The success of athletes across a multitude of individual and team sports that incorporate sprinting relies heavily on explosive leg power. Sprint running is essentially representative of 3 distinct phases namely: (a) the initial starting phase, (b) acceleration phase, and (c) the maximum speed running phase (8). For the purpose of this study, it is important to pay particular attention to the acceleration phase in generating the appropriate power in completing a linear sprint. The start and acceleration phases of the sprint are initiated through an explosive concentric force production of the hip and knee extensor muscles (8). It is therefore of utmost importance to use a representative training method as part of a warm-up that will assist in generating sufficiently greater force in the acceleration phase to enhance sprint performance (15), hence the focus on assessing the use of depth jumps (DJs) on 20-m sprint performance.
To date, a number of studies have examined the acute effects of heavy back squats on sprint performance and have shown significant improvements over distances from 10 to 40 m (6,14,25). Although heavy squats having been found to be effective in improving sprint performance, further methods of initiating postactivation potentiation (PAP) have also been investigated. One study to date has examined the effect of a plyometric exercise in the form of a tuck jump on sprint performance (23). Till and Cooke found that 5 tuck jumps were not effective in significantly improving 10- and 20-m sprint performance. In respect of using DJs, 1 study to date (12) has examined the use of a modified DJ on jumping performance. The study found that modified DJs led to a significant improvement on countermovement jump (CMJ) power output. To date, no study has examined the use of DJs to attempt to elicit PAP to improve sprint performance.
Postactivation potentiation is an increase in muscle twitch and low-frequency tetanic force following contractile activity, initiated through the use of a conditioning activity such as back squats (18). The principal mechanism behind PAP is believed to be the phosphorylation of myosin regulatory light chains, which enables the actin-myosin interaction to become more sensitive to Ca2+ that is released from the sarcoplasmic reticulum leading to an augmented level of myosin cross-bridge activity (19). A second mechanism has also been proposed, which suggests that increasing the Hoffman reflex (H-reflex) leads to an increase in the recruitment of higher order motor units, subsequently causing greater muscle force production (11).
This study aimed to examine the acute potentiating effects of including DJs as part of a dynamic flexibility warm-up protocol on potentially improving 20-m sprint performance. It was hypothesized that the addition of 3 DJs would lead to a significant improvement in 20-m sprint time in comparison with a dynamic flexibility warm-up and a cardiovascular warm-up. The bounce DJ technique was used for drop height identification and the depth jumps to a dynamic warm-up (DYNDJ) protocol as it is an example of a fast stretch-shortening cycle and may therefore be more appropriate in training for sports, which frequently have a short and limited ground contact time (21) and rapid eccentric force development (24).
Experimental Approach to the Problem
A randomized crossover design was used to examine the effect of 3 experimental warm-up protocols on 20-m sprint performance. Bounce DJs were chosen to determine what possible potentiating effect they may have on sprint performance. The study consisted of 2 parts where all 29 subjects participated. The first part of the study aimed to identify each athlete's optimal drop height for the warm-up intervention that included 3 DJs through the use of the maximum jump height (MJH) method. Optimal drop height determination was conducted 1 week before the second part of the study. For the second part of the study, all subjects carried out 3 different warm-up protocols that were followed 1 minute later by a 20-m sprint. The 3 warm-up protocols were conducted 1 week apart. For the purpose of randomizing, the group was subdivided into 3 further groups, all of which carried out the 3 protocols in a randomized order.
Twenty-nine physically active male students from varied sporting disciplines (i.e., basketball, rugby, hurling, Gaelic football, and soccer) at higher education intercollege level volunteered for this study (Table 1). All subjects were encouraged to continue their normal training that comprised 2 team training sessions and a match per week during the in-season. To be included in the study, subjects needed to be free from lower limb injury for the preceding 6 months and never to have undergone lower limb surgery. Subjects were informed of the experimental risks and signed an informed consent document before the investigation. The study was approved by the Institutional Ethics Committee.
Subjects participated in one familiarization session one week before drop height determination commenced. The content of the familiarization session included dynamic flexibility exercises used in part 2 of the study and depth jumps used in parts 1 and 2 of the study. Subjects were required to wear standard running shoes and to be well hydrated for both parts of the study. Subjects had abstained from training the day before testing and were asked to maintain a consistent dietary intake on each day of testing. Consumption of water was permitted during drop height determination and testing of the 3 warm-up protocols (500 ml).
Part 1: Drop Height Determination
Each subject was first tested to determine their optimal DJ drop height. The determination of drop height was performed on a wooden sprung floor in the college Physiology Laboratory between 1400 and 1600 hours. The optimal drop height for each subject was determined from the MJH method (5). This method determines the drop height from the corresponding highest jump height achieved by the subject from incremental testing (2,20).
The test protocol began with a 10-minute cardiovascular warm-up on a stationary bicycle, cycling at 80 rpm with a workload of 40 W. Subjects then performed 3 practice bounce DJs from each of 5 drop heights (0.20, 0.30, 0.40, 0.50, and 0.60 m) using aerobic steps (Reebok, Lancaster, United Kingdom). A previous study (5) determined that using 0.10-m increments for the drop heights was effective in identifying optimal drop height as athletes may find a 0.15- or 0.20-m increase to large an increase for their neuromuscular system. The test involved each subject performing 3 maximal effort bounce DJs from the 5 different drop heights. A 15-second rest (16) was allowed between DJs, with an additional 2-minute rest (4) between the different drop heights to reduce any negative effects of fatigue. The optimal DJ height was determined from the highest jump achieved from the corresponding drop height with a ground contact time of less than 0.250 seconds (21).
When performing the bounce DJ, subjects were instructed to jump as high as possible and to spend the least amount of time in contact with the ground when landing from the drop height. Subjects had to keep their hands on their hips to eliminate any contribution of arm swing. Feedback on ground contact time was provided immediately after the completion of every bounce DJ to ensure that an appropriate technique was being used (26). Ground contact time, which is the duration between the foot initially contacting the ground and the end of the take-off phase, and flight time were measured using the Optojump system (Microgate, Bolzano, Italy).
Part 2: Warm-up Protocols
Once optimal DJ height had been established during a separate testing occasion, all subjects carried out 3 different warm-up protocols, 1 week apart, each followed by a 20-m sprint 1 minute after the completion of the intervention. The subject group was subdivided into 3 further groups, all of which carried out the 3 interventions in a randomized order.
The 3 protocols are represented in Figure 1 and consisted of a cardiovascular control (C) warm-up protocol, a dynamic warm-up (DYN) protocol, and the same dynamic warm-up protocol with the addition of 3 DJs (DYNDJ). The 3 warm-up protocols were performed between 1400 and 1600 hours in a college sports hall on a wooden sprung floor.
The C warm-up protocol required the subjects to perform a 5-minute jog in a linear direction up and down a 20-m distance. The DYN warm-up protocol consisted of a 5-minute jog as per the C warm-up protocol with the addition of 10 dynamic stretches, which were individually performed for 30 seconds with a 10-second rest in between each exercise (9) (Table 2). Subjects were instructed to maintain good posture while performing the dynamic stretches. The total time for the DYN protocol was approximately 11 minutes. The DYNDJ protocol consisted of the C and DYN warm-up protocols with the addition of 3 DJs with a rest period of 2 minutes between the end of the dynamic stretches and the commencement of the DJs. A 15-second rest (16) was allowed between the 3 DJs. The total time for the DYNDJ protocol was approximately 13.5 minutes.
20-m Sprint Performance
Timing gates (Newtest, Ouhu, Finland) were set up at 0- and 20-m positions to measure 20-m sprint time. Subjects started each sprint from a standard 2-point starting position with the subjects' front foot placed on a line 0.5 m behind the first set of timing gates and were instructed to perform the sprint with maximal effort. This procedure was conducted to ensure subjects did not set off the timing gates before the start of each sprint. The gates were set at a height of approximately 80 cm off the ground to minimize the chance of the light beams being broken by the lower leg or lower arm during the sprinting action. The intraclass correlation coefficient for 20-m sprint over the 3 protocols was 0.895 (p = 0.0001).
Descriptive statistics (mean ± SD) were calculated for age, mass, and height. Twenty meters time was expressed as the mean ± SD after a test for normality of distribution was conducted. The Friedman test was conducted to determine whether sprinting performance was significantly different between the 3 warm-up protocols. Post hoc tests were carried out using paired comparisons to determine where the significant differences existed. Effect size and power were determined for treatment interaction from the parametric version of the Friedman test, the repeated-measures analysis of variance for 20-m sprint time. The level of significance was set at an alpha level of p ≤ 0.05. All statistical analyses were conducted using the Statistical Package for Social Sciences (SPSS) version 15.0 software (SPSS, Inc., Chicago, IL, USA).
The Friedman test displayed a significant difference between the 3 warm-up protocols (p = 0.0001). Pairwise comparisons displayed significant improvements in sprint time of 2.2% between the control (C) protocol and the DYN protocol (3.300 ± 0.105 vs. 3.227 ± 0.116 seconds, p = 0.001), 5.01% between the C and the DYNDJ protocols (3.300 ± 0.100 vs. 3.132 ± 0.12 seconds; p = 0.001), and 2.93% between the DYN protocol (3.227 ± 0.116 seconds) and the DYNDJ protocol (3.227 ± 0.116 vs. 3.132 ± 0.116 seconds; p = 0.001); Figure 2. The use of the DYNDJ protocol resulted in a high effect size for treatment interaction (0.84) and a power of 1.0.
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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