Relation between differences in electromyographic adaptations during static contractions and the muscle function

This study determines whether changes in the EMG values of two important mu scles of the shoulder and neck region, the anterior deltoid and the upper t rapezius, are due to changes in torque production or due to fatigue process es during sustained activity. Contractions at 20, 40, 60, 80 and 100% MVC w ere performed during a flexion of the arm in the sagittal plane at 90 degre es, to examine the relation between torque and EMG. A sustained contraction at 20% MVC was performed to endurance point in the same position. RMS, a n ew parameter called activity, (ACT), and MPF of the deltoid anterior and th e upper trapezius were analysed. The amplitude values correlated highly wit h increasing torque production, both for the deltoid muscle (range r = 0.95 -0.96), and the trapezius muscle (range r = 0.83-0.87), whereas no signific ant difference was found for MPF. For the endurance task, the decrease in M PF was far more pronounced for the deltoid than for the trapezius, whereas the opposite occurred with RMS (P less than or equal to 0.01). Furthermore, there was no significant difference over time for the ACT values of the de ltoid, whereas there were significant increases in ACT for the trapezius (P less than or equal to 0.01). The RMS/ACT ratio correlated highly (r = 0.81 ) with the MPF. Regression coefficients of these parameters differed signif icantly for the trapezius muscle but not for the deltoid muscle. Therefore, the RMS/ACT ratio may be extremely important in analysing the fatigue effe cts during sustained efforts, independent of torque variations, which can i nfluence indicators of fatigue. (C) 1999 Elsevier Science Ltd. All rights r eserved.