Myofascial trigger points: does recent research gives new insights into the pathophysiology?

The paper by Ge et al (see page 150) may be an important milestone in the investigation of the pathophysiological substrate of the myofascial trigger point (MTrP). The paper appears to show that there is a significantly reduced threshold and increased amplitude of an electrophysiological reflex (similar to the Hoffman (H-) reflex) when it is stimulated electrically from within an MTrP compared with normal muscle nearby. It appears to be the first paper in which an electrophysiological reflex similar to the H-reflex has been recorded following an intramuscular electrical stimulus. The important finding of the paper is that the amplitude of this reflex as a proportion of the maximum direct muscle stimulus effect (measured peak to peak from the M-wave—the electromyography (EMG) recording of the direct muscle or motor nerve stimulus) appears to be significantly greater when elicited from within an MTrP than from within normal muscle tissue (non-MTrP). The H-reflex was first described in 1910 by the German physiologist Paul Hoffmann. It is an electrophysiological equivalent of the stretch reflex (usually elicited as a tendon jerk in the physical examination), but it bypasses the muscle spindle. It is used in research as a noninvasive neurophysiological probe to study the neural control of movement. 4 The H-reflex is stimulated by a transcutaneous electrical impulse (square wave of short duration, ie, 0.5–1 ms) over a major nerve (eg, the tibial nerve behind the knee) and recorded with EMG electrodes over a muscle in the motor distribution of the same nerve (eg, the gastrocnemius or soleus muscle). The electrical stimulus over the nerve is increased until the H-reflex appears (typically 4–10 mA), which corresponds to stimulation of the Ia muscle spindle afferents (MSAs)—Ia MSAs are the largest diameter and fastest conducting nerves. A monosynaptic (or oligosynaptic) reflex results in stimulation of the associated Aa motor nerves. At higher stimulus intensities the M-wave appears—this is caused by direct stimulation of the Aa motor nerves to the muscle. The H-reflex appears before the M-wave because the Ia MSAs have a lower electrical threshold than the Aa motor nerves. In the paper by Ge et al the M-wave was stimulated from within muscle, either by intramuscular stimulation of Aa motor nerves or by direct depolarisation of muscle cell walls. Despite this, the latency of the M-wave recorded from surface EMG was similar whether elicited by stimulation of the tibial nerve or elicited by intramuscular stimulation— presumably because the speed of transmission of a muscle action potential is some 13 times slower than the nerve action potential in the Aa motor nerves. We need to be cautious in our interpretation of the result in Ge et al, since the H-reflex is not generally stimulated from