Kawakami Y, Amemiya K, Kanehisa H, Ikegawa S, Fukunaga T

J Appl Physiol 88:1969-1975, 2000

Nine healthy men (22-45 yr) completed 100 repetitive maximal isometric contractions of the ankle plantar flexor muscles in two knee positions of full extension (K0) and flexion at 90� (K90), positions which varied the contribution of the gastrocnemii. Electromyographic (EMG) activity was recorded from the medial (MG) and lateral (LG) gastrocnemii and soleus (Sol) muscles using surface electrodes. Plantar flexion torque in K0 was greater than, and decreased more rapidly than in K90. The EMG amplitude decreased over time, and there was no significant differences between muscles and knee joint positions. The level of voluntary effort, assessed by a supramaximal electrical stimulation during every 10th contraction, decreased from 96% to 70% (P smaller than 0.05) with no difference between K0 and K90. It was suggested that a decrease in plantar flexion torque was attributable to both central and peripheral fatigue, and that greater fatigability in K0 than in K90 would result from a greater contribution and hence more pronounced fatigue of the gastrocnemius muscle. Further support for this possibility was provided from changes in twitch torque and mechanomyographic responses.