Time course of inhibition of the forearm flexor H-reflex by a single motor threshold conditioning stimulus applied at t = 0 msec to the radial nerve. The timing of the test stimulus to the median nerve at the elbow after the radial nerve shock is given on the x-axis. The first two phases of inhibition can be seen on the average (AV) curve from 10 normal subjects (mean ± SE) at 0–1 msec and 10–40 msec. Only the first phase of inhibition is evident in the patient with simple writer's cramp (Mrs S).