A pilot study to investigate shoulder muscle fatigue during a sustained isometric wheelchair-propulsion effort using surface EMG.

OBJECTIVE: The primary aim of this pilot study was to develop a testing protocol for gathering shoulder muscle surface electromyography (EMG) data during a sustained submaximal isometric wheelchair-propulsion effort. Of special interest was analysis of the median frequency of the surface EMG signal power spectrum; a negative median frequency shift during a sustained effort is an indicator of muscle fatigue. The long-term goal is to improve our understanding of how fatigue and muscle imbalance might play a role in the development of a broad array of upper-extremity overuse syndromes. METHOD: Participants were a convenience sample of seven male manual-wheelchair users with spinal cord injury and 14 able-bodied males. Surface EMG in six right-side shoulder muscles was recorded while subjects resisted a static force equal to 60% of their maximum wheelchair-propulsion strength until fatigued. Percent of maximum voluntary contraction and the median frequency shift for each muscle were analyzed. RESULTS: The able-bodied group used a higher percent of maximum than the manual-wheelchair group (p< .05), and a more pronounced negative median frequency shift was evident in the able-bodied group (p < .10). CONCLUSION: Patterns that emerged suggest that spectral analysis of the surface electromyographhic signal has promise as a clinically useful tool to authenticate muscle fatigue in the upper extremities during occupational performance. Such data in the future could be used to justify the need for and benefits of adapted performance techniques and assistive technology.