3-DIMENSIONAL STRUCTURE OF THE HUMAN TEMPORALIS MUSCLE

Background: The maximal force a muscle is capable of producing is prop ortional to its physiological cross-sectional area and its excursion r ange to the length of the muscle fibers. The length of the sarcomeres is a major determinant for both force and excursion range. The human t emporalis muscle is an architecturally complex muscle, and little is k nown regarding the possible heterogeneous distribution of these parame ters throughout the muscle. The objective of this study was to determi ne this distribution for different muscle portions and to examine the functional consequences. Methods: In eight cadavers, sarcomere lengths , fiber lengths, and physiological cross-sectional areas were measured for the closed mouth position in six different anteroposterior portio ns of the temporalis muscle. To determine the spatial position of the muscle portions, the three-dimensional coordinates of attachment sites of a number of fiber bundles were registered. These parameters were u sed as input for a mathematical model with which sarcomere length chan ges and the consequences for the production of active force at differe nt open positions of the jaw were estimated. Results: At the closed-ja w postion, average sarcomere length ranged between 2.26 and 2.34 mu m and did not differ significantly among the muscle portions. Average fi ber bundle length ranged between 21.7 and 28.9 mm and differed signifi cantly among the muscle portions. The physiological cross-sectional ar ea ranged between 1.82 and 2.93 cm(2); the smallest values were found posteriorly, and the largest values anteriorly. The line of pull of th e anteriormost muscle portion was slightly inclined anteriorly and med ially, whereas the posteriormost portion was relatively strongly incli ned backwardly and laterally. The model predicted that during jaw open -close movements a nonuniform change in length of the sarcomeres would occur; sarcomere excursions were smaller posteriorly than anteriorly. Different muscle portions seemed to function along different parts of the active length-force relationship. Conclusions: The temporalis mus cle is an architecturally heterogeneous muscle. Different muscle porti ons are capable of producing different maximum force and excursion ran ge, and the portions have the capability of performing different mecha nical actions. (C) 1996 Wiley-Liss, Inc.