38 Research Report Isolated Activation of the Infraspinatus Muscle in Four Manual Muscle Testing Positions In-cheol Jeon, PT, Ph.D1; Sung-min Ha, PT, Ph.D2; Sung-hoon Jung, PT, Ph.D3 1Department of Physical Therapy, College of Life and Health Science, Hoseo University, South Korea 2Department of Physical Therapy, College of Health Science, Sangji University, South Korea 3Department of Physical Therapy, College of Health Science, Yonsei University, South Korea Background To normalize infraspinatus muscle activity among different exercises or positions, it JMST 2018; 2(2): 38-42 is necessary to isolate the infraspinatus from the surrounding musculature. Published Online Dec 31, 2018 Purpose We sought to determine the best position in which to obtain an isolated infraspinatus pISSN 2635-8573 eISSN 2635-8581 electromyography (EMG) signal for normalization of the infraspinatus muscle in four selected positions. Article History Received 02 October 2018 Revised 02 October 2018 Study design Comparative, repeated measures design. (1st) Revised 01 November 2018 (2nd) Methods To measure infraspinatus maximal voluntary muscle contraction (MVIC), 13 healthy Accepted 13 November 2018 participants performed four test positions (side-lying with the shoulder adducted, prone, shoulder abducted, and side-lying wiper external rotation [SWE]). EMG activity was measured in the CONTACT [email protected] infraspinatus, middle trapezius, and posterior deltoid muscles. Sung-min Ha, Department of Physical Results The EMG ratio was significantly higher for SWE than for the other test positions. To Therapy, College of Health Science, Sangji maximize the EMG ratio, high infraspinatus activity and low surrounding muscle (middle University, South Korea trapezius and posterior deltoid) activity are required. This is an Open-Access article distributed under the terms of the Creative Commons Att- Conclusions We recommend switching from prone external rotation to SWE to normalize the ribution Non-Commercial Li- cense (http://creativecommons. infraspinatus EMG signal. org/licenses/by-nc/4.0) which permits unrestricted non-co- mmercial use, distribution, and reproduction in any me- Key words Electromyography; Infraspinatus; Normalization; Side-lying wiper external rotation. dium, provided the original work is properly cited. INTRODUCTION commonly take measurements while the subject is in the prone position5–8. However, the prone position can lead to The most common way to generate a reference level for over-activation of the posterior deltoid, and sub-optimal muscle activation to normalize infraspinatus electromyo- activation of the infraspinatus contributes to an increase in graphy (EMG) data is to record the maximum voluntary anterior humeral gliding in the glenoid9,10. From this point isometric contraction (MVIC) to identify the effects of of view, increased translation of the instantaneous center of exercise or intervention on infraspinatus muscle strengthen- rotation of the humeral head may be associated with muscle ing1–3. Unlike other normalization methods, including sub- imbalance between the rotator cuff (e.g., infraspinatus) and MVIC and reference voluntary contraction (RVC), MVIC is the scapulohumeral (e.g., posterior deltoid and middle advantageous because it has physiological meaning; derived trapezius) muscles8. data are expressed relative to the maximum4. Recent shoulder external rotation exercises focus on For infraspinatus normalization, clinicians or researchers decreasing the contribution of the posterior deltoid to https://doi.org/10.29273/jmst.2018.2.2.38 Journal of Musculoskeletal Science and Technology Isolated Activation of the Infraspinatus Muscle in Four Manual Muscle Testing Positions 39 shoulder external rotation to strengthen the infraspinatus Procedure 2, 11 muscle isolately . Adducted shoulder position isolates the The dominant arm, defined as the upper extremity used to infraspinatus from the posterior deltoid, and side-lying eat and write, was tested for all participants. All participants external rotation exercise may be the most effective ma- were right-arm dominant, as determined by a questionnaire. 5,6 neuver to increase infraspinatus activity in isolation . To A target bar was placed such that the radial border of the normalize infraspinatus muscle activity among different participant’s wrist touched it in each test position. To exercises or positions, it is necessary to isolate the infra- measure MVIC in the side-lying with the shoulder adducted spinatus from the surrounding musculature. (ADD) position, subjects lay on their sides with the Several studies have been conducted to identify methods shoulder in a neutral position and the elbow at 90° flexion. 7,12 to normalize the infraspinatus muscle . However, there From the starting position, with the forearm resting on the has been no attempt to investigate the optimal MVIC test abdomen, subjects moved the tested arm to a position of position for the isolation of the infraspinatus muscle. The external rotation (45°) until the dorsal forearm touched the objective of this study was to determine the optimal position target bar and maintained the shoulder at 45° external among four tested positions in which to obtain an isolated rotation6. The subject then performed maximal contraction infraspinatus EMG signal for the normalization of in- of the dominant side arm by applying manual resistance to fraspinatus muscle EMG activity. pressure by an examiner to the subject’s wrist. All subjects were given consistent verbal encouragement during maxi- Subjects mal contraction. In the prone position, the subject was The participants in this study were 13 subjects with no required to lie prone with the shoulder abducted at 90° and known shoulder dysfunction. The mean age, weight, and the elbow flexed to 90°, while the forearm was in neutral height of the subjects were 25.2±3.9 years, 60.3±1.2 kg, and position5. Additionally, the subject’s elbow was fastened to 168.4±5.3 cm, respectively. Before the initial measurements the table using a non-elastic belt to prevent compensatory were conducted, each subject completed a health question- shoulder motion. To measure MVIC in the prone position, naire and provided informed consent. A screening evalua- the subject moved to a position with shoulder external tion of the cervical spine and both upper extremities was rotation to 90°. The subject then performed maximal performed by a physical therapist. contraction of the dominant side arm by applying manual resistance to pressure by an examiner to the subject’s wrist. Instruments All subjects were given consistent verbal encouragement EMG data were collected using a Noraxon TeleMyo during maximal contraction. To measure MVIC in shoulder 2400T and analyzed using MyoResearch Master Edition abducted (ABD) position, the participant stood with the 1.06 XP software (Noraxon Inc., Scottsdale, AZ, USA). dominant arm abducted and internally rotated to 90° and Skin preparation of the electrode sites involved shaving and the elbow flexed to 90°. From the starting position, the cleaning with rubbing alcohol. Surface electrode pairs were participant moved the extremity to a position of full lateral positioned at an interelectrode distance of 2 cm. The rotation until the wrist joint touched the target bar. The reference electrode was placed on the ipsilateral clavicle. subject then performed maximal contraction of the domi- EMG data were collected for the following muscles: middle nant side arm by applying manual resistance to pressure by trapezius (midway on a horizontal line between the root of an examiner to the subject’s wrist. All subjects were given the spine of the scapula and the third thoracic spinous consistent verbal encouragement during maximal contrac- process), upper trapezius (midway between C7 and the tion. To measure MVIC in the side-lying wiper external acromion), posterior deltoid (lateral border of the spine of rotation (SWE) position, the participant lay in the side-lying the scapula and angled on an oblique angle toward the arm position with the extremity flexed and internally rotated to running parallel to the muscle fiber), and infraspinatus (4 90° and the elbow flexed to 90°. The participant supported cm below the spine of the scapula, on the lateral aspect over the distal humerus of the dominant arm with the palm of the the infrascapular fossa of the scapula). The sampling rate opposite hand. From the starting position, the participant was 1,000 Hz. The raw signal was full-wave rectified and moved to a position of full lateral rotation until the wrist filtered using a Lancosh FIR digital filter. The band-pass joint touched the target bar. We instructed the participant to filter frequency was between 20 and 300 Hz. EMG data use the opposite hand to support the dominant arm during were converted to root mean square (RMS) values, which external rotation to maintain the center of rotation in the were calculated from 50 ms windows within the data points. same position. Vol. 2, No. 2, Dec. 2018 Journal of Musculoskeletal Science and Technology 40 Isolated Activation of the Infraspinatus Muscle in Four Manual Muscle Testing Positions The familiarization period was completed when the EMG activity among four selected positions. Kelly et al. subject was able to maintain three normalization methods (1996) reported that standardizing normalization test for 5 s. All subjects were comfortable after the familiariza- positions for MVIC is one step toward the elimination of tion period, and none reported