Stretch and unloading reflexes in a human hand muscle.

Stretch and unloading reflexes were demonstrated in the first dorsal interosseous muscle by averaging the electromyographic responses to brief mechanical stimuli. Subjects were not able to abolish these reflexes by altering their preparatory set. When subjects were prepared to contract or relax the muscle in response to a stretching force, the size of the stretch reflex was not affected significantly. When they were prepared to contract or relax in response to unloading, the "silent period" was diminished. In general, these reflexes were not modulated appropriately to improve the performance of an intended voluntary movement. Our results do not support the notion that spinal reflexes of the intrinsic hand muscles are "dominated" by corticospinal inputs.