Basic Principles and Technique

F responses are evoked from muscle as the result of antidromic activation of motor neurons by a peripheral electrical stimulus. The stimulus should be of greater intensity than is required to elicit a maximal CMAP and may not always elicit an F response. F responses are small (usually less than 5 percent) compared with the

CMAP ( Flg..?!-^ ), and their latency and amplitude vary considerably even when the stimulating and recording arrangements remain constant, presumably because different anterior horn cells are activated antidromically. Various parameters of F waves can be measured, but the most popular is the minimum latency of 10 or more responses.

The H reflex, named after Hoffmann who first described it, can be recorded easily only from the gastrocnemius-soleus muscle in the legs and the flexor carpi radialis muscle in the forearm in response to stimulation of the nerve to these muscles. The stimulus required to elicit the response is lower in intensity than is necessary to elicit CMAPs of maximal size (...Fig 24-23 ). The afferent pathway subserving the H reflex consists of spindle afferent (la) fibers, and the efferent arc is via the alpha motor axons. The H reflex is not easily obtained from other muscles except in infants or patients with pyramidal lesions, limiting its clinical utility.

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