Synergistic behaviour of triceps surae muscles (medial gastrocnemius-MG, lateral gastrocnemius-LG, soleus-SOL) during sustained submaximal plantarflexions was investigated in this study. Six male subjects were asked to sustain an isometric plantar flexor effort to exhaustion at two different knee angles. Exhaustion was defined as the point when they could no longer maintain the required tension. The loads sustained at 0 and 120 degrees of knee flexion represented 50% and 36% of their maximum voluntary contraction (MVC) respectively. MVC was measured at 0 degree knee flexion. During the contractions, electromyograms (EMG) from the surface of the triceps surae muscles were recorded. Changes in the synergistic behaviour of the triceps surae were assessed via partial correlations of the average EMG (AEMG) between three muscle combinations; MG/LG, MG/SOL, LG/SOL, and correlation between SOL/MG + LG and MG/SOL + LG. The latter combinations were based on either common fibre type or innervation properties. Two types of synergisms were identified: trade-off and coactivation. Trade-off and coactivation synergies were defined by significant (p less than 0.05) positive and negative correlations respectively. Coactivation synergism was found to occur predominantly under conditions of high load or reduced length of the triceps surae, and increased with the duration of the contraction. Trade-off synergism was evident when the muscles were at their optimum length and the loads sustained were submaximum.(ABSTRACT TRUNCATED AT 250 WORDS)