Fine control of acceleration and force at the cat´s toe

Conventional wisdom is that proximal muscles will have larger motor units. However, a mechanical perspective suggests that precise control is more important for proximal than for distal muscles. Our study uses an established model of cat hindlimb to study how single motor units may affect force and acceleration at the toe. Motor unit counts were obtained from published data. Moment arms and inertia matrices were used to calculate joint isometric torques, and force or accelerations at the toe. A planar analysis was shown to be a valid approximation (errors < 5%) for study of planar forces in the 7 d.o.f. cat hindlimb model. Relative changes in force or acceleration were generally < 6% per motor unit. Motor unit forces acted along several different directions. In contrast, with the foot in the central workspace, accelerations from different muscles tended to fall along two distinct lines as a result of the effect of the inertia tensor. Percentage changes in muscle force as a result of single motor units were on the average smaller in proximal muscles. We conclude that there is finer-grained control of proximal muscle force, perhaps as a result of motor units of similar sizes in all muscles.