POST-ACTIVATION POTENTIATION - A CLUE FOR SIMPLIFYING MODELS OF MUSCLE DYNAMICS

Post-activation potentiation is a phenomena that occurs only in fast-t witch muscle fibers. Its main effect is to enhance muscle force at sub -maximal activation levels for a short duration of time following prev ious muscle activation, We characterized this phenomenon in feline cau dofemoralis (CF) muscle (composed of 100% fast-twitch muscle fibers) t o understand its importance during physiological patterns of activatio n. During such patterns (e.g., 43 pps, 8 pulse trains delivered at 1 s ec intervals) CF potentiated rapidly and apparently maximally, When CF was allowed to relax, potentiation decayed slowly with a time constan t 20-40x slower than the rise-time. The level of potentiation reached during the potentiating paradigm was stable in response to a wide rang e of stimuli, including various stimulation rates (15-120 pps) and var ious inter-train intervals (up to 10 sec). The shape of the twitch for ce-length curve for potentiated CF was similar to that of the tetanic force-length curve in either the potentiated or unpotentiated state. I n contrast, the shape of the twitch force-length curve for unpotentiat ed CF was shifted markedly to the right accompanied by a narrowing of the curve's peak, We conclude from our observations that fast-twitch m uscle fibers operate and should be modeled in a state of full potentia tion, and that modeling the potentiated state may actually be simpler than modeling the unpotentiated state.