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 fiber length. The length of the sarcomeres is a major determinant of both force and velocity. The goal of this study was to characterize the architecture of the human pterygoid muscles and to evaluate possible functional consequences for muscle force and muscle velocity. For the heads of the lateral and medial pterygoid, the length of sarcomeres and of fiber bundles, the PCSA, and the three-dimensional coordinates of origin and insertion points were determined. Measurements were taken from eight cadavers, and the data were used as input for a model predicting sarcomere length and active muscle force as a function of mandibular position. At the closed-jaw position, sarcomeres in the lateral pterygoid (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 (anterior 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 capable of producing maximum active force was estimated to be between 5 degrees and 10 degrees. The lateral pterygoid was characterized by relatively long fibers (inferior, 23 +/- 2.7 mm; superior, 21.4 +/- 2.2 mm) 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 PCSA (anterior, 2.47 +/- 0.57 cm(2); posterior, 3.53 +/- 0.97 cm(2)). The mechanical consequence is that the lateral pterygoid is capable of producing 1.7 times larger displacements and velocities than the medial pterygoid, whereas the medial pterygoid is capable of producing 1.6 times higher forces. The model showed that jaw movement had a different effect on active force production in the muscles.