Surface EMG Amplitude and Countermovement Jump Height Dissociate in the Minutes Following a Conditioning Activity in Youth Soccer Players

Post-activation performance enhancement (PAPE) is often attributed to elevated neural drive, yet neuromuscular and mechanical responses are rarely tracked concurrently. This study examined whether the two scale proportionally over the 1–4 min following a conditioning activity, with a maximal countermovement jump (CMJ) at each minute. Twenty-two male youth soccer players performed a baseline CMJ, a conditioning activity, then a CMJ at each minute to 4 min, with bilateral vastus lateralis and semitendinosus EMG (fixed 300 ms pre-take-off window) and jump height recorded simultaneously. Jump height varied across time, owing to a transient decrease at 1 min (−1.41 cm); from 2 min onward, it was equivalent to baseline (TOST, ±1.5 cm, all p ≤ 0.005). The composite EMG index instead rose almost exclusively at 4 min (dz = 3.22), as did each channel individually, producing a signal × time interaction (F(3, 63) = 108.85, p < 0.001) and a decoupling index of +120.4%; the within-participant association between the two measures was weak (rrm = 0.11). Surface EMG amplitude can therefore rise sharply where mechanical output is equivalent to baseline within ±1.5 cm, and cannot be assumed to index a proportional mechanical gain. With no control condition, the observed time course reflects the conditioning activity together with the intervening maximal jumps.