Torque-step perturbations were applied to flex or extend the wrists of normal human subjects. The electromyographic activity (EMG) of two of the stretched muscles, flexor carpi radialis and extensor carpi radialis, was monitored. 2. Based on functional characteristics and temporal bursting patterns, the EMG responses were partitioned into four distinct temporal intervals : 30-60, 60-120, 120-200, and greater than 200 ms after the onset of the torque step. The last interval continues for the duration of the step input: 200- 400 ms was chosen to represent activity in this interval. 3. EMG responses in the first two intervals show short, stable latencies and amplitudes that depend on the level of muscle contraction prior to the torque step. They are facilitated by any instruction requiring a reaction by the subject. They are reflexes that cannot be voluntarily suppressed by instruction to the subject. 4. The third EMG response is a triggered response. It is not a reflex because its appearance or absence is absolutely under voluntary control. Unlike true voluntary responses, there exists no dichotomy in responses latency or variability between known versus unknown directions of torque steps. 5. We consider that a truly voluntary response to a torque perturbation does not begin until about 200 ms after the step, which is on the order of visual or auditory reaction times. 6. The EMG responses were similar in both the wrist flexor and extensor studied. Ankle flexors and extensors do not show such similarity. 7. The EMG responses at the wrist and ankle are compared and shown to have many similarities. A general scheme for their classification is discussed.
