This study analyses the relative contribution of the triceps surae and tibialis anterior (TA) muscles to tension development with reference to voluntary plantarflexion at two articular positions of the knee joint (extended and flexed at 90 degrees) for various inertial loads. Subjects were instructed to perform plantarflexions at various sub-maximal and maximal velocities with no intention of stopping the movement. Whereas in one series of experiments the subjects were informed of the load countering the movement, in the other they were not. The average electromyographic (EMG) activity of the different muscles was recorded. The main results were that with loading: (a) greater maximal plantarflexion velocities were recorded in flexed as compared to extended-knee positions; (b) greater durations and amplitudes of agonist and antagonist EMG bursts were recorded; (c) the co-activation of the TA and triceps surae muscles was enhanced; (d) unexpected sub-maximal loads induced greater EMG activity and speed of movement. It is concluded that increasing the load during plantarflexion in humans brings about changes in neuromuscular strategies that contribute to the efficiency of contractile activity during rapid movements. The results also indicate that unexpected sub-maximal loading induces a potentiated neuromuscular activity which increases the speed of movement.