Measured and modeled properties of mammalian skeletal muscle: IV. Dynamicsof activation and deactivation

The interactive effects of length and stimulus frequency on rise and fall t imes and on sag were investigated in fast-twitch feline caudofemoralis at n ormal body temperature. The length and stimulus frequency ranges studied we re 0.8-1.2 L-0 and 15-60 pps. Isometric rise times were shortest under two sets of conditions: short lengths + low stimulus frequencies and long lengt hs + high stimulus frequencies. In contrast the isometric fall time relatio nship showed a single minimum at short lengths + low stimulus frequencies. Velocity was shown to have an additional effect on fall time, but only at h igher stimulus frequencies (40-60 pps): fall times were shorter during move ment in either direction as compared to isometric. The effects of sag were greatest at shorter lengths and lower stimulus frequencies during isometric stimulus trains. Potential mechanisms underlying this last effect were inv estigated by comparing isometric twitches elicited prior to and immediately following a sag-inducing stimulus train. Post-sag twitches produced less f orce, reached peak force earlier and initially decayed more quickly compare d to pre-sag twitches. However, the final rate of force decay and the initi al rate of force rise (during the first 15 ms) were unaffected by sag. We c onstruct a logical argument based on these findings to hypothesize that the predominant mechanism underlying sag is an increase in the rate of sarcopl asmic calcium ion removal. All of the above findings were used to construct a model of activation dynamics for fast-twitch muscle, which was then extr apolated to slow-twitch muscle. When coupled with a previous model of kinem atic dynamics, the complete model produced accurate predictions of the forc es actually recorded during experiments in which we applied concurrent dyna mic changes in length, velocity and stimulus frequency.