Knowledge of muscle actions is essential for understanding biomechanics in running. In this study, 17 young runners were investigated at 13 different running speeds. Telemetric surface electromyograms from lower leg muscles were recorded continuously, and they were synchronized with the recordings of 3-dimensional ground reaction forces from a 10-m-long force platform. As expected, the rate of force production and the peak forces increased with increasing running speed. In the lateral forces, there was a short-duration inward force at the beginning of the contact followed by a longer-lasting outward force. The results revealed further the importance of preactivity and eccentric activity of the leg extensor muscles and the role of the hamstring muscles. The preactivity appears to be a preparatory requirement both for the enhancement of electromyographic activity during the braking phase and for timing of muscular action with respect to the ground contact. The increased force production with increased running speed is, furthermore, partly due to high and long-lasting activity of the hip extensor muscles during the contact phase.
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