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Downloaded from aim.bmj.com on January 5, 2010 - Published by group.bmj.com Commentary soleus muscle). The electrical stimulus over Myofascial trigger points: does recent the nerve is increased until the H-reflex research gives new insights into the appears (typically 4–10 mA), which corre- sponds to stimulation of the Ia muscle pathophysiology? spindle afferents (MSAs)—Ia MSAs are the largest diameter and fastest conducting nerves. A monosynaptic (or oligosynaptic)5 Mike Cummings reflex results in stimulation of the asso- ciated Aa motor nerves. At higher stimulus intensities the M-wave appears—this is caused by direct stimulation of the Aa The paper by Ge et al1 (see page 150) may be nerve stimulus) appears to be significantly motor nerves to the muscle. The H-reflex an important milestone in the investigation greater when elicited from within an appears before the M-wave because the Ia of the pathophysiological substrate of the MTrP than from within normal muscle MSAs have a lower electrical threshold myofascial trigger point (MTrP). The paper tissue (non-MTrP). than the Aa motor nerves. appears to show that there is a significantly The H-reflex was first described in 1910 In the paper by Ge et al the M-wave reduced threshold and increased amplitude by the German physiologist Paul was stimulated from within muscle, of an electrophysiological reflex (similar to Hoffmann.2 It is an electrophysiological either by intramuscular stimulation of the Hoffman (H-) reflex) when it is stimu- equivalent of the stretch reflex (usually Aa motor nerves or by direct depolarisa- lated electrically from within an MTrP elicited as a tendon jerk in the physical tion of muscle cell walls. Despite this, the compared with normal muscle nearby. examination), but it bypasses the muscle latency of the M-wave recorded from It appears to be the first paper in which spindle.3 It is used in research as a non- surface EMG was similar whether elicited an electrophysiological reflex similar to invasive neurophysiological probe to by stimulation of the tibial nerve or the H-reflex has been recorded following study the neural control of movement.3 4 elicited by intramuscular stimulation— an intramuscular electrical stimulus. The The H-reflex is stimulated by a transcu- presumably because the speed of trans- important finding of the paper is that the taneous electrical impulse (square wave of mission of a muscle action potential is amplitude of this reflex as a proportion of short duration, ie, 0.5–1 ms) over a major some 13 times slower than the nerve the maximum direct muscle stimulus nerve (eg, the tibial nerve behind the action potential in the Aa motor nerves.6 effect (measured peak to peak from the knee) and recorded with EMG electrodes We need to be cautious in our inter- M-wave—the electromyography (EMG) over a muscle in the motor distribution of pretation of the result in Ge et al, since the recording of the direct muscle or motor the same nerve (eg, the gastrocnemius or H-reflex is not generally stimulated from Figure 1 This figure shows a monosynaptic reflex arc including a muscle spindle Ia afferent nerve (green) and a Aa motor efferent nerve (blue). The insulated EMG needle is likely to be closer to muscle spindle Ia afferent nerves when placed in an MTrP than when placed in non-MTrP muscle. 148 Acupunct Med December 2009 Vol
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