Identification of Muscular Imbalance in Upper Limb: an Implication for Elbow Enthesopathies

Objectives: the etiology of tennis elbow has remained uncertain for more than a century. to examine muscle imbalance as a pathophysiological factor requires a reliable method of assessment. this study describes the development of such method and its performance in healthy subjects. a combination of surface-electrode and fine-wire electromyography (emg) of shoulder and forearm muscles and wrist strength measurements will be a reliable tool for assessing muscle fatigue/imbalance relevant to the pathophysiology of tennis elbow. materials and methods: six healthy volunteers participated. an emg study in normal subjects of forearm fatigue during a constant grip task and shoulder fatigue was performed. emg data was acquired at 50% maximal voluntary contraction from five forearm muscles during a constant grip and three shoulder muscles during external rotation and abduction, and analyzed using relative median frequency slopes as a fatigue index. wrist extension/flexion strength was also measured. results: significant negative slope of median frequency was found for all muscles, with good reproducibility in subjects studied twice. (amplitude slope showed high variability and was therefore unsuitable for this purpose). wrist flexion was (mean ± sem) 27% ± 8.4 stronger than extension. conclusion: this is a reliable method in measuring muscle fatigue in forearm and shoulder. electromyographic and wrist strength studies together can provide a reliable method for muscle imbalance assessment. identifying the normal relationship (muscle balance) and fatigue allocation in wrist-forearm-shoulder chain is of great importance for understanding muscle imbalance in the pathogenesis of tennis elbow.