In normal adults, H reflexes are commonly found only in the antigravity calf and flexor carpi radialis (FCR) muscles. However, these responses may be more widespread if agonist muscles contract. This study demonstrates that H reflexes may be found in the peroneal (P) and anterior tibialis (TA) muscles in the leg and the extensor digitorum communis (EDC) muscle in the arm on contraction of the antagonistic calf and flexor carpiradialis muscles, respectively. H reflexes were also elicited regularly in the EDC by passive extension of the wrist. The data indicate physiologic unity of comparably acting human antigravity muscles and their antagonists in both the upper and lower extremities, and imply an important functional role of group II afferent fibers in normal motor system activity.