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.