study, the “core” was divided into the trunk and hip joints because the main actions of the muscles crossing the pelvis within the “core” act at either the trunk or the hip. The purpose of this study was to differentiate between the strength of the hip and trunk joints during flexion and extension to determine the relationship between the two joints. The secondary purpose was to determine if there is a decrease in torque as contraction speed increases. We hypothesized that the trunk joint will be significantly stronger than the hip joint in both flexion and extension. We also hypothesized that peak and average torque would decrease with increased contraction speed for both the trunk and hip joints during all isokinetic tests. 9

Methods Subjects

Subjects consisted of a sample of Division 1 collegiate athletes from Georgia Southern

University. There were a total of 28 subjects that completed testing. Refer to the table below for other demographic information. Subjects were excluded from the study if they were under 18 years old, if they were not a student-athlete within the university, or if they were currently withheld from athletic activity due to an injury. Subjects were included in the study if they were healthy student athletes currently participating in sport and over the age of 18.

Subject Demographics n=28 (12 males, 16 females) Height (in) Weight (Lb) Age (years)

Mean 69.14 171.57 20.82

Standard Deviation 4.81 45.54 1.31

Procedures

The Biodex Isokinetic Dynamometer was used to measure hip and trunk strength. Before beginning the testing, all participants completed a warm up on the stationary bike for ten minutes. Hip flexion/extension and trunk flexion/extension were tested isometrically and concentrically using the hip and back attachments of the Biodex. Subjects were given 3 minutes of rest in between testing speeds for both the hip and the trunk.18 They were also given ten minutes of rest in between testing for the different muscle groups.19

Measurement of Hip Flexion/Extension Strength

Hip flexion/extension strength was tested on the Biodex using the hip attachment in a supine position. The greater trochanter was used as the bony landmark for matching the axis of rotation of the hip joint with the axis of the dynamometer lever arm.20,21 Subjects were strapped into the dynamometer with a Velcro strap along the distal thigh. They were also secured into the machine with Velcro straps across the pelvis and chest. Subjects completed 2-3 submaximal practice trials in order to familiarize themselves with how testing would feel.21 After completing practice trials, each subject performed 3 maximal-effort contractions for hip flexion and extension isometrically and at speeds of 60 deg/s, 120 deg/s, and 180 deg/s.22 Our protocol was consistent with previous literature. Subjects were tested at 60 degrees of knee flexion in order to match knee angle when performing trunk flexion/extension in the back attachment of the Biodex. At least 3 minutes recovery were given in between test speeds.

Measurement of Trunk Strength

Subjects were tested isometrically at 0 degrees of extension and concentrically at speeds of 60,

120 and 180 deg/s.18,23,24 Subjects were tested on the Biodex using the back attachment in a seated upright position. The body was strapped to the back of the chair with two straps coming across the chest, one across the pelvis, and two across the legs.25 Subjects were instructed to hold the straps across the chest with their arms in order to minimize the use arm muscles for the tested task. Axis of rotation was the anterior superior iliac spine.26 Following previous literature, subjects performed 2-3 submaximal practice trials to familiarize themselves with the testing protocol. They then performed 3 maximal-effort contractions for trunk flexion and extension 11 isometrically and at speeds of 60 deg/s, 120 deg/s, and 180 deg/s with at least 3 minutes of rest in between each testing speed.18,24

The intraclass correlation coefficients (ICCs) for Biodex biomechanical measurements of hip flexion and extension in the supine position ranged from 0.91 to 0.95 for peak torque, 0.89 to

0.95 for total work, and 0.87 to 0.95 for average power.20 Claiborne et al. found ICCs for concentric hip flexion and extension to be 0.82 and 0.85 respectively.21 ICC values ranged from

0.74 to 0.88 for measurements of peak torque and 0.88-0.93 for total work during trunk flexion and extension.24

Data Analysis

The independent variables were joint, contraction speed, and flexion/extension. The dependent variables were peak and average torque. The means for peak and average torque for both hip and trunk measurements were compared in order to determine whether or not there is a significant difference in strength between the two joints. Multiple ANOVAs were ran in order to compare means between the two joints.

One-factor ANOVAs with repeated measures were ran in order to compare hip and trunk joint peak isometric torque in both flexion and extension. Two-factor ANOVAs with repeated measures were ran in order to compare hip and trunk peak and average torque in flexion and extension at the different contraction speeds. Tukey’s post-hoc analyses were ran in order to limit the amount of error within the data. Cohen’s d was calculated to determine effect size.

Alpha level for statistical significance will be set at 0.05.

Results

There were 16 females and 12 males from a variety of sports (football, men’s and women’s soccer, baseball, women’s volleyball, women’s track and field, cheer, and women’s swim and dive). The mean age of the subjects was 20.82 ± 1.31, the mean height in inches was

69.14 ± 4.81, and the mean weight in pounds was 171.57 ± 45.54. There was a significant interaction between joint and speed for peak isokinetic hip flexion torque (F(1,28)=22.75, p< 0.05), average isokinetic hip flexion torque (F(1,28)=13.93, p< 0.05,), peak isokinetic hip extension torque (F(1,28)=32.72, p<0.05), and average isokinetic hip extension torque (F(1,28)=37.90, p<

0.05). For the isometric tests, there was significance between joints for both flexion

(F(1,28)=86.15, p< 0.05) and extension (F(1,28)=66.58, p< 0.05).

When reviewing the results, significant interactions or main effects were separated into homogenous groups by letters. If one of the bars within the graph has a different letter than the other, then there is a significant difference between the two values. For example, group A is significantly different from group B. In the case of a double letter, that means that the specific group is not significantly different from either of the two groups it represents, but those two groups are significantly different from each other. For example, group AB would not be significantly different from either group A or B, but there is a significant difference between groups A and B when compared to each other.

For all post hoc comparisons of isokinetic tests, trunk strength was significantly different between the different test speeds. In Figure 1, A was greater than B (194.7336 +/- 2.2761,

161.1233 +/- 2.2761, respectively, d=0.6651) and B was greater than C (161.1233 +/- 2.2761,

126.0352 +/- 2.2761, respectively, d=0.6581). In Figure 2, A was greater than B (144.1215 +/-

2.035, 122.4152 +/- 2.035, respectively, d=0.563) and B was greater than C (122.4152 +/- 2.035, 13

98.0786 +/- 2.035, respectively, d=0.6312). In Figure 3, A was greater than B (285.5586 +/-

4.6505, 226.2966 +/- 4.6505, respectively, d=0.8745) and B was greater than C (226.2966 +/-

4.6505, 157.1237 +/- 4.6505, respectively, d=1.0208). In Figure 4, A was greater than B

(224.046 +/- 3.8831, 172.7967 +/- 3.8831, respectively, d=0.9869) and B was greater than C

(172.7967 +/- 3.8831, 125.6149 +/- 3.8831, respectively, d=0.9086).

Figure 1: Peak Trunk vs. Hip Isokinetic Flexion Torque

Figure 2: Average Trunk vs. Hip Isokinetic Flexion Torque For post hoc comparisons of peak isokinetic extension torque seen in Figure 3, hip strength was significantly different from trunk strength at 60 and 120 deg/s [A was greater than

C (285.5586 +/- 4.6505, 157.1237 +/- 4.6505, respectively, d=1.8953) and B was greater than C

(226.2966 +/- 4.6505, 157.1237 +/- 4.6505, respectively, d=1.0208)], but there was no significant difference between hip and trunk strength at 180 deg/s as well as hip strength at the different speeds. The same relationship can be seen for the post hoc comparison of average isokinetic extension torque in Figure 4 [A was greater than C (224.046 +/- 3.8831, 125.6149 +/-

3.8831, respectively, d=1.8955) and B was greater than C (172.7967 +/- 3.8831, 125.6149 +/-

3.8831, respectively, d=0.9086)].

Figure 3: Peak Trunk vs. Hip Isokinetic Extension Torque

Figure 4: Average Trunk vs. Hip Isokinetic Extension Torque

For post hoc comparisons of peak isokinetic flexion torque, hip strength was significantly different when compared to the trunk at all testing speeds [A was greater than D (194.7336 +/-

2.2761, 92.7774 +/- 2.2761, respectively, d=2.0175), A was greater than E (194.7336 +/- 2.2761,

78.0399 +/- 2.2761, respectively, d=2.3091), B was greater than D (161.1233 +/- 2.2761,

92.7774 +/- 2.2761, respectively, d=1.3524), B was greater than E (161.1233 +/- 2.2761, 78.0399

+/- 2.2761, respectively, d=1.644), C was greater than D (126.0352 +/- 2.2761, 92.7774 +/-

2.2761, respectively, d=0.6581), and C was greater than E (126.0352 +/- 2.2761, 78.0399 +/-

2.2761, respectively, d=0.9497)]. Hip strength was also significantly different at 60 deg/s when compared to 120 and 180 deg/s for peak isokinetic flexion torque [D was greater than E (92.7774

+/- 2.2761, 78.0399 +/- 2.2761, respectively, d=0.2916)].

When looking at post hoc comparisons for average isokinetic flexion torque, hip strength was significantly different when compared to trunk strength at all testing speeds [A was greater than D (144.1215 +/- 2.035, 65.3089 +/- 2.035, respectively, d=2.0442), A was greater than DE

(144.1215 +/- 2.035, 57.6215 +/- 2.035, respectively, d=2.2436), A was greater than E (144.1215

+/- 2.035, 48.3885 +/- 2.035, respectively, d=2.4831), B was greater than D (122.4152 +/- 2.035,

65.3089 +/- 2.035, respectively, d=1.4812), B was greater than DE (122.4152 +/- 2.035, 57.6215

+/- 2.035, respectively, d=1.6806), B was greater than E (122.4152 +/- 2.035, 48.3885 +/- 2.035, respectively, d=1.9201), C was greater than D (98.0786 +/- 2.035, 65.3089 +/- 2.035, respectively, d=0.8499), C was greater than DE (98.0786 +/- 2.035, 57.6215 +/- 2.035, respectively, d=1.0494), and C was greater than E (98.0786 +/- 2.035, 48.3885 +/- 2.035, respectively, d=1.2889)]. Hip strength at 60 deg/s was significantly different than hip strength at

180 deg/s [D was greater than E (65.3089 +/- 2.035, 48.3885 +/- 2.035, respectively, d=0.4389)], but hip strength at 120 deg/s was not significantly different than hip strength at 60 or 180 deg/s. 17

When looking at the post hoc comparison for peak isometric flexion torque in Figure 5, trunk strength is significantly different when compared to hip strength [A was greater than B

(216.0196 +/- 5.4007, 119.8934 +/- 5.4007, respectively, d=1.4142)]. Findings were the same when looking at the post hoc comparison for peak isometric extension torque in Figure 6 [A was greater than B (360.114 +/- 11.74, 176.4438 +/- 11.74, respectively, d=1.4142)].

Throughout the data, there were medium to large effect sizes, meaning there was a significantly large difference in the results found within the data set. This also helps us to determine that the sample size for this research was adequate enough to determine significance that can be present within the target population.

Figure 5: Peak Trunk vs. Hip Isometric Flexion Torque 18

Figure 6: Peak Trunk vs. Hip Isometric Extension Torque

Discussion

The purpose of this study was to differentiate between the strength of the hip and trunk joints during flexion and extension. Hip and trunk joint was tested using the Biodex Isokinetic

Dynamometer isometrically and isokinetically at varying contraction speeds. The data gathered partially supports our hypothesis that trunk joint strength would be significantly greater than hip joint strength. Unless moving at high speeds into extension, the trunk and hip joints are very different when comparing peak and average torque. The trunk was significantly different across all speeds when compared to the hip during flexion, but the trunk was not significantly different than the hip when moving into extension at 180 deg/s.

Trunk joint strength was significantly greater than hip joint strength in both flexion and extension when observing peak torque during an isometric contraction. Trunk joint strength was significantly greater when comparing all isokinetic tests at 60 deg/s to 120 deg/s and when comparing 120 deg/s to 180 deg/s. Trunk joint strength was also significantly greater than hip joint strength at all contraction speeds when observing peak and average torque in flexion.

Interestingly, trunk joint strength at 180 deg/s was not significantly greater than hip joint strength at 60, 120, and 180 deg/s during isokinetic extension tests.

Hip joint strength was significantly greater at 60 deg/s when compared to 120 and 180 deg/s during peak isokinetic flexion testing. When observing average isokinetic flexion, hip joint strength at 60 deg/s was significantly greater than hip joint strength at 180 deg/s. Hip joint strength at 120 deg/s of the same testing condition was not significantly different from either 60 or 180 deg/s. Hip joint strength was not significantly different between contraction speeds for either peak or average isokinetic extension tests. 20

Grabiner et al. performed isokinetic trunk flexion and extension testing on 10 healthy women at 60, 120, and 180 deg/s.26 When comparing peak torque of their sample to ours, we observe that our numbers are much higher. Woodhouse et al. also performed isokinetic trunk flexion and extension testing at speeds of 60 and 120 deg/s on 10 “well-conditioned” men.27

When comparing results, our peak torque measures were slightly lesser than their numbers.

These differences are most likely due to the differences in samples between the studies. For our study, we used both male and female collegiate athletes who would be stronger than sedentary, healthy females. Due to the female athletes in our sample, we would expect our numbers to be slightly lesser than the results from the results of physically active males. Waldhelm et al. performed isometric trunk flexion and extension testing on a sample of 15 active college-aged males, and our isometric results were much higher.28 This is most likely due to fact that our sample contained collegiate athletes who would train harder and more frequently than a regular active male and the greater sample size.

When comparing peak torque measures from our study to other research, our numbers seem to be less than others. Nesser et al conducted a study testing hip flexion and extension peak torque in 20 young men involved in sprint-type sports such as football, baseball, and lacrosse.29

Their measures of peak torque were much greater than ours. Dowson et al. also had greater numbers of peak torque in hip flexion and extension with a sample of 8 male rugby players, 8 male track sprinters, and 8 male active, competitive sportsmen.19 Similarly, their results were much greater than ours. This is most likely due to the higher number of female athletes that participated in our study. Claiborne et al. conducted a study of peak torque hip flexion and extension at 60 deg/s with 15 male and 15 female participants.21 When comparing our results to this study, our hip flexion results fell between the average peak torque for men and women, but 21 our results of hip flexion were just under their results for women. This difference is most likely due to more females being included in our study.

Based on our findings, the muscles of the trunk joint and hip joint are significantly different when flexing, but, when moving at higher speeds into extension, they act similarly. We also found that, as speed increased, there was a significant decrease in peak and average torque in the trunk, but there was no significant difference in peak or average torque as speed increased at the hip. This was most likely due to the fact that the muscles of the trunk act more in maintain posture and stability while the muscles of the hip are constantly used for prime movements involved in walking, running, and jumping. It is important to make the differentiation between trunk and hip musculature because these muscles act very different when observing strength in flexion and extension at varying contraction speeds. Therefore, researchers should be careful when grouping the muscles surrounding these two joints into the “core”.

This study was limited to a specific population of collegiate athletes at a single university in southern Georgia. Therefore, this study cannot be generalized to a specific population.

Although our sample size may have seemed to limit our study, our effects sizes were large, meaning that we were still able to find large significant differences within our data. This study was limited to only select sports within the university because some athletes from other sports such as basketball and softball did not wish to participate. This study is also limited because we did not examine differences between genders or sports. Future studies could expand the sample to include other sports, recreationally active subjects, or normal subjects. Future studies could also look at the difference between the muscular endurance of the muscles of the trunk and hip to see if there is a similar pattern that we observed in this study.

Conclusion

The muscles of the trunk joint and hip joint are significantly different when flexing, but, when moving at higher speeds into extension, they act similarly. We also found that, as speed increased, there was a significant decrease in peak and average torque in the trunk, but there was no significant difference in peak or average torque as speed increased at the hip. This was most likely due to the fact that the muscles of the trunk act more in maintain posture and stability while the muscles of the hip are constantly used for prime movements involved in walking, running, and jumping. When studying the core, researchers should be careful about how to define the “core” and how to test the strength of the core because some tests may focus more on trunk or hip musculature. The trunk and hip joints act differently in flexion and extension, and grouping the muscles of these two joints together can cause confusion when determining how to test or strengthen the actual “core”. Researchers must be able to clearly define, measure, and interpret the difference between trunk and hip musculature.

APPENDIX A RESEARCH OUTLINE

Limitations: • The study cannot be generalized to a specific population. • The study only represents collegiate athletes. • The study did not examine differences between genders or sports.

Delimitations: • Only collegiate athletes from Georgia Southern University.

Assumptions: • Athletes will give maximum effort during strength testing.

Research Questions: • What is the relationship between hip and trunk joint strength in both flexion and extension? • How does the increase in contraction speed affect peak and average torque?

Hypotheses: • The trunk joint will be significantly stronger than the hip joint in both flexion and extension. • Peak and average torque will decrease with increased contraction speed for both the trunk and hip for all isokinetic tests.

Operational Definitions: • The trunk joint is defined as the angle of movement between the pelvis and rib cage. • The hip joint is defined as the angle of movement between the pelvis and the femur.

APPENDIX B REVIEW OF LITERATURE

Research Question: What are the effects of hip flexor strains on hip and trunk flexion/extension strength? How does the increase in contraction speed affect peak and average torque?

Anterior Hip Musculature

The muscles of the anterior hip are comprised of the quadriceps femoris muscle, which consists of the rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis, and the iliopsoas muscle, which consists of the psoas major and the iliacus muscles, and the sartorius.30

These muscles play a key role in almost all human movements in order to help support body weight during walking, running, and jumping.31

The proximal rectus femoris has two tendinous origins: the direct (straight) head, arising from the anterior–inferior iliac spine, and the indirect (reflected) head, arising slightly more inferiorly and posteriorly from the superior acetabular ridge and hip joint capsule. The two heads form a conjoined tendon a few centimeters below their origins.32 The direct head, making up most of the superficial component of the conjoined tendon, blends more distally with the anterior fascia of the rectus femoris. The indirect head, forming most of the posterior component of the conjoined tendon, becomes intrasubstance and forms a long, deep musculotendinous junction extending approximately two thirds of the length of the muscle.32 The rectus femoris extends the knee in conjunction with the other three muscles of the quadriceps femoris, but it also works to steady the hip joint and flex the hip in conjunction with the iliopsoas. The iliacus muscle originates off of the superior two thirds of the iliac fossa, the ala of sacrum, and the anterior sacroiliac ligaments and inserts into the lesser trochanter of the femur and the psoas 25 major tendon. The psoas major originates on the transverse processes of the lumbar vertebrae, and the lateral sides of the T12-L5 vertebrae and inserts into the lesser trochanter. These two muscles are generally referred to as the iliopsoas since they act together in order to flex the hip and stabilize the trunk.30

According to Paschalis et al., better motor recruitment is associated with greater torque outputs of muscle, which will bring the activated muscle closer to its damage limit.33 Muscle fatigue is heavily dependent on the intensity of muscle contraction.34 Muscle fatigue is closely related to the firing pattern of recruited motor units within each muscle35 and the activation strategy among synergistic muscles within a given muscle group. 36–39 The rectus femoris muscle is known to be a commonly injured muscle during athletic activities40 and is sometimes considered an “at-risk” muscle,41,42 which could be due to the rectus being easily fatigued compared to the other quadriceps femoris muscles.34,37,43,44

Sahrmann et al determined that increased anterior gliding of the femoral head results from weakness or decreased utilization of the gluteal muscles during hip extension and the iliopsoas muscles during hip flexion.45 This increased anterior gliding can lead to increased anterior hip joint forces, hip instability, and anterior hip pain.45 The rectus femoris muscle has a lot in common with its bi-articular antagonist, the biceps femoris, which is part of the hamstring muscle group. The actions of these bi-articular muscles contribute to power transfer actions from proximal to distal joints during explosive leg extensions.46 This power transfer allows high power output at distal joints despite the smaller distal muscles. Activation of rectus femoris was proposed to decrease the angular acceleration of the trunk, which, combined with the onset of knee extension, ensued in power being transferred from hip to knee joint.47,48 The biceps femoris transfers a certain amount of energy between the knee and hip joints in the opposite direction 26 than energy transferred through the rectus femoris muscle.46 Both of these muscles also help distal muscles to dissipate the mechanical energy of the body during the shock-absorbing phase of movement.46 This reciprocal activation between hamstrings and rectus femoris muscles was found specifically in observations of sprinting.48 During gait, the rectus femoral and iliac muscles are very active with swing-phase hip flexion, while the hamstring muscles act eccentrically to control hip flexion and decelerate knee extension.49

The kinetic chain is defined as a combination of several successively arranged joints constituting a complex motor unit.50 In other words, activation of one muscle creates a movement that, in turn, creates a chain of events that will affect the movement of joints and muscles that are in the surrounding region of the body. Along the kinetic chain of the lower extremity, the gluteus muscles can affect the muscles of the anterior and posterior thigh. An

American Academy of Orthopaedic Surgery conference consensus statement said that hip position and motion influence knee position, loads, and stiffness. The moment that is developed at the knee is likely to be strongly influenced by the moment at the hip, suggesting that the gluteal muscles help facilitate the activation of the bi-articular knee musculature (the rectus femoris and the biceps femoris).51 Decreased eccentric gluteal activation and increased quadriceps activity can contribute to altered kinetic energy absorption during landing activities.52

Trunk/Core

According to Borguis et al., the center of the functional kinetic chain is the core.1 The core is seen as a muscular corset that works as a unit to stabilize the body, particularly the spine, both with and without limb movement.3 Core stability is typically defined as the ability to control the position and motion of the trunk over the pelvis and leg to allow optimum production, 27 transfer, and control of force and motion to the terminal segment in integrated kinetic chain activities.7 Stability of the core is related to body’s ability to control the trunk in response to internal and external disturbances generated from the distal body segments.53 When moving distal extremities, reactive forces are imposed on the spine parallel to the action and opposing the movement.54 Due to its multi-segmental nature and the requirement for muscle contraction to provide stability of the spine, the spine is particularly prone to the effect of these reactive forces.

This indicates the importance of muscular control of the trunk during limb movement.54

There are multiple factors included in the research behind core stability. For optimal core stability, both the smaller, deeper core muscles and the larger, superficial core muscles must contract in sequence with appropriate timing and tension.10,11 Most dynamic stability of the body, or any specific joint, depends on neuromuscular control of the displacement of all contributing body segments during movement.53 The muscle actions must be precisely coordinated to occur at the right time, for the correct duration and with the right combination of forces.55 Imbalanced muscle activation can lead to inappropriate magnitudes of muscle force and stiffness, thereby loading the spine incorrectly and inducing low back pain and musculoskeletal injury, especially in regards to the trunk flexor-to-extensor strength ratio.10,56 An inefficient neuromuscular system may not adapt well to demands of the distal extremities, resulting in impaired performance or even injury.57 Because the core is considered the center of the kinetic chain, neuromuscular control of core muscles is very important in creating proximal stability for distal mobility during throwing, kicking, and running activities seen in all sports.8,58

Some researchers believe the “core” involves musculature of the lumbopelvic-hip complex, which involves deeper muscles, such as the internal oblique, transversus abdominis, transversospinalis (multifidus, rotatores, semispinalis), quadratus lumborum, and psoas major and 28 minor, and superficial muscles, such as the rectus abdominis, external oblique, erector spinae

(iliocostalis, spinalis, longissimus), latissimus dorsi, gluteus maximus and medius, hamstrings, and rectus femoris (Table 1).4–6 Other researchers believe the core is a muscular corset about the trunk that works as a unit to stabilize the body, particularly the spine, both with and without movement.2,3

The muscles of the trunk and pelvis are responsible for the maintenance of stability of the spine and pelvis and help in the generation and transfer of energy from large to small body parts during many sports activities.8,9 The muscles and joints of the hip, pelvis, and spine are centrally located to be able to perform many of the stabilizing functions that the body will require in order for the distal segments to do their specific function.7

Table 1:

Muscle Origin Insertion Innervation Action External Oblique External surfaces of Linea alba, pubic Thoracoabdominal Compresses and 5th-12th ribs tubercle, and anterior nerves (T7-T11 spinal supports abdominal half of iliac crest nerves) and subcostal viscera, flexes and nerve rotates trunk Internal Oblique Thoracolumbar Inferior borders of Thoracoabdominal Compresses and fascia, anterior two 10th-12th ribs, linea nerves (anterior rami of supports abdominal thirds of iliac crest, alba, and pecten T6-T12 spinal nerves) viscera, flexes and and connective deep pubis via conjoint and first lumbar nerves rotates trunk to lateral third of tendon inguinal ligament Tranversus Abdominis Internal surfaces of Linea alba with Thoracoabdominal Compresses and 7th-12th costal aponeurosis of nerves (anterior rami of supports abdominal cartilages, internal oblique, T6-T12 spinal nerves) viscera thoracolumbar fascia, pibuc crest, and and first lumbar nerves iliac crest, and pecten pubis via connective tissue conjoint tendon deep to lateral third of inguinal ligament Rectus Abdominis Pubic symphysis and Xiphoid process and Thoracoabdominal Flexes trunk (lumbar pubic crest 5th-7th costal nerves (anterior rami of vertebrae), and cartilages T6-T12 spinal nerves) compresses abdominal viscera; stabilizes and controls tilt of pelvis (antilordosis) Psoas major Transverse processes By a strong tendon to Anterior rami of lumbar Acting inferiorly of lumbar vertebrae; lesser trochanter of nerves L1-L3 with iliacus, it flexes sides of bodies of femur the thigh; acting T12-L5 vertebrae and superiorly it flexes intervening vertebral column intervertebral discs laterally; it is used to 29

balance the trunk, when sitting it acts inferiorly with iliacus to flex trunk Psoas Minor Sides of T12-L1 Pectineal line, Anterior rami of lumbar Act in flexing thigh vertebrae and iliopectineal nerves (L1, L2) at hip joint and in intervertebral discs eminence via stabilizing this joint iliopectineal arch Iliacus Superior two thirds of Lesser trochanter of Femoral nerve Flexes thigh and iliac fossa, ala of femur and shaft stabilizes hip joint; sacrum, and anterior inferior to it, and to acts with psoas sacroiliac ligaments psoas major tendon major Quadratrus Lumborum Medial half of Iliolumbar ligament Anterior branches of Extends and laterally inferior border of 12th and internal lip of T12 and L1-L4 nerves flexes vertebral ribs and tips of iliac crest column; fixes 12th rib lumbar transverse during inspiration processes Iliocostalis Arises by a broad Lumborum, thoracis, Posterior rami of spinal Bilaterally: extend tendon from posterior cervicis; fibers run nerves vertebral column and part of iliac crest, superiorly to angles head; as back is posterior surface of of lower ribs and flexed, control sacrum, sacroiliac cervical transverse movement via ligaments, sacral and processes eccentric contraction Longissimus inferior lumbar Thoracis, cervicis, Posterior rami of spinal spinous processes, capitis; fibers run nerves Acting unilaterally: and supraspinous superiorly to ribs laterally flex ligament between tubercles vertebral column and angles to transverse processes in thoracic and cervical regions, and to mastoid process of temporal bone Spinalis Thoracis, cervicis, Posterior rami of spinal capitis; fibers run nerves superiorly to spinous processes in the upper thoracic region and to cranium Transversospinalis Transverse processes Spinous processes of Posterior rami of spinal Extension more superior nerves vertebrae Semispinalis Arises from Thoracis, cervicis, Posterior rami of spinal Extends head and transverse processes capitis; fibers run nerves thoracic and cervical of C4-T12 vertebrae superomedially to regions of vertebral occipital bone and column and rotates spinous processes in them contralaterally thoracic and cervical regions, spanning 4-6 segments Multifidus Arises from posterior Thickest in lumbar Posterior rami of spinal Stabilizes vertebrae sacrum, posterior region; fibers pass nerves during local superior iliac spine of obliquely movements of ilium, aponeurosis of supermedially to vertebral column erector spinae, entire length of sacroiliac ligaments, spinous processes, mammillary located 2-4 segments processes of lumbar superior to proximal vertebrae, transverse attachment processes of T1-T3, articular processes of C4-C7 30

Rotatores (brevis and Arise from transverse Fibers pass Posterior rami of spinal Stabilize vertebrae longus) processes of superomedially to nerves and assist with local vertebrae; best attach to junction of extension and developed in thoracic lamina and rotatory movements region transverse process or of vertebral column; spinous process of may function as vertebra immediately organs of (brevis) or 2 proprioception segments (longus) superior to vertebra of attachment Latissimus Dorsi Spinous processes of Floor in Thoracodorsal nerve Extends, adducts, inferior 6 thoracic intertubercular sulcus (C6-C8) and medially rotates vertebrae, of humerus humerus; raises body thoracolumbar fascia, towards arms during iliac crest, and climbing inferior 3-4 ribs Rectus Femoris Anterior inferior iliac Via common Femoral nerve Extend leg at knee spine and ilim tendinous joint; steadies hip superior to (quadriceps tendon) joint and helps acetabulum and independent iliopsoas flex thigh Vastus Lateralis Greater torochanter attachments to base Femoral nerve Entends leg at knee and lateral lip of linea of patella; indirectly joint aspera of femur via patellar ligament Vastus Medialis Intertrochanteric line to tibial tuberosity; Femoral nerve and medial lip of medial and lateral linea aspera of femur vasti also attach to Vastus Intermedius Anterior and lateral tibia and patella via Femoral nerve surfaces of shaft of aponeuroses (medial femur and lateral patellar retinacula) Adductor longus Body of pubis Middle third of linea Obturator nerve, branch Adducts thigh inferior to pubic crest aspera of femur of, anterior division Adductor brevis Body and inferior Pectineal line and (L2, L3, L4) Adducts thigh; to ramus of pubis proximal part of some extent flexes it linea aspera of femur Adductor magnus Adductor part: Adductor part: Adductor part: obturator Adducts thigh; inferior ramus of gluteal tuberosity, nerve (L2-L4), branches pubis, ramus of linea aspera, medial of posterior division Adductor part: flexes iscium supracondylar line thigh Hamstring part: tibial Hamstrings part: Hamstring part: part of sciatic nerve Hamstring part: ischial tuberosity adductor tubercle of extends thigh femur Gluteus Maximus Ilium posterior to Most fibers end in Inferior gluteal nerve Extends thigh posterior gluteal line; iliotibial tract, which (L5, S1, S2) (especially from dorsal surface of inserts into lateral flexed position) and sacrum and coccyx; condyle of tibia; assists in its lateral sacrotuberous some fibers insert on rotation; steadies ligament gluteal tuberosity thigh and assists in in rising from sitting position Gluteus Medius External surface of Lateral surface of Superior gluteal nerve Abduct and medially ilium between greater trochanter of (L5, S1) rotate thigh; keep anterior and posterior femur pelvis level when gluteal lines ipsilateral limb is Gluteus Minimus External surface of Anterior surface of weight bearing and ilium between greater trochanter of advance opposite anterior and inferior femur (unsupported) side gluteal lines during its swing phase 31

Semitendinosus Ischial tuberosity Medial surface of Tibial division of sciatic Extends thigh; flex superior part of tibia nerve part of tibia (L5, leg and rotate it Semimembranosus Ischial tuberosity Posterior part of S1, S2) medially when knee medial condyle of is flexed; when thigh tibia; reflected and leg are flexed, attachment forms these muscles can oblique popliteal extend trunk ligament (to lateral femoral condyle) Biceps Femoris Long head: ischial Lateral side of head Long head: Tibial Flexes leg and tuberosity of fibula; tendon is division of sciatic nerve rotates it laterally split at this site by part of tibia (L5, S1, S2) when knee is flexed; Short head: linea fibular collateral extends thigh aspera and lateral ligament of knee Short head: common (accelerating mass supracondylar line of fibular division of during first step of femur sciatic nerve (L5, S1, gait) S2)

Stabilization of the trunk is crucial for maintaining static or dynamic balance, especially to provide a solid base when attempting to exert forces upon external objects, which forms the base of success in the majority of sports and it is a necessity in the activities of daily living.8 The position, motion, and contributions of the core are also important when evaluating and treating extremity injuries since core activity is heavily involved with almost all extremity activities such as running, kicking, and throwing.7 The muscles of the trunk are activated first when completing athletic lower extremity activities and act as muscular constraints to minimize the degrees of freedom within one or several joints in order to stabilize excessive mobility of the extremities.57

Hip musculature plays an important role in the kinetic chain in regards to walking and running activities as well as in stabilization of the trunk and pelvis when transferring forces from the lower extremities to the pelvis and spine.3 Strength in both core and hip musculature is especially important for athletes because the wide variety of movements associated with various sport activities requires sufficient strength in these muscle groups in order to provide stability in all three planes of motion.59 Decreased proprioception and increased instability of the core at the trunk may contribute to decreased active neuromuscular control of the lower extremity, which may lead to lower extremity instability and faulty alignment during athletic movements.59,60 32

Some researchers believe that decreased core stability may predispose to injury and that appropriate training may reduce injury suggested to contribute to the etiology of lower extremity injuries.61 The influence of proximal stability on lower extremity structure and pathology remains largely unknown.59 The relationship between relative movement and muscle activity of the thigh, hip, and lumbar spine during sagittal flexion and extension is also unknown.62

Trunk vs Hip

Most of the prime mover muscles for the lower extremity such as the hamstrings, quadriceps, and iliopsoas muscles attach in the similar anatomical areas as the trunk musculature, leading clinicians to believe that the hip extensors and posterior hip muscles influence the lower back.63 Some researchers include the hip extensor and knee flexor muscles in the definition of the core. Considering the wide variety of movements associated with athletics, athletes must possess sufficient strength in hip and trunk muscles to provide stability in all three planes of motion.59 Neuromuscular control of the trunk59,64 and hip musculature65–67 also plays an important role in lower extremity mechanics. An alteration in the kinetic chain such as inflexibility of the hip flexors, hamstrings or rectus femoris, leg length discrepancy, and muscular adaptations68–73 can lead to injury of the low back or the lower extremities.

The primary contribution of the active muscular elements of the core to the stability of the lumbo-pelvic-hip complex is to increase the stiffness of the hip and trunk. Co-contraction of antagonistic trunk muscles occurs both in preparation for and in response to spinal loading.15

However, in the absence of spinal loading or anticipated spinal loading, the muscles that increase stiffness of the hip and trunk are relatively inactive, and the stability of the system rests largely on passive elements.74 Using muscles in the hip and trunk to increase core stiffness must be 33 highly coordinated to balance the demands of the intended physical task while limiting excessive loading.15 Trunk muscle activity before the activity of the lower extremities occurs in order to stiffen the spine to provide a foundation for functional movements.75 Rapid fatigue of the trunk musculature and low back dysfunction have been related to impaired neuromuscular control of the body’s center of mass, inhibition of lower extremity muscles, and elevated risk for lower extremity injury.53,76–79

Typically, the transverse abdominus activates prior to initial movement of a lower limb in order to prepare the body for the disturbance that is produced by the movement.80 Bouisset et al. suggested that motor activity in the form of postural support must occur before the initiation of voluntary extremity movements.81 It is possible that individuals with delayed trunk muscle response to perturbation have greater potential for core instability and may be at greater risk for chronic low back pain.80,81 According to Nadler et al., injury migrates from distal to proximal structures and, conversely, low back pain and abnormal strength or neuromuscular control of the core can increase the likelihood of distal injury.78

Plenty of research supports the relationship between the musculature of the hip and lower extremity injuries to core stability at the trunk. The hip extensors and abductors play a major role in all ambulatory activities, stabilizing the trunk and hip and helping to transfer force from the lower extremities to the pelvis.82,83 The gluteus maximus plays a major role in stabilizing the pelvis during trunk rotation or when the center of gravity is grossly shifted, while the hamstrings play a more significant role during activities such as running or jumping.82,83 Kankaanpaa et al. and Leinonen et al. demonstrated poor endurance of the gluteus maximus in those with chronic low back pain.62,84 In some cases of low back pain, the biceps femoris will compensate when the gluteal muscles are not strong enough. If the hamstrings are tight, it can cause the subject to have 34 postural shortening and increased low back pain.85 Poor endurance and delayed firing of the hip extensor and abductor muscles has previously been noted in individuals with chronic low back pain or lower extremity instability.84,86–89 Devlin et al. also reported that fatigue of the abdominals was a contributing factor in hamstring injuries.90 A gap in the literature occurs when researching the relationship between the anterior hip musculature and the core.

With regard to muscular influences on low back pain, the hip musculature plays a significant role in transferring forces from the lower extremity up toward the spine during upright activities and thus theoretically may influence the development of low back pain.83,91

Conversely, the trunk musculature, as well as proximal thigh muscles, can have an influence on lower extremity injuries. Because the pelvis is the origin attachment site for the hamstring muscles, it has been suggested that neuromuscular control of the lumbopelvic region, including anterior and posterior pelvic tilt, is needed to create optimal function of the hamstrings in sprinting and high-speed skilled movement.92 Changes in pelvic position could lead to changes in length-tension relationships or force-velocity relationships, such as an excessive lordotic curve in the lumbar spine.92,93 This positioning puts the hamstring in a mechanically disadvantaged position.94

Core Strength

Core strengthening exercises are very popular within rehabilitation and strength training programs despite little evidence existing as to the efficacy on improving subsequent performance for athletes after injury or during the competitive season.12 When strengthening the core, the three most important exercises include the curl up, the side bridge, and the leg/arm extension.2

For the purposes of this study, the curl up and the leg/arm will be discussed. 35

During the curl up, the rectus abdominis is the most active during the initial elevation of the head, neck, and shoulders while maintaining a neutral position of the spine.2 As the shoulders are raised off of the ground, there is a significant increase in hip flexor activation.95

The leg/arm extension exercise is done in a hands-and-knees position, eventually leading to the bird-dog exercise where one arm is extended at the same time as the opposite leg while maintaining a neutral spine.2 Ekstrom et al. found that, during this specific exercise, the hamstring muscles were significantly activated as well as the trunk extensor muscles.96

Most research typically uses the McGill protocol, which consists of four different isometric holds to test trunk flexors, trunk extensors, and lateral musculature.13 Although this testing protocol is popular among researchers, it tests muscular endurance rather than actual strength of the trunk musculature. Over the past 30 years, isokinetic strength testing of the core has gained popularity. Isokinetic testing is considered unique compared to other testing methods because it measures muscle torque at constantly changing angles and associated muscle moment arms, which is presumed to more closely represent a dynamic spinal loading event.15

Research has also found that, as the speed of contraction increases for different joints and muscle groups, peak and average torque decreases.16,17 This is most likely due to the fact that the dynamometer can only register external torque when the machine’s lever arm can be made to exceed the preselected machine speed. If the subject is unable to accelerate his or her limb to a speed greater than the machine speed, the machine’s internal load cell records zero torque being produced. This recording does not mean that the contracting muscle is generating zero force, but rather that all of the resultant force is being transformed into limb movement. As the machine’s speed setting is progressively increased, more of the muscle’s force production goes into generating limb speed and less is responsible for producing externally measured torque.16 Very 36 little research reports on whether or not this same pattern of decreased torque with faster contraction speeds exists at the hip or trunk.

Trunk and Hip during Athletic Activities

It has been suggested that precise control of the trunk position and motion over the pelvis could optimize the energy transfer in the kinetic chains from torso to extremities for performing athletic activities composed of highly loaded movements.97 This necessary body control depends on the output of the musculature that mainly function at the lumbo-pelvic-hip region as well as the proximal lower limbs.1 The core is actively involved in providing stiffness that helps stabilize running form, and maximize the kinetic chains of upper and lower extremity function during running activities.97,98 This critical role of core function in the kinetic chain may help to explain possible limitations of athletic performance in sports activities if the athlete has some sort of dysfunction in core stability. There is also a possibility that global function of the core is being reduced due to fatigue during intense running.1

Both the hip flexors and extensors are responsible for increased power generation in running and sprinting.99 As gait speed increases, the pelvis and trunk tilt further forward and the center of mass is lowered, maximizing the horizontal force produced in the propulsion phase.99

The position and acceleration of center of mass determines the direction and magnitude of the ground reaction forces produced.99 With an increase in speed also comes an increase in power and magnitude of joint moments.99,100 The relative contribution from the different muscle groups changes so that more power is generated from the proximal structures.99 The increase in pace also results in an increase in the eccentric activity of the muscles at work, causing these structures to withstand more rapid and severe lengthening.101 37

Biodex

When testing the strength of muscles within the body using an isokinetic dynamometer, there are multiple factors that can exert a major effect on the data that the researcher is trying to collect. These include positioning and stabilization of the subjects' body, testing posture performed by subjects, velocity of test movements, and types of contraction modes.102 Torque is usually defined as the force measured about a joint’s axis of rotation, and peak torque is the point in the range of motion tested at which the greatest torque is produced.103 The tool that will be used to conduct research for this project is the Biodex Isokinetic Dynamometer. In order to test rectus femoris strength compared to the biceps femoris, patients will be tested in hip flexion and extension isometrically and isotonically at varying degrees of knee flexion. Isometric and isotonic strength of the trunk will also be tested in flexion and extension.

In certain tasks of daily life, such as lifting up an object, trunk extensors should be contracting concentrically, and trunk flexors eccentrically. These contractions are vice versa for putting an object down.102 Eccentric muscular contraction of the trunk plays a significant role in the activities of daily living and exercise, for instance, in deceleration of the body during walking, running, and lifting.104–108 Based on these co-contractions that occur with most activities of daily living and the increased eccentric activity during walking, running and lifting, the relationship between concentric and eccentric torque is important in evaluating the muscle strength of the trunk.104,109

For trunk flexion and extension testing, the anterior superior iliac spine will be used as the bony landmark for matching the axis of rotation against the dynamometer lever arm.26

Subjects will be positioned supine in the dynamometer with the knees slightly flexed with the lower extremity appropriately strapped down in order to make sure these muscles do not 38 influence testing.18,102 After a proper warm up lasting between ten and fifteen minutes, subjects will be tested at isokinetic speeds of 60, 120, and 1 80°/sec through a standardized maximum range of motion of 100°.18,22,26,102 Subjects will have the opportunity to complete two to three sub-maximal and maximal familiarization repetitions before actual testing.21 Concentric and eccentric contractions will be measured at each speed for three repetitions with a minimum of one minute rest between sets.102 Ten minutes recovery will separate the different joint actions.103

For isokinetic testing at the hip, the greater trochanter was used as the bony landmark for matching the axis of rotation of the hip joint with the axis of the dynamometer lever arm.20 Each set of three contractions will be reciprocal, such that motion in one direction will immediately be followed by motion in the opposite direction.21 Subjects will be strapped into the attachment with a resistance pad fixed at the distal end of the thigh.110 They will also have a stable surface to hold on to in order to support the rest of the body during testing. Range of motion for testing will be set from maximal hip extension to 120° of flexion.110 Subjects will complete three repetitions at

60, 120, and 180°/sec.22,29,103

According to Drouin et al., the Biodex System 3 isokinetic dynamometer is a mechanically reliable instrument for the valid measurement of angular position, isometric torque and slow to moderately high velocities les than 300° /s in comparison to previous reports of mechanical reliability and validity of isokinetic dynamometry.111–113 Zawadzki et al. also found the Biodex System 3 allows valid measurements of muscular torque under static conditions and that the results obtained can be used in scientific analyses.114 In general, reliability coefficients above 0.75 are indicative of good reliability.115 Cahalan et al. reported a high intraclass correlation coefficient (ICC) of 0.96 for standing isokinetic concentric and eccentric hip flexion 39 and extension.23 Karatas et al. found an ICC value of 0.89-0.95 for peak torque of the trunk flexors and 0.80-0.92 for the trunk extensors.116

Plenty of research is available regarding the topic of core stability and the posterior, medial, and lateral musculature as well as the relationship between hip extensor muscles and low back pain. Sherry and Best were also able to find a connection between core stability and hamstring strains.92 The rectus femoris is the antagonist to the biceps femoris, and these two muscles work together during gait and athletic activities such as running and jumping. Because both of these muscles have a high injury rate during sprinting, and there is a connection between the stability of the core with hamstring strains, then it can inferred that core stability has some sort of connection to the rectus femoris. There is limited research that actually studies the relationship between core stability and the rectus femoris, especially after an injury.

APPENDIX C IRB

COLLEGE OF Health and Human Sciences

DEPARTMENT OF Health and Kinesiology

CONSENT TO ACT AS A SUBJECT IN AN EXPERIMENTAL STUDY

Title of Project: The Effect of Hip Flexor Strains on Hip and Trunk Flexion/Extension Strength

Investigator’s Name: Jasmin Brown, ATC, LAT Phone: (404)-432-5021

Participant’s Name:______Date:______

Data Collection will be in the Biomechanics Laboratory, Georgia Southern University

We are attempting to investigate whether or not a previous history of hip flexor strain has an effect on hip or trunk strength. We are also attempting to see if the possible loss of strength is only present in a previously injured grouped of people compared to people who have not had this injury before. The results of this study will help medical professionals better understand strength deficits in athletes with a previous history of a hip flexor strain in order to aid in returning athlete to play.

You are invited to participate in this study because you have met the qualification criteria for this study. Further, you have no history of lower extremity injury other than a hip flexor strain within the last six months, you are currently participating in your sport, and you have progressed out of the acute phase of healing for your injury. An injury is considered to be in the acute phase of healing from the initial time of injury and while the pain, bleeding, and/or swelling is at its worst. A typical time frame for the acute phase of healing is between two to four days after the injury, depending on how the injury has been treated.

You are unable to participate in the study if you answer “yes” to any of the following questions:

1. Has your doctor ever said that you have a heart condition and that you should only do physical activity recommended by a doctor? 2. Do you feel pain in your chest when you do physical activity? 3. In the past month, have you had chest pain when you were not doing physical activity? 4. Do you lose your balance because of dizziness or do you ever lose consciousness? 5. Do you have a bone or joint problem (for example, back, knee, or hip) that could be made worse by a change in your physical activity? 6. Is your doctor currently prescribing drugs (for example, water pills) for your blood pressure or heart condition? 7. Do you know of any other reason why you should not do physical activity?

If you agree to participate in the study, you will be asked to attend a testing session that will last approximately 45- 60 minutes. You will be asked to refrain from physical activity 24 hours prior to testing. Prior to actual testing, you complete a warm up on the stationary bike for a minimum of ten minutes. During the session, you will be strapped into the Biodex Isokinetic Dynamometer in order to test both hip and trunk strength. During testing, you will be asked to perform three maximal-effort contractions isometrically and isokinetically at speeds of 60, 120, and 180 degrees per second. During testing for hip strength, you will also be tested at different knee angles during each testing speed. Data will be collected using the Biodex Software. The best attire to wear for testing would be athletic clothing that is easy to move in, such as running shorts and a T-shirt.

The data that we collect will be analyzed in a software program; no one will know that you have participated in the study and all information will be kept confidential. 42

There is minimal risk of physical injury during this session. There is a risk of minor muscle injury due to maximal- effort muscle contractions. To help reduce this risk, you will warm up prior to testing and you will be allowed at least three minutes of rest in between testing speeds and ten minutes of rest in between testing of the hip and the trunk in order to minimize fatigue. A trained individual will observe testing for proper procedure to help minimize risk of injury. You understand that medical care is available if needed, but neither financial compensation nor free medical treatment is provided. Should medical attention be required, contact Health Services at (912) 478-5641.

You will not receive any direct benefit from participating in this study.

You will attend one testing session after 24 hours of rest from physical activity.

You understand that you will not receive compensation for your participation in this project, and you will not be responsible for any costs.

You understand that you do not have to participate in this project and your decision is voluntarily. At any time, you can choose not to participate by telling the primary investigator. You can terminate participation in this study without any prejudice to future care.

All the data concerning yourself will be kept confidential. You understand that any information about your records will be handled confidentially. A case number will indicate your identity on all records and all data will be stored in a password protected computer within a locked office. Signed informed consent will be kept on file in a locked cabinet within a locked office. You/ will not be mentioned in any publications. Your records will be kept for a period of 3 years after the completion of this study as required by the Georgia Board of Regents policy.

You understand that you can decline to answer specific questions.

You understand that there is no deception involved with this project.

You certify that you are 18 years of age or older, and you have read the preceding information, or it has been read to you, and understand its contents. If you have any questions regarding this research, they can be answered by the investigator listed at the beginning of this consent form or you can call the Office of Research Integrity at (912) 478- 5465.

You have been provided a copy of this form. This project has been reviewed and approved by the GSU Institutional Review Board under tracking number H17141.

Principal Investigator Advisor

Jasmin Brown, ATC, LAT Li Li, Ph.D. 1205 Hanner Fieldhouse 0107C Hollis Building (404)-432-5021 (912)-478-0200 [email protected] [email protected]

______Participant Signature Date

I, Jasmin Brown, verify that the above informed consent procedure has been followed.

______Investigator Signature Date 43

APPENDIX D STATISTICS Peak Trunk vs. Hip Isokinetic Extension Torque

Analysis of Variance Table for PT

Source DF SS MS F P Subj 13 208605 16047 0.85 0.6109 Group 1 6824 6824 0.36 0.5574 Error Subj*Group 13 244651 18819 Joint 1 218470 218470 29.07 0.0001 Error Subj*Joint 13 97688 7514 Speed 2 91654 45827 61.33 0.0000 Error Subj*Speed 26 19427 747 Group*Joint 1 874 874 0.13 0.7203 Error Subj*Group*Joint 13 84835 6526 Group*Speed 2 837 419 0.39 0.6793 Error Subj*Group*Speed 26 27732 1067 Joint*Speed 2 39630 19815 32.72 0.0000 Error Subj*Joint*Speed 26 15745 606 Group*Joint*Speed 2 1584 792 0.80 0.4591 Error Subj*Group*Joint*Speed 26 25659 987 Total 167 1084215

Grand Mean 128.41 CV(Subj*Group) 106.83 CV(Subj*Joint) 67.51 CV(Subj*Speed) 21.29 CV(Subj*Group*Joint) 62.91 CV(Subj*Group*Speed) 25.43 CV(Subj*Joint*Speed) 19.16 CV(Subj*Group*Joint*Speed) 24.46

Tukey HSD All-Pairwise Comparisons Test of PT for Joint*Speed

Joint Speed Mean Homogeneous Groups 2 60 210.62 A 2 120 166.91 B 2 180 115.89 C 1 60 102.05 C 1 120 92.55 C 1 180 82.45 C

Comparisons of means for the same level of Joint Alpha 0.05 Standard Error for Comparison 6.9507 Critical Q Value 4.345 Critical Value for Comparison 21.356 Error terms used: Subj*Speed and Subj*Joint*Speed Comparisons of means for the same level of Speed Alpha 0.05 Standard Error for Comparison 14.414 Critical Q Value 4.646 Critical Value for Comparison 47.349 Error terms used: Subj*Joint and Subj*Joint*Speed Comparisons of means for different levels of Joint and Speed Alpha 0.05 Standard Error for Comparison 14.588 44

Critical Q Value 4.639 Critical Value for Comparison 47.848 Error terms used: Subj*Joint and Subj*Speed and Subj*Joint*Speed There are 3 groups (A, B, etc.) in which the means are not significantly different from one another.

Means of PT for Joint*Speed

Joint Speed Mean 1 60 102.05 1 120 92.55 1 180 82.45 2 60 210.62 2 120 166.91 2 180 115.89 Observations per Mean 28 Standard Error of a Mean 4.6505 Error term used: Subj*Joint*Speed, 26 DF

Average Trunk vs. Hip Isokinetic Extension Torque

Analysis of Variance Table for AT

Source DF SS MS F P Subj 13 142048 10927 0.89 0.5853 Group 1 6001 6001 0.49 0.4979 Error Subj*Group 13 160459 12343 Joint 1 130482 130482 24.44 0.0003 Error Subj*Joint 13 69407 5339 Speed 2 42876 21438 31.24 0.0000 Error Subj*Speed 26 17842 686 Group*Joint 1 1320 1320 0.33 0.5756 Error Subj*Group*Joint 13 52039 4003 Group*Speed 2 549 275 0.33 0.7241 Error Subj*Group*Speed 26 21846 840 Joint*Speed 2 32001 16001 37.90 0.0000 Error Subj*Joint*Speed 26 10977 422 Group*Joint*Speed 2 1615 808 1.16 0.3278 Error Subj*Group*Joint*Speed 26 18027 693 Total 167 707491

Grand Mean 100.58 CV(Subj*Group) 110.46 CV(Subj*Joint) 72.65 CV(Subj*Speed) 26.04 CV(Subj*Group*Joint) 62.90 CV(Subj*Group*Speed) 28.82 CV(Subj*Joint*Speed) 20.43 CV(Subj*Group*Joint*Speed) 26.18

Tukey HSD All-Pairwise Comparisons Test of AT for Joint*Speed

Joint Speed Mean Homogeneous Groups 2 60 165.25 A 2 120 127.45 B 2 180 92.65 C 1 120 76.60 C 1 60 73.53 C 1 180 68.01 C

Comparisons of means for the same level of Joint Alpha 0.05 Standard Error for Comparison 6.2918 Critical Q Value 4.345 Critical Value for Comparison 19.332 Error terms used: Subj*Speed and Subj*Joint*Speed Comparisons of means for the same level of Speed Alpha 0.05 Standard Error for Comparison 12.134 Critical Q Value 4.646 Critical Value for Comparison 39.866 Error terms used: Subj*Joint and Subj*Joint*Speed Comparisons of means for different levels of Joint and Speed Alpha 0.05 Standard Error for Comparison 12.516 Critical Q Value 4.628 Critical Value for Comparison 40.962 Error terms used: Subj*Joint and Subj*Speed and Subj*Joint*Speed There are 3 groups (A, B, etc.) in which the means 46 are not significantly different from one another.

Means of AT for Joint*Speed

Joint Speed Mean 1 60 73.53 1 120 76.60 1 180 68.01 2 60 165.25 2 120 127.45 2 180 92.65 Observations per Mean 28 Standard Error of a Mean 3.8831 Error term used: Subj*Joint*Speed, 26 DF

Peak Trunk vs. Hip Isokinetic Flexion Torque

Analysis of Variance Table for PT

Source DF SS MS F P Subj 13 94474 7267 1.13 0.4119 Group 1 21 21 0.00 0.9549 Error Subj*Group 13 83288 6407 Joint 1 152945 152945 110.25 0.0000 Error Subj*Joint 13 18034 1387 Speed 2 34980 17490 129.25 0.0000 Error Subj*Speed 26 3518 135 Group*Joint 1 506 506 0.29 0.5971 Error Subj*Group*Joint 13 22414 1724 Group*Speed 2 37 19 0.07 0.9293 Error Subj*Group*Speed 26 6604 254 Joint*Speed 2 6599 3299 22.75 0.0000 Error Subj*Joint*Speed 26 3772 145 Group*Joint*Speed 2 165 82 0.55 0.5838 Error Subj*Group*Joint*Speed 26 3903 150 Total 167 431260

Grand Mean 88.305 CV(Subj*Group) 90.64 CV(Subj*Joint) 42.18 CV(Subj*Speed) 13.17 CV(Subj*Group*Joint) 47.02 CV(Subj*Group*Speed) 18.05 CV(Subj*Joint*Speed) 13.64 CV(Subj*Group*Joint*Speed) 13.87

Tukey HSD All-Pairwise Comparisons Test of PT for Joint*Speed

Joint Speed Mean Homogeneous Groups 2 60 143.63 A 2 120 118.84 B 2 180 92.96 C 1 60 68.43 D 1 120 57.56 E 1 180 48.41 E

Comparisons of means for the same level of Joint Alpha 0.05 Standard Error for Comparison 3.1644 Critical Q Value 4.345 Critical Value for Comparison 9.7227 Error terms used: Subj*Speed and Subj*Joint*Speed Comparisons of means for the same level of Speed Alpha 0.05 Standard Error for Comparison 6.3196 Critical Q Value 4.634 Critical Value for Comparison 20.707 Error terms used: Subj*Joint and Subj*Joint*Speed Comparisons of means for different levels of Joint and Speed Alpha 0.05 Standard Error for Comparison 6.2919 Critical Q Value 4.636 Critical Value for Comparison 20.627 Error terms used: Subj*Joint and Subj*Speed and Subj*Joint*Speed There are 5 groups (A, B, etc.) in which the means are not significantly different from one another. 48

Means of PT for Joint*Speed

Joint Speed Mean 1 60 68.43 1 120 57.56 1 180 48.41 2 60 143.63 2 120 118.84 2 180 92.96 Observations per Mean 28 Standard Error of a Mean 2.2761 Error term used: Subj*Joint*Speed, 26 DF

Average Trunk vs. Hip Isokinetic Flexion Torque

Analysis of Variance Table for AT

Source DF SS MS F P Subj 13 56897 4376.7 1.17 0.3903 Group 1 61 61.4 0.02 0.9000 Error Subj*Group 13 48601 3738.5 Joint 1 94847 94846.9 113.06 0.0000 Error Subj*Joint 13 10906 838.9 Speed 2 15124 7561.9 61.62 0.0000 Error Subj*Speed 26 3191 122.7 Group*Joint 1 324 324.2 0.28 0.6051 Error Subj*Group*Joint 13 15009 1154.6 Group*Speed 2 143 71.5 0.52 0.6033 Error Subj*Group*Speed 26 3606 138.7 Joint*Speed 2 3230 1615.1 13.93 0.0001 Error Subj*Joint*Speed 26 3015 116.0 Group*Joint*Speed 2 136 67.9 0.53 0.5930 Error Subj*Group*Joint*Speed 26 3310 127.3 Total 167 258399

Grand Mean 65.880 CV(Subj*Group) 92.81 CV(Subj*Joint) 43.96 CV(Subj*Speed) 16.81 CV(Subj*Group*Joint) 51.58 CV(Subj*Group*Speed) 17.88 CV(Subj*Joint*Speed) 16.35 CV(Subj*Group*Joint*Speed) 17.13

Tukey HSD All-Pairwise Comparisons Test of AT for Joint*Speed

Joint Speed Mean Homogeneous Groups 2 60 106.30 A 2 120 90.29 B 2 180 72.34 C 1 60 48.17 D 1 120 42.50 DE 1 180 35.69 E

Comparisons of means for the same level of Joint Alpha 0.05 Standard Error for Comparison 2.9195 Critical Q Value 4.345 Critical Value for Comparison 8.9704 Error terms used: Subj*Speed and Subj*Joint*Speed Comparisons of means for the same level of Speed Alpha 0.05 Standard Error for Comparison 5.0493 Critical Q Value 4.619 Critical Value for Comparison 16.490 Error terms used: Subj*Joint and Subj*Joint*Speed Comparisons of means for different levels of Joint and Speed Alpha 0.05 Standard Error for Comparison 5.0731 Critical Q Value 4.616 Critical Value for Comparison 16.559 Error terms used: Subj*Joint and Subj*Speed and Subj*Joint*Speed There are 5 groups (A, B, etc.) in which the means are not significantly different from one another. 50

Means of AT for Joint*Speed

Joint Speed Mean 1 60 48.17 1 120 42.50 1 180 35.69 2 60 106.30 2 120 90.29 2 180 72.34 Observations per Mean 28 Standard Error of a Mean 2.0350 Error term used: Subj*Joint*Speed, 26 DF

Peak Trunk vs. Hip Isometric Flexion Torque

Analysis of Variance Table for PT

Source DF SS MS F P Subj 13 45626 3509.7 0.87 0.5986 Group 1 141 141.4 0.03 0.8545 Error Subj*Group 13 52545 4041.9 Joint 1 70361 70361.2 86.15 0.0000 Error Subj*Joint 13 10617 816.7 Group*Joint 1 634 633.8 1.22 0.2901 Error Subj*Group*Joint 13 6774 521.1 Total 55 186698

Grand Mean 123.88 CV(Subj*Group) 51.32 CV(Subj*Joint) 23.07 CV(Subj*Group*Joint) 18.43

Tukey HSD All-Pairwise Comparisons Test of PT for Joint

Joint Mean Homogeneous Groups 2 159.33 A 1 88.43 B

Alpha 0.05 Standard Error for Comparison 7.6377 Critical Q Value 3.057 Critical Value for Comparison 16.510 All 2 means are significantly different from one another.

Means of PT for Joint

Joint Mean 1 88.43 2 159.33 Observations per Mean 28 Standard Error of a Mean 5.4007 Std Error (Diff of 2 Means) 7.6377 Error term used: Subj*Joint, 13 DF

Peak Trunk vs. Hip Isometric Extension Torque

Analysis of Variance Table for PT

Source DF SS MS F P Subj 13 92621 7125 0.85 0.6117 Group 1 113 113 0.01 0.9094 Error Subj*Group 13 108748 8365 Joint 1 256962 256962 66.58 0.0000 Subj*Joint 13 62264 4790 1.24 0.3514 Group*Joint 1 90 90 0.02 0.8810 Error Subj*Group*Joint 13 50171 3859 Total 55 570968

Grand Mean 197.87 CV(Subj*Group) 46.22 CV(Subj*Group*Joint) 31.40

Tukey HSD All-Pairwise Comparisons Test of PT for Joint

Joint Mean Homogeneous Groups 2 265.61 A 1 130.14 B

Alpha 0.05 Standard Error for Comparison 16.603 Critical Q Value 3.057 Critical Value for Comparison 35.890 All 2 means are significantly different from one another.

Means of PT for Joint

Joint Mean 1 130.14 2 265.61 Observations per Mean 28 Standard Error of a Mean 11.740 Std Error (Diff of 2 Means) 16.603 Error term used: Subj*Group*Joint, 13 DF

References

1. Borghuis J, Hof AL, Lemmink KA. The importance of sensory-motor control in providing core stability. Sports Med. 2008;38(11):893–916.

2. Seidenberg P, Bowen JD, eds. The Hip and Pelvis in Sports Medicine and Primary Care. New York, NY: Springer New York; 2010.

3. Akuthota V, Nadler SF. Core strengthening. Arch Phys Med Rehabil. 2004;85:86–92.

4. Axler CT, McGill SM. Low back loads over a variety of abdominal exercises: searching for the safest abdominal challenge. Med Sci Sports Exerc. 1997;29(6):804-810.

5. McGill S, Juker D, Kropf P. Appropriately placed surface EMG electrodes reflect deep muscle activity (psoas, quadratus lumborum, abdominal wall) in the lumbar spine. J Biomech. 1996;29(11):1503–1507.

6. McGill SM. Distribution of tissue loads in the low back during a variety of daily and rehabilitation tasks. J Rehabil Res Dev. 1997;34(4):448.

7. Kibler WB, Press J, Sciascia A. The role of core stability in athletic function. Sports Med. 2006;36(3):189–198.

8. Putnam CA. Sequential motions of body segments in striking and throwing skills: descriptions and explanations. J Biomech. 1993;26:125–135.

9. Haff GG, Triplett NT. Essentials of Strength Training and Conditioning 4th Edition. Human kinetics; 2015.

10. McGill SM. Low back stability: from formal description to issues for performance and rehabilitation. Exerc Sport Sci Rev. 2001;29(1):26–31.

11. McGill SM, Grenier S, Kavcic N, Cholewicki J. Coordination of muscle activity to assure stability of the lumbar spine. J Electromyogr Kinesiol. 2003;13(4):353–359.

12. Hibbs AE, Thompson KG, French D, Wrigley A, Spears I. Optimizing Performance by Improving Core Stability and Core Strength: Sports Med. 2008;38(12):995-1008.

13. Nesser TW, Huxel KC, Tincher JL, Okada T. The Relationship Between Core Stability and Performance in Division I Football Players: J Strength Cond Res. 2008;22(6):1750-1754.

14. Nesser TW, Lee WL. The relationship between core strength and performance in division I female soccer players. J Exerc Physiol Online. 2009;12(2):21–28.

15. Willson JD, Dougherty CP, Ireland ML, Davis IM. Core stability and its relationship to lower extremity function and injury. J Am Acad Orthop Surg. 2005;13(5):316–325.

16. Barnes WS. The Relationship of Motor-Unit Activation to Isokinetic Muscular Contraction at Different Contractile Velocities. Phys Ther. 1980;60(9):1152-1158. 54

17. Wyatt MP, Edwards AM. Comparison of quadriceps and hamstring torque values during isokinetic exercise. J Orthop Sports Phys Ther. 1981;3(2):48–56.

18. Delitto A, Crandell CE, Rose SJ. Peak torque-to-body weight ratios in the trunk: a critical analysis. Phys Ther. 1989;69(2):138–143.

19. Dowson MN, Nevill ME, Lakomy HKA, Nevill AM, Hazeldine RJ. Modelling the relationship between isokinetic muscle strength and sprint running performance. J Sports Sci. 1998;16(3):257-265.

20. Bertocci GE, Munin MC, Frost KL, Burdett R, Wassinger CA, Fitzgerald SG. Isokinetic Performance After Total Hip Replacement: Am J Phys Med Rehabil. 2004;83(1):1-9.

21. Claiborne TL, Timmons MK, Pincivero DM. Test–retest reliability of cardinal plane isokinetic hip torque and EMG. J Electromyogr Kinesiol. 2009;19(5):e345-e352.

22. Sugiura Y, Saito T, Sakuraba K, Sakuma K, Suzuki E. Strength deficits identified with concentric action of the hip extensors and eccentric action of the hamstrings predispose to hamstring injury in elite sprinters. J Orthop Sports Phys Ther. 2008;38(8):457–464.

23. Cahalan TD, Johnson ME, Liu S, Chao EYS. Quantitative measurements of hip strength in different age groups. Clin Orthop. 1989;246:136–145.

24. DeLitto A, Rose SJ, Crandell CE, Strube MJ. Reliability of isokinetic measurements of trunk muscle performance. Spine. 1991;16(7):800–803.

25. Ripamonti M, Colin D, Rahmani A. Torque–velocity and power–velocity relationships during isokinetic trunk flexion and extension. Clin Biomech. 2008;23(5):520–526.

26. Grabiner MD, Jeziorowski JJ, Divekar AD. Isokinetic measurements of trunk extension and flexion performance collected with the biodex clinical data station. J Orthop Sports Phys Ther. 1990;11(12):590–598.

27. Woodhouse ML, Heinen JR, Shall L, Bragg K. Concentric isokinetic trunk extension/flexion testing of rigid and semirigid lumbar/sacral supports. J Athl Train. 1993;28(2):106.

28. Waldhelm A, Li L. Endurance tests are the most reliable core stability related measurements. J Sport Health Sci. 2012;1(2):121-128.

29. Nesser TW, Latin RW, Berg K, Prentice E. Physiological Determinants of 40-Meter Sprint Performance in Young Male Athletes. J Strength Cond Res. 1996;10(4):263–267.

30. Clinically Oriented Anatomy, 6th Edition, Anne M. R. Agur, Arthur F. Dalley, Kei.

31. Akima H, Saito A. Activation of quadriceps femoris including vastus intermedius during fatiguing dynamic knee extensions. Eur J Appl Physiol. 2013;113(11):2829-2840.

32. Gyftopoulos S, Rosenberg ZS, Schweitzer ME, Bordalo-Rodrigues M. Normal Anatomy and Strains of the Deep Musculotendinous Junction of the Proximal Rectus Femoris: MRI Features. Am J Roentgenol. 2008;190(3):W182-W186. 55

33. Paschalis V, Koutedakis Y, Baltzopoulos V, Mougios V, Jamurtas AZ, Giakas G. Short vs. long length of rectus femoris during eccentric exercise in relation to muscle damage in healthy males. Clin Biomech. 2005;20(6):617-622.

34. Ebenbichler GR, Kollmitzer J, Glöckler L, Bochdansky T, Kopf A, Fialka V. The role of the biarticular agonist and cocontracting antagonist pair in isometric muscle fatigue. Muscle Nerve. 1998;21(12):1706–1713.

35. Moritani T, Muro M, Nagata A. Intramuscular and surface electromyogram changes during muscle fatigue. J Appl Physiol. 1986;60(4):1179–1185.

36. Akima H, Saito A, Watanabe K, Kouzaki M. Alternate muscle activity patterns among synergists of the quadriceps femoris including the vastus intermedius during low-level sustained contraction in men. Muscle Nerve. 2012;46(1):86–95.

37. Kouzaki M, Shinohara M, Fukunaga T. Non-uniform mechanical activity of quadriceps muscle during fatigue by repeated maximal voluntary contraction in humans. Eur J Appl Physiol. 1999;80(1):9–15.

38. Kouzaki M, Shinohara M, Masani K, Kanehisa H, Fukunaga T. Alternate muscle activity observed between knee extensor synergists during low-level sustained contractions. J Appl Physiol. 2002;93(2):675–684.

39. Tamaki H, Kitada K, Akamine T, Murata F, Sakou T, Kurata H. Alternate activity in the synergistic muscles during prolonged low-level contractions. J Appl Physiol. 1998;84(6):1943–1951.

40. Hasselman CT, Best TM, Hughes C, Martinez S, Garrett WE. An explanation for various rectus femoris strain injuries using previously undescribed muscle architecture. Am J Sports Med. 1995;23(4):493–499.

41. Garrett W. Muscle strain injuries. Am J Sports Med. 1996;24(6 Suppl):S2-S8.

42. Ryan AJ. Quadriceps strain, rupture and charlie horse. Med Sci Sports Exerc. 1969;1(2):106– 111.

43. Akima H, Takahashi H, Kuno SY, et al. Early phase adaptations of muscle use and strength to isokinetic training. Med Sci Sports Exerc. 1999;31(4):588-594.

44. Akima H, Takahashi H, Kuno S, Katsuta S. Coactivation pattern in human quadriceps during isokinetic knee-extension by muscle functional MRI. Eur J Appl Physiol. 2004;91(1):7–14.

45. Sahrmann S. Diagnosis and Treatment of Movement Impairment Syndromes. Elsevier Health Sciences; 2002.

46. Prilutsky BI, Zatsiorsky VM. Tendon action of two-joint muscles: Transfer of mechanical energy between joints during jumping, landing, and running. J Biomech. 1994;27(1):25-34.

47. Bobbert MF, van Ingen Schenau GJ. Coordination in vertical jumping. J Biomech. 1988;21(3):249–262. 56

48. Jacobs R, van Ingen Schenau GJ. Intermuscular coordination in a sprint push-off. J Biomech. 1992;25(9):953–965.

49. Montgomery WH, Pink M, Perry J. Electromyographic Analysis of Hip and Knee Musculature During Running. Am J Sports Med. 1994;22(2):272-278.

50. Ellenbecker TS, Davies GJ. Closed Kinetic Chain Exercise: A Comprehensive Guide to Multiple Joint Exercise. Human Kinetics; 2001.

51. Griffin LY, Agel J, Albohm MJ, et al. Noncontact anterior cruciate ligament injuries: risk factors and prevention strategies. J Am Acad Orthop Surg. 2000;8(3):141–150.

52. Zazulak BT, Ponce PL, Straub SJ, Medvecky MJ, Avedisian L, Hewett TE. Gender comparison of hip muscle activity during single-leg landing. J Orthop Sports Phys Ther. 2005;35(5):292–299.

53. Zazulak BT, Hewett TE, Reeves NP, Goldberg B, Cholewicki J. Deficits in neuromuscular control of the trunk predict knee injury risk a prospective biomechanical-epidemiologic study. Am J Sports Med. 2007;35(7):1123–1130.

54. Ebenbichler GR, Oddsson LI, Kollmitzer J, Erim Z. Sensory-motor control of the lower back: implications for rehabilitation. Med Sci Sports Exerc. 2001;33(11):1889–1898.

55. Comerford MJ, Mottram SL. Movement and stability dysfunction–contemporary developments. Man Ther. 2001;6(1):15–26.

56. McGill SM. Low Back Disorders, 3E. Human Kinetics; 2015.

57. Anderson K, Behm DG. The impact of instability resistance training on balance and stability. Sports Med. 2005;35(1):43–53.

58. Zattara M, Bouisset S. Posturo-kinetic organisation during the early phase of voluntary upper limb movement. 1. Normal subjects. J Neurol Neurosurg Psychiatry. 1988;51(7):956–965.

59. Leetun DT, Ireland ML, Willson JD, Ballantyne BT, Davis IM, others. Core stability measures as risk factors for lower extremity injury in athletes. Med Sci Sports Exerc. 2004;36(6):926– 934.

60. Zazulak BT, Hewett TE, Reeves NP, Goldberg B, Cholewicki J. The effects of core proprioception on knee injury a prospective biomechanical-epidemiological study. Am J Sports Med. 2007;35(3):368–373.

61. Shimpi AP, Kharkar SA, Talreja AA, Rairikar SA, Shyam A, Sancheti P. Effect of Induced Muscular Fatigue on Balance and Core Strength in Normal Individuals. Indian J Physiother Occup Ther - Int J. 2014;8(3):182.

62. Leinonen V, Kankaanpää M, Airaksinen O, Hänninen O. Back and hip extensor activities during trunk flexion/extension: effects of low back pain and rehabilitation. Arch Phys Med Rehabil. 2000;81(1):32–37. 57

63. Ambegaonkar JP, Mettinger LM, Caswell SV, Burtt A, Cortes N. Relationships between core endurance, hip strength, and balance in collegiate female athletes. Int J Sports Phys Ther. 2014;9(5).

64. Alentorn-Geli E, Myer GD, Silvers HJ, et al. Prevention of non-contact anterior cruciate ligament injuries in soccer players. Part 1: Mechanisms of injury and underlying risk factors. Knee Surg Sports Traumatol Arthrosc. 2009;17(7):705–729.

65. Claiborne TL, Armstrong CW, Gandhi V, Pincivero DM, others. Relationship between hip and knee strength and knee valgus during a single leg squat. J Appl Biomech. 2006;22(1):41.

66. Palmieri-Smith RM, Wojtys EM, Ashton-Miller JA. Association between preparatory muscle activation and peak valgus knee angle. J Electromyogr Kinesiol. 2008;18(6):973–979.

67. Lawrence RK, Kernozek TW, Miller EJ, Torry MR, Reuteman P. Influences of hip external rotation strength on knee mechanics during single-leg drop landings in females. Clin Biomech. 2008;23(6):806–813.

68. Salminen JJ, Oksanen A, Mäki P, Pentti J, Kujala UM. Leisure time physical activity in the young. Int J Sports Med. 1993;14(7):406–410.

69. Biering-sørensen FIN. Physical measurements as risk indicators for low-back trouble over a one-year period. Spine. 1984;9(2):106–119.

70. Fairbank JC, Pynsent PB, Van Poortvliet JA, Phillips H. Influence of anthropometric factors and joint laxity in the incidence of adolescent back pain. Spine. 1984;9(5):461–464.

71. Mierau D, Cassidy JD, Yong-Hing K. Low-back pain and straight leg raising in children and adolescents. Spine. 1989;14(5):526–528.

72. Giles LGF, Taylor JR. Low-back pain associated with leg length inequality. Spine. 1981;6(5):510–521.

73. Hoikka V, Ylikoski M, Tallroth K. Leg-length inequality has poor correlation with lumbar scoliosis. Arch Orthop Trauma Surg. 1989;108(3):173–175.

74. Cholewicki J, McGill SM. Mechanical stability of the in vivo lumbar spine: implications for injury and chronic low back pain. Clin Biomech. 1996;11(1):1–15.

75. Leetun DT, Ireland ML, Willson JD, Ballantyne BT, Davis IM. Core Stability Measures as Risk Factors for Lower Extremity Injury in Athletes: Med Sci Sports Exerc. 2004;36(6):926- 934.

76. Hammill RR, Beazell JR, Hart JM. Neuromuscular consequences of low back pain and core dysfunction. Clin Sports Med. 2008;27(3):449–462.

77. Hart JM, Fritz JM, Kerrigan DC, Saliba EN, others. Reduced quadriceps activation after lumbar paraspinal fatiguing exercise. J Athl Train. 2006;41(1):79. 58

78. Nadler SF, Malanga GA, DePrince M, Stitik TP, Feinberg JH. The relationship between lower extremity injury, low back pain, and hip muscle strength in male and female collegiate athletes. Clin J Sport Med. 2000;10(2):89–97.

79. Suter E, Lindsay D. Back muscle fatigability is associated with knee extensor inhibition in subjects with low back pain. Spine. 2001;26(16):E361–E366.

80. Hodges PW, Richardson CA. Contraction of the abdominal muscles associated with movement of the lower limb. Phys Ther. 1997;77(2):132–142.

81. Bouisset S. [Relationship between postural support and intentional movement: biomechanical approach]. Arch Int Physiol Biochim Biophys. 1991;99(5):A77–92.

82. Radebold A, Cholewicki J, Panjabi MM, Patel TC. Muscle response pattern to sudden trunk loading in healthy individuals and in patients with chronic low back pain. Spine. 2000;25(8):947–954.

83. Lyons K, Perry J, Gronley JK, Barnes L, Antonelli D. Timing and relative intensity of hip extensor and abductor muscle action during level and stair ambulation. Phys Ther. 1983;63(10):1597–1605.

84. Kankaanpää M, Taimela S, Laaksonen D, Hänninen O, Airaksinen O. Back and hip extensor fatigability in chronic low back pain patients and controls. Arch Phys Med Rehabil. 1998;79(4):412–417.

85. McGregor AH, Hukins DWL. Lower limb involvement in spinal function and low back pain. J Back Musculoskelet Rehabil. 2009;22(4):219-222.

86. Beckman SM, Buchanan TS. Ankle inversion injury and hypermobility: effect on hip and ankle muscle electromyography onset latency. Arch Phys Med Rehabil. 1995;76(12):1138– 1143.

87. Bullock-Saxton JE. Local sensation changes and altered hip muscle function following severe ankle sprain. Phys Ther. 1994;74(1):17–28.

88. Devita P, Hunter PB, Skelly WA. Effects of a functional knee brace on the biomechanics of running. Med Sci Sports Exerc. 1992;24(7):797–806.

89. Jaramillo J, Worrell TW, Ingersoll CD. Hip isometric strength following knee surgery. J Orthop Sports Phys Ther. 1994;20(3):160–165.

90. Devlin L. Recurrent posterior thigh symptoms detrimental to performance in rugby union. Sports Med. 2000;29(4):273–287.

91. Joseph J, Nightingale A. Electromyography of muscles of posture: thigh muscles in males. J Physiol. 1954;126(1):81–85.

92. Sherry MA, Best TM. A comparison of 2 rehabilitation programs in the treatment of acute hamstring strains. J Orthop Sports Phys Ther. 2004;34(3):116–125. 59

93. Clark RA. Hamstring injuries: risk assessment and injury prevention. Ann Acad Med Singap. 2008;37(4):341.

94. Hennessey L, Watson AW. Flexibility and posture assessment in relation to hamstring injury. Br J Sports Med. 1993;27(4):243–246.

95. Konrad P, Schmitz K, Denner A. Neuromuscular evaluation of trunk-training exercises. J Athl Train. 2001;36(2):109-118.

96. Ekstrom RA, Donatelli RA, Carp KC. Electromyographic analysis of core trunk, hip, and thigh muscles during 9 rehabilitation exercises. J Orthop Sports Phys Ther. 2007;37(12):754– 762.

97. Tong TK, Wu S, Nie J, Baker JS, Lin H. The occurrence of core muscle fatigue during high- intensity running exercise and its limitation to performance: the role of respiratory work. J Sports Sci Med. 2014;13(2):244.

98. Cook SD, Kester MA, Brunet ME, Haddad Jr RJ. Biomechanics of running shoe performance. Clin Sports Med. 1985;4(4):619–626.

99. Novacheck TF. The biomechanics of running. Gait Posture. 1998;7(1):77–95.

100. Prilutsky BI, Gregor RJ, Ryan MM. Coordination of two-joint rectus femoris and hamstrings during the swing phase of human walking and running. Exp Brain Res. 1998;120(4):479-486.

101. Perry J, Burnfield JM. Gait analysis: normal and pathological function. 1992.

102. Shirado O, Ito T, Kaneda K, Strax TE. Concentric and eccentric strength of trunk muscles: influence of test postures on strength and characteristics of patients with chronic low-back pain. Arch Phys Med Rehabil. 1995;76(7):604–611.

103. Perrin DH. Isokinetic Exercise and Assessment. Human Kinetics; 1993.

104. Shirado O, Kaneda K, Ito T. Trunk-muscle strength during concentric and eccentric contraction: a comparison between healthy subjects and patients with chronic low-back pain. J Spinal Disord Tech. 1992;5(2):175–182.

105. Smidt GL, Amundsen LR, Dostal WF. Muscle Strength at the Trunk*. J Orthop Sports Phys Ther. 1980;1(3):165–170.

106. DeLisa JA. Rehabilitation Medicine: Principles and Practice. Lippincott Williams & Wilkins; 1988.

107. Ellenbecker TS, Davies GJ, Rowinski MJ. Concentric versus eccentric isokinetic strengthening of the rotator cuff objective data versus functional test. Am J Sports Med. 1988;16(1):64–69.

108. Komi PV, Buskirk ER. Effect of Eccentric and Concentric Muscle Conditioning on Tension and Electrical Activity of Human Muscle*. Ergonomics. 1972;15(4):417–434. 60

109. Reid JG, Costigan PA. Trunk muscle balance and muscular force. Spine. 1987;12(8):783–786.

110. Requiao LF, Nadeau S, Milot MH, Gravel D, Bourbonnais D, Gagnon D. Quantification of level of effort at the plantarflexors and hip extensors and flexor muscles in healthy subjects walking at different cadences. J Electromyogr Kinesiol. 2005;15(4):393–405.

111. Drouin JM, Valovich-mcLeod TC, Shultz SJ, Gansneder BM, Perrin DH. Reliability and validity of the Biodex system 3 pro isokinetic dynamometer velocity, torque and position measurements. Eur J Appl Physiol. 2004;91(1):22–29.

112. Farrell M, Richards JG. Analysis of the reliability and validity of the kinetic communicator exercise device. Med Sci Sports Exerc. 1986;18(1):44–49.

113. Timm KE, Gennrich P, Burns R, Fyke D. The mechanical and physiological performance reliability of selected isokinetic dynamometers. Isokinet Exerc Sci. 1992;2(4):182–190.

114. Zawadzki J, Bober T, Siemienski A. Validity analysis of the Biodex System 3 dynamometer under static and isokinetic conditions. Acta Bioeng Biomech. 2010;12(4):25–32.

115. Portney L. G.; Watkins, M., P.:“Foundations of Clinical Research “. Prentice-Hall, New Jersey; 2000.

116. Karatas GK, Gögüs F, Meray J. Reliability of isokinetic trunk muscle strength measurement. Am J Phys Med Rehabil. 2002;81(2):79–85.