Introduction: Restoring optimal strength and biomechanics of a pathologic shoulder knowledge of activity patterns of healthy glenohumeral muscles is mandatory. Yet, data on normal shoulder muscle activity are not always conclusive. The study was undertaken (a) to evaluate muscle activity patterns in the healthy shoulder using surface and fine-wire electromyography (EMG), and (b) to assess method's suitability in the clinical setting especially regarding painfulness and practicability.
Materials and methods: Surface and fine-wire EMG was performed on 11 healthy subjects (2f/9 m, Ø age 28 years) to assess 14 muscles including rotator cuff muscles during 8 planar standardised shoulder movements (abduction, forward flexion, internal and external rotation in neutral, 45° and 90° abduction). Pain was assessed using the visual analogue scale before testing, after inserting the fine-wire electrodes, after maximal voluntary contraction, before and after exercises, and after electrode removal.
Results: The most important finding regarding EMG activity patterns in the healthy shoulder was that the subscapularis activity was found to play a major role in abduction and forward flexion. Furthermore, this study was able to show that EMG measurements, especially fine-wire EMG, is prone to high failure rates (up to 32%); however, pain was not a limiting factor.
Conclusion: The present study (1) revealed a new insight, especially finding the subscapularis activity playing a major role in abduction and forward flexion of the healthy shoulder; and (2) motion analysis system and the use of fine-wire electrodes were prone to failure; however, pain was not a limiting factor.
Level of evidence: Basic Science, Electrodiagnostic Study.