EMG Signs of Fatigue in Shoulder and Forearm Muscles: a Method for the Assessment of Muscle Imbalance

Objectives: muscle imbalance has increasingly become an important topic in the aetiology of many musculoskeletal conditions. the role of imbalance and fatigability of forearm and shoulder muscles has not been sufficiently studied in the aetiology of elbow enthesopathies including tennis elbow. a combination of surface-electrode and fine-wire electromyography (emg) of shoulder and forearm muscles will be a reliable tool for assessing muscle fatigue/imbalance relevant to the pathophysiology of tennis elbow. material and methods: muscle fatigue in forearm and shoulder during constant isometric contractions in normal subjects was measured. six healthy volunteers participated. 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. results: for all muscles significant negative slope of median frequency was found, with good reproducibility in subjects studied twice. there was a high inconsistency with amplitude slope making it an unsuitable parameter for measuring muscle fatigue. conclusion: muscle fatigue in forearm and shoulder can be reliably measured with this method. electromyographic studies over wrist-elbow-shoulder chain can provide a reliable method for muscle imbalance assessment of musculoskeletal disorders in the area. identifying the muscle balance and fatigue allocation in wrist-forearm-shoulder kinetic chain is of great essence for understanding muscle imbalance in the pathogenesis of tennis elbow and other tendinopathies in upper limb.