Length changes in the human masseter muscle after jaw movement

The human masseter is a multilayered, complex muscle contributing to jaw mo tion. Because variations in stretch may cause muscle fibers to function ove r different portions of their length-tension curves, the aim of this study was to determine how parts of the masseter lengthen or shorten during volun tary jaw movements made by living subjects. Magnetic resonance (MR) imaging and optically-based jaw-tracking were used to measure muscle-insertion pos itions for four parts of the muscle with six degrees of freedom (DOF), befo re and after maximum-opening, jaw protrusion and laterotrusion in four adul t males. Muscle part lengths and intramuscular tendon lengths were calculat ed, and these data, with fiber-tendon ratios published previously, were use d to estimate putative changes in fiber-length. During maximum jaw-opening, the largest increases in muscle length (34-83%) occurred in the medial par t of the deep masseter, whereas the smallest changes occurred in the poster ior-most, superficial masseter (2-19%). Smaller changes were found during m ovement to the ipsilateral side, than during protrusion and movement to the contralateral side. On maximum opening, putative fibers in the deep masset er lengthened up to 83%, whereas those of the superficial masseter stretche d up to 72%. The masseter muscle does not stretch uniformly for major jaw m ovement. Jaw motion to the ipsilateral side causes little length change in any part, and the effect of tendon-stretch on estimated fiber lengths is no t substantial. The stretch that occurs infers there are task-related change s in the active and passive tensions produced by different muscle regions. Anat Rec 262: 293-300, 2001. (C) 2001 Wiley-Liss, Inc.