Fig. 5. Ground forces relative to epaxial EMG activity, produced by combining the mean force trace with mean EMG activity traces. The primary burst of epaxial muscle activity begins just after peak vertical (vert.) force, activity occurs throughout the acceleration (accel.) phase of fore–aft force production, and activity ceases at approximately the same time that vertical and horizontal forces return to zero. The secondary bursts are shorter in duration, such that they generally correspond to the second half of the acceleration phase of fore–aft force production. decel., deceleration.