Mechanical properties of single motor units in speech musculature

Electromyograms were recorded from three subjects during steady, low-rate discharge of single motor units of anterior belly of digastric, accompanying maintenance of isometric tension. The overall tension developed in muscles subserving mandibular depression was recorded from a force tansducer situated inferior to the mandibular symphysis. The force transducer output from the 90 ms period immediately following each of the several hundred discharges of a motor unit was averaged to obtain a characteristic tension curve for the contractile response (twitch) of that unit. Peak twitch tensions and time taken to develop peak tension (contraction time) were recorded for 27 motor units. The obtained median twitch tension of 0.26 g and contraction time of 29 ms were compared with values obtained for units in other human muscles and in animal muscles. As expected from the size principle, later recruited units possessed higher twitch tensions. Contrary to some statements of the size principle, contraction times were not shorter in later recruited units. Data on muscles elevating the mandible were also evaluated.