# Power Up: Comparing PAPE Responses in Free-Weight vs. Flywheel Resistance Training Systems
## 1 Introduction  
Post-activation potentiation enhancement (PAPE) is a physiological phenomenon that leads to an acute improvement in muscular performance following a resistance training protocol [1, 2, 3, 4, 5, 6]. This temporary increase in performance is believed to arise from a combination of neuromuscular, mechanical, and biochemical changes [3]. The most widely supported explanation suggests that the phosphorylation of myosin regulatory light chains during muscle contraction increases the rate of cross-bridge attachment, thereby improving muscle performance [7]. Additionally, PAPE is thought to result from an enhanced sensitivity of contractile proteins to calcium released from the sarcoplasmic reticulum, triggering a cascade of events that lead to a stronger muscular response [7, 8]. PAPE can be induced by using resistance training exercises prior to sport-specific activities [3, 5, 6]. Specifically, in conditioning activities that involve traditional weightlifting exercises (i.e., free weights) with high relative loads (>80% of one-repetition maximum (1 RM)), a PAPEresponse is observed, as evidenced by improvements in subsequent jumping and sprinting performance [9]. PAPE-induced performance improvements tend to peak from 3 min after the last conditioning activity and start to decline progressively after 10 min [10; 11]. In addition to the positive effects of free-weight training on PAPE, recent studies have also observed positive effects in athletes using flywheel training devices [1; 5; 6; 12]. Research comparing the effects of resistance training with these two systems on physical performance is becoming increasingly popular [1; 6; 12; 13; 14; 15; 16]. However, attempting to match the training stimulus between both training methods is not an easy task and several factors must be considered [17]. Unlike traditional resistance training systems, flywheel devices do not rely on gravity, using the moment of inertia from a rotating flywheel to generate resistance throughout the entire movement [18; 19]. In contrast to gravity-dependent methods, where variations in biomechanical levers and muscle length contribute to the concentric \"sticking point\", flywheel devices provide resistance consistently throughout the concentric phase [18], achieving near-maximal muscle activation with each repetition [14]. Additionally, flywheel training has been shown to induce greater eccentric muscle activation and force production than free weights due to the need to decelerate the flywheel [14; 19]. In fact, a recent study showed that, despite training with similar intensity and volumes controlled through mean propulsive velocity (MPV), force and velocity responses in both the eccentric and concentric phases were significantly different between devices [17]. In this regard, two studies showed vertical jump potentiation with flywheel devices compared to free weights four minutes post-exercise [12; 20], while other studies showed positive effects with both methods without differences between them [1; 6; 15]. Finally, another study found no PAPE effect on vertical jump performance [16]. Regarding the enhancement in sprinting ability, [PERSON] et al. [20] demonstrated a PAPE effect on the time to run 30 m after a half-squat training protocol using flywheel devices. However, when comparing this effect with training using free weights, some studies show no effect on sprint performance on 30 and 5 m [1; 16], while others indicate that both protocols have a positive effect but without differences between them [6]. Discrepancies in findings between studies may be partially due to the parameter used to match training intensities across devices. One of the main difficulties when programming flywheel resistance exercises is the absence of a maximal load (1 RM) due to the dependency of external resistance upon the user-generated flywheel angular acceleration [21]. The results of previous comparisons where free-weight loads were matched via %RM [14; 16] should therefore be taken with caution.