[Activation of agonist and antagonist muscles during plantar flexion of the ankle].

The aim of this study was to examine isokinetic moment produced by well-trained and sedentary subjects, during plantar flexion, at 13 angular velocities (from -120 to 300 degrees.s-1), under maximal voluntary and superimposed electrical conditions. In order to verify the level of activation of agonist (Soleus (SOL) and Gastrocnemius Medialis (GM)) and antagonist muscles (Tibialis Anterior (TA), their myoelectrical activities were detected and quantified as Root Mean Square (RMS). Whatever the angular velocity considered, the moments developed at 90 degrees by well-trained subjects were significantly (p < 0.05) higher than those developed by sedentary subjects, except at -10 degrees.s-1. Eccentric superimposed moments of sedentary subjects were significantly (p < 0.05) higher than those developed by the same subjects under maximal voluntary conditions. For a given velocity, the RMS's of S and GM muscles were significantly (p < 0.05) lower, during eccentric than during concentric actions, in sedentary, but not in well-trained subjects. The TA co-activation in well-trained subjects was significantly (p < 0.05) lower than in sedentary subjects, whatever the angular velocity considered. We concluded that a co-activation phenomenon could partly explain differences in isokinetic performances. Differences between voluntary and superimposed eccentric moment as well as lower agonists' RMS during eccentric action, support the presence of a tension-regulating mechanism in sedentary subjects.