Research Review by Dr. Shawn Thistle©



Study Title:

Reflexive muscle activity alterations in shoulders with anterior glenohumeral instability


Myers JB, Ju YY, Hwang JH, McMahon PJ, Rodosky MW, Lephart SM

Publication Information:

American Journal of Sports Medicine 2004; 32(4): 1013-1021


Shoulder stability refers to proper alignment of the humerus in the glenoid fossa by equalizing forces around the shoulder, hence preventing excessive movement. Shoulder instability can be associated with trauma, but in many cases, it is atraumatic. Either of these etiologies can become recurrent, which can drastically affect athletic performance. Therefore, it is an important condition to be aware of from a clinical and rehabilitation standpoint for manual medicine practitioners.

It has been well established that people who suffer recurrent shoulder instability have reduced proprioceptive capacity around the shoulder. That is, they have diminished ability to sense, hence control the shoulder through various ranges of motion. This study aimed to quantify specific muscle activation deficiencies that may accompany this lack of proprioception in patients with anterior glenohumeral instability.

Eleven subjects diagnosed with traumatic, recurrent (defined as 3-25 episodes) anterior shoulder instability took part in the study. All patients were non-surgical at the time of the study and overhead athletes were excluded due to the neuromuscular adaptations these patients undergo due to their sports activities.

Experimental subjects were matched to a control group by age, height, weight and limb dominance and involvement. Electromyographic (EMG) analysis was performed using indwelling electrodes in the supraspinatus, infraspinatus and subscapularis as well as surface electrodes on the sternal portion of pectoralis major, anterior deltoid, latissimus dorsi and biceps brachii.

Subjects were placed in a shoulder apprehension apparatus with the affected arm at 90 abduction and 30 of horizontal adduction in the scapular plane. After a baseline of maximum active external rotation from this position was established, the starting position was set at 35 before that angle. A perturbation was then introduced into the apparatus by a lever arm striking the limb at 180 /second (trying to force the arm into further external rotation - a position that normally aggravates the condition and brings on symptoms of shoulder instability).

All visual, auditory and tactile cues were removed so the patients could not brace for these impacts. Muscle reflex latency (i.e. how long it took the muscle to activate) and peak activation was measured for each of the seven muscles, and patterns of activation were also noted.

Patients with instability demonstrated suppressed pec major and biceps activation and increased peak activation of the subscapularis, supraspinatus and infraspinatus compared to the control subject. Further, the patient group exhibited significantly longer biceps reflex latency and had significantly less supraspinatus/subscapularis co-activation than the control group.

Conclusions & Practical Application:

This study found that "…patients diagnosed with recurrent traumatic anterior instability displayed altered neuromuscular control of the shoulder dynamic stabilizers. Shoulder muscle dysfunction like that seen in the current study can have dramatic effects on the resulting glenohumeral joint forces, thereby affecting joint stability and possibly contributing to the recurrence commonly seen in patients with glenohumeral instability" (p. 1017).

Some specific implications for rehabilitation of this condition become evident when looking at the data from this study. First, the reduced biceps activation and increased reflex latency should be addressed in a rehab program, due to the recent finding that the biceps is an important anterior stabilizer for the shoulder. The pec major is also thought to be an important anterior stabilizer by some authors, although this theory remains controversial due to conflicting evidence.

Second, the suppressed coactivation of the supraspinatus/subscapularis may represent some disconnect between the force couple mechanism that should exist between the rotator cuff muscles and the deltoid. This force couple is essentially responsible for centering the humeral head in the glenoid fossa during movement and normally involves contraction of the entire rotator cuff.

The specific deficiency noted in this study (supraspinatus/subscapularis) would contribute to a lack of anterior-posterior stability in these patients, which may help to explain the recurrent nature of their instability. This deficiency could be addressed with closed - or open - chain exercises, rhythmic stabilization protocols etc.

Some limitations of this study which should be noted are:
  1. the direction of perturbation - was only into external rotation, whereas usual forces that put the shoulder at risk could include a component of horizontal abduction.
  2. the setup position - was in the scapular plane and not the traditional "apprehension" position (which is in the frontal plane). The authors did acknowledge this and referred to previous research that established no significant EMG patterns between the two positions.