Linear sprinting is an important performance factor in many field sports. The initial acceleration phase and maximum velocity phase are components in developing peak velocity in athletes (15). Training programs incorporate resisted sprint training to improve these essential components of speed and acceleration. Sled towing is a common form of overload training to develop muscular strength for increased sprint speed. This type of training leads to acute or chronic outcomes. Acute training leads to postactivation potentiation (PAP), which is when subsequent muscle performance is enhanced after a preload stimulus. The first mechanism of PAP is thought to be phosphorylation of myosin regulatory light chains that causes an influx of calcium to create more binding sites. The second mechanism is activation of higher-order motor units that allows type II fibers to produce high force and high velocity (3,7,17,21,22). After a preload stimulus, fatigue increases and acute performance decreases. However, during the recovery period, there is an increase in performance because of increased potentiation and decreased fatigue. Critical variables that affect PAP include exercise selection, intensity, volume, rest, and subject population. The precise application of exercise to elicit PAP after a preloaded stimulus remains unclear (26). There is limited research regarding the optimal rest interval to enhance sprint performance after a sled tow preload stimulus.
Towing with lighter loads has been shown to improve the acceleration phase (8,15), whereas towing with heavy loads tends to improve the maximum velocity phase (21). Many studies have examined the chronic effects of sled towing, but acute responses to sled towing are limited (8,12). Previous research that has examined PAP on sprint performance has implemented traditional heavy resistance exercises (3,22) and found that the optimal rest period is approximately 8 minutes, while assisted sprinting has shown optimal rest periods to be much shorter at 1–2 m (19). To properly implement sprint training programs, further research is needed to examine the optimal rest intervals after moderate sled towing. Therefore, the purpose of this study was to determine differences in rest intervals after sled towing on acute sprinting.
Experimental Approach to the Problem
This study used a repeated-measures design to compare 30-m bodymass (BM) sprint times after sled towing between rest conditions (baseline [BL], 2, 4, 6, 8, and 12 minutes). Before data collection, all subjects were notified of potential risks and gave written informed consent, approved by the University Institutional Review Board. Subjects were required to attend 6 days of testing separated by 24–48 hours to avoid fatigue. Day 1 consisted of BL BM sprints to compare across conditions, while subsequent days consisted of a 30% BM sled tow, a random rest interval then a final BM only sprint.
Twenty healthy recreationally trained men (age = 22.3 ± 2.4 years, range = 20–28, height = 176.95 ± 5.46 cm, mass = 83.19 ± 11.31 kg) who were currently active in a field sport twice a week for the last 6 months volunteered to participate. A field sport was classified as any sport that required sprinting and running on grass with cleats. Participants were free from lower-body injuries in the last year. All participants were notified of potential risks and provided written informed consent approved by the University Institutional Review Board before data collection.
Day 1: Testing and Familiarization
On arrival, participants read and signed an informed consent document. Body mass and height were obtained using an electronic scale (ES200L; Ohaus Corporation, Pinebrook, NJ, USA) and electronic stadiometer (Seca, Ontario, CA, USA). Participants performed a dynamic warm-up consisting of a 400-m jog and 20 m of A-skips, high knees, butt kickers, cariocas, back pedal, and three 30-m submaximal sprints, 1 at 70, 80 and 90% perceived maximum velocity. A 5-minute recovery period was provided after the warm-up. Every subject started with a staggered stance (11), then performed 2 maximum 30-m BM sprints with 5-minute rest between trials. Reliability, using the intraclass correlation coefficient (ICC), for BM sprints has previously been shown (19) to be very high (ICC = 0.91). After completion of the 2 BM sprints, participants underwent familiarization with sled towing with increasing loads of 15, 20, and 30% BM. On completion of familiarization, participants randomly selected the rest periods for testing days 2–6. Sessions were scheduled during the same time of the day. Cleats were required for all testing sessions and grass levels were maintained throughout data collection (approximately 1.5 inches).
Days 2–6 Testing
Participants followed the same warm-up procedures as day 1 followed by a 5-minute recovery period. Participants then sprinted 30 m towing a sled loaded with 30% of their BM then rested for 2, 4, 6, 8, or 12 minutes (Figure 1). According to Smith et al. (21), heavier loads might provide potentiation for acute sprint performance. Reliability for sled towing was high (ICC = 0.85). After the rest period, they performed a maximum 30-m post-test BM sprint. All test days were identical except for the rest period.
Participants were attached to a weighted sled (Power Systems, Knoxville, TN, USA) via a waist belt. Sprint times were collected at 4 splits using wireless electronic timing gates (Brower Timing Systems, Draper, UT, USA). Infrared beam sensors in the timing gates were attached to tripods and placed at the starting line, 5, 10, 20, and 30 m. Participants were positioned immediately behind the starting line with a self-selected 2-point standing start position (11). The timing gates were initiated by the participants' feet crossing the initial timing gate at the starting line and split times were recorded as they crossed all subsequent gates. Total 30-m sprint times and 4 split times were recorded for BL, each sled tow and each post sled tow condition and used for analysis. In addition, each individual's best time was recorded, regardless of rest interval.
A 1 × 7 repeated-measures analysis of variance (ANOVA) analyzed total 30-m sprint time between conditions (BL, 2, 4, 6, 8, 12 minutes, and best). A 2 × 4 (condition × split) repeated-measures ANOVA analyzed BL and best split times. A 1 × 5 repeated-measures ANOVA analyzed sled tow total 30 m sprint time between conditions (2, 4, 6, 8, 12 m). Interactions and main effects were followed up with simple ANOVAs. An alpha level of 0.05 was used to determine statistical significance. All analyses were performed with SPSS version 23.0 software (SPSS Inc., Chicago, IL, USA).
For post sled tow BM 30 m total time, there was a main effect for condition where best was significantly less than BL (Table 1). For post sled tow BM split times, there was a significant interaction where best was less than BL only for the 0–5-m split (Table 2). Individual 0–5-m delta times between BL and best times were calculated showing that 17 of 20 subjects potentiated (Figure 2), but at different rest intervals (9 at 2 minutes, 5 at 4 minutes, 0 at 6 minutes, 2 at 8 minutes, and 1 at 12 minutes).
For sled tow 30-m total time, there were no differences between conditions (Table 3).
The purpose of this study was to determine the acute effect of different rest intervals after a 30% BM sled tow on BM sprint times. The major finding was that post sled tow BM sprint times were less than BL times on an individualized rest interval basis. This decrease only occurred in the acceleration phase over the first 5 m and was likely due to PAP and the complex relationship between fatigue and potentiation relative to intensity of the sled tow and rest interval.
Acute training can lead to PAP, where subsequent muscle performance is enhanced after a preload stimulus. The first mechanism proposed for PAP is phosphorylation of myosin regulatory light chains that causes an influx of calcium resulting in more binding sites. The second mechanism is activation of higher-order motor units that allows for type II fibers to produce high force and high velocity (3,7,17,21,22). After a preload stimulus, fatigue increases and performance decreases. However, as fatigue declines, potentiation is realized and performance increases. The precise application of exercise to elicit PAP is unclear and some studies have shown no effect after a preload stimulus. An assisted jump study by Beaudette et al. resulted in no PAP and no change in muscle activation (2). Critical variables that affect PAP include exercise selection, intensity, volume, rest, and subject population. Wilson et al. concluded that potentiation was increased with increased training status and was optimal after performing a preload activity of multiple sets with moderate intensity and rest intervals (26). Manipulation of these variables may help explain findings of this study.
Exercise selection plays an important role in PAP as transfer of the preload stimulus may be greatest to a similar exercise. Turner et al. (22) examined the influence of walking, plyometrics, and weighted plyometrics and found that plyometric exercise optimally enhanced sprint acceleration. McBride et al. examined the effects of heavy back squats or loaded countermovement jumps on running speed and found that heavy back squats resulted in faster sprint times, whereas Weber et al. found that back squats acutely increased squat jump performance (18,23). Leyva et al. (14) examined hex bar deadlift vs. back squat on vertical jump and found a decrease after back squats but no change after hex bar deadlift. White et al. examined the acute effects of dumbbell lateral squats vs. control on agility into-a-sprint and found that pre dumbbell lateral squats enhanced speed (25). Cazas et al. (4) performed assisted vertical jumps followed by BM only jumps and found that assisted jumping acutely enhanced vertical jump. In the same way, Nealer et al. (19) performed an assisted sprint followed by a BM sprint over 20 m and also found enhanced acute sprint performance. Similarly, the current study found that a sled tow sprint acutely enhanced a BM sprint, therefore, exercise specificity may have positively transferred to the sprint time decrease.
Preload intensities ranging from light to heavy have been used to elicit a PAP response. Lowery et al. (16) demonstrated that moderate and high intensities resulted in PAP while lower intensities did not. Whelan et al. (24) investigated the effects of resisted sprinting on sprint performance after sled towing with loads of 25–30% BM. There was evidence of initial fatigue and no evidence of PAP. Therefore, the intensity of the sled tow may not have been adequate for every individual to potentiate as a group. Smith et al. (21) analyzed the effects of PAP on sprint performance using sled resistances of 0, 10, 20, or 30% BM over 40 years. They showed that all conditions were faster; however, resistances were not significantly different. It was suggested that heavier loads might provide potentiation for acute improvements in sprint performance. The previous studies analyzed PAP results as a group only, while the current study used a similar intensity of 30% BM load and found PAP during acceleration by analyzing results on an individual basis.
A major component for eliciting PAP is the optimal rest interval between the preload stimulus and the final exercise activity. Jo et al. (10) used rest intervals of 5, 10, 15, or 20 minutes and a Wingate test was performed after each rest period. They found that PAP was individualized by rest interval, similar to this study. Chattong et al. (5) used only a 2-minute rest interval after 5 jumps onto a box and found increased subsequent vertical jump performance. Seitz and Haff (20) reviewed the influence of rest periods after a conditioning activity on subsequent jump, sprint, throw, and upper-body ballistic performances and found moderate PAP effects in sprint performance with longer rest intervals between 5 and 7 minutes resulting in the greatest PAP effect. Comyns et al. (7) investigated the optimal rest interval after heavy resistance exercise on a slow stretch-shortening cycle exercise and demonstrated the greatest improvement in jump performance after a 4 minutes rest. In the current study, 17 of 20 subjects potentiated over the first 5 m at rest intervals between 2 and 12 minutes, with 14 of the 20 potentiating between 2 and 4 minutes. This was probably due to the complex and individualized relationship between fatigue and PAP. After the initial stimulus, fatigue predominates acutely then potentiation is realized. The time course of these events seems to be individualized, leading to subject specific rest intervals.
The population and training status of individuals must be considered when attempting to elicit a PAP response. Arias et al. (1) tested resistance-trained men and found that heavy deadlifts did not elicit a PAP response. A review article by Healy and Comyns (9) concluded that to achieve a positive PAP response, protocols should be considered on an individual basis rather than as a group to enhance the post-conditioning activity. Chiu et al. (6) compared athletes to recreationally trained individuals by having them perform rebound and concentric only jump squats. They concluded that PAP acutely enhanced explosive performance and was greater in athletic populations compared with recreationally trained individuals. The current study used field sport athletes, which may have positively influenced the acceleration PAP outcome relative to their training age, albeit on an individualized basis.
Volume is another component to consider when attempting to acutely enhance performance via PAP. When sprinting, volume may be considered as multiple foot strikes over a specific distance. Whelan et al. (24) performed 3 sled tows over 10 m followed by BM sprints but did not show evidence of PAP. Therefore, volume may have been too high, leading to fatigue and masking any acute PAP response. Khamoui et al. examined the effect of high-force back squat volume on vertical jump in recreationally trained men. The experimental conditions consisted of sets of 1 × 2, 1 × 3, 1 × 4, or 1 × 5 with no potentiating response of any volume (13). The current study used a 30-m distance with approximately 20 steps (10 per foot) which resulted in an acute sprint acceleration increase. Although this step volume seems to be adequate to elicit a PAP response, no 1 critical variable should be analyzed alone but always within the context of the interrelationship with all others.
This study illustrates that PAP after sled towing is the result of the complex relationship between the critical variables of exercise selection, intensity, volume, rest, and subject population. These findings demonstrate that 30 m of 30% BM sled towing followed by 2, 4, 6, 8, or 12 minutes rest acutely decreases the initial 5 m acceleration sprint time for trained field sport athletes on an individualized basis. Therefore, those interested in enhancing acute 5-m sprint acceleration should test their athletes on an individualized basis to determine optimal rest times after a 30% BM sled tow.
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