In order to investigate muscle activities during sustained isometric contractions, electromyogram(EMG) patterns of the triceps surae muscles(medial gastrocnemius: MG, lateral gastrocnemius: LG, and soleus: SOL), which are synergists for plantar flexion, were examined. Experimental loads were set to obtain constant impulses at three different intensities(20% maximal voluntaly contraction(MVC), 40% MVC, and 60% MVC). Surface EMGs were recorded from the triceps surae muscles in five healthy male subjects. The EMGs were integrated every l s, and the integrated EMGs(iEMGs) were divided by the iEMG at the MVC, which was recorded before application of the experimental loads. The results can be summarized as follows: 1)Changes in relative iEMGs tended to decrease significantly with time, especially in the MG, not only to increase. 2)IEMGs increased significantly with time not only in the MG(so-called fatigable muscle), but also in the SOL(so-called fatigue-resistant muscle). 3)Complementary changes in EMG activities during sustained isometric contractions at 20% MVC were shown. These results suggest that the activity of the triceps surae muscles dose not always increase in each muscle during sustained isometric contraction at constant force. It can be assumed that control mechanisms in the central nervous system might play an important role in synergistic movement.