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North American Congress on Biomechanics Canadian Society for Biomechanics - American Society of Biomechanics University of Waterloo Waterloo, Ontario, Canada August 14-18, 1998 |
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In order to rehabilitate an injured shoulder effectively, the clinician must understand the demands placed on specific muscles by the various exercises available. In this study, the activity of five shoulder muscles was quantified during seven rehabilitation exercises.
Patients with posterior shoulder injuries are commonly encountered by physicians, therapists, and athletic trainers in both the general orthopaedic and sports medicine population (Wilk et al., 1997). Since shoulder muscles function in both arm movement and dynamic shoulder stabilization, rehabilitation is often difficult. Various external rotation and horizontal abduction maneuvers are generally accepted as good exercises for strengthening the posterior shoulder, especially the rotator cuff muscles. However, there is a controversy about which variations are optimal for shoulder strengthening and testing (Malanga et al., 1996). The purpose of this study was to compare posterior shoulder muscle activity during various rehabilitation exercises.
Ten subjects (5 male, 5 female) with no history of shoulder injury volunteered as subjects. Pairs of 27-gauge fine wire electrodes (Nicholet Biomedical, Madison, WI) were inserted into the supraspinatus (SUP), infraspinatus (INF), teres minor (TM), posterior deltoid (POS), and middle deltoid (MID) muscles of the dominant shoulder of each subject. Synchronized EMG signals from the five muscles were recorded at 1000 Hz with a Myosystem 2000 (Noraxon USA, Inc., Scotsdale, AZ). Signals were converted from analog to digital data and stored with an ADS system (Motion Analysis Corp., Santa Rosa, CA). Three-dimensional motion of reflective markers attached to bony landmarks on the dominant wrist, dominant elbow, both shoulders, and both hips were captured with a motion analysis system (Motion Analysis Corp., Santa Rosa, CA). The EMG and motion analysis systems were synchronized electronically.
Data collection for each subject began with a series of isometric exercises selected to solicit maximum voluntary isometric contraction (MVIC) of all muscles tested. Then, the subject performed seven exercises with dumbbells, in random order. Six of these were the subject performing external rotation, with 90° abduction (ER@90), in prone position with 90° abduction (PRONE), with 0° abduction (ER@0), with a towel pressed between elbow and hip (TOWEL), while lying on side (SIDE), and with the arm in the scapular plane (SCAP). (When the arm is in the scapular plane, it is abducted approximately 45° and horizontally adducted approximately 30°.) The other exercise, first defined by Blackburn et al. (1990), was horizontal abduction in prone position, with the arm in 100°abduction and full external rotation (BLACK). Tempo was set by a metronome, resulting in approximately three full repetitions per ten-second trial.
EMG data were rectified, integrated, and normaluzed as a percentage of the activity achieved during MVIC (%MVIC). Shoulder abduction, external rotation, and flexion, and elbow flexion were calculated from the motion analysis data. Maximum EMG values for each muscle were averaged for the multiple repetitions within each trial, identified by arm motion, and then averaged among all subjects.
Maximum activity of each muscle during each exercise is shown in Table 1.
| SUP | INF | TM | POS | MID | |
|---|---|---|---|---|---|
| ER@90 | 55 | 45 | 50 | 55 | 60 |
| PRONE | 80 | 55 | 50 | 80 | 55 |
| ER@0 | 50 | 35 | 25 | 15 | 10 |
| TOWEL | 35 | 50 | 35 | 25 | 5 |
| SIDE | 60 | 65 | 65 | 75 | 50 |
| SCAP | 35 | 55 | 65 | 60 | 25 |
| BLACK | 100 | 50 | 50 | 90 | 95 |
Table 1. Maximum muscle activity, averaged for all subjects, expressed as %MVIC.
The greatest activity for the supraspinatus and deltoid muscles were observed during BLACK. This was consistent with the data from Malanga et al. (1996), which also showed high activity of these muscles during the Blackburn exercise (Table 2).
| SUP | INF | TM | POS | MID | |
|---|---|---|---|---|---|
| Current study | 100 | 50 | 50 | 90 | 95 |
| Malanga et al. | 94 | 72 | 66 | 96 | 111 |
Table 2. Comparison of maximum muscle activity (%MVIC) during Blackburn exercise, between the current study and Malanga et al. (1996).
High activity of the SUP and POS were also observed during PRONE. The high activity during both BLACK and PRONE may be in part due to stabilization of the prone trunk by the table. Like PRONE, ER@90 is an external rotation exercise performed with the arm abducted 90°. These exercises have functional advantages for rehabilitation, as several activities (such as overhand throwing) are performed in a similar position.
The moderate activity of all muscles tested during ER@90 supports the use of this exercise when a 90° functional position is desired. While ER@90 does have a functional advantage, the combination of abduction and external rotation places high strain on the shoulder's capsule, particularly the anterior band of the inferior glenohumeral ligament (Wilk et al., 1997). When the arm is not abducted, external rotation places less strain on this ligament. Therefore, although muscle activity was low to moderate during ER@0, this rehabilitation exercise may be worthwhile when strain of the inferior glenohumeral ligament is of concern. SIDE may be a better choice than ER@0, as both include external rotation with 0° of abduction, but all five muscles demonstrated higher activity during SIDE.
Theoretically, SCAP may be the best shoulder external rotation exercise, as it offers a compromise between the functional benefit of ER@90 and the reduced capsular strain of ER@0. Furthermore, since the long axis of the humerus intersects near the center of the glenoid fossa of the scapula when the arm is in this position, humeral head translation during external rotation may be minimized.
One problem with trying to maintain glenohumeral stability during external rotation is imbalance in force generation between superior and inferior shoulder muscles. Theoretically, TOWEL provides both the low capsular strain offered by ER@0 and also a good balance between the superior shoulder muscles that externally rotate the arm and the inferior shoulder muscles that adduct the arm to hold the towel. However, this study showed minimal difference in muscle activity between TOWEL and ER@0.
In summary, results from this study can help clinicians choose the exercises that best fit their objectives. Exercises in the prone position (BLACK and PRONE) produced maximum muscle activity of the SUP and POS muscles. All exercises produced moderate activity of the INF and TM muscles. Of the exercises designed to produce minimal capsular strain (ER@0, TOWEL, SIDE), SIDE produced the greatest muscle activity. SCAP is a good choice to be incorporated into most rehabilitation programs, as it offers a good balance among functionality, joint stability, and muscle activity.
While muscle activity during different exercises was measured in this study, muscle force was not quantified. Also, this study investigated only a limited number of exercises. Future research on other exercises, such as those with rubber tubing or plyometrics, would be helpful.
Blackburn T.A. et al., Athl Training, 25, 40-45, 1990.
Malanga G.A. et al. Med Sci Sports Exerc, 28, 661-664, 1996.
Wilk K.E. et al. JOSPT, 25, 364-379, 1997.
The authors would like to thank David Downs for his assistance in data collection.