The electrically elicited muscle twitch has been used to identify mechanical muscle properties in relaxed muscles. We attempted to characterize the mechanical muscle properties in an active muscle. Each subject was seated and his/her left foot was strapped to a platform. The ankle torque and electromyogram (EMG) of the ankle extensors and flexors were measured while the subject was asked to match the ankle torque to a pseudo randomized rectangular tracking signal. A system identification technique was used to determine the impulse response from EMG to torque at various contraction levels. The amplitude of the impulse response decreased markedly with the contraction level when the amplitude of the tracking signal was constant, whereas the amplitude of the impulse response increased with the amplitude of the tracking signal. An explanation for these findings could be seen in the results from the properties of individual motor units. Our results suggest that the rate modulation that occurs during rapid changes in the force in an already isometric contracted muscle is very efficient in generating force in the newly recruited motor units, but inefficient in motor units approaching tetanus.