[Organization of reflex responses elicited in man by muscle stretch applied to uni- and bi-functional muscles]

The organization of human forearm reflexes following the application of extension torque perturbations has been studied in three flexors of the elbow: brachioradialis (BR), biceps (B), and pronator teres (PT). Two of these flexors are also involved in wrist supination: (B) and pronation (PT). During the perturbation, the wrist was fixed at various angles: from full supination to full pronation. The results show that the amplitude of the reflex responses of B and PT is highly dependent on wrist position. By eliminating biceps length changes as a source of observed reflex variation, it has been shown that the normalized reflex amplitude of B increases by 300% when the wrist position is varied from full pronation to full supination. In contrast, the BR muscle has a reflex response which is independent of wrist position. These results are discussed in relation to the role of reciprocal innervation and the possible action of receptors encoding wrist angle.