Locomotor patterns of running were studied using computerization to synchronize electromyography (EMG) and cinematography (CMG). Surface electrodes monitored the muscle action potentials from rectus femoris, vastus lateralis, vastus medialis, biceps femoris, semitendinosus, semimembranosus, triceps surae and tibialis anterior muscles as 10 female subjects ran on a treadmill at speeds of 2.5 m/s and 3.5 m/s. Averaged integrated electromyograms were formulated to represent action potential levels for various sub-sections of the running cycle. Beginning at foot contact, the running cycle was dominated initially by muscle activity concerned with stabilization. The co-contraction of vastus medialis, vastus lateralis, semimembranosus, tibialis anterior, biceps femoris and triceps surae were associated with clockwise rotation (running from left to right) of the thigh, leg and foot in providing a stable base during the early support phase. Lower limb stabilization then gave way to the powerful driving thrust of the mid and late support phases. This period was characterized by increases in the activity levels from triceps surae and biceps femoris. The co-ordination of inertial effects and secondary muscular activity was associated with leg flexion as the thigh changed direction and with leg extension during the swing phase of running. This conclusion was supported by both EMG and resultant muscle moment of force data. Increased activity from semimembranosus and semitendinosus occurred with cessation of thigh flexion and leg extension prior to the subsequent heel strike. Tibialis anterior also eccentrically contracted to place the foot on the treadmill under control. The increase in the running speed was related to an increase in muscle action potential (in parts of the cycle) where the particular muscle was functional. This increase was paralleled Kinetically by an increase in the resultant muscle moment of force level.