Electrical stimulation of muscle has been proposed as a technique to restore function to paralyzed muscles. But, from a control stand-point, little is known about how such artificial activation interacts with the still intact spinal reflex loops. We have developed instrumentation to measure and compare ankle compliance and muscle EMG activity when the ankle is subjected to perturbations in torque or angular position from bias positions that are achieved volitionally or via electrical muscle stimulation. We deliver precise torque or position perturbations (step, ramp, sinusoidal, random) to the ankle via a pivoting footplate driven by a computer-controlled torque motor. Angular displacement, torque, acceleration, and 2 to 4 channels of electromyographic (EMG) data are collected on analog (VHS) tape and simultaneously digitized and stored. Torque or position biases to normal ankle equilibrium position are applied volitionally (for the neurologically intact) or via electrical stimulation of the Gastroc/Soleus or the Tibialis Anterior muscle (for either the neurologically intact or spinal cord injured). A special stimulator/recording amplifier permits the recording of EMG signals from the muscle being stimulated. An overview of the features and response characteristics of the perturbation system and a comparison from preliminary studies of responses at different biases achieved volitionally versus those achieved by stimulation are presented.