Shoulder musculature activity and stabilization during upper extremity weight-bearing activities

Upper extremity weight-bearing exercises are routinely used in physical therapy for patients with shoulder pathology. However, little evidence exists regarding the demand on the shoulder musculature.

To examine changes in shoulder muscle activity and center of pressure during upper extremity weight-bearing exercises of increasing difficulty.

Electromyographic (EMG) and kinetic data were recorded from both shoulders of 15 healthy subjects (10 male and 5 female). Participants were tested in a modified tripod position under three conditions of increasing difficulty: (1) hand directly on the force plate, (2) on a green Stability Trainer(™) and (3) on a blue Stability Trainer(™). Ground reaction forces were recorded for each trial. Surface EMG was recorded from the serratus anterior, pectoralis major, upper trapezius, lower trapezius, infraspinatus, anterior deltoid, posterior deltoid, and the lateral head of the triceps muscles.

Mean deviation from center of pressure significantly increased when using the Stability Trainer(™) pads. The activities of the triceps, serratus anterior, and anterior deltoid muscles significantly increased as each trial progressed, irrespective of stability condition. Additionally, activity in the anterior deltoid, lower trapezius, and serratus anterior muscles significantly decreased with increasing difficulty, whereas activity in the triceps muscles significantly increased.

Balancing on a foam pad made it more difficult to maintain the upper extremity in a stable position. However, this activity did not alter the proprioceptive stimulus enough to elicit an increase in shoulder muscle activation. While the results on this study support the use of different level Stability Trainers(™) to facilitate neuromuscular re-education, a less compliant unstable surface may produce larger training effects.