In relation to subject responses to the warm-up protocols, 93% of the subjects (27 of 29 subjects) produced their best 20-m sprint performance after completing the DYNDJ protocol. However, 7% of the subjects (subjects 6 and 26) produced their best sprint performance after completing the DYN protocol (Figure 3).
The results of this study have shown that the addition of bounce DJs to a dynamic flexibility (DYNDJ) warm-up protocol produced a significantly better 20-m sprint performance when compared with the use of a dynamic warm-up protocol (DYN) and a cardiovascular warm-up that acted as the control (C). A 2.2, 5.01, and 2.93% improvement in sprint time was observed between the C protocol and the DYN protocol, the C protocol and the DYNDJ protocol, and the DYN and DYNDJ protocols, respectively.
A significant improvement in sprint time is in agreement with several studies that induced PAP and found a significant improvement in sprint performance over distances for 10, 30, and 40 m and split times for 10–20 m and 30–40 m (6,14,25). These studies examined the use of the squat exercise with the number of repetitions, intensities, and rest periods varying among these studies. This study contrasts to that of Till and Cooke (23), which examined the use of tuck jumps and found no significant improvement in 10- and 20-m sprint performance. The selection of DJ as a means to improve sprint performance was justified in a study conducted by Hilfiker et al. (12) who investigated the effect of DJ on jump height and maximum power output of subsequent CMJs and squat jumps. The study demonstrated that the use of DJ within a warm-up routine does improve explosive force development in athletes. According to Young et al. (27), CMJ performance correlates with sprint velocity thereby highlighting the need to use activities, such as DJ, which improve jump performance as a means of improving sprint performance.
Because identifying the cause for the improvement in sprint performance was beyond the scope of this study, we can only speculate. According to Stieg et al. (22), DJ can be considered a form of a maximal muscle action that may elicit PAP. Postactivation potentiation may have been elicited because of an increase in the H-reflex after the completion of the DYNDJ protocol that included 3 DJs, which improves the production of power through an increase in the neural stimulation of the muscle by increasing the level of excitation of active motor units (11). The H-reflex response may have been the cause of PAP in this study, which has been found to be present during fast concentric muscle actions at high stimulation frequencies (1). This generation of greater concentric force produced in a short time interval can enhance the ability of the athlete to accelerate at the beginning of the sprint and overcome the resistance provided by bodyweight in serving to improve sprint performance (15). The increase in concentric force during sprint running is highly likely to be because of an increase in the muscle-tendon unit stiffness attributable to an increase in a reflex response (13) such as the H-reflex. This increase in muscle stiffness enables elastic energy storage in the series elastic component, especially the tendon (3). Finni et al. (10) displayed that in the concentric phase of a drop jump, the stretch and shortening occurred in the quadriceps tendon with little change in muscle length. This energy stored in the tendon is used during tendon recoil at very high speeds and with a large restoring force to amplify power output (3).
Another possible mechanism that may have served to enhance sprint performance includes the phosphorylation of light chain myosin (19). Greater muscle activation is because of a greater duration of calcium ions in the muscle cell environment and therefore the greater the phosphorylation of the light chain myosin (17). The result of greater phosphorylation translates into faster contraction rates and faster rates of tension development (7).
In terms of individual subject responses, 93% of the subjects (27 of 29 subjects) produced their best 20-m sprint performance after completing the DYNDJ protocol. However, 7% (2 subjects) produced their best sprint performance after completing the DYN protocol. A possible reason for these 2 subjects performing better on the DYN protocol may be because of their individual optimal drop heights for the DJs set too high resulting in the overload of their muscle stretch tolerance during the amortization phase of the DJ.
Although this study found that 3 DJs were effective in improving sprint performance, the optimal number of DJs and recovery time is unknown. Future research could examine the optimal number of DJs used and the optimal recovery time between the completion of the warm-up routine to elicit PAP to maximize sprint performance.
In summary, this study determined that the addition of bounce DJs to a dynamic warm-up routine is effective in improving sprint performance over a distance of 20 m in sporting events, such as soccer, rugby, Gaelic football, hurling, and athletics, 1 minute before the performance.
This study has shown that 3 DJs using a bounce technique as part of a dynamic flexibility warm-up routine (including 5 minutes of jogging) for intercollegiate athletes involved in sports events, such as soccer, rugby union, Gaelic football, hurling, and athletics, can significantly enhance sprint performance over 20 m in comparison with a dynamic flexibility warm-up. However, as 2 subjects had their worst performance for the 20-m sprint when the DJ was included as part of the warm-up, this highlights the need to measure individual responses to ensure that the appropriate warm-up is used for each athlete. It is important to note that a significant improvement in 20-m sprint time was only determined for a time period of 1 minute after the performance of 3 DJs. In addition to the rest period employed, coaches for these types of sports need to consider the format of the warm-up before a 20-m sprint. It is recommended that the format should comprise a 5-minute jog, 10 dynamic stretching exercises, and 3 DJs. When designing a warm-up routine that includes DJs, it is important to individualize the drop height (training load). To individualize the drop height, athletes and coaches are recommended to use the MJH method in a separate testing session to identify an athlete's optimal drop height. The individualization of the drop height is to meet the athlete's neuromuscular capacity so as to maximize speed performance improvements and minimize injury.
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