Research suggests that performing a preloading exercise facilitates post-activation potentiation (PAP), a phenomenon characterised by the facilitation of volitional force production in response to previous activation of skeletal muscle (4). The phenomenon of PAP underpins the practice of complex training, a resistance training method incorporating heavy load weight-lifting exercises with plyometric exercise (1) that is often prescribed to enhance neuromuscular activation and the development of power. The effectiveness of PAP depends on performing the preloading or conditioning stimulus (CS) in complex training sets at an ideal time period prior to performance of the plyometric task, yet consideration of the time-course of potentiation is largely ignored in the application of complex training research. Although subjecting muscle to periods of brief stimulation can improve force production, continued stimulation may also impair or attenuate the contractile response, resulting in increased fatigue (6). To observe increases in performance, fatigue is required to dissipate more rapidly than PAP decays (7). Investigation of the temporal profile of potentiation highlights the optimal time at which peak force is greatest and fatigue is minimal. While some laboratory research has attempted to characterise the temporal profile of muscle potentiation (2,5), these investigations failed to consider that evoking repeat muscular contractions to measure PAP in a continuous trial may inadvertently act as a conditioning stimulus causing an additive effect on the magnitude of PAP and fatigue. Previous studies have reported changes in muscle twitch characteristics (2,3) confirming the presence of PAP. Other studies have assessed the influence of preloading on countermovement jump performance (2,5), yet failed to confirm the presence of PAP using mechanistic measures. Therefore, this investigation aims to investigate the time-course of PAP via the assessment of evoked muscle twitch characteristics, and the efficacy of jump squats as a valid and practical measure of muscle potentiation. Methods Muscle twitch characteristics and kinetic jump squat variables of eight recreationally trained males were assessed at three different time-intervals (4, 8, and 12mins) post the application of the CS (5RM back squat) on separate occasions in a randomised order, and in two continuous trials consisting of either muscle twitch or jump squat assessment only. Evoked twitches were elicited via percutaneous electrical stimulation of intramuscular branches of the femoral nerve. Measurement of kinetic variables from the jumps was collected using a commercially available force plate and linear position transducer. Force plate data was collected using a sampling rate of 200Hz and subsequently analysed using purpose-build laboratory software.
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