Unloading reflex in patients with hemiparesis

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Abstract
When a muscle is unloaded during voluntary contraction, the electromyogram shows a biphasic pattern. Phase 1 is the "silent period," a sharp decrease in amplitude of the muscle action potentials. Phase 2 is a volley of large action potentials which are normally larger than those preceding phase 1. The unloading reflex was studied in 5 patients with hemiparesis due to cerebro-vascular disease. Phase 1 was demonstrated in all of the limbs tested, including the paretic ones. The phase of the unloading response is believed to result from a pause in spindle afferent discharge and provides indirect evidence that the voluntary motor discharge is facilitated by afferent impulses from the muscle spindles. According to this interpretation, proprioceptive feedback may have a supporting role in voluntary motor activity, even if the cortico-spinal pathway has been damaged. The 2nd phase of the unloading response was found to be absent or greatly reduced in all of the paretic limbs. While the origin and functional significance of phase 2 have not been determined, the data suggest that a corticospinal tract lesion tends to damage the neural mechanism which underlies this response.

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