LOAD, LENGTH, AND VELOCITY OF LOAD-MOVING TIBIALIS ANTERIOR MUSCLE OFTHE CAT

Three-dimensional relationships of load, length, and velocity of short ening of the tibialis anterior muscle in the cat were derived experime ntally and fitted with an analytic model. Gravitational loads were app lied to the isolated muscle, which arrived at an equilibrium with the passive forces before supramaximal tetanic stimulation was delivered t o its nerve. Recordings of initial passive muscle length at equilibriu m and length changes throughout the shortening phase up to the final l ength at active equilibrium were taken and numerically differentiated to obtain each load's instantaneous velocity. A three-dimensional surf ace was constructed by using instantaneous length and the correspondin g velocity for each of several loads. Maximal velocity of shortening w as shown to gradually decrease, occurring earlier in the shortening ph ase (at larger muscle lengths) as loads increased. Whereas load-veloci ty curves were hyperbolic for middle and short muscle lengths, they we re nonmonotonic during shortening above the optimal length. The model was found to correlate well with the experimental data (R = 0.98) and allowed for prediction of both muscle performance boundaries and insta ntaneous shortening velocity for a given length across the physiologic al load spectrum, thus offering a realistic estimation of the contract ile properties exhibited by the tibialis anterior muscle in functions similar to naturally occurring movements against gravitational loads, which are accelerated and decelerated during the movement.