Load compensation in human masseter muscles.

The reaction of masseter and digastric muscles to changes in the load applied during a movement was studied in human volunteers. 2. Rapid stretch of the jaw-closing muscles evoked a monosynaptic jaw-jerk reflex response in the masseteric electromyogram (e.m.g.) after a delay of 6-8 msec. 3. The averaged integrated e.m.g. activity of the masseter was increased 5-10 msec after the rapid application of a load during voluntary closure of the jaw. It was not necessary to stretch the muscle to obtain this apparently monosynaptic response. 4. A compensatory increase in the velocity of movement followed 6-10 msec after the e.m.g. peak. 5. Unloading the masseter muscle during contraction was followed by a fall in e.m.g. activity, beginning after a latency of 6-5--1 msec, with later phases of depression beginning 27--36 and 60--70 msec after unloading. 6. The spindle-poor digastric muscles were not monosynaptically activated during loading, but responded with a latency of 24--34 msec. 7. It is concluded that monosynaptic e.m.g. changes are a prominent feature of the load compensation system in human jaw-closing muscles but not in jaw-opening muscles.