 Reflexes during supine, prone, sitting, and standing. H-reflex elicited at the back or leg muscle. The H-reflex is analogous to the stretch reflex muscle spindles activate Ia afferents that synapse on alpha-motor neurons, but is elicited by an electrical stimulus to the Ia fibers in the afferent nerve, circumventing the muscle spindle. The H-reflex will be elicited using electrical stimulation to the tibial nerve using an AgCl cathode in the popliteal fossa and a 40-mm-diameter anode placed over the patella. The H-reflex can also be elicited using electrical stimulation from stimulating electrodes placed on the subjects back. Concurrently we will record EMG from leg muscles using bipolar surface electrodes placed on the muscle bellies. The EMG signals propagate through long shielded wires to custom-built amplifiers frequency response 20 Hz-10 kHz. The stimulus will be a 1 ms square wave pulse 0.5 Hz - 200 Hz delivered by a custom-built computer-controlled constant current stimulator. This response will be dependent on the stimulus strength. Eliciting a stimulus with a strength that is just above the threshold for the alpha-motoneurons will also result in direct stimulation of the leg muscle or back, producing the M-wave. The peak to peak amplitude of the H-wave will be plotted versus the peak to peak amplitude of the M-wave. The maximum H-wave amplitude divided by the maximum M-wave amplitude will be calculated. 