The ability to develop high levels of muscular power is considered an essential component of many key activities performed in team sports (e.g., sprinting and change of direction). For example, Sleivert and Taingahue (13) reported negative correlations between relative peak power output (PPO) during the split squat and 5-m sprint time (r = −0.65) and relative PPO during the traditional squat and 5-m sprint time (r = −0.66), which may indicate that increasing PPO will lead to an improvement in sprinting performance, a primary performance outcome in many team sports. Consequently, training methods aimed at improving an athletes PPO have received significant attention in the strength and conditioning literature recently. These training methods have included athletes trying to develop power while working against their body mass (e.g., plyometrics) and also while working against external loads that equate to various intensities of their 1 Repetition Maximum (RM) (e.g., 70-80% for Olympic-style weightlifting movements) (e.g., 10). Recently, a training method that requires an athlete to work against a heavy load (e.g., preload stimulus, >80% 1 RM) followed by a light load (body mass) has been proposed to be an effective training method for enhancing power output in athletes (e.g., 2, 6). This method commonly referred to as contrast training is based on the physiological condition, namely, postactivation potentiation (PAP), with PAP defined as an acute enhancement of muscle function after a preload stimulus (8).
The literature regarding an athlete's ability to harness PAP has been conflicting and can in part be explained by numerous potential methodological differences in the various studies (8). Recently, researchers have sought to investigate the optimal conditions to observe an enhancement in muscle performance after a preload stimulus. For example, studies by Kilduff et al. (11,12) and Bevan et al. (3) have investigated the optimal recovery time to observe enhanced performance after a preload stimulus and have reported that on average an 8-minute recovery is required between the preload stimulus and the explosive activity. However, although researchers now have a better understanding of the exact experimental design required to observe enhanced performance with PAP during squat jumps and ballistic bench throws (3,10,12), research still needs to be carried out to see if PAP can be harnessed to enhance performance in more functional activities such as sprinting.
Therefore, because of the lack of research regarding PAP and its effect on activities directly transferable to sport, the aim of the present study was to investigate the effects of a preload stimulus on 5- and 10-m sprint times of professional rugby players.
Experimental Approach to the Problem
During this within-subject design study, each subject was required to attend the laboratory on 2 occasions. The objective of the first testing session was to determine the subjects 3 RM on the squat and familiarize the subjects to the study procedures that were to follow. During the main experimental trial, subjects completed a baseline 10-m sprint (with a 5-m split), then after a 20-minute recovery period, subjects were required to complete 1 set of a preload stimulus (1 set of 3 repetitions at 91% of the subjects estimated 1 RM) on the squat. After the preload stimulus, subjects completed a 10-m sprint (with 5-m split) every 4 minutes up to and including 16 minutes (4, 8, 12, and 16 minutes).
Sixteen professional rugby players (Table 1) from whom written informed consent had been obtained, volunteered to take part in the present study, which was approved by the a local ethics committee and carried out during the preseason (August-September). At the time of entry into the study, subjects completed a power phase that incorporated Olympic lifts, the various derivates of the Olympic lifts, and complex/contrast exercises (including sprinting). The average resistance training experience of the present group of subjects was 2.1 ± 1.4 years.
Before the commencement of the main experimental trial, subjects visited the laboratory to become familiar with the testing methods and to have their 3 RM squats measured. Forty-eight hours after the familiarization and strength-testing period, all subjects performed the main experimental trial.
Subjects reported to the laboratory on the morning of testing after having refrained from alcohol, caffeine, and strenuous exercise, 48 hours before the study. After the measurement of each subject's stature and body mass, subjects underwent a standardized warm-up, which comprised of progressive 10-m sprints with players performing dynamic mobility exercise at set intervals throughout the warm-up with an emphasis on warming-up the musculature associated with the squat and sprinting. After the warm-up, subjects completed baseline 10-m sprints (with a 5-m split). After a 20-minute recovery period, subjects completed the preload stimulus on the Squat. After 4, 8, 12, and 16 minutes of recovery from the preload stimulus, players completed a 10-m sprint (with a 5-m split). To ensure that any effect observed during this experiment was because of the preload stimulus, 10 subjects were required to complete 4 10-m sprints after a standardized warm-up with 4-minute recovery between each one. This was carried out to ensure that during the main experimental trial there was no warm-up effect or fatigue effect from the subsequent 10-m sprint. A repeated measures 1-way analysis of variance (ANOVA) revealed no significant time effect over the duration of the study (F = 1.382, Effect Size = 0.090, p = 0.252).
Consumption of water (500 ml) was permitted during each test. Room temperature was maintained between 20 and 24 °C. Verbal encouragement was given to maximize performance.
Before the start of the strength-testing session, all subjects underwent a standardized warm-up that comprised of light intensity rowing for 5 minutes, followed by a series of dynamic movements with an emphasis on warming up the musculature associated with the Squat. Subjects then performed 3 warm-up sets of 8 repetitions at 50% 1 RM, 4 repetitions at 70% 1 RM, and finally 2 repetitions at 80% of their 1 RM. After the final warm-up set, subjects attempted 3 repetitions of a set load (3 RM), and if successful, the lifting weight was increased until the subject could not lift the weight through the full range of motion. All subjects had been previously exposed to 3 RM testing for the Squat. A 5-minute rest was imposed between all attempts to allow subjects adequate time to replenish energy stores. The 3 RM was determined after 3-4 attempts in all subjects. The Squat movement was carried out according to the International Powerlifting Federation rules (9).
Five- and 10-m Sprint Performance
Timing gates (Brower Timing Systems, Salt Lake City, UT, USA) were set up at 0-, 5-, and 10-m positions to measure 5- and 10-m sprint times. Subjects started each sprint from a standard 2 point starting position with the subjects' front foot placed on a line 30 cm behind the first set of timing gates. This procedure was carried out to ensure the subjects did not set off the timing gates before the start of each sprint. The timing gates were set at a height of approximately 80 cm off the ground (around hip height), which from previous experience was necessary to minimize the chance of light beams being broken by the lower leg or lower arm during the sprinting action.
Subjects completed 5 10-m sprints at the following times: baseline, 4, 8, 12, and 16 minutes after the preload stimulus. The preload stimulus consisted of 1 set of 3 repetitions at 91% of the subject's estimated 1 RM on the squat. Test-retest reliabilities (intraclass correlations) for 10-m sprint were 0.976.
After a test for the normality of distribution, data were expressed as the mean ± SD. Statistical analysis was carried out using a repeated measures 1-way ANOVA to determine whether sprinting performance changed throughout the testing session. When significant F values were observed (p ≤ 0.05), paired comparisons were used in conjunction with Holm's Bonferroni method for control of type I error to determine significant differences. A Pearson correlation analysis was used to assess the relationship between strength and changes in PPO after potentiation.
The level of significance was set at p ≤ 0.05 in the present study.
Five- and 10-m Sprint Times
A repeated measures ANOVA revealed no significant time effect over the duration of the study with regard to 5 m (F = 1.650, ES = 0.105, p = 0.175) and 10-m sprint times (F = 1.028, ES = 0.068, p = 0.401).
However, based on previous research completed in our laboratory (3,11,12) on the influence of recovery time on PAP, we reported large intraindividual responses with regard to the optimal recovery between the preload stimulus and the subsequent explosive activity. Based on this research, we examined the current data for individual response to PAP.
With regard to the 5-m sprint time, 47% of the subjects performed their best 5-m sprint 8 minutes after the preload stimulus, 27% after 12 minutes, and 13% at 4 and 16 minutes. A paired sample t-test revealed a significant decrease in sprint time over 5 m after the subjects best 5-m sprint after the preload stimulus compared with baseline (Baseline: 1.09 ± 0.06 seconds vs. Best time: 1.05 ± 0.05 seconds, p = 0.007) (Figures 1 and 2).
A similar finding when comparing 10-m sprint times at baseline compared with the best 10-m sprint time after the preload stimulus (Baseline: 1.83 ± 0.08 seconds vs. Best time: 1.79 ± 0.08 seconds, p = 0.003) (Figures 1 and 2). Similarly to the 5-m results, a majority of subjects (53.3%) performed their best sprint times at the 8-minute time point, with the remainder evenly distributed between the 4 and 16-minute time points.
The results of the present study demonstrate that PAP can be harnessed to enhance sprinting performance in professional rugby players providing adequate and individualized recovery is given between the preload stimulus and subsequent sprint activity (Figures 1 and 2).
In the present study, the subjects observed a 5.0 ± 1.0 and 8.0 ± 1.0% improvement in sprint performance over 5 (Baseline: 1.09 ± 0.06 seconds vs. Best time: 1.05 ± 0.05 seconds, p ≤ 0.007) and 10 m (Baseline: 1.83 ± 0.08 seconds vs. Best time: 1.79 ± 0.08 seconds, p ≤ 0.003), respectively, compared with baseline after the preload stimulus. Recently, we have demonstrated the effectiveness of PAP in enhancing performance in professional rugby players performed both the squat jump (11,12) and ballistic bench press (3,11). Because electromyography recordings were not obtained in this study, we can only speculate on the potential mechanism for the observed improvement in performance after PAP. However, 2 primary theories have been proposed to date: (a) the preload stimulus acts to enhance motor-unit excitability, possibly affecting a number of processes such as increased motor-unit recruitment, increased motor-unit synchronization, decreased presynaptic inhibition or greater central input to the motor neuron; and (b) enhanced phosphorylation of the myosin light chain (MLC), where the preload causes an increase in sarcoplasmic Ca2+, which activates MLC kinase, which in turn increases actin-myosin crossbridging (8).
As previously indicated, recent research from our laboratory has demonstrated a very individual response in terms of recovery time between the preload stimulus and the subsequent explosive activity (3,11,12), probably because of the varying strength levels of the subjects within this study. The current study found that a majority of subjects performed their best sprint times 8 minutes post preload stimulus (5 m: 46.7%; 10 m 53.3%), and this agrees with previous research performed on lower body and upper body PAP and elite rugby union players (3,11,12). The findings of Gullich and Schmidtbleicher (7) support the current study findings in that they reported the greatest increase in H-reflex activity (32%) after their preload stimulus occurred after 8.7 ± 3.6-minute recovery an period, which leads to a significant enhancement of explosive force production in plantar flexions after this recovery period. As mentioned previously, Hodgson et al. (8) indicated that training history and strength levels of the subjects seem to be important factors in the outcome of PAP studies. Studies to date have used subjects of varying strength levels (from recreationally trained to power athletes), and in some studies, it was only when subjects were differentiated into “strong” and “weak” subjects based on their strength levels (2) or training experience (4) that a performance effect was observed. For example, Chiu et al. (4) initially reported no change in performance after a preload stimulus when the group was considered as a whole; however, once the group was divided on the basis of strength, performance increases were observed. In support of this, the studies by Young et al. (14) and Duthie et al. (5) found significant correlations between performance changes after the preload stimulus and measures of strength (e.g., 1 RM) (r = 0.73 and r = 0.66, respectively), which indicated that stronger subjects had greater potential for performance gains after heavy resistance training (HRT). Although the exact reason behind this relationship between strength and potentiation remains unclear, it has been demonstrated that resistance-trained athletes have greater activation of the musculature involved during HRT, which would affect the H-reflex and myosin regulatory light chain phosphorylation the 2 mechanisms involved in the PAP phenomenon (1). In addition, Gullich and Schmidtbleicher (7) reported differences between speed-strength athletes (highly trained) and sports students (trained) in that the highly trained athletes showed a significantly higher and longer lasting potentiation effect compared with the less trained sports students. In addition, this study reported differences between the level of potentiation between the soleus muscle (predominantly slow-twitch muscle fiber) and the gastronomies muscle (predominantly fast-twitch muscle fiber), with the gastronomies muscle having a greater level and longer lasting potentiation effect compared to the soleus muscle.
In conclusion, the current study observed improvements in performance in 5- and 10-m sprint after a heavy preload stimulus when adequate and individualized recovery was given between the preload stimulus and the sprint.
The current findings indicate that after a preload stimulus, sprinting performance is improved providing adequate recovery is given between the preload stimulus and sprinting performance. This finding may have important application in the training of speed in many team sports although research still needs to be conducted to assess if this method produces positive results if incorporated into a speed-training phase. This study further highlights the need for individual determination of the optimal recovery time required for enhanced performance. Research still needs to determine the effectiveness of this method of enhancing power output as a training method.
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