Changes in the stretch reflex of the human first dorsal interosseous muscle during different tasks.

1. Subjects flexed the interphalangeal joint of the index finger against a lever which was mounted on the shaft of a torque motor. 2. There were two different tasks. In one, the subject attempted to maintain a constant finger position in the face of changing forces, whereas in the other the subject attempted to maintain a constant force while the motor moved the lever. 3. Each of the tasks was interrupted by ramp extensions. These evoked stretch reflexes which were recorded in the first dorsal interosseous (FDI) muscle electromyogram (EMG). 4. Long-latency (55-90 ms) reflex responses were larger during the 'maintain position' task than during the 'maintain force' task, although the ramp extensions began from a similar finger position, a similar flexing force, and with a similar amount of FDI EMG activity. 5. It is concluded that the nature of the task has an effect on the magnitude of the long-latency stretch reflex.