Interlimb reflexes evoked in human arm muscles by ankle displacement.

Interlimb reflexes evoked by ankle displacements were studied in arm muscles of 6 normal subjects. EMGs from grastrocnemius (G), tibialis anterior (TA), biceps brachii (BB), and triceps brachii (TB) were amplified, rectified and low-pass filtered before recording. Averaging and Wiener filtering were used to detect changes in tonic EMG activity evoked by dorsiflexing or plantarflexing displacements of the ankle. A consistent pattern of response was observed in all subjects. In the leg muscles, the responses to stretch were consistent with previous reports. In the arm muscles, the response of TB was dominant. Dorsiflexing displacements of the ankle evoked a small excitation followed by a more marked decrease in TB activity but had no effect on BB. In contrast, plantarflexing displacements of the ankle resulted in a large, early period of excitation followed by a decreased level of activity in TB. A similar but smaller pattern of activity was observed in BB. It is notable that the TB responses to displacement were sizable, often modulating the tonic EMG activity by as much as 80%. Interlimb reflexes evoked by ankle displacement were larger and of shorter latency than those evoked by cutaneous electrical stimulation of the foot reported previously. This suggests that proprioceptive afferents may have stronger and more direct interlimb reflex connections than cutaneous afferents and may therefore play an important role in the coordination of movement.