Introduction.

Current Anterior Cruciate Ligament Reconstruction (ACLR) rehabilitation protocols often focus on three major tenets: 1) protection and facilitation of the graft, 2) improving knee range of motion and strength to “within normal limits” and 3) passing of “established” return to sport testing, most notably single-leg hop testing, quad strength greater than 85% of uninvolved limb, etc. Despite hundreds of articles high-lighting these points, incidence of re-injury continues to be high and further, it has been demonstrated that even if there is no re-injury, only 63% of athletes return to their pre-injury level of performance (Ardern CL, 2011). This suggests that current rehabilitation models and return to sport parameters may not be addressing the appropriate deficits. At Nevada PT, we are evidence-led and the evidence suggests that the key to successful rehab lies in appreciating the multi-factorial nature of ACL injury and re-injury. Improved hamstring strength (in addition to established quadriceps protocols), equal lower extremity activation during athletic movements such as jumping, cutting, etc., improved core strength, more stringent return to sport benchmarks are but a few of the multi-pronged approach we use. Additionally, the timeline of graft healing may be much longer than previously thought as some grafts show healing continuing past the two-year mark. Finally, the establishment of an acceptable “chronic workload” as an athlete or patient is resuming athletic participation is an update on the “envelope of function” concept (with new objective risk factor ratios) and may become a cornerstone of reducing injury risk throughout not only ACL injuries, but rehabilitation in its entirety.

(6 month versus 10 month ACL graft on MRI, Rabuck SJ, 2013 Jan). While the following protocol borrows many tried-and-true rehabilitation theories from the best in sports medicine, it also incorporates new progressive models to address the above-mentioned goals. Phase 1 Progressions: The patient needs to be seen frequently and aggressively immediately after surgery to facilitate the completion of phase 1 goals. It is imperative to manage excessive inflammation and swelling as early as possible post-op while progressing extension as tolerated. Symmetrical extension should be achieved as early as possible, with a goal of achieving equal bilateral hyperextension within the first 1-2 weeks. Additionally, normalizing patellar mobility, preventing or minimizing scar tissue formation and facilitating improved quadriceps activation (Wilk KE, 2012 Mar) should be emphasized at this time. If abnormal scar tissue is allowed to form, extension deficits may persist or if the inflammatory cycle is allowed to continue after the first few weeks, the patient may have continued difficulty throughout the ACL rehabilitation program. Aggressive pursuit of phase 1 goals often sets the table for a smooth rehabilitation progression in the next several months. Improved Hamstring Activation/Decreased Hamstring Laxity: Hamstring laxity has been suggested as a primary indicator for ACL injury susceptibility, specifically in the female population (Hewett TE, February 2006). Additionally, hamstring activation unloads shear forces on the ACL, especially during athletic movements such as cutting and sidestepping (Weinhandl JT, June 2014) and it would follow that a program emphasizing ACL repair protection would have a heavy hamstring component while continuing the well-established quadriceps and hip musculature strengthening of established programs. The following protocol addresses this in multiple approaches: primarily in the form of deadlift progressions (Escamilla RF, 2002 Apr) such as assisted band-work, bodyweight and single-leg movements, kettlebells and finally barbell work as appropriate. While squat progressions are well known (yet amazingly still under-utilized), very few protocols incorporate deadlifts and if they do, it is often in a single-leg platform with a balance emphasis. Additionally, hamstring weakness when matched to their male counterparts, as opposed to quadriceps weakness, may be a bigger predictor for ACL injury and subsequent re-injury in females (G.D. Myer, 2009). Not only is the deadlift when performed correctly safe for ACL repairs, it is a compound multi-joint hamstring dominant movement requiring strong core activation. Hamstring and core weakness are well-documented risk factors for ACL injury and are often underemphasized components of many rehabilitation programs. Improved Core Strength: It has been theorized that poor core strength translates to poor lower body control, specifically when change of direction occurs. The deadlift (and barbell squat) addresses core strength to a greater extent than conventional “core” exercises (Hamlyn N, 2007) and in addition, the Selective Functional Movement Assessment has created a platform on which to progress core stability from the ground to an overhead squat, for example (clinical confession: although we recognize the lack of evidence supporting this specific application, we find the system one we are able to implement early and believe it does an excellent job of highlighting weak points outside of just the knee!). In short, the SFMA maintains that functional movement is built on developmental layers: rolling supine to prone, prone to supine; quadruped movement, high kneeling stability, and finally, bipedal movement. Gray Cook and the SFMA suggest many protocols or strength programs often skip over the normal developmental progressions and this, in turn, may translate to dysfunctional athletic movement. This protocol incorporates these theories in an attempt to approach core strength as more than sit-ups on a BOSU ball but rather as facilitating and strengthening the core itself on a pattern consistent with our biological development combined with heavy barbell work. Symmetrical Lower Extremity Activation: Isokinetic testing has long been the norm for measuring lower extremity strength, specifically with return to sport following ACL repair. Consistent decrease in knee extension values are shown as far as 12-24 months after surgery (Xergia SA, 2013 Mar) suggesting current quadriceps-specific directed therapeutic exercise is falling short. The problem with this type of testing, in our opinion, is that it is single-joint and as such, its predictive implications on athletic movements and the multi-joint systems they require may be over-stated. Additionally, access to the extremely expensive Biodex units the research is performed on may limit accessibility to smaller clinics. That being said, it is important to establish an objective quadriceps/hamstring ratio as this has been suggested to be continued risk factor for re-injury for Return to Sport (RTS) athletes (Kyritsis et al., 2016, Knapik et al, 1991). While the single-leg hop test offers a more complete picture of return to sport deficits in the ACLR athlete, specifically decreased knee flexion angles in load production and increased plantar flexion angles in load absorption (Xergia SA, 2013 Mar), again current rehab protocols may not be addressing the complete athlete. We implement the Keiser Leg extension/flexion machines that rely on pneumatic resistance and provide objective work outputs to establish these ratios. Compound multi-joint training with an objective measure on individual leg output may address the above-mentioned deficits with return to sport and is another testament to the multi-factorial approach it takes for comprehensive ACLR management.

Workload Capacity: One of the most overlooked components in ACL rehabilitation is the establishment of load capacity. Too often the decision for Return to Sport (RTS) is based entirely off subjective reporting of the athlete and time from surgery. While it has been established that the risk of re-injury decreases by 50% for every month the decision to clear an athlete is delayed after the 6-month mark (Grindem et al., 2016), more and more evidence is emerging that the RTS decision is far more complex than that. For this protocol, we will focus on the components we can influence or modify; the non- modifiables such as gender, sport-specific risks, exposure, etc. should be considered as well. Acute:Chronic ratio establishment has recently been gaining traction in the re-injury risk conversation and while it applies directly to ACL rehabilitation, (Blanch. P, Gabbett. T, 2015) we also feel it is the foundation of all non-contact injuries whether they be muscle strains and re-strains, tendinopathy or even osteoarthritis. Exceeding the load capacity beyond the levels these specific structures have adapted too over time has been well documented in the degenerative cascades seen in tendinopathy (Cook et al., 2015). Essentially, the Acute:Chronic ratio is an objective number comparing the athlete’s chronic workload (in this article, the previous 4 weeks although we feel it can be applied to months or even years) to an uptick in volume over a much shorter period of time (e.g. current week). Gabbett et al. established that a ratio greater than 1.5 significantly increases an athlete’s risk for re-injury. This concept integrates seamlessly with our RTS programming as Return to Sport must be treated as a continuum or a curve rather than a single inflection point. Over the months prior to an athlete or patient’s projected discharge, these parameters will be determined and progressed accordingly. For example, if we are treating a point guard for the University, we may monitor “minutes played” x “rate of perceived exertion (RPE)” x “frequency”. Weeks 1-4 may be 10 minutes at an RPE of 6, three times per week for example. This would then be a chronic ratio of 10x6x3=180. Week 5 might then be progressed to no greater than 270 which may be broken down as possibly 15 minutes played x RPE 6 x 3/week (270) or even 10 minutes x RPE 7 x 3-4/week (210-280) or further, RPE 9x 10 min x 3/week (270). It is important to note that while week 5 might allow a workload of 270, week 6 is not 270x 1.5 (405) but rather the average of weeks 2-5 (180, 180, 180, 270= 202). Subsequently, week 6 work load would be approximately 300. Although this example utilizes basketball, it can just as easily be applied to weight-lifting, running, or even workplace activity. While it has yet to be studied in this parameter, this concept also serves as our foundation to reducing injuries early in season when athletes return from summer break and likely have not established an appropriate chronic work load for their given sport but that is a conversation for a different protocol! External Cueing and Jump Mechanics: Additionally, many clinicians have had the displeasure of coaching an athlete or individual for months and observing that while they demonstrate textbook jumping mechanics in the clinic, as soon as they hit the field they often revert to their default programming. After digging into this concept, we have found a wealth of research proposing a mechanism to correct this and it lies in our brains, not our joints (Wulf. 2013). While this is given many names (cerebellar training, reflexive training, CNS facilitation etc.), one thing has become clear: for an individual to truly learn a new movement, it must be coached entirely different. What we have strived to do in this version of our protocol is establish an external focus for an individual when performing a movement rather than an internal focus. How many times have you been told to keep your knee over your 2nd and 3rd toe when performing a step-down? Or to “land with your hips and knees flexed” when jumping? An external locus of focus would be coaching the above movements by telling an athlete to “try and touch the cone when you step down” or to “push the ground away and land soft” when jumping. This maybe a simplification but it is perhaps one of the biggest changes we can implement with our ACL patients to ensure a new movement pattern that may reduce their risk of injury is not only able to be demonstrated in the clinic, but will also show up under duress in a competitive environment (Wulf, G. 2013). In summary, Nevada Physical Therapy and the University of Nevada-Reno Sports Medicine Department hope to advance the “ACL rehabilitation for athletes” conversation by adding a deadlift progression model to facilitate appropriate hip musculature activation and functional hamstring strengthening while protecting the graft, an evidence-based core/functional movement assessment and exercise prescription consistent with the Selective Functional Movement Assessment, and prioritizing equal lower extremity output during compound multi-joint movements more consistent with strengthening the platforms all athletic movements require. The goal of this rehab model is to improve the athlete to not only their prior level of function but also beyond it as their pre-existing risk factors need to be identified, addressed and corrected.

Bone-tendon-Bone Anterior Cruciate Ligament Rehabilitation Protocol for Athletes

Common Risk Factors for ACL Injury

• Hamstring Weakness/Decreased Hamstring to Quad Strength ratio (>66% for males, >75% for females)

• Valgus collapse/Poor Jumping and Landing Mechanics

• History of ACL injury (contralateral/ipsilateral)

• Core Weakness

Pre-Operative Status. (21 days post-op)

“Because of the large impact that quadriceps strength may have on knee function, the identification and treatment of quadriceps weakness prior to ACL reconstruction are paramount in maximizing patient outcomes (Adams D, 2012 Mar 8. See also Tables 1 and 2).”

The primary goals of the pre-operative patient are as follows:

• Decrease knee effusion and pain to within normal levels

• Improve Range of Motion to uninvolved limb

• Normalize gait

• Maximize voluntary quadriceps contraction and strength Therapeutic Exercise

• Wall slides

• Isometric Quad sets/Straight leg raises

• Supine hamstring curls

• 4-way hip strengthening

• Clamshells/lateral theraband shuffle

• Double-leg/Single-leg leg press as able

PO Days 1-6:

Goals

• Protect Graft Fixation • Control inflammation. • Facilitate full extension • Gait Training • Rehab progression education

Therapeutic Exercise

• Weight bearing as tolerated. • Supine Wall Slides, limited to 90° degrees flexion first 2 weeks, 130° first 4 weeks. • Quad Sets with small towel under knee • 4-way ankle TheraBand strengthening • Hip abduction if able to achieve appropriate quad contraction • Standing Hamstring curls if no pain reported • Blood Flow Restriction (BFR) implementation as tolerated

End of Week AROM: PROM: 1 0-80° 0-90° 2 0-105° 0-120 3 0-120° 0-125°

Weeks 2-4

Goals

• Continue to facilitate normal gait. • VMO activation improvement/Normalize Straight Leg Raise • Avoid knee over toes in all closed-chain exercises. • Do not progress flexion greater than 125° • Weight-bearing exercises should only be progressed once adequate quad function is restored. • Normalize Patella Mobility

Therapeutic Exercise

• Bike as ROM allows, use for ROM facilitation if flexion less than 100° • Straight leg raise if able to perform with no extensor lag o Begin core cue work, TA/oblique activation in conjunction • 4-way hip strengthening o Avoid lax VMO activation with accessory planes (abd, add, ext) • Bilateral balance/weight shift work. • Standing Terminal Knee Extension • Single-leg balance work, progress as able • Hamstring bridging if ROM permits • Double-leg squats 0-30° • Stool scoots • Step-ups/downs (week 3-4) with attention to minimal anterior translation of tibia over toes. • Lateral Gait Progressions (Hurdle step over, banded step over, etc.) • Lateral Agilities, band around knee

SFMA/Accessory Work:

• Initiate upper extremity (2x1) rolling patterns • Assisted (2x1) hip flexion with SLR • Assisted hamstring/glute bridge (2x2) as appropriate • Assisted Squat and deadlift patterns as appropriate with ROM limitations • McGill Sit-ups with concurrent quad set, Overhead TA drills

Weeks 4-8

Remember: Progressions to weight bearing activities should come only after full extension and adequate quad function have been achieved although let clinical presentation dictate progressions.

Goals

• Decrease knee pain and effusion. • ROM within Normal limits • Goals as above.

Therapeutic Exercise

• As above, progressed as appropriate • Continue BFR programming, begin leg press progressions. • Single-leg squats 0-45° with forward trunk flexion. • Good Mornings, Hip Hinge patterns (BPTB) • Single leg Romanian Deadlifts (with trunk flexion) with can touch, progress weight as appropriate • KB Romanian Deadlifts, avoid anterior tibial translation as mentioned • Single-leg balance work with external stimuli • May begin swimming (week 6), avoid whip kick

SFMA/Accessory Work: • Quadruped work as appropriate • Bridge progressions. • Lower Extremity rolling patterns

Weeks 8-12

Goals

• Progress gait to include stair-climbing • Increase lower extremity strength, Discharge BFR protocol. • Increase proprioception, reactive progressions • Optimize force production/absorption • Begin Power Phase (2-day split, Hip Dominant vs Knee Dominant)

Therapeutic Exercise

• Progressive bike resistance/intensity • Knee Dominant Day: Front/Goblet KB squats, lunge progressions, step-up height etc. • Hip Dominant Day: Double Leg RDLs, Single Leg RDLs, Side Planks, Elevated/Rack/Box Barbell Deadlifts as appropriate • Nordic Trak/Elliptical • Initiate lateral lunge work, attention to limit tibia over toes • Progress previous therapeutic exercise above • Begin pool running (10-12 weeks) • Light leg sled plyometrics (30% of body weight)

SFMA/Accessory Work: • Progress above • Initiate kettlebell top-down deadlift patterns by week 4-6 if BPTB, later if HS graft Month 3-4

Progression Criteria: • No patella-femoral pain • 0-120 degrees of motion • Sufficient strength and proprioception to initiate running. • Minimal swelling/inflammation.

Goals

• Full Range of Motion • Begin light sport-specific strength training, see jump protocol • Continue Power Phase, Strength 70% of uninvolved lower extremity. • Avoid unnecessary graft strain.

Therapeutic Exercise

• As above • Leg press to tolerance • Initiate Barbell Squat Progressions (Box squat-> low bar-> High bar-> Front Squat) • Begin swimming • Advanced proprioception exercises. • Begin Jump Protocol if patient is able to complete greater than 15 single leg box squats to parallel with minimal valgus collapse. NOTE: Blocks are 2-4 weeks, 3-6 reps x 3-6 sets, 3x/week.

Block 1 No gravity (jump to elevation, step down) Day 1 Day 2 Rest Day 4 Day 5 Rest Box-Jumps SL Box Hops Recovery SL Lateral Jump Lateral Box Hops Recovery 3-6 sets, 3-6 reps, Block is 2-4 weeks Block 2 Gravity Introduction Day 1 Day 2 Rest Day 4 Day 5 Rest Hurdle Jump SL Hurdle Hop Recovery SL Lateral Line Hops Lateral Hurdle Hops Recovery 3-6 sets, 3-6 reps, Block is 2-4 weeks Block 3 Gravity/Elastic (begin secondary movement intorduction) Day 1 Day 2 Rest Day 4 Day 5 Rest Hurdle Jump Acc. SL Clock Jump Recovery SL Ski Hop Progression SL Hurdle Hop (Multi) Recovery 3-6 sets, 3-6 reps, Block is 2-4 weeks Block 4 Sport Specific (begin sport training) Day 1 Day 2 Rest Day 4 Day 5 Rest Hurdle Jump Acc. SL Clock Jump w. Rotation Recovery SL Ski Hop Progression SL Hurdle Hop (Multi) Recovery 3-6 sets, 3-6 reps, Block is 2-4 weeks Criteria for Progression: No increase in swelling, no pain, no subjective complaint of mechanical symptoms. Athlete must be able to control landing, normalized jump mechanics: hip and knee flexion, valgus correction, etc.

SFMA/Accessory Work • Front planks/Side plank progression • High Kneel Chops/Lifts • Kettlebell box step downs/ups from plyo box • Kettlebell rack walks/core work/suitcase carry Months 4-6

Goals

• Symmetric performance of basic and sport specific agility drills • Single hop and 3 hop tests 85% of uninvolved lower extremity at 6 months post-op (if able to demonstrate greater than 90% uninvolved leg strength symmetry) • Quadriceps/Hamstring strength at least 85% of uninvolved lower extremity • Establish Chronic Workload parameters (4-6 weeks Accumulation Phase) • Phase 3 Progression (Continued Power progressions, sport-specific training)

Therapeutic Exercise

• Agility Progressions o Side steps/crossovers/shuttle run/single leg and double leg jumping o Ladder drills/acceleration/deceleration sprints • Side plank progressions • Begin level-ground running (see progression/soreness models) • Continue Barbell Squat/Deadlift and Single Leg Patterns

Months 6-Return to Sport • Continue barbell squat and deadlift training o Patient should demonstrate greater than 75% of uninvolved limb by month 7 as measured by Keiser Machines/Biodex. o Patient should demonstrate greater than 85% of uninvolved limb by month 8 as measured by Keiser Machines/Biodex. o Single Hop (single hop. Crossover hop, triple hop) testing greater than 85% of uninvolved limb by month 9. • Jump Progressions with external cueing focus, fatigue protocols, progressing single-leg and change of direction tasks. • Acute: Chronic ratio should be near 75% of in-season workload by end of month 8.

Additional Considerations:

Athlete Progression Timeline

Weeks 1-8 (Or Until Phase 1 goals Nevada Physical Therapy Achieved)

Weeks 9-16 Co-treat Nevada PT/Sports Medicine (monthly check-ups with NVPT)

Weeks 17-24 Nevada Sports Medicine/Strength (monthly check-ups with NVPT)

Months 6-9 UNR Strength and Conditioning (monthly check-ups with NVPT)

Table 1 Soreness Rules*

Soreness during warm-up that continues 2 Soreness during warm-up that goes away days off, drop down 1 level Stay at level that led to soreness

Soreness during warm-up that goes away but Soreness the day after lifting (not muscle redevelops 2 days off, drop down 1 level soreness) 1 day off, do not advance during session program to the next level

No soreness Advance 1 level per week or as professional instructed by healthcare

*Reprinted with permission from SAGE Publications: Fees M, Decker T, Snyder-Mackler L, Axe MJ. Upper extremity weight-training modifications for the injured athlete. A clinical perspective. Am J Sports Med. 1998;26(5):735. Copyright ©1998 SAGE Publications.

Table 2 Running Progression*

Level 1 0.1-mi walk/0.1-mi jog, Jog straights/walk curves (2 mi) repeat 10 times

Level 2 Alternate 0.1-mi Jog straights/jog 1 curve every other lap (2 mi) walk/0.2-mi jog (2 mi)

Level 3 Alternate 0.1-mi Jog straights/jog 1 curve every lap (2 mi) walk/0.3-mi jog (2 mi) Level 4 Alternate 0.1-mi Jog 1.75 laps/walk curve (2 mi) walk/0.4-mi jog (2 mi)

Level 5 Jog full 2 mi Jog all laps (2 mi)

Level 6 Increase workout to Increase workout to 2.5 mi 2.5 mi

Level 7 Increase workout to 3 Increase workout to 3 mi mi

Level 8 Alternate between Increase speed on straights/jog curves running/jogging every 0.25 mi

*Progress to next level when patient is able to perform activity for 2 mi without increased effusion or pain. Perform no more than 4 times in 1 week and no more frequently than every other day. Do not progress more than 2 levels in a 7-day period. Conversion: 1 mi = 1.6 km. Reprinted with permission from Tara Manal, University of Delaware Physical Therapy Clinic.

Return to Sport Criteria for Division I athletes:

Dr. Freddie Fu recently presented MRI studies documenting graft type healing rates and makes the argument that early return to sport clearance should be supported by imaging to support an adequately healed graft. The below images were give during a recent webinar (Freddie Fu, MD, 2014):

Keeping this new information on graft healing rates and the above timeline progression in mind, we are able to form a more complete return to sport model. The following parameters use established return to sport baselines as well as goals unique to Nevada Physical Therapy and the University of Nevada-Reno Sports Medicine Department.

Itemized Return to Sport Testing List: (Pass, Fail, risk description)

1) Quad Girth. PASS/FAIL: Persistent quadriceps strength has been linked to poor Return to Sport testing; “QF strength deficits predicted hop test performance beyond the influences of graft type, presence of meniscus injury, knee pain, and knee symptoms.” (Scmitt et al., 2012) (Knapik et al., 1991)

2) Valgus with Jumping Tasks, PASS/FAIL “knee abduction moment, which directly contributes to dynamic lower extremity valgus, was a significant predictor for future ACL injury risk” (Hewett et al., 2005)

3) Core Weakness, PASS/FAIL Decreased Single Leg Squat and/or Decreased Side Plank/Lateral Chain Strength may be predictive of ACL re-injury (Hegedus et al. 2016)

4) Quad:Hamstring Ratio, PASS/FAIL Research suggests decreased quadriceps to hamstring strength may be a predictor for ACL re-injury (Knapik et al., 1991) and “for every 10% decrease in the hamstring to quadriceps strength ratio there was a 10.6 times higher risk of sustaining an ACL graft rupture (Kyritsis et al., 2016)

5) IKDC, PASS/FAIL. “reduced IKDC score distinguish patients who are unable to return to preinjury sports participation because of fear of reinjury/lack of confidence” (Lentz et al., 2015)

6) Single, Triple Hop. PASS/FAIL; Decreased single and triple hop testing is correlated with increased risk of re-injury. (Kyritsis et al., 2015)

7) Anterior Y-Balance PASS/FAIL; Athletes who had a greater than 4cm difference in anterior Y-balance scores at 12 weeks were shown to not achieve 90% or greater lower limb symmetry at return to sport testing. (Garrison et al., 2015) 8) T-test, PASS/FAIL; Decreased T-test speeds compared to normative data are associated with increased ACL re-injury. (Kyritsis et al., 2015)

9) Chronic Workload Established, PASS/FAIL “when an athlete's training and playing load for a given week (acute load) spikes above what they have been doing on average over the past 4 weeks (chronic load), they are more likely to be injured” (Gabbett et al. 2015) “tendons are at lowest risk with consistent workloads and susceptible to injury with sudden upgrades in workload” (Orchard et al., 2015)

10) Complete Nevada PT Return to Sport Testing, PASS/FAIL. Completing the established Return to Sport Criteria detailed above may decrease re-injury by 84% (Grindem et al., 2016) and has been show to facilitate a 4x higher rate of return to elite athletics (Kyritsis et al., 2015).

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Anderson AF, I. J., & Committee., I. K. (2006 Jan). The International Knee Documentation Committee Subjective Knee Evaluation Form: normative data. . Am J Sports Med, 34(1):128-35.

Ardern CL, W. K. (2011). Return to sport following anterior cruciate ligament reconstruction surgery: a systematic review and meta-analysis of the state of play. . Br J Sports Med, 45:596-606.

Blanch P, Gabbett T., (2015) Has the athlete trained enough to return to play safely? The acute:chronic workload ratio permits clinicians to quantify a player's risk of subsequent injury. Brit J Sports Med, 2015

Escamilla RF, F. A. (2002 Apr). An electromyographic analysis of sumo and conventional style deadlifts. Med Sci Sports Exerc, 34(4):682-8.

Garrison et al., (2015) Y Balance Test Anterior Reach Symmetry at Three Months is Related to Single Leg Functional Performance at Time of Return to Sports Following Anterior Cruciate Ligament Reconstruction. Int J Sports Phys Ther,

Grindem et al., (2016) Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study. Brit J Sports Med; Myer, K. F. (2009). The relationship of hamstrings and quadriceps strength to anterior cruciate ligament injury in female athletes. Clin. J. Sport Med, 19, pp. 3–8.

Hamlyn N, B. D. (2007 Nov). Trunk muscle activation during dynamic weight-training exercises and isometric instability activities. J Strength Cond Res, 21(4):1108-12.

Hegedus et al., (2016) Physical performance tests predict injury in National Collegiate Athletic Association athletes: a three-season prospective cohort study. Brit J Sports Med;

Hewett et al., (2005) Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study, Am J Sports Med Hewett TE, M. G. (February 2006). Anterior Cruciate Ligament Injuries in Female Athletes: Part 1 mechanisms and risk factors. Am J Sports Med, vol. 34 no. 2 299-311.

Knapik et al., (1991) Preseason strength and flexibility imbalances associated with athletic injuries in female collegiate athletes, Am J Sports Med;

Krosshaug T, Nakamae A, Boden BP, et al., (2006) Mechanism of anterior cruciate ligament injury in basketball: video analysis of 39 cases. Am J Sports Med;

Kyritsis et. al, (2015) Likelihood of ACL graft rupture: not meeting six clinical discharge criteria before return to sport is associated with a four times greater risk of rupture. Brit J Sports Med;

Lentz et al., (2015) Comparison of physical impairment, functional, and psychosocial measures based on fear of reinjury/lack of confidence and return-to-sport status after ACL reconstruction. Am J Sports Med;

Orchard et al., (2015) Cricket fast bowling workload patterns as risk factors for tendon, muscle, bone and joint injuries. Br J Sports Med. Rabuck SJ, B. M. ( 2013 Jan). Anterior cruciate ligament healing and advances in imaging. Clin Sports Med, 32(1):13-20.

Scmitt et al., (2012) The impact of quadriceps femoris strength asymmetry on functional performance at return to sport following anterior cruciate ligament reconstruction. J Orthop Sports Phys Ther; Weinhandl JT, E.-B. J. (June 2014). Reduced hamstring strength increases anterior cruciate ligament loading during anticipated sidestep cutting. Clin Biomech.

Wilk KE, M. L. (2012 Mar). Recent Advances in the Rehabilitation of Anterior Cruciate Ligament Injuries. J Orthop Sports Phys Ther. , 42(3):153-71.

Xergia SA, P. E. (2013 Mar). Asymmetries in functional hop tests, lower extremity kinematics, and isokinetic strength persist 6 to 9 months following anterior cruciate ligament reconstruction. J Orthop Sports Phys Ther., 43(3):154-62.