Is coordination of two-joint leg muscles during load lifting consistent with the strategy of minimum fatigue?

The purpose of this study was to examine if strong correlations reported fo r a back lift task between activity (EMG) of two-joint rectus femoris (RF), hamstrings (HA), and gastrocnemius (GA) and the difference in the joint mo ments could be predicted by minimizing an objective function of minimum fat igue. Four subjects lifted barbell weights (9 and 18 kg) using a back lift technique at three speeds - normal, slow, and fast. Recorded ground reactio n forces and coordinates of the leg joints were used to calculate the resul tant joint moments. Surface EMG of five muscles crossing the knee joint wer e also recorded. Forces of nine muscles were calculated using static optimi zation and a minimum fatigue criterion. Relationships (i) (RF EMG-HA EMG) v s (knee moment-hip moment) and (ii) GA EMG vs, (ankle moment - knee moment) were closely related (coefficients of determination were typically 0.9 and higher). Qualitatively similar relationships were predicted by minimizing fatigue. Gastrocnemius and hamstrings had the agonistic action at both join ts they cross during load lifting, and their activation and predicted force s increased with increasing flexion knee moments and extension ankle and hi p moments. The rectus femoris typically had the antagonistic action at the knee and hip, and its activation and predicted force were low. Patterns of predicted muscle forces were qualitatively similar to the corresponding EMG envelopes (except in phases of low joint moments where accuracy of determi ning joint moments was presumably poor). It was suggested that muscle coord ination in load lifting is consistent with the strategy of minimum muscle f atigue. (C) 1998 Elsevier Science Ltd. All rights reserved.