Introduction
Over the past decade researchers have investigated the use of complex training to enhance power output (1,7,8,12,20,25). Complex training involves the execution of a heavy resistance exercise before performing an explosive movement with similar biomechanical characteristics, referred to as a complex pair (19). The proposed mechanisms to enhance explosive performance after performing a heavy contractile activity are phosphorylation of the light myosin chain and neural excitability (11,16). The force the muscle is able to generate after previous contractile activity is the result of the net balance of fatigue and potentiation (15,30). A sufficient amount of recovery is needed after performing the heavy contractile activity for replenishment of phosphocreatine stores (28) and this varies among individuals (23). This gives a relatively brief window of opportunity (4–12 minutes) (16,23) to use the potentiating effect as it dissipates over time.
Reviews on complex training (post activation potentiation [PAP]) have shown that the squat exercise has been widely used across research to elicit PAP responses in either highly trained, recreationally trained, or untrained populations (11,13,19). Although researchers have reported significant increases in jump performance measures after the heavy squat exercise (6,8,14,23,25,31), the potentiating effect of complex training on sprint performance is not clear. Chatzopoulos et al. (4) found that 10-single squat repetitions (with a 90% 1 repetition maximum [RM] load) significantly improved selected running phases (0–10 m (effect size [ES = 2.0]) and 0–30 m [ES = 1.2]) in 15 amateur team game players (basketball, volleyball, handball, and soccer players), 5 minutes after the heavy resistance stimulus. Comyns et al. (7), using a complex training protocol consisting of 3 RM back squats, reported a significant improvement (p ≤ 0.05) among 11 rugby players from session 1 to session 4 in the pretest to posttest changes in instantaneous velocity at 20 m (ES = 0.37) and 30 m (ES = 0.32). Comyns et al. (7) hypothesized that the rugby players may be able to learn to apply the potentiating effects of complex training. In contrast, Crewther et al. (8) reported no significant performance improvements in 5, 10, and 20 m sprint times in rugby players (n = 9) for any retesting at ∼15 seconds, 4, 8, 12, and 16 minutes after performing 3 RM back squats. Similarly, Deutsch and Lloyd (10) found no significant performance improvements in 5, 10, and 20 m sprint times in rugby players (n = 8) for retesting at 10 minutes after performing 3 RM squats. Deutsch and Lloyd (10) concluded that sprint training should be performed independently to minimize any potential interference from previous resistance training.
The variability observed among many of the complex training studies may be due to the lack of movement specificity between the PAP stimulus and the testing exercises (8). Recently, Whelan et al. (36) investigated the use of resisted sprinting using a sled load of 25–30% body mass, hypothesizing that the sled pull may be better suited as a complex pair with sprinting because of the biomechanical similarity between the movements. Interestingly, the results did not provide strong evidence of PAP in 10 m sprint performance after resisted sprinting at 1, 2, 4, 6, 8, and 10-minute rest periods. The authors concluded that the preload of 30% body mass was not sufficient to cause a fatigue effect, and without sufficient fatigue the PAP response was unlikely to occur. Currently, no researchers have examined if sled loads exceeding 30% body mass can be used as a PAP stimulus to improve sprint performance. It could be argued that heavier loads in the sprint-style sled pull could induce greater muscle fiber recruitment of the sprint-specific motor units and therefore elicit greater neural and muscular mechanisms that could lead to acute increases in explosive sprinting capability. Such a contention is supported by biomechanical studies of heavy sprint-style sled pulling, which have demonstrated kinematic and kinetic similarities to the acceleration phase of sprinting (22,37). Furthermore, weighted sled towing with heavier loads may help improve sprint acceleration performance by teaching athletes to produce larger horizontal or resultant ground reaction force (GRF) impulse (21).
Researchers have reported that heavy sled pulls (68.0 ± 44.3 kg and 60.9 ± 38.9% body mass) are the most common strongman-type implement used by coaches in strength and conditioning practice (38), however no study has investigated the use of heavy sled pulls as a form of complex training to improve sprint performance. Because sprinting speed is an activity that is vital to successful performance in many sports (9) there is a clear need to evaluate whether heavy sled pulling can induce a PAP response. Such data would give practitioners a greater understanding of the applications and likely chronic adaptations to this form of training. Therefore, the purpose of this study was to investigate the acute potentiating effects of heavy sprint-style sled pulls on sprint performance. Two training loads (75 and 150% body mass) were used to determine whether sled load influences the potentiating response and what sled load and rest interval may be optimal to elicit a PAP effect. Such an analysis was important as researchers have demonstrated that greater sled towing loads have a negative influence on acceleration kinematics (26,27), which may negate the potentiating training effect. However, because PAP effects have generally been reported with heavy loads (4,6), it was hypothesized that the heavy sled pull condition (150% body mass) with the rest period of 8 minutes would induce a greater potentiating effect (i.e., faster sprint times) than the lighter sled condition (75% body mass).
Methods
Experimental Approach to the Problem
A randomized crossover and counterbalanced design was used to examine the effects of the heavy sprint-style sled pull, with loads of 75% (over 15 m) and 150% (over 7.5 m) body mass, on 5, 10, and 15 m split times (at 4, 8, and 12 minutes after the resisted sprint). Twenty-two experienced resistance-trained rugby players volunteered to participate in this study. Data were collected for each participant over 2 sessions separated by 1 week. Baseline and postintervention sprint times were compared using two-way repeated measures analysis of variance (ANOVA) and effect statistics.
Subjects
Twenty-two experienced resistance-trained rugby athletes volunteered to participate in this study, a summary of the participant's characteristics is presented in Table 1. All subjects had an extensive strength training background, including experience with the heavy sled pull. The study was conducted in the participant's off-season where the majority of subjects were at the start of a training cycle aimed at improving strength and power performances. Subjects were excluded if: any medical problems were reported that compromised their participation or performance in this study; and, athletes were taking or had previously taken any performance-enhancement drugs of any kind. All testing for this study was undertaken at a similar time of day with subjects instructed to maintain their normal dietary intake before and after each workout. We did not control for nutrition or hydration levels, but subjects were told not to make any changes in the above during the testing period. Subjects were asked to refrain from undertaking any other types of training 48 hours before each testing session. All subjects provided written informed consent after having being briefed on the potential risks associated with this research. Previous ethical approval was granted by the AUT University Ethics Committee, Auckland, New Zealand.
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Table 1: Demographics and training characteristics (mean ± SD) for rugby players.
Testing Procedures
This study was conducted over a 21-day period at the end of the rugby off-season. The experimental procedure was performed in 3 sessions (Figure 1). Before the start of the first session, subjects had their height and body mass recorded. Subjects were then familiarized with the experimental conditions, which included maximal sprints and heavy sled pulls with 75 and 150% body mass. One week after the familiarization session, performance testing occurred and was randomized across 2 sessions each separated by 7 days of rest and supplementary training. Subjects performed a 10-minute standardized warm-up before testing that consisted of dynamic stretching and light jogging interspersed with bodyweight exercises. Testing commenced 5 minutes after the warm-up. Each testing session involved the determination of the subjects 5, 10, and 15 m sprint times (s) from a 15-m sprint at 8 and 4 minutes before a single heavy sprint-style sled pull (75% body mass load × 15 m; 150% body mass load × 7.5 m), followed by unloaded 15 m sprint retesting at 4, 8, and 12 minutes after heavy sled. Sled pull loads and distances were equated based on the equation; mechanical work = load × distance.
Figure 1: Experimental design.
For the heavy sprint-style sled pull subjects were instructed to start in a four-point power position (Figure 2) and accelerate the sled (Strongman pulling sled, 11.5 kg, length 600 mm, width 400 mm, Getstrength, Auckland, New Zealand) 15 and 7.5 m (load 75 and 150% body mass, respectively) as quickly as possible over the indoor artificial turf surface using powerful triple extension of the lower body. Two baseline sprint performance tests were performed before the heavy sled pulls to determine whether a learning or warm-up effect occurred and these were averaged for analysis. Subjects were seated between each trial to reduce any fatigue effects. All sprint testing were performed indoors on artificial turf (15 mm underlay/10 mm overlay) at the same time of day.
Figure 2: Heavy sprint-style sled pull 4 point starting position.
Performance Testing
The sprints were measured using SpeedlightV2 wireless dual-beam timing lights (Swift Performance Equipment, Brisbane, Australia) that recorded times to an accuracy of 0.01 seconds. Timing started when the first beam was broken. Subjects started in a standing split stance, with the toes of the back foot in line with the heel of the front foot, 50 cm before the start line. No rocking or backward steps were allowed before the start. Subjects were instructed to sprint at maximal effort through the timing lights placed at 5, 10, and 15 m from the first set. Timing light beams were set at 92.5 cm (top beam) and 68 cm (bottom beam) for all performance test undertaken in this study. The assessment of sprint times in this population has been found highly reliable (Coefficient of Variation [CV] = 0.6%) (39). Subjects also wore Polar Team2 heart rate monitors (New York, NY, USA) throughout the warm-up and testing procedure. From the heart rate data (expressed as a percentage of estimated HR maximum [%HR Max]) the following variables were analyzed; (a) average heart rate during the testing period (time before first baseline sprint to peak heart rate after the 12-minute sprint), (b) peak heart rate after the sled pull (peak maximal heart rate after sled pull) and heart rate before the post 4-minute sprint (point before the post 4-miute sprint). This was performed so to provide a crude measure of subjects recovery and fatigue.
Statistical Analyses
Descriptive statistics were calculated and reported as mean and SDs. Within-trial and between-trial reliability testing was conducted for baseline sprint times at 8 minutes and 4 minutes. All sprint measures were log transformed to reduce bias arising from nonuniformity of error. The changes in sprint performance (vs. baseline values) were examined using ANOVA with repeated measures and Cohen's effect statistics. A post hoc Bonferroni test determined which measures differed significantly. Effect sizes (ES = mean change/SD of the sample scores) were calculated to quantify the magnitude of the PAP effects associated with the heavy sled pull conditions. Cohen (5) applied qualitative descriptors for the ES with ratios of 0.2, 0.5, and 0.8 indicating small, moderate, and large changes, respectively. A paired t-test was used to determine the peak change in sprint time (as a percentage) compared with baseline values at each corresponding distance and time point between the 2 sled conditions. Significance was accepted at the p ≤ 0.05 level. All statistical analyses were performed using SPSS 20.0 for Windows (SPSS Inc., Chicago, IL, USA).
Results
The results between the baseline and post sled pull sprint times are presented in Table 2. High levels of within-trial and between-trial reliability were observed between the 15-m baseline sprints at pre-8 minutes and pre-4 minutes (CVs of 1.15 and 1.13%; and intraclass correlation coefficient [ICC] of 0.86 and 0.81, respectively).
Table 2: Baseline and post sprint times (mean ± SD) before and after heavy sled pulls (75 and 150% body mass).*
75% Body Mass Sled Load
A significant time effect on 15 m (p = 0.023) sprint times for the 75% body mass load was noted, with a faster (p = 0.036) 15 m group sprint time being observed at 12 minutes. Small nonsignificant improvements in 5, 10, and 15 m group sprint times (compared with baseline value) were observed at 8 minutes (ES = 0.33, 0.31, and 0.24, respectively) and 12 minutes for 5 and 10 m sprint times (ES = 0.24 and 0.26, respectively) (Table 2).
150% Body Mass Sled Load
There were no significant differences from baseline to 12 minutes for 5, 10, and 15 m sprint times after completing the 150% body mass sled pulls. Small negative changes were associated with sprint times after the 150% sled pull, slower group sprint times noted at 4 minutes (5 and 10 m; ES = −0.28), and 12 minutes (10 m; ES = −0.22).
Peak Changes (%) in Sprint Times
A graphical representation of the peak differences (%) in group sprint times between baseline sprints and sprints after the 75 and 150% body mass sled pulls is presented in Figure 3. Significant differences in the percentage of change in group sprint times between the 2 sled pull conditions were at 4 minutes (5 m, p = 0.044, ES = 0.49; 10 m, p = 0.047, ES = 0.51; and 15 m, p = 0.044, ES = 0.52) and 8 minutes (10 m, p = 0.050, ES = 0.59) and 12 minutes (15 m, p = 0.05, ES = 0.64).
Figure 3: Peak changes (expressed as a percentage) to sprint times (mean ± SD) after the 75 and 150% body mass sled pulls. *Significantly different (p ≤ 0.05) to other level of variable. Note: Positive values represent positive changes to sprint times (i.e., faster sprint times) compared with baseline values.
Additional Analysis
Analysis of the 75% sled pull data showed that 13 (62%) subjects improved their 15 m sprint time at the 4-minute time point, 17 (81%) subjects improved their 15 m sprint time at the 8-minute time point, and 16 (76%) subjects improved their 15 m sprint time after 12 minutes. Nine (43%) subjects obtained their best 15 m sprint time at the 8-minute time point and 7 (33%) subjects obtained the best 15 m time at 12 minutes. Four subjects (19%) produced their best 15 m time after only 4 minutes of recovery.
In contrast, no improvements were shown in 15 m sprint time after the 150% sled pull at the 4-minute time point. Nine (43%) subjects improved their 15 m sprint time at the 8-minute time point and 12 (57%) subjects improved their 15 m sprint time after 12 minutes. Nine (43%) subjects obtained their best 15 m sprint time at the 8-minute time point and 6 (29%) subjects obtained the best 15 m time at 12 minutes.
When compared with unloaded sprinting, the 75% body mass sled pull reduced baseline sprint velocity by 34 ± 5%, 35 ± 4%, and 37 ± 3% for the 5, 10, and 15 m times (respectively), and the 150% body mass sled load reduced sprint velocity by 56 ± 5% at 5 m. Sled pull times were significantly different (p < 0.001) with the 75% body mass sled load distance (15 m) taking significantly longer (4.04 ± 0.32 seconds vs. 3.48 ± 0.53 seconds) to complete than the 7.5-m distance associated with the 150% body mass sled load. Heart rate data (expressed as %HR Max) showed no significant differences between the 75 and 150% sled conditions for average heart rate during the testing period (62.2 ± 5.98%HR Max vs. 62.6 ± 8.89%HR Max; ES = −0.05) and peak heart rate after the sled pull (81.2 ± 10.06%HR Max vs. 79.6 ± 8.21%HR Max; ES = 0.17) and peak heart rate before the post 4-minute sprint (60.3 ± 7.33%HR Max vs. 57.7 ± 8.94%HR Max; ES = 0.32) (respectively).
Discussion
Because the heavy sled pull is the most commonly used strongman implement used by coaches in strength and conditioning practice as a means of performance enhancement (38), it is important to obtain data on the applications of heavy sled pulling as a conditioning stimulus. This study examined the potentiating effects of heavy sprint-style sled pulling on sprint performance. Two sled load conditions (75 and 150% body mass) were used to determine whether sled load influences the potentiating response and what sled load and rest interval may be optimal to elicit a PAP effect. It was hypothesized that the 150% body mass sled pull with the rest period of 8 minutes would induce a greater potentiating effect (i.e., faster sprint times) than the lighter sled condition (75% body mass).
Contrary to the initial hypothesis, the main findings of this study was that the 75% sled load demonstrated small improvements (ES = >0.2) in 5, 10, and 15 m group sprint times after 8 and 12 minutes recovery, with a significantly faster 15 m group sprint time observed after 12 minutes recovery. Slower group sprint times (ES = −0.22 to −0.28) were associated with the 150% sled pull for 5 and 10 m group sprint times after 4 minutes recovery and 10 m group sprint times after 12 minutes of recovery. Such findings support previous researchers who have cited that at least 8 minutes of recovery is required between the heavy resistance training exercise and the subsequent explosive movement to observe enhanced power output (3,8,23). Interesting, variability existed in this study with subjects improving their 15 m sprint time at the 4, 8, and 12-minute recovery time points. The results support previous researchers who have indicated that PAP is a highly individualized phenomenon and may be influenced by the amount (18) and type of loading (24,32,34), muscle fiber composition of the muscles exercised (35), and the specificity of the athletic background (17).
From the significant differences in the percentage of change in sprint times between the 75 and 150% body mass sled pull conditions, it can be concluded that the lighter sled load (75% body mass) was a significantly more effective preload stimulus for improving subsequent sprint performance. Such a result was not initially expected, as traditionally, studies have reported PAP effects using very heavy loads (3 RM and 90–93% 1 RM) (4,6–8). Our results do however align with researchers who have used lighter loads and found evidence of potentiation. Baker (2) found that after subjects performed 6 repetitions at 65% of a 6-RM bench press load, power in the bench press throw (with a resistance of 50 kg) increased by 4.5%. Radcliffe and Radcliffe (29) investigated the effects of 5 different warm-up protocols on horizontal jump performance using a variety of preloads and found a significant improvement only occurred after the warm-up protocol that included 4 power snatches at 75–85% of a 4-RM load (29). Smith et al. (33) examined the effects of sled towing using loads of 10, 20, and 30% body mass on subsequent 40-yrd sprint performance. The subjects 40-yrd sprint times improved after the 10% load by 1.2% and improvements were greater than 2% for the 20 and 30% body mass loads. Such findings are in contrast to those of Whelan et al. (36) who reported that 30% body mass sled load, although being a relatively high load for resisted sprinting, was not sufficient to produce a PAP effect. The differences between these 2 studies may be due to the number and training age of the subjects. Training status has been suggested to have an effect on the ability of individuals to use the potentiated effects of the preload activity on subsequent explosive performance (12,16).
It would seem from the results of this study that a 75% body mass sled load or a load that reduces sprint velocity by 34–37% can be an effective acute stimulus to enhance subsequent sprint performance. Recently, Kawamori et al. (21) compared the effects of weighted sled towing with 2 different external loads (that reduced sprint velocity by 30% [heavy load] and 10% [light load]) over 8 weeks among 21 physically active men. Interestingly, the heavy group significantly improved both the 5 and 10-m sprint time by 5.7 ± 5.7% and 5.0 ± 3.5%, whereas only the 10 m sprint time was improved significantly by 3.0 ± 3.5% in the light group. From the results of the present study and that of Kawamori et al. (21) it could be suggested that loads for weighted sled towing, that slow down sprinting compared with an unresisted condition, by ∼30–40%, may have potential benefits over lighter or heavier training loads for the subjects represented in these studies (competitive and recreational team-sport athletes).
Although the authors sought to equate work using “load × distance,” significant differences in time taken to complete the sled pulls were apparent with the 75% body mass sled distance (15 m) taking significantly longer (4.04 ± 0.32 seconds vs. 3.48 ± 0.53 seconds) to complete than the 7.5-m distance associated with the 150% body mass sled load. Although no statistical differences were observed between the sled pull conditions for average and peak heart rates (expressed as a %HR Max) recorded during the testing period, “time under tension” (i.e., stance time) may have been different between the 2 sled pull conditions which may have influenced PAP responses. The use of video analysis (to measure ground contact time) in pilot studies may help to better equate time under tension using different sled loads. Measuring blood lactate could also give further insight into physiological stress imposed by various sled pull loads.
Because of the individual variability of sprint velocity decrement observed in the present study for the 75% body mass load (23–44%) and 150% body mass load (44–67%), the prescription of training load based on decrement in sprint velocity may be the best approach to the loading of athletes. Heavier sled towing loads can increase the training overload but will also decrease the specific training movement associated to unresisted sprinting (21), which may explain the findings in this study. Practitioners may have to use sled loads that will balance “training overload” and “specificity” to determine the optimal training outcome. Practitioners should also be advised that harness type and sled type, chain length and coefficient of friction of the pulling surface will all influence the training overload (37).
Practical Applications
The current findings have practical applications for team-sport athletes. Our findings indicate that a 75% body mass sled load or a load that reduces group's sprint velocity by 34–37% at 5, 10, and 15 m can be an effective acute stimulus to enhance subsequent sprint performance, especially when the recovery period is individualized for each athlete. Practitioners should however be advised that PAP is a highly individualized phenomenon and sled loads may need to be individualized for the athlete. Future research could use video analysis to determine time under tension and use a range of sled loads to determine their effect on subsequent sprint performance.
Acknowledgments
The authors would like to thank each of the rugby athletes for participating in this study and the Bay of Plenty Rugby Union (BOPRU) for the usage of their training facility for the duration of the study.
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