Measured and modeled properties of mammalian skeletal muscle. II. The effects of stimulus frequency on force-length and force-velocity relationships

Interactions between physiological stimulus frequencies, fascicle lengths a nd velocities were analyzed in feline caudofemoralis (CF), a hindlimb skele tal muscle composed exclusively of fast-twitch fibers. Split ventral roots were stimulated asynchronously to produce smooth contractions at sub-tetani c stimulus frequencies. As described previously, the peak of the sub-tetani c force-length relationship was found to shift to longer lengths with decre ases in stimulus frequency, indicating a length dependence for activation t hat is independent of filament overlap. The sub-tetanic force-velocity (FV) relationship was affected strongly both by stimulus frequency and by lengt h; decreases in either decreased the slope of the FV relationship around is ometric. The shapes of the force transients following stretch or shortening revealed that these effects were not due to a change in the instantaneous FV relationship; the relative shape of the force transients following stret ch or shortening was independent of stimulus frequency and hardly affected by length. The effects of stimulus frequency and length on the sub-tetanic FV relationship instead appear to be caused by a time delay in the length-d ependent changes of activation. In contrast to feline soleus muscle, which is composed exclusively of slow-twitch fibers, CF did not yield at sub-teta nic stimulus frequencies for the range of stretch velocities tested (up to 2 L-0/s). The data presented here were used to build a model of muscle that accounted well for all of the effects described. We extended our model to account for slow-twitch muscle by comparing our fast-twitch model with prev iously published data and then changing the necessary parameters to fit the data. Our slow-twitch model accounts well for all previous findings includ ing that of yielding.