Post-Activation Potentiation—A Clue for Simplifying Models of Muscle Dynamics

SYNOPSIS. Post-activation potentiation is a phenomena that occurs only in fasttwitch muscle fibers. Its main effect is to enhance muscle force at sub-maximal activation levels for a short duration of time following previous muscle activation. We characterized this phenomenon in feline caudofemoralis (CF) muscle (composed of 100% fast-twitch muscle fibers) to understand its importance during physiological patterns of activation. During such patterns (e.g., 43 pps, 8 pulse trains delivered at 1 sec intervals) CF potentiated rapidly and apparently maximally. When CF was allowed to relax, potentiation decayed slowly with a time constant 20–40 x slower than the rise-time. The level of potentiation reached during the potentiating paradigm was stable in response to a wide range of stimuli, including various stimulation rates (15–120 pps) and various inter-train intervals (up to 10 sec). The shape of the twitch force-length curve for potentiated CF was similar to that of the tetanic force-length curve in either the potentiated or unpotentiated state. In contrast, the shape of the twitch force-length curve for unpotentiated CF was shifted markedly to the right accompanied by a narrowing of the curve's peak. We conclude from our observations that fast-twitch muscle fibers operate and should be modeled in a state of full potentiation, and that modeling the potentiated state may actuaUy be simpler than modeling the unpotentiated state.