FORCE VECTORS OF SINGLE MOTOR UNITS IN A MULTIPENNATE MUSCLE

The masseter muscle of the rabbit has a complex architectural design. Restricted motor unit territories in the muscle provide an anatomic ba sis for accurate control of the force vector through selective activat ion. In addition, the muscle shows regional differences in fiber type composition, The main objective of the present study was to measure th e force vectors of single motor units within the rabbit masseter muscl e by a direct mechanical approach to test the hypothesis that: (1) mot or units within the masseter muscle are capable of generating differen t force vectors; and (2) different motor unit types are distributed he terogeneously throughout the muscle. We used a force transducer, capab le of measuring both the magnitude and the position of the line of act ion of a force in a single plane. Motor units in the masseter muscle s howed a large range of twitch contraction times and force magnitudes. There was also a large variation in the direction and moment arm of th e lines of action. The variation of the lines of action was (almost) a s large as the range of fiber directions found inside the muscle. Larg est forces, with relatively slow contraction velocities, were produced by motor units in the anterior masseter. Smaller forces and fastest t witch contractions were produced by motor units in the posterior deep masseter. In addition, motor units in the anterior masseter showed mor e variability in force production than in the posterior masseter. Our results support the idea that the masseter muscle is divided into func tionally different parts.