The relationship between sarcomere length and activation pattern in the rabbit masseter muscle.
The sarcomere lengths of muscle fibres in 12 locations of the masseter were determined for 7 mandibular positions characteristic of jaw excursions during normal chewing and biting. In symmetric jaw postures, the superficial masseter had relatively long and the deep masseter short sarcomeres. Sarcomere length changes due to jaw opening could be predicted by a geometric model that included degree of gape, initial sarcomere length, orientation of fibre bundle relative to hinge axis and ratio of fibre to muscle (origin/insertion) length as parameters. Lateral deviations of the jaw caused lengthening of ipsilateral and shortening of contralateral fibres in the superficial masseter. The reverse length changes occurred in the deep masseters. The contraction pattern of the masseter was different from region to region and there was a correlation between timing of activity and timing of sarcomere shortening. Muscle fibres became active at sarcomere lengths of 2.5-2.8 micron, reached peak electromyograms at 2.2-2.5 micron and stopped firing at 2.2-2.4 micron. Hence contraction occurred at sub-optimal sarcomere lengths. By comparison with published length-tension diagrams, the isometric muscle force at molar occlusion was estimated to be 65 per cent of the force at optimum length. During initial jaw closing, the rate of shortening was high relative to the presumed maximum intrinsic speed of the muscle (13-19 micron/sarcomere/s). It was lower during loaded closing and the occlusal phase (3-7 micron/sarcomere/s). Hence during each firing period, initially a high speed of contraction at a low force developed and subsequently a near-isometric force at a low speed of contraction.
D000818:Animals; D004576:Electromyography; D008297:Male; D008406:Masseter Muscle / Q000502:physiology* / Q000648:ultrastructure; D008409:Mastication; D008410:Masticatory Muscles / Q000502:physiology*; D008954:Models, Biological; D009119:Muscle Contraction; D011817:Rabbits