The masticatory muscles in human beings and in other mammals show a number of specific adaptations. Their muscle fibres contain at least four different isoforms of myosin heavy chain (MHC) and many fibres express more than one kind simultaneously. This implicates a continuous range of fibre contraction speeds for these muscles. Most or all fibres have a high oxidative capacity and consequently are very resistant against fatigue. The fast muscle fibres of the masticatory muscles appear to have smaller calibers than the slow ones. This condition is particular and points to mild atrophy. The motor units of the jaw muscles are characterised by large fibre numbers per unit, but concentrated into small subvolumes of muscle. Because of the anatomical heterogeneity of the muscles, motor units are capable of generating quite a variety of force directions. The fibres of jaw muscle motor units often belong to different fibre types, as far as their MHC-composition is concerned. For this reason, the units cannot be subdivided into clear-cut types, but show a continuous range of contraction times.
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