Recent research has shown that performing muscular contractions under near-maximal load conditions improves subsequent performance during movements requiring large muscular power outputs of the stimulated muscle groups (8,14,18,23). These increases in performance have been attributed to a postactivation potentiation (PAP) effect within the stimulated muscle groups, whereby the force produced by the muscles is increased as a result of previous contractile activity. Different underlying mechanisms have been proposed to cause PAP. For example, phosphorylation of myosin light chains resulting from the initial muscle activity, which would render the actin and myosin molecules more sensitive to Ca2+ availability, is related to increased rates of force development and maximal isometric force (19). Others have proposed neural factors, such as the excitability of α-motoneurons as being responsible for increased contractile performance after previous muscular activity (9). Although the exact mechanisms responsible remain to be fully delineated, the acute benefits of performing heavy resistance exercises before explosive movements have been demonstrated in a variety of upper and lower body exercises (4,8,9,18,23).
Despite the evidence supporting the acute potentiating effects of heavy resistance exercises on improving subsequent explosive performance, there is little research investigating the effects of heavy resistance exercises on subsequent sprint running performance. Recently, McBride et al. (14) reported that 40-m sprint time during a single trial was significantly faster in football players after three repetitions of back squats using a load of 90% of 1 repetition maximum (1RM). Whereas the 40-m sprint times were improved by the squat exercise, 10-m and 30-m times were not significantly affected. These differential effects reflect the multidimensional nature of sprint running (5). Specifically, sprint running comprises different phases, including initial acceleration, attainment of maximal velocity, and maintenance of maximal velocity, each with specific mechanical demands (15).
A fast sprint start relies strongly on the total amount of muscle mass that can be activated to increase the energy of the body center of mass, particularly during the first push-off (20). However, Jacobs and Van Ingen Schenau (13) noted that the initial acceleration phase of sprinting requires a specific muscle activation pattern that optimizes the interaction between horizontal and vertical impulses during stance. In contrast, Weyand et al. (21) noted that during maximal velocity sprinting, large vertical impulse should be generated during each stance to allow sufficient time to reposition the swinging leg. Other investigators have identified the eccentric strength of the hip extensors as a limitation to maximal velocity sprint running by reducing the braking forces experienced during stance (22). The mechanical demands of the maintenance of maximal velocity phase also require a reduction in braking forces (15), so the eccentric strength of hip extensors is likely to be important. As a result of the differing mechanical demands, particular resistance exercises may well have a potentiating effect during specific phases within a sprint. For example, front squats have been reported to produce a greater hip extensor moment compared with back squats (17), so they may have a greater effect on the maximal velocity and maintenance of maximal velocity phases of sprint running than back squats. Despite the importance to sprint coaches and athletes, there is no research investigating the effects of different heavy resistance exercises on subsequent sprint performance. The purpose of the present study was to investigate the effects of different heavy resistance exercises (back and front squats) on the average speed during each 10-m interval of 40-m sprint trials.
Experimental Approach to the Problem
This study used a randomized, cross-over design to investigate the effects of three treatments [heavy back squats (HBS), heavy front squats (HFS), and control (C)] on the average speed during each 10-m interval of 40-m sprint trials. Three 40-m sprint trials, with 3 minutes' rest between each, were performed after each of the treatment conditions. Split times (0-10 m, 10-20 m, 20-30 m, and 30-40 m) were collected during each of the sprint trials. The times during each of these 10-m intervals were averaged among the three trials and converted to sprinting speeds. This design allowed for the determination of the effectiveness of the HBS and HFS treatments as PAP methods on the different phases of the sprint: initial acceleration, attainment of maximal velocity, and maintenance of maximal velocity.
Ten men volunteered to participate in this study, which was approved by the Institutional Review Board for the Protection of Human Subjects of East Stroudsburg University. The subjects (age 22.3 ± 0.8 years, height 1.77 ± 0.06 m, mass 89.2 ± 7.3 kg) were involved in sports such as football, weightlifting, and track and field, and were all considered to be strength trained based on the number of years that they had engaged in resistance training. However, the subjects' current use of resistance training protocols was varied. For example, some subjects focused on resistance training sessions to develop power utilizing predominantly multi-joint movements, whereas others were engaged in hypertrophy sessions involving mainly single-joint movements. The subjects' involvement in sprint-type activities at the time of the study was similarly varied. After being informed of the risks associated with participating in the study, the subjects signed informed consent forms before the testing. All subjects were asked to refrain from intense exercise and standardize their diet 48 hours before the testing sessions.
The subjects participated in four testing sessions: a 1RM parallel back squat determination session and three PAP testing sessions (HBS, HFS, and C). All subjects first performed the 1 RM determination session before any of the PAP sessions. The order of the PAP sessions was randomized across the subjects.
1RM Back Squat Determination
A 1RM for the parallel back squat was determined for each subject using the protocol outlined by Baechle et al. (2). A lift was deemed successful if the top of the thighs were parallel to the ground during the lowest point of the descent and the bar continued to move upward throughout the ascent without assistance. Spotters were used during each squat attempt, and a standard 20-kg Olympic barbell and Olympic disks (Ivanko, Reno, NV) were used during the exercise. Three days' rest was provided between the 1RM procedure and the first data collection session.
Heavy Back Squat, Heavy Front Squat, and Control Treatments
For the HBS treatment, subjects warmed up on a cycle ergometer (Monark Ergomedic 828E, Varberg, Sweden) for 5 minutes at an intensity of 300 kp. After this, the subjects walked for 4 minutes to a free-weights room where they performed a series of parallel back squats. The subjects first performed five repetitions at 30% of their 1 RM, then four repetitions at 50% 1 RM, and finally three repetitions at 70% 1 RM. Two minutes' rest was provided between each of the loading sets. The subjects then walked for 4 minutes to an indoor track where they performed the sprint trials (Figure 1). This protocol is similar to that used in a previous PAP investigation involving sprint running (14).
For the HFS treatment, the subjects followed the same procedure as for the HBS treatment except that they performed parallel front squats rather than parallel back squats. The 1 RM for the front squat exercise for each subject was determined by calculating a load equivalent to 80% of their 1 RM back squat. This value was deemed appropriate for the sample based on previous recommendations (1) and the subjects' current resistance training regimens. The subjects then performed five repetitions at 30% of their estimated 1 RM, then four repetitions at 50% estimated 1 RM, and finally three repetitions at 70% estimated 1 RM. Two minutes' rest was provided between each of the sets. The subjects then walked for 4 minutes to the indoor running track to perform the sprint trials (Figure 1).
For the control treatment, the subjects performed the 5-minute warm-up on the cycle ergometer, then walked for 4 minutes to the running track and performed the sprint trials (Figure 1). During all treatment conditions, an investigator remained with the subjects to ensure that no other warm-up exercises were performed.
40-m Sprint Trials
After the HBS, HFS, and C treatments, the subjects performed three 40-m sprint trials on an indoor rubber track. The subjects began each trial when they were ready from a three-point crouched start (Figure 2), and they were instructed to run at maximal effort throughout each trial. Three minutes' rest was provided between each trial.
Photocells (Brower Timing Systems, Draper, UT) were used to record the following split times: 0-10 m, 10-20 m, 20-30 m, and 30-40 m. These intervals were considered appropriate to capture the three distinct sprint phases for the present sample (initial acceleration, attainment of maximal velocity, maintenance of maximal velocity) (5). The first set of photocells was set at a height of 0.85 m, and the other four pairs were set at a height of 1 m. The reliability of this protocol for 10-m and 20-m sprint times has been reported previously (16). Using this protocol, there was no need to perform familiarization trials with the present sample (16). The average split times were calculated among the three trials for each subject. The average split times were then converted to speeds for each 10-m interval (distance/time) for the subsequent analyses.
All statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS for Windows, version 14.0; SPSS Inc., Chicago, Ill.). Measures of central tendency and spread of the data are presented as means and standard deviations A general linear model with repeated measures on one factor [treatment (HBS, HFS, and C)] was used to assess the differences in average speed during each 10-m interval caused by the different treatment. Pairwise comparisons with Bonferroni corrections for multiple comparisons were used to identify the magnitude of the differences in average speed caused by the different treatments. The α value was set at P ≤ 0.05 for all analyses.
1 RM Back Squat Determination
The 1 RM parallel back squat protocol produced a mean load of 142.2 ± 32.1 kg. This provided mean 30%, 50%, and 70% 1 RM values of 42.7 ± 9.6, 71.1 ± 16.1, and 99.5 ± 22.5 kg, respectively. Accordingly, the estimated 1 RM for the front squat was 113.8 ± 25.7 kg, providing mean 30%, 50%, and 70% 1 RM values of 34.1 ± 7.7, 56.9 ± 12.9, and 79.6 ± 18.0 kg, respectively.
40-m Sprint Trials
Table 1 shows the average speeds achieved during each 10-m interval of the 40-m sprint trials after the HBS, HFS, and C treatments.
Figure 3 shows the percent change in speed (relative to C) during each 10-m interval for the HBS and HFS treatments.
There were no significant differences in average speed during the 0- to 10-m interval caused by the different treatments (P > 0.05).
A significant difference was found for the average speed during the 10- to 20-m interval (P = 0.004) with the HBS treatment producing faster speeds compared with the C treatment (mean difference, 0.12 m·s−1; 95% likely range, 0.05-0.18 m·s−1; P = 0.001). There was no significant difference between the average speed produced by the HBS and HFS treatments during this interval (P > 0.05).
There were no significant differences in average speed during the 20- to 30-m interval caused by the different treatments (P > 0.05).
A significant difference was found for the average speed during the 30- to 40-m interval (P = 0.009) with the HBS treatment producing faster speeds compared with both the HFS treatment (mean difference, 0.24 m·s−1; 95% likely range, 0.02-0.45 m·s−1; P = 0.034) and the C treatment (mean difference, 0.18 m·s−1; 95% likely range, 0.03-0.32 m·s−1; P = 0.021).
The purpose of the present study was to investigate the effects of different heavy resistance exercises (back and front squats) on the average speed during each 10-m interval of 40-m sprint trials. Previously, McBride et al. (14) reported that heavy back squats (3 × 90% 1 RM) performed by strength-trained subjects significantly reduced subsequent 40-m sprint time during a single trial, whereas 10-m and 30-m sprint times were not significantly changed. Similar findings were reported in the present study using a similar subject sample, with the HBS treatment producing significantly greater speeds than the C treatment during the 30- to 40-m interval. For the present subjects, this interval represented the maximal velocity or the maintenance of maximal velocity phase of the sprint. The magnitude of the increase recorded in the present study (2.3%) was greater than that reported by McBride et al. (0.9%). In the present study, the volume of the HBS treatment was greater than that used by McBride et al. (14), although the maximal load lifted was less. It is possible that the volume associated with the heavy resistance exercise is of greater importance than the load when trying to elicit a PAP effect to improve maximal sprinting velocity.
The HBS treatment also produced significantly greater average speeds during the 30- to 40-m interval when compared with the HFS treatment. Russell and Phillips (17) reported that the joint moments during back and front squats differed about the hip joint, with the front squat producing a greater hip extensor moment, while the knee and ankle moments were similar. Some investigators have identified the eccentric strength of the hip extensors as a limiting factor during maximal velocity sprinting by reducing the braking forces experienced (22). This may be equally important during the maintenance of maximal velocity phase. As such, the front squat may be expected to have a greater PAP effect on maximal velocity and maintenance of maximal velocity phases of sprinting than the back squat. However, the lower loads used during the HFS treatment may have limited the activation levels of the hip extensors and therefore the possible PAP effect. It should be noted that the low loads may well have been caused by the fact that the front squat loads were calculated from an estimated 1 RM rather than from a directly measured 1 RM. Future research should investigate the effects of HFS on sprinting performance using loads calculated directly from a 1 RM.
Improvements in the 30- to 40-m interval produced by the HBS may have been as a result of greater vertical impulses generated during each stance during this phase. Weyand et al. (21) reported that faster maximal sprint velocities were produced by greater vertical impulses, which allowed more time to reposition the swinging leg. The induction of a potentiation effect of the lower limb extensor musculature as a result of the stimulation during the back squat exercise is a possible explanation. Despite the improvement during the 30- to 40-m interval, the speed during the 0- to 10-m interval remained unaffected after HBS. These intervals represent distinct phases within the sprint and as such have different associated mechanical demands (15). Therefore, a heavy resistance exercise that improves performance in one phase may not influence performance in another phase. Therefore, the mechanical demands of each phase should be considered when selecting possible heavy resistance exercises to improve performance. Jacobs and Van Ingen Schenau (13) noted that the initial acceleration phase of sprinting requires a specific muscle activation pattern allowing for the rotation of the body center of mass over the stance foot before the explosive extension of the lower limb joints. This activation pattern allows for the optimal interaction between stride length and stride frequency by limiting the vertical impulse and maximizing the horizontal impulse at toe-off (12). It may be that the HBS treatment, while stimulating the muscles involved in this action, did not provide the stimulation in the specific activation pattern; therefore, the initial acceleration performance was not improved. Importantly, however, the HBS treatment did not seem to interfere significantly with the initial acceleration phase. It is likely that the HBS stimulated the active musculature in a manner that was specific to the mechanical demands of maximal velocity sprinting but not the initial acceleration phase. However, the reliance on kinematic measures in the present study prohibits anything further than conjecture. Future research should investigate the mechanical similarities between the heavy resistance exercises and sprint performance by measuring kinetic variables (joint moments and powers) while including electromyographic data to assess muscle activation patterns.
The HBS treatment produced significantly greater speeds than C during the 10- to 20-m interval, although the improvements were not significantly different from the HFS treatment. It is difficult to explain this improvement in terms of the mechanical similarities between the back squat movement and the sprinting movement during this phase. The improvements by HBS during the 10- to 20-m interval were less than those recorded during the 30- to 40-m interval. It is possible that the improvements reflect the importance of the knee and ankle musculature during the 10- to 20-m interval of sprinting.
Despite the significant increase in sprinting speed during the 30-40 m after the HBS treatment, there was considerable variation in the subjects' responses (see error bars in Figure 3). For example, one subject actually recorded a very small reduction in speed (0.6%) compared with C. It is worth noting that the weakest and strongest subjects recorded a small (1.4%) and large (5.4%) increase in sprinting speed during the 30- to 40-m interval, respectively. Güllich and Schmidtbleicher (9) noted that PAP effects were only reported for well-trained subjects. Others have reported that the PAP effects are greatest in those subjects demonstrating the greatest percentage of type II fibers in the stimulated muscles (10). Strength-trained subjects were included in the present study for these very reasons. However, the variation in the present subjects' use of resistance training protocols at the time of the study may account for some of the variation in the responses, as could the subjects' use of sprint-type training activities. The variation in responses is further compounded by the fact that the 30- to 40-m interval represented the attainment of maximal velocity phase for some subjects but the maintenance of maximal velocity for other subjects. It should be noted that the variation in the responses of the subjects during the 10- to 20-m interval after the HBS treatment was much less pronounced. Future research should attempt to identify the specific variables that predispose an athlete to a PAP effect in response to the treatments employed in the preset study, providing coaches with greater confidence in the efficacy of such treatments with their athletes.
The possibility exists for using the acute effects reported in the present study to produce chronic adaptations as has been proposed through the use of complex training (3,6,7). The present results possibly extend the scope of complex methods to include sprint training. This has significant implications for sprint coaches and athletes. Based on the present findings, long-term improvements in acceleration and maximal velocity or maintenance of maximal velocity phases of sprint running may be gained from performing HBS (30%, 50%, and 70% 1 RM) as part of the athletes' warm-up 4 minutes before sprint trials. This could lead to long-term benefits after sprint training by allowing the athlete's neuromuscular system to perform at a higher level during each training session. Such conjecture is appealing, but there is a paucity of research investigating the long-term effects of using potentiating resistance exercises during a training regimen. At present it is unknown how the neuromuscular system adapts to the long-term use of potentiating heavy resistance exercises and whether the response diminishes with repeated exposure to the treatment. It is clear that research involving training studies is required.
Although it is appealing to ascribe the improvements recorded in the present study to the potentiation of the stimulated muscles caused by the HBS treatment, such a conclusion should be viewed with caution given the absence of definitive measures such as twitch response and H-reflex (11). Including neuromuscular measures of twitch responses and H-reflex in future studies would ensure that the appropriate muscles are potentiated and provide information about the underlying mechanisms of PAP, which would aid future investigations.
The use of a regimen of HBS (30%, 50%, and 70% 1 RM) performed 4 minutes before multiple sprint trials can significantly increase the speed achieved during specific 10-m intervals (10-20 m and 30-40 m) in strength-trained men. These findings suggest that coaches could incorporate such exercises into the warm-up of similar athletes to improve sprinting performance. However, the considerable variation in responses in the present subjects means that coaches should be cautious in prescribing HBS for all athletes. Given the multidimensional nature of sprint running and the unique mechanical demands associated with the distinct sprint phases, it is likely that heavy resistance exercises other than back or front squats may influence performance during the initial acceleration phase of sprint running (0-10 m).
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