Introduction

The swim start is a crucial component of competitive swimming, especially in sprint events where performance differences are often decided within the first few meters of the race. Because of the high- intensity and explosive nature of the movement, minimizing the duration of the start phase is recognized as a critical factor by both coaches and sports scientists striving to maximize competitive performance. Researchers have investigated biomechanical and physiological factors influencing swimming start performance, highlighting key variables such as take-off velocity, take-off and entry angles, underwater trajectory, and 10-meter entry time (Tor et al., 2015). Based on these findings, various training interventions have been developed and applied to specifically target and improve these performance determinants (Waddingham et al., 2021). Among these interventions, plyometric training has been shown to enhance neuromuscular function, thereby increasing take-off power and improving start performance in competitions (Bishop et al., 2009). In addition, warm-up protocols play a crucial role in preparing athletes for competition by elevating core temperature, enhancing neural activation, and optimizing muscle-tendon function (Bishop, 2003).

Among various warm-up strategies, plyometric exercises such as drop jumps have demonstrated potential for acutely enhancing explosive movements through mechanisms collectively known as acute neuromuscular enhancements. These mechanisms include increased muscle-tendon stiffness, enhanced motor unit recruitment, and faster cross-bridge cycling, all of which contribute to greater force production in dynamic tasks (Li et al., 2023). Drop jumps, which utilize the stretch-shortening cycle, have been studied as effective pre-performance activities due to their ability to elicit acute power enhancements (Byrne et al., 2021). When combined with extra loadings (e.g., 0% to 10% of body weight via a weighted vest), drop jumps — which stimulate greater neuromuscular activation while minimizing fatigue — are potentially effective in improving subsequent explosive movements such as sprinting or jumping (Maloney et al., 2014).

Although drop jump warm-ups have demonstrated performance benefits in land-based sports, their application in aquatic sports, such as swimming starts, remains underexplored. Previous studies investigating drop jump or loaded drop jump protocols in swimming have focused exclusively on adult athletes (Wilson et al., 2013). While similar protocols have been examined in youth athletes participating in sports like soccer and weightlifting (Haris et al., 2021; Werfelli et al., 2021), research specifically targeting youth swimmers is scarce (Đurović et al., 2022). This gap is particularly important because warm-up strategies proven effective in adults may not yield the same benefits in younger athletes due to age-related differences in physiological and neuromuscular development. Consequently, there is a clear need to investigate age-appropriate warm-up protocols to ensure their effectiveness and safety for adolescent swimmers.

To address these gaps, the present study aimed to investigate the acute effects of weighted drop jump warm-up protocols on swim start performance in youth competitive swimmers. This study established two primary hypotheses. Our initial hypothesis proposed that a drop jump warm-up (DW) intervention would lead to greater improvements in swimming start performance compared to a conventional warm-up (CW). Our second hypothesis suggested the existence of an optimal loading condition within different DW protocols that would maximize start performance. To test this hypothesis, we implemented four distinct warm-up conditions with adolescent swimmers: the conventional warm-up that athletes typically perform, a drop jump warm-up without extra weight, and drop jump warm-ups with 5% and 10% added body weight.

Methods

Experimental Setup

Swim start performance was evaluated by dividing the motion into two distinct phases. The first phase (1st P) included the movement sequence from the initiation of the start until water entry. This phase was recorded using a high-speed camera (Exilim EX-F1, Casio, Japan) operating at a sampling rate of 300 frames per second. The second phase (2nd P) encompassed the underwater dolphin kick performed immediately after water entry, extending to the 10-meter mark. Motion data for 2nd P were collected using an inertial measurement unit (IMU) sensor (Blue Trident, Vicon, UK) attached to the back side of the swimmer’s pelvis, sampled at 228 Hz (Figure 1). The X-axis was defined as the forward direction of the swimmer’s movement underwater, while the Y-axis represented the medio-lateral (M-L) axis of the body, corresponding to the axis of pelvic pitching motion during underwater movement. Additionally, total movement time covering both phases was recorded using a smartphone camera (Galaxy S22, Samsung, Korea) positioned at the 10-meter mark. Video calibration was performed before and after the data collection sessions using a custom-made calibration frame constructed from fabric material. This frame featured a grid pattern with 0.5-meter spacing and measured 5.44 meters in height and 2.2 meters in width. The high-speed camera for the 1st P was positioned 7 meters from the pool’s edge, providing an optimal field of view to capture the swimmer’s motion from the starting block to water entry.

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Figure 1

Experimental protocol and measurement setup for assessing swim start performance.

Experimental Procedures

This study employed a repeated-measures, withinsubjects design to investigate the acute effects of four different warm-up conditions on swim start performance: conventional warm-up (CW), drop jumps warm-up (DW) without additional weight (0 BW), and DWs with 5% (5 BW) and 10% (10 BW) added body weight. All warm-up conditions were randomly assigned in a counter-balanced order, with a minimum of 48 hours separating each testing session. The experiments were conducted in a 50-meter indoor swimming facility equipped with starting blocks. At the start of each session, participants completed the assigned warm-up protocol. The conventional warm-up referred to each swimmer’s habitual pre-competition routine, which typically included dynamic stretching of major joints (such as ankle and shoulder rotations), neural activation or tactile stimulation by slapping the skin, and light aerobic activities (e.g., on-the-spot jumping).

For the overweight drop jump warm-up conditions, participants performed one set of five trials of drop jump from a 0.3 m-high box while wearing a weighted vest (0%, 5%, or 10% of body weight), with 15 seconds of rest between jumps. A 30 cm platform was created by stacking two 15 cm step boxes. To ensure a consistent drop height, a reference rope was set at 30 cm. Participants stepped forward with one foot over the rope before dropping from the platform. Immediately upon landing, they performed a maximal vertical jump. Arm swing was allowed to maximize performance during the jump. Following the warm-up, a five-minute passive rest period was provided, based on prior evidence suggesting optimal recovery for post-activation performance enhancement (Bishop, 2003; Gray et al., 2002). The durations of DWs and resting intervals were set equal to those of CW conditions. After the rest period, participants completed a single swim start trial with maximal efforts.

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Figure 2

Drop jump warm-up protocol with a weighted vest.

Results

Discussion

This study aimed to determine whether an overweight drop jump warm-up could enhance swim start performance in youth swimmers aged 14 to 19 compared to a conventional warm-up. The hypothesis was based on previous research suggesting that appropriately loaded drop jumps could activate the neuromuscular system through post-activation potentiation (PAP), potentially leading to improved subsequent performance (de Poli et al., 2020; Seitz & Haff, 2016; Abbes et al., 2018; Al Kitani et al., 2021). The experimental design of this study differed from previous studies in the following key aspects. First, unlike prior studies that rarely validated findings with adolescent participants, this study specifically involved subjects within the adolescent age range. Second, the task employed was not a simple vertical jump, which is commonly used to directly examine the effects of post-activation potentiation (PAP), but rather the swim start—a complex movement that includes a jumping component but also involves other dynamic and sport-specific elements. The DW showed improvement only in the horizontal distance of the 1st phase compared to the CW. Contrary to expectations on the first hypothesis, the CW revealed better performance in average horizontal velocity of the 2nd P, total movement time, and the main frequency of the angular velocity in the M-L axis (the 2nd P).

The absence of significant differences in dominant performance variables between the two warm-up conditions may be attributed to several reasons. Firstly, it could be partially ascribed to the inherent characteristics of the swim start motion. The swimming start comprises multiple phases—such as the block phase, flight phase, water entry, and underwater propulsion—making it challenging to isolate the specific impact of additional loading on each phase. Secondly, individual differences in fatigue resistance and responsiveness to post-activation potentiation (PAP) may have attenuated the effects of the DW condition. Specifically, despite exhibiting a longer horizontal distance during the 1st phase, the inferior performance observed in terms of average horizontal velocity of the 2nd phase and total movement time—compared to the CW condition—suggests that the participants may not have fully benefited from the warm-up or PAP stimuli. The findings of this study partially align with those of Cuenca-Fernández et al. (2020), who reported that while resistance-based warm-ups can enhance start performance, they may adversely affect subsequent 50-meter performance due to accumulated fatigue.

Another potential explanation for the observed results is the muscular fatigue likely induced by the execution of overweight drop jumps, although direct verification through electromyographic (EMG) analysis was not conducted in this study. Repetitive stretchshortening cycle exercises may enhance immediate jump performance but can also cause fatigue, leading to decreased lower limb muscle activation and impaired performance (Lesinski et al., 2016). This study assumed the optimal time window for the post-activation potentiation (PAP) effect as a 5-minute resting time when designing the warm-up-to-start transition for adolescent athletes, but the fatigue induced by overweight drop jumps may have offset the benefits of PAP.

Additionally, inconsistencies in previous research regarding optimal recovery time, drop jump height, and weight conditions for maximizing the PAP effect posed challenges in study design. Wilson et al. (2013) suggested that a recovery period of 7–10 minutes is appropriate, whereas Waddingham et al. (2021) found that 6 minutes was effective in improving swimming start performance. Although this study incorporated insights from these studies when setting experimental conditions, identifying significant differences based on weight conditions remained challenging.

In this study involving youth swimmers aged 14 to 19, the absence of significant improvements in swim start performance following the overweight drop jump warm-up may be partially attributed to the ongoing neuromuscular development characteristic of adolescents. During adolescence, the neuromuscular system is still maturing, which can influence the efficacy of interventions like post-activation potentiation (PAP). Compared to adults, adolescents generally exhibit lower muscle-tendon stiffness and reduced efficiency in neuromuscular coordination, which can lead to a delayed or diminished response to potentiation stimuli (Mersmann et al., 2017). Furthermore, their recovery capacity from high-intensity loads such as weighted plyometric exercises may be limited, making them more susceptible to fatigue-related performance declines (Falk & Dotan, 2006). These developmental factors should be taken into account when applying PAP-based warm-up protocols to youth athletes, as the optimal balance between potentiation and fatigue may differ significantly from that of adults (Sanchez-Sanchez et al., 2018).

Another potential factor is the negative transfer effect caused by differences in movement characteristics between the warm-up and the actual swimming start. The drop jump is a discrete motor skill with a clear beginning and end, whereas the underwater dolphin kick is a continuous motion requiring rhythmic, repetitive movements. Studies on motor control and learning suggest that transfer between discrete and continuous skills is limited (Levac et al., 2019; Woltz et al., 2000).

This study aimed to identify the optimal loading condition for enhancing swim start performance. Based on findings from previous research (Halteman et al., 2018), DWs with additional loads of 0%, 5%, and 10% of body weight were implemented. However, contrary to the initial hypothesis, no significant differences were observed in swim start performance across the different loading conditions. This suggests that the loading conditions employed may not have been optimal for maximizing swim start performance. The findings of this study partially align with the study by Halteman et al. (2018), which reported that adding 5 to 15% external load during drop jumps did not affect jump height, mean velocity, or mean power.

This study has a couple of limitations. First, the variability of the conventional warm-up (CW) protocol poses a challenge. Because participants favored different CW styles, we could not standardize a specific type for the control condition. This inherent variability may have reduced the study's internal validity. While our intention was to enhance external validity by allowing participants to perform the CW as they would in a competitive setting, we speculate that narrowing the CW conditions might have been beneficial. A more constrained CW protocol could have improved statistical power and allowed for a clearer focus on the hypothesized outcomes, thereby strengthening internal validity.

Second, limitations existed in applying variables uniformly across all participants, such as drop jump height, recovery time, and exercise intensity (absolute percentage of body weight). Because adolescents are in a period of rapid growth, their strength and power develop significantly year by year. We regret not being able to implement individually tailored experimental conditions based on personal characteristics (e.g., strength, power, fatigue recovery ability) or swimming style. If follow-up research is conducted in the future, it will be essential to incorporate personalized experimental conditions according to individual characteristics.

Future investigations could also examine whether combining sport-specific dynamic movements with moderate plyometric loading can achieve a better balance between potentiation and fatigue. Longitudinal studies assessing the chronic adaptation effects of repeated PAP interventions in youth swimmers would further advance understanding of optimal warm-up strategies for this population.

Conclusion

This study aimed to determine whether an overweight drop jump warm-up protocol is better than a conventional warm-up in terms of swim start performance for adolescent swimmers. After data collection and analysis, the following conclusions were induced.

First, when comparing kinematic variables between warm-up protocols, the drop jump warm-up was superior only for the horizontal distance of the 1st phase, while no significant differences were observed, or the conventional warm-up showed better performance, for the remaining variables. The conventional warm-up showed a faster underwater average velocity, a reduced movement up to the 10-m mark, and a higher pelvic pitching frequency of the dolphin kick.

Second, when comparing kinematic variables among the three different overweight drop jump warm-up protocols (additional 0%, 5%, and 10% body weight), no statistically significant differences were observed in any variables between conditions.

In conclusion, implementing an overweight drop jump warm-up protocol to enhance swim start performance in youth swimmers aged 14 to 19 may not be effective. In fact, a conventional warm-up routine might yield better results. Future studies investigating the acute effects of warm-ups should comprehensively consider various complex factors, including an adolescent athlete’s individual physical characteristics, muscle development, muscle fatigue, weight selection, exercise intensity, jump height, recovery time, and technical aspects of movement.