Fig. 1. Experimental apparatus. Subject performed swinging of one leg while securely strapped to a rigid frame. Leg angle was measured using an optical encoder. Subjects attempted to swing at a target amplitude, displayed through visual feedback. The target amplitude was varied with frequency to maintain a constant average rate of positive mechanical work on the leg. A force plate measured ground reaction forces, which were used with to calculate the leg torque about the hip. Electromyographic (EMG) activity of the medial hamstring was also recorded to assess duration of muscle force production. Oxygen consumption was measured to provide indirect calorimetry of metabolic energy expenditure.