Kilduff, LP, Cunningham, DJ, Owen, NJ, West, DJ, Bracken, RM, and Cook, CJ. Effect of postactivation potentiation on swimmingstarts in international sprint swimmers. J Strength Cond Res 25(9): 2418-2423, 2011-The aim of this study was to investigate the effects of postactivation potentiation (PAP) on swim start performance (time to 15 m) in a group of international sprint swimmers. Nine international sprint swimmers (7 men and 2 women) volunteered and gave informed consent for this study, which was approved by the university ethics committee. Initially, swimmers performed a countermovement jump (CMJ) on a portable force platform (FP) at baseline and at the following time points; 15 seconds, 4, 8, 12, and 16 minutes after a PAP stimulus (1 set of 3 repetitions at 87% 1 repetition maximum [RM]) to individually determine the recovery time required to observe enhanced muscle performance. On 2 additional days, swimmers performed a swim start to 15 m under 50-m freestyle race conditions, which was preceded by either their individualized race specific warm-up or a PAP stimulus (1 set of 3 repetitions at 87% 1RM). Both trials were recorded on 2cameras operating at 50 Hz with camera 1 located at the start and camera 2 at the 15-m mark. Peak vertical force (PVF) and peak horizontal force (PHF) were measured during all swim starts from a portable FP placed on top of the swim block. A repeated measures anal sis of variance revealed a significant time effect with regard to power output (PO) (F = 20.963, p <0.01) and jump height (JH) (F = 14.634, p <0.01) with a paired comparison indicating a significant increase in PO and JH after 8 minutes of recovery from the PAP stimulus. There was a significant increase in both PHF and PVF after the PAP stimulus compared to the swim-specific warm-up during the swim start (PHF 770 ± 228 vs. 814 ± 263 N, p = 0.018; PVF: 1,462 ± 280 vs. 1,518 ± 311 N, p = 0.038); however, time to 15 m was the same when both starts were compared (7.1 ± 0.8 vs. 7.1 ± 0.8 seconds, p = 0.447). The results from this study indicate that muscle performance during a CMJ is enhanced after a PAP stimulus providing adequate recovery (∼8 minutes) is given between the 2 activities. In addition, this study demonstrated that swimmers performed equally well in terms of time to 15 m when a PAP stimulus was compared to their individualized race specific warm-up and indicates that PAP may be a useful addition to a warm-up protocol before races. However, more research is required to fully understand the role PAP plays in swim performance. © 2011 National Strength and Conditioning Association.