Speed is one of the most vital elements a successful athlete can possess to be competitive in their sport. Therefore, strength and conditioning coaches are always seeking the optimal way to increase their athlete's level of sprint performance. When training for sports like soccer, basketball, football, and rugby where success relies heavily on speed, coaches should focus on increasing velocity and acceleration of their athletes (14). Nonetheless, many athletes, such as those in soccer, do not normally run long enough distances to attain peak speed. Although soccer player's average total mileage run is high in a single game, approximately 11% of the distance covered is in short sprints (3). Accordingly, various training methods have been examined in an attempt to find ways to maximize speed and acceleration for these athletes. Two of these are resisted and assisted sprinting.
One mechanism that has gained attention in training is postactivation potentiation (PAP) as an acute outcome of resisted or assisted sprinting (1,3,11,29,31). Resisted is characterized by running and towing a load while assisted is running and being pulled by an elastic cord. Assisted vertical jumping has also been examined using PAP to induce an acute performance increase (4–6,28). To elicit PAP, an athlete undertakes either a resistive or assistive stimulus, rests for a specific amount of time, then follows up with a subsequent explosive movement like a jump or sprint (7,12,13,16,17,19,21,25,27). Currently there are 2 common physiological mechanisms that can explain PAP. The first is increased phosphorylation of myosin regulatory light chains (12,21,22,25) causing an increase in CA2+ sensitivity, which leads to greater force output due to increased actin and myosin crossbridges (12,13,21,22,25). The second is an increase in α-motorneuron excitability seen with a change in H-reflex activity (13,21,22). However, the full explanation of PAP is not fully understood (4,13).
Numerous studies have examined a variety of protocols to elicit the greatest PAP effect (6–8,10), however, the significance of PAP and its influence on performance has been inconsistent. Discrepancies might be attributed to differences in the training age of the subject population, individualized responses, methodology, and or training status (3,16,18,19,21,27,30). Manipulation of rest periods, loading, volume, resistive, or assistive stimulus and training age have all been explored. The majority of the PAP literature has used some form of overload stimulus to increase performance. However, minimal research has incorporated assisted or supramaximal methods (4–6,28).
Previous research has investigated an assisted stimulus on vertical jump, sprint performance and PAP. Bartolini (3) and Tran (28) investigated several elastic cord assistance levels on vertical jump and sprint performance and found 30–40% bodyweight assistance to be optimal. Cazas et al. (6) had recreationally trained men perform one set of 5 successive assisted vertical jumps with 30% bodyweight assistance then rest for 30 seconds, 1, 2, or 4 minutes before completing 3 unassisted bodyweight vertical jumps. Their results demonstrated that take-off velocity and relative peak power increased after 1-min rest. In this manner, supramaximal speeds are achieved, resulting in an acute PAP response.
The majority of prior sprint research has examined PAP effects in a male population with a heavy overload stimulus, with little attention to assisted protocols or rest times. There is also a lack of research on assisted sprinting and PAP in female athletes. Therefore, the purpose of this study was to examine the acute effects of different rest periods after an assisted sprint on bodyweight sprint performance in female soccer athletes vs. recreational players.
Experimental Approach to the Problem
This study used a between and within repeated measures mixed design to compare varied rest intervals of 30 seconds, 1, 2 and 4 minutes with their postassisted sprint time to baseline sprint times between athletes and recreational subjects. For each 20 m sprint, total time and 5 m split times were compared between assisted and postassisted sprints. Subjects attended 5 days of testing 24–48 hours apart. They completed baseline testing and each rest interval protocol on separate days.
Twenty-four female soccer players were divided into 2 groups: recreational (n:11; age:20 ± 1.67 years; 19–24 years old; ht:162.30 ± 4.35 cm; mass:61.02 ± 8.78 kg) and Division I collegiate athletes (n:13; age:19.76 ± 0.83 years; ht:166.85 ± 5.98 cm; mass:61.23 ± 3.77 kg). They attended 5 separate sessions, separated by 24–48 hours (3,6,28). Testing time of day was controlled for each individual subject. They were instructed to refrain from any additional physical activity other than their ordinary routine. At the time of collection, the athletes were in their spring soccer season and were given resistance training 3 times a week with 5 days of soccer training and intermittent games throughout the semester. The recreational players were also instructed to continue with their normal routine of physical activity in addition to the 90 minutes per week of soccer in which they were currently participating. Before data collection, all subjects were notified of potential risks and gave written informed consent, approved by the University Institutional Review Board.
For assisted sprinting, bodyweight assistance (BWA) was accomplished by stretching a 20′ elastic cord (3/16″ Premium Black 3-Strand Nylon, 1080 breaking strength, #356964; New England Ropes, Fall River, MA, USA) (3) with an elastic modulus of 6.17 Nm2, calculated by dividing delta force by delta length (3). It was attached to a Crane Scale (Model #ICS-CCS-500; Industrial Commercial Scales, LLC, North Charleston, SC, USA), to determine the appropriate amount of tension to achieve 30% of each participant's BWA. To adjust the amount of tension, the other end of the Crane Scale was attached to a hand crank (600 lb. zinc-plated trailer winch with solid gears, Model# BR59230, Internet # 203494921). Each participant wore a nylon belt, where the elastic cord was attached to their waist by an open D-ring (Figure 1). Once they began to sprint, the elastic cord became slack and fell from the D-ring, and was then pulled out of the way by an additional rope held by one of the researchers. These procedures were previously used to establish 30% BWA as optimal (3).
Sprint times were measured in 5 m splits (0–5, 5–10, 10–15, 15–20 m) using an electronic timing system (Speedtrap II; Brower, Salt Lake City, UT, USA) (3). It uses an infrared laser-timing device accurate within 0.01 and was triggered at the start by breaking the initial sensor. All subsequent splits were measured by separate sensors (Figure 1). Sensor tripod legs were extended to full length at 106 cm (9). All sprints were performed on a natural grass soccer field cut to approximately 4 cm and began with a staggered foot placement of the subject's choice (15). Subjects wore their own soccer cleats, shorts, and a loose fitting shirt for all sprints. These were recorded and used for every subsequent session.
Subjects signed the informed consent form then body mass and height were obtained using an electronic scale (ES200L; Ohaus Corporation, Pinebrook, NJ, USA) and stadiometer (Seca, Ontario, CA, USA). They also randomly chose their rest periods for the following 4 sessions, but it was not revealed to them. They then performed a dynamic warm-up consisting of jogging 300 m followed by 2 repetitions of 20 m of A-skips, high knees, butt kickers, lunges, carioca, and backwards running (3,24). After this, they performed 2 submaximal bodyweight sprints at approximately 75 and 90% of their perceived maximal sprint speed then rested for 5 minutes (3,24). To determine baseline sprint time, each participant completed 2 maximal effort 20 m sprints. Consistent verbal encouragement was provided but no knowledge of results. Sprint distance was divided into 5 m splits and the average of the 2 baseline times used for analysis. Test retest reliability for sprint times was ICC of 0.91. Finally, they were familiarized with assisted running by performing 3 sprints at 10, 20, and 30% BWA.
On each of the following 4 sessions, subjects completed the same dynamic warm up as session 1 then rested 5 minutes before testing. Each session consisted of one 20 m sprint with 30% BWA followed by a randomized rest period of 30 seconds, 1, 2, or 4 minutes. They were driven back to the start line in a golf cart and remained seated till just before rest was over. After rest, they completed a subsequent unassisted bodyweight 20 m sprint.
Two 5 × 4 × 2 (condition × split × group) mixed factor ANOVAs compared baseline to postassisted and assisted split times. Two 5 × 2 (condition × group) mixed factor ANOVAs compared baseline to postassisted total 20 m times. Interactions were followed-up with simple ANOVAs. The Statistical Package for the Social Sciences (SPSS 22.0 for Windows, SPSS, Inc., Chicago, IL, USA) was used for all analyses. An apriori Alpha was set at 0.05.
For postassisted split times, there was a 3-way interaction. This was followed up with two 5 × 4 (condition × split) repeated measures ANOVAs, one for each group. For athletes, there was an interaction. This was followed up with four 1 × 5 ANOVAs, one for each split. For 0–5 m, there was a main effect where 1 minute was less than baseline, 30 seconds, and 4 minutes while 2 minutes was less than baseline and 30 second. No other times were different (Table 1). For recreational players, there was no interaction but there was a main effect for split. Split 0–5 m was greater than all other splits, whereas split 5–10 m was greater than 10–15 m and 15–20 m. No other splits were different (Table 2).
For postassisted total 20 m sprint times, there was no interaction. However, there was a main effect for group. Athletes' times were significantly less than those of recreational players (Table 3).
For assisted sprint split times, there was a 2-way interaction of condition by split. This was followed up with four 1 × 5 ANOVA's, one for each split. All times for all splits and conditions were significantly less than baseline, but the differences for 0–5 m were greater than all other splits. (Table 4).
For assisted sprint total 20 m times, there was no interaction. However, there were main effects for condition and group. All rest condition times were significantly less than baseline and athletes' times were significantly less than those of recreational players (Table 5).
The purpose of this study was to examine the effects of different rest periods after an assisted sprint on bodyweight sprint times in female soccer players. The major finding was that sprint time was significantly decreased from 0 to 5 m after 1 and 2 minutes of rest only in collegiate athletes. Possible reasons for this could be PAP, increased ground reaction force at foot strike or the elastic band having full tension only at the sprint start. Furthermore, differences between groups could be explained by athletes' ability to produce greater force and power than untrained (8).
The present study only found PAP effects in the trained soccer athletes, whereas the recreational players showed no change. These findings are supported by earlier studies that also found that athletically trained individuals exhibited the greatest PAP response (8,23,26,32). Chiu et al. examined the effects of heavy weighted back squats on athletes and recreationally trained individuals and found that athletes demonstrated greater potentiation. Wilson et al. and Seitz et al. reviews concluded that strength training increases neural activation, therefore, higher threshold motor units are recruited by athletes (26,32). Another explanation could be that athletes are more resistant to fatigue, which could alter the balance between fatigue and potentiation (26). Additionally, trained individuals have greater myosin phosphorylation than do untrained individuals (32) suggesting that the ability to potentiate and produce greater power is directly related to training experience.
The traditional protocol for PAP occurs with a heavy resisted exercise, rest for a specific amount of time, then a subsequent explosive exercise resulting in enhanced performance (19). Prior research suggests that PAP may increase force production after adequate rest as enhancement occurs when muscular fatigue has subsided but potentiation still exists (13,19,21,23). Therefore, one of the most important variables to consider with PAP is optimal rest. Potentiation can be attributed to 2 main physiological mechanisms; phosphorylation of myosin regulatory light chains and an increase in motor unit recruitment and firing rate (6,13,21,22). At completion of the initial exercise stimulus, the myosin regulatory light chains become phosphorylated and more calcium is released allowing greater myosin and actin crossbridges resulting in increased force production. Simultaneously, the increase in motorneuron excitability leads to greater motor unit activation and synchronization (6,21,22). The traditional method to elicit PAP uses a rest period ranging from 8 to 12 minutes and finally followed with a subsequent explosive activity (18,21,32). Due to the initial stimulus being heavy, rest time must account for the balance between fatigue and potentiation. However, a new model of PAP uses an overspeed stimulus. Cazas et al. examined the influence of rest periods after assisted jumping using elastic bands at 30% BWA on bodyweight vertical jumps. After completing 5 consecutive assisted jumps they rested for 30 seconds, 1, 2, or 4 minutes followed by 3 bodyweight vertical jumps (6). Their results revealed that jump variables were enhanced after 1 minute of rest. Therefore, when an assisted stimulus is used, the rest time needed to induce PAP is less than the traditional heavy resisted stimulus, presumably due to less fatigue.
The findings of the current study demonstrated a decrease in postassisted sprint time but only in the first 5 m and only in the trained soccer athletes. However, the total postassisted 20 m sprint time did not change in either group. Only at the start of the sprint was the elastic band pulled to the full 30% BWA. Through empirical observation, the elastic band fell off between 5 and 10 m of the start, which could explain the significant decrease only in 0–5 m split time. Similar results were found by Upton et al., where assisted sprint training showed the greatest improvements from 5 to 15 yd of a 40 yd sprint. However, they did not examine PAP as it was a training study (29). Another explanation could be an increase in velocity due to the overspeed stimulus of the elastic band. Though the current study did not measure velocity, Mero and Komi examined velocity at submaximal, maximal, and supramaximal (overspeed) running in sprinters and found running velocity increased significantly after supramaximal sprint training (20). They attributed this to increased activation of higher order motor units and eccentric forces during supramaximal sprinting (20). The results of the current study demonstrating a decrease in 0–5 m sprint time may be due to the same mechanisms.
A key component of eliciting PAP is determining the optimal rest period between the initial stimulus and the final exercise. A recent review (32) found that trained athletes demonstrated performance improvements at rest periods between 3–7 and 7–10 minutes. Therefore, fatigue may dominate with shorter rest periods after a heavy overloaded stimulus and reduce the ability to elicit PAP (32). However, only a few studies have examined the effects of a nonfatiguing stimulus to produce PAP (18,26). Another review (26) found that PAP exists earlier after a bodyweight or ballistic stimulus with the greatest effect at approximately 0.3–4 minutes rest (26). Dabbs et al. studied varied short rest intervals (30 seconds, 1, 2, and 4 minutes) after bouts of whole-body vibration on vertical jump and found jump height was potentiated at different rest times specific to the individual (10). Thus, stimulating the neuromuscular system can be accomplished through a nonfatiguing activity like whole-body vibration (10). Cazas et al. (6) found similar results in a study examining different rest periods on vertical jump following assisted jumping and saw potentiation at 1 minute of rest. Collectively, these findings support the current study in that 1 minute rest is adequate to elicit potentiation after a relatively nonfatiguing bout of assisted exercise such as a short supramaximal sprint.
Another component of eliciting PAP is the optimal stimulus required. Although the concept of using an overspeed stimulus to elicit PAP is relatively new, overspeed training has been used for some time (1–4,6,11,29). Previous studies have examined ways to increase jump height or decrease sprint time through the means of overspeed training (2,3,28). Although the current study did not focus on training, the stimulus used was similar to previous work. Examining the optimal stimulus for assisted vertical jumps, Tran et al. (28) found that jump height was increased across all conditions of 0, 10, 20, 30, and 40% BWA. However, relative ground reaction force did not increase until 20% BWA and takeoff velocity until 30% BWA. Therefore, 30% BWA seems to be optimal for eliciting the greatest performance effect. Similar results were found by Bartolini et al. (3) investigating the optimal elastic cord assistance level for sprinting where sprint times decreased up to 30% BWA. However, neither Tran et al. nor Bartolini et al. examined the effects of BWA on PAP. Cazas et al. and the current study both found that 30% BWA through elastic cords resulted in performance enhancement after minimal rest. In a training study, Upton et al. (29) examined the effects of assisted and resisted sprinting on acceleration and velocity in collegiate female soccer players using an average 15% BWA and found the greatest increase in the assisted sprint trained group for acceleration from 5 to 15 yd. In contrast, the resisted sprint trained group showed an increase in max velocity from 15 to 25 yd of a 40 yd sprint (29,31). The difference between overspeed and overload protocols seems to explain these results related to acceleration and top speed.
In summary, previous studies have concluded that an overspeed stimulus can increase sprint performance both acutely and chronically. Only one previous study examined the effects of bodyweight assistance on PAP and vertical jump but not in linear sprinting (6). Therefore, the current findings add to the literature in that an acute PAP response can be elicited using an overspeed stimulus in linear sprinting after short rest periods in trained athletes. Future research should investigate the effects of chronic training using an overspeed stimulus on long-term sprint performance.
These findings demonstrate that 20 m of 30% bodyweight assisted supramaximal running with an elastic band followed by 1 or 2 minutes rest acutely decreases an athlete's initial 5 m accelerative sprint time. Therefore, strength coaches should consider adding overspeed sprints for trained athletes to increase acute sprint speed. This technique can be implemented by using elastic bands that pull horizontally at 30% bodyweight assistance resulting in supramaximal speeds. Conversely, recreational soccer players should use other forms of speed development techniques.
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