Electromyographic Analysis of Rotator Cuff Muscle Activity During Shoulder Immobilization: Standard Sling versus Abduction Sling in Filipinos Standard Sling versus Abduction Sling in Filipinos
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Abstract
Objectives. To compare supraspinatus and infraspinatus muscle activity using needle electromyography (EMG) between a standard arm sling and an abduction sling in healthy Filipino adults.
Methodology. This experimental pilot study included 34 healthy Filipino adults aged 18–40 years without prior shoulder pathology. Needle electromyography was performed by a single licensed and experienced electromyographer. Baseline electromyographic recordings obtained during active shoulder forward flexion, abduction, and external rotation confirmed appropriate muscle activation and correct needle placement before immobilization testing. Motor unit action potential (MUAP) recruitment and amplitude were evaluated under two immobilization conditions: a standard arm sling with the shoulder positioned in internal rotation and adduction, followed by an abduction sling positioned at approximately 30° of shoulder abduction with slight external rotation.
Results. Thirty-four participants completed the study. The study population consisted of 21 males (61.8%) and 13 females (38.2%), with a mean age of 28.9 ± 5.5 years. Twenty-eight participants (82.4%) were right-hand dominant, while six (17.6%) were left-hand dominant. Baseline electromyographic recordings demonstrated normal motor unit recruitment in both target muscles. During resting immobilization, complete absence of detectable motor unit recruitment and MUAP amplitude was observed in both the supraspinatus and infraspinatus muscles under both the standard arm sling and abduction sling conditions. Identical electromyographic findings were demonstrated in all participants regardless of the immobilization method.
Conclusion. Both the standard arm sling and the abduction sling demonstrated complete electromyographic quiescence of the supraspinatus and infraspinatus muscles during resting immobilization. Neither immobilization method demonstrated a measurable advantage over the other with respect to resting electromyographic muscle activity. However, because this study evaluated muscle electrical activity rather than passive tendon tension, these findings should not be interpreted as evidence of biomechanical equivalence between sling positions. Further prospective studies involving postoperative rotator cuff repair patients are warranted.
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