Postactivation potentiation (PAP), also known as activity-dependent potentiation, is an increase in muscle isometric twitch and low frequency tetanic force following a “conditioning” activity.^1,2 Examples of conditioning activity are a series of evoked isometric twitches (staircase or treppe), an evoked isometric tetanic contraction (post-tetanic potentiation), a sustained isometric maximal voluntary contraction (MVC), and a series of dynamic contractions. In fact, any type of contractile activity is likely to activate the mechanism of PAP—that is, phosphorylation of myosin regulatory light chains, which increases Ca²⁺ sensitivity of the myofilaments.² The result is an amplified level of myosin cross bridge activity in response to submaximal concentrations of myoplasmic Ca²⁺. A notable feature of PAP is that it has no effect on the force of high frequency tetanic isometric contractions, because in such contractions a “saturating” concentration of Ca²⁺ is attained, making any increase in Ca²⁺ sensitivity inconsequential. Although less studied, PAP also increases the force of shortening (concentric) contractions,³ and the highest frequency at which PAP is effective is greater for rapid shortening (concentric) contractions than for isometric contractions.⁴

On the basis of the foregoing, it would appear that PAP …