Nonlinear summation of force in cat tibialis anterior: a muscle with intrafascicularly terminating fibers.

The complex connective tissue structure of muscle and tendon suggests that forces from two parts of a muscle may not summate linearly, particularly in muscles with intrafasciculary terminating fibers, such as cat tibialis anterior (TA). In four anesthetized cats, the TA was attached to a servomechanism to control muscle length and record force. The ventral roots were divided into two bundles, each innervating about half the TA, so the two parts could be stimulated alone or together. Nonlinear summation of force (F(nl)) was measured during isometric contractions. F(nl) was small and negative, indicating less than linear summation of the parts, which is consistent with the predicted F(nl) of muscle fibers connected in series. F(nl) was more significant when smaller parts of the muscle were tested (21.8 vs. 8% for whole muscle). These data were fit to a model where both parts of the muscle were assumed to stretch a common elasticity. Compensatory movements of the servomechanism showed the common elasticity is very stiff, and the model cannot account for F(nl) in cat TA.