ARCHITECTURE OF THE HUMAN PTERYGOID MUSCLES

Muscle force is proportional to the physiological cross-sectional area (PCSA), and muscle velocity and excursion are proportional to the fib er length. The length of the sarcomeres is a major determinant of both force and velocity. The goal of this study was to characterize the ar chitecture of the human pterygoid muscles and to evaluate possible fun ctional consequences for muscle force and muscle velocity. For the hea ds of the lateral and medial pterygoid, the length of sarcomeres and o f fiber bundles, the PCSA, and the three-dimensional coordinates of or igin and insertion points were determined. Measurements were taken fro m eight cadavers, and the data were used as input for a model predicti ng sarcomere length and active muscle force as a function of mandibula r position. At the closed-jaw position, sarcomeres in the lateral pter ygoid (inferior head, 2.83 +/- 0.1 mu m; superior head, 2.72 +/- 0.11 mu m) were significantly longer than those in the medial pterygoid (an terior head, 2.48 +/- 0.36 mu m; posterior head, 2.54 +/- 0.38 mu m). With these initial lengths, the jaw angle at which the muscles were ca pable of producing maximum active force was estimated to be between 5 degrees and 10 degrees. The lateral pterygoid was characterized by rel atively long fibers (inferior, 23 +/- 2.7 mm; superior, 21.4 +/- 2.2 m m) and a small PCSA (inferior, 2.82 +/- 0.66 cm(2); superior, 0.95 +/- 0.35 cm(2)), whereas the medial pterygoid had relatively short fibers (anterior, 13.5 +/- 1.9 mm; posterior, 12.4 +/- 1.5 mm) and a large P CSA (anterior, 2.47 +/- 0.57 cm(2); posterior, 3.53 +/- 0.97 cm(2)). T he mechanical consequence is that the lateral pterygoid is capable of producing 1.7 times larger displacements and velocities than the media l pterygoid, whereas the medial pterygoid is capable of producing 1.6 times higher forces. The model showed that jaw movement had a differen t effect on active force production in the muscles.