Fatigue of jaw muscles and speech mechanisms.

Histochemical studies show that the distribution of fiber types in human jaw muscles is different from that in various limb muscles, no doubt representing different functional demands as well as a different embryological derivation. Jaw-closing muscles appear more resistant to fatigue than limb muscles with intermittent maximal contractions. Endurance of continuous isometric biting is limited by pain. Masseter motor unit fatigability in sub-maximal contractions is similar to the limb muscles. There are few physiological data for the jaw-opening muscles. The distribution of fiber types in human speech muscles is consistent with the high speeds of contraction that must be used in phonation. Although clinical syndromes of fatigue of speech muscles are recognized, there is little direct information on the fatigability of the muscle fibers themselves.