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Original Research

Postactivation Potentation Effects From Accommodating Resistance Combined With Heavy Back Squats on Short Sprint Performance

Wyland, Timothy P.1; Van Dorin, Joshua D.2; Reyes, G. Francis Cisco1

Author Information
Journal of Strength and Conditioning Research: November 2015 - Volume 29 - Issue 11 - p 3115-3123
doi: 10.1519/JSC.0000000000000991
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Abstract

Introduction

Postactivation potentiation (PAP) is characterized by an increase in muscle twitch and force due to previous contractile exercises (34). An example would be an increase in explosive lower-body power in the vertical jump after a set of heavy squats. Researchers attribute this increase in neuromuscular performance to higher sensitivity of actin and myosin molecules to Ca2+ availability (36), increased excitability of α-motorneurons (22), increased synchronization of motor unit firing, and reduced peripheral inhibition from Golgi tendon organs, and enhanced reciprocal inhibition of antagonist muscles (3,4,17,18). Although the exact physiological mechanisms for the increase in performance from PAP is still unclear, there are many sources of evidence where muscular power, both upper body and lower body, has been acutely increased after sets of heavy resistance exercise (4,6–8,21,22,28,46).

In regards to lower-body effects of PAP, the majority of the research has examined vertical jump performance, with some examining PAP effects on sprinting abilities. Previous research reported significant decreases in sprint times over longer distances, such as 30 and 40 m (10,11,27,45), as well as shorter distances such as 0–10 m (7,10,39,45) after sets of heavy squats with resistances ranging from 70 to 90% of one's squat 1 repetition maximum (1RM).

A sprint run comprises 3 different phases: initial acceleration, reaching of maximal velocity, and maintaining maximal velocity (29). With many sports requiring high ability in the acceleration phase to cover 0–10 m as fast as possible, many coaches will focus on improving an athlete's acceleration ability. It is believed that acceleration depends highly on the total amount of muscle mass that can be recruited to increase the energy of the athlete's center of mass, especially during the first step (41). In the concepts of basic physics and Newton's second law of acceleration, an object's acceleration is directly influenced by the force acted on it and inversely proportional to the object's mass. If coaches can find ways to improve the amount of force an athlete creates into the ground for their respective body mass, the greater their ability to accelerate. To further enhance their acceleration abilities, an athlete must be able to express maximum force rapidly, otherwise known as muscular power.

Recently, the use of accommodating resistance has been a modality implemented by practitioners to increase muscular power. The general thought behind the use of accommodating resistance, such as combining elastic bands with free weights, is to minimize the “sticking point” of an athlete's lift. This sticking point is the most disadvantageous position during the movement, which can then limit the maximal amount of load lifted (20). If the sticking point can be minimized during a lift, through the use of elastic bands, the athlete will be able to accelerate the weight through the full range of motion, which can develop strength and power over a large range of motion. Wallace et al. (43) demonstrated increases in peak power and peak forces when performing back squats with elastic bands, compared with back squats without elastic bands. Israetel et al. (24) provided further evidence of higher force, velocity, and power during the latter portion of the concentric phase during squats when combined with elastic bands.

To date and knowledge, there has been no published research examining the effects of accommodating resistance during a complex training pair of exercises, especially when attempting to manipulate linear sprint performance. With evidence of neuromuscular change during lifting with elastic bands, it was thought of by the researchers to use this strategy with complex training. Complex training is when heavy loads are alternated with light loads in order elicit that PAP response to maximize power output in the light resistance set (4). What makes this study unique is the utilization of accommodating resistance into complex training; previous research has only used standard isoinertial load in the complex pair of exercise to elicit greater power output. With research presenting improvements in power when lifting with elastic bands, this study attempts to investigate the PAP effects of elastic bands when they are removed. Therefore, the purpose of this study was to determine whether linear sprinting speed can be acutely enhanced after several sets of heavy back squats with or without the addition of elastic bands. The researchers hypothesize sprint time to be improved after performance of heavy squats with either elastic bands or standard load, when compared with the control group. Also, because of evidence of altered mechanics and neuromuscular stimulation through the use of accommodating resistance, it is hypothesized by the researchers that the exercise using elastic bands combined with standard load would enhance sprint time more so than the control and standard load-only group.

Methods

Experimental Approach to the Problem

For this study, a within-subject, repeated-measures design was implemented, with random treatment order. This specific design was implemented to counter any sort of statistical bias and neuromuscular adaptations that may occur during the course of data collection. All 20 subjects possessed previous back squat and sprinting experience to reduce any change in performance due to acquisition of new skills. Before testing, all subjects directly measured their 1RM on the barbell back squat. Based on their 1RM performance, specific percentages were then calculated and used to determine relative load from subject to subject. Independent variables included the resistance training squat treatment, as well as the time interval of sprints after the sets of squats. Different squat treatments were used to assess the effect accommodating resistance has on power when used in a complex training method. It was important to the researchers to have different time intervals of posttesting sprints to assess the duration of potentiation from the squatting sets. This study examined the dependent variable of 9.1-m sprint times before and after several sets of heavy back squats. In other words, the 9.1-m sprints performed before the squat sets served as a benchmark for assessing the influence of the different squatting conditions on the sprints performed after the squats. Sprint time was chosen as the dependent variable due to the availability of testing equipment, as well as the practical function that coaches use to assess sprinting ability. On completing a standardized dynamic warm-up, subjects completed three 9.1-m sprints, which served as their pretest. Then, subjects either sat quietly for 10 minutes (CTRL), performed 5 sets of 3 repetitions of back squats at 85% of their 1RM (STND), or performed 5 sets of 3 repetitions of back squats at 85% of their 1RM, with 30% of that total load coming from accommodating resistance (BAND). On completion of the final repetition of the squat, the 9.1-m sprints were performed at intervals of immediately after and 1, 2, 3, and 4 minutes posttest. All data were collected through 4 testing sessions, separated by at least 72 hours. As a result, differences in 9.1-m sprint times can be attributed to the effects of several sets of heavy squats and the testing time interval, as opposed to the previous sets of sprints.

T1-16
Table 1:
9.1-m sprint times across testing conditions and time intervals.*

Subjects

Permission to conduct the study was granted by the university's institutional review board. Before any participation, the experimental procedures and the benefits and risks of the research were verbally explained to all participants, as well as physically read by all participants. They then gave their voluntary written informed consent and understood that they were free to withdraw from the study at any time. All subjects agreed to the procedures, benefits, and risks by signing a permission form/informed consent. Because of the use of heavy resistance training and high-intense sprinting, all subjects also completed a health-history questionnaire to assess their current risk of cardiovascular disease, as suggested by the American College of Sports Medicine, as well as for any musculoskeletal injuries.

Twenty recreationally resistance-trained males (age: 23.3 ± 4.4 years; height: 178.9 ± 6.5 cm; weight: 88.3 ± 10.8 kg) volunteered to be participants in this study. All subjects were fully-enrolled undergraduate students at a public state university and were all Kinesiology and Physical Education majors. They all reported at least 12 months of consistent resistance training experience, especially in the back squat before participating in the study. None of them were competing in a varsity sport but were active in their own unofficial competitions, such as local parks and recreation leagues and university intramurals. The sporting background reported by the subjects included football, hockey, baseball, and basketball.

Procedures

Preliminary Strength Testing

To ensure proper relative loading for the acute testing sessions, the initial session consisted of all subjects completing a 1RM test for the back squat. The procedures for measuring their muscular strength followed the recommendations set forth by Baechle and Earl (2). All subjects underwent a standardized warm-up, which included self-myofascial release for improvement of muscle quality, mobility and stability work, and dynamic stretching. They then performed 10 repetitions of the back squat with only the Olympic barbell (20 kg) inside a squat rack. The loads on the barbell were then progressively increased until the subjects could only perform 1 successful repetition. It was emphasized that all subjects squat to parallel or below. Esformes and Bampouras (21) demonstrated a greater PAP effect when subjects performed parallel squats vs. quarter squats. To ensure consistent parallel depth, a box was set up behind the squatter. The height of the box was set at the length of their lower leg, which was defined as the distance from the ground to the subjects' tibial tuberosity during a standing position. During every repetition performed on the back squat throughout the entire study, the subjects were instructed to squat down until their gluteus maximus muscles touched the box before beginning the ascent of their squat. An unsuccessful repetition was defined as the inability to fully stand erect with the load after touching the box or the failure to descend deep enough that the subject touched the box. That specific weight was recorded as their 1RM. At least 3 minutes of active rest was allowed between sets to ensure proper metabolic and neuromuscular recovery.

Sprint Testing

All testing occurred between the months of April and May. Time of testing on each testing day was controlled for, whereas hydration status and diet was not. All subjects reported to the laboratory in the late afternoon, accommodating to their class and work schedules. In addition, it was required that the subjects wear the same type of athletic shoes for all testing sessions, shoes that allowed for optimal squatting and sprinting performance. Finally, it was required that they refrained from any type of high-intense exercise at least 48 hours before testing days.

Sprint time was tested by electronic timing system (Brower Timing System, Draper, UT, USA). Before pretesting, subjects performed the same standardized warm-up as their 1RM preliminary testing session (muscle quality, lengthening, stability, and dynamic stretching). They then performed 3 trials of the 9.1-m sprint, separated by 60 seconds of rest. Subjects started in a 2-point stance, with their lead foot behind the starting line. One pair of photogate lights were erected onto tripods and were placed 9.1 m away from the starting line. They were instructed to sprint the 9.1 m as fast as possible and were given verbal encouragement throughout each trial. The electronic timing system initiated the timing on the subjects' first movement leading into the sprint. The physical disruption of the photogate light on crossing the finish line stopped the timing system, displaying the time it took the subjects to run the 9.1 m. The lowest time of their 3 pretest sprints was selected and recorded for analysis.

Treatment Interventions

Three different intervention strategies were used to manipulate sprint performance. During the control session (CTRL), the subjects sat in a chair and rested for 5 minutes after their pretest in the sprint. During the standard weight session (STND), the subjects performed 5 sets of 3 repetitions at 85% of their back squat 1RM using traditional isoinertial load with plates. Each set was separated by 2 minutes of rest. During every repetition, the subjects were verbally encouraged and reminded to descend deep enough that their gluteus maximus touched the box and ascend as explosively as possible to a fully erect position. The last intervention was the elastic band plus standard weight session (BAND). During this session, approximately 30% of their measured 85% of 1RM comprised accommodating resistance through elastic bands (EliteFTS, London, OH, USA). The manufacturers supplied a table that corresponded different lengths of the bands to equivalent tension. The bands were looped around the barbell until the desired 30% of 85% 1RM tension was achieved. The subjects performed 5 sets of 3 repetitions of this accommodating resisted load with 2 minutes of rest between sets. Squat depth was ensured by using the box strategy, as mentioned above, as well as verbally encouraged and reminded during each repetition to ascend as explosively as possible to a fully erect position.

Posttesting of the 9.1-m sprint was performed 5 times, with each trial being separated by 1 minute. During the CTRL session, subjects performed the 9.1-m sprints immediately after their 5-minute resting period, and then on the minute, every minute, for 4 minutes. During the STND and BAND sessions, the first sprint trial occurred immediately after their last squat repetition of their last set (Post-Immediate). The remaining sprint trials occurred 1 minute after each other, up to 4 minutes after the last squat repetition (Post-1min, Post-2min, Post-3min, and Post-4min; Figure 1).

F1-16
Figure 1:
Testing protocol. 1RM = 1 repetition maximum.

Statistical Analyses

Mean values and SDs were calculated for the pretesting sprint times and the 5 posttesting sprint trials to describe the variables. A repeated-measures analysis of variance (3 conditions × 6 time points) was used to examine statistical significant differences between the 3 conditions (CTRL, STND, and BAND) and the testing (Pre-Test, Post-Imm, Post-1min, Post-2min, Post-3min, and Post-4min). If significant main effects were present, a Bonferroni post hoc analysis was used to determine specific individual differences. Cohen's d effect sizes (ESs) were also calculated and reported with small ESs being less than 0.4, moderate between 0.4 and 0.7, and large being greater than 0.7. All statistical significance was set at p ≤ 0.05. All statistical calculations were performed using the IBM SPSS statistical software for Windows (version 21.0; SPSS, Inc., Chicago, IL, USA).

Reliability of sprint time was determined during the pretest of the CTRL treatment, in which no squatting was applied. Intraclass correlation coefficient was 0.855 between sprints 1 and 2, and 0.875 between sprints 2 and 3 during the pretest of the CTRL session. The coefficient of variation between all sprint trials was calculated at 3%.

Results

The assumptions of linear statistics were met as each data point was separated and not influenced by other data points. No outliers were reported when analyzing the standardized z-scores of the raw data. Finally, the data assumed a bell-shaped curve with little deviation, according to the skewness and kurtosis calculations. Therefore, no data transformation was needed as the data represented linear statistics.

Pearson's product-moment correlation revealed no significant relationship between squatting strength (1RM) and sprint time (−0.33, p = 0.28) and body mass (in kilograms) and sprint time (0.17, p = 0.57).

The changes in sprint performance across all conditions and time frames are presented in Table 1. There were no significant changes in sprint time across posttesting times during the CTRL and STND condition (Figures 2 and 3). During the BAND condition, when comparing the “Post-Test Immediate” sprint time to “Post-4min,” 9.1-m sprint time significantly decreased by 3.5% (p = 0.002; Figure 4). The standardized ES, d, was 0.83, indicating a large ES. Post hoc power analysis based on alpha-level sample size, and ES was calculated to be 1.0.

F2-16
Figure 2:
Sprint performance during CTRL protocol.
F3-16
Figure 3:
Sprint performance during STND protocol.
F4-16
Figure 4:
Sprint performance during BAND protocol. †Statistically significant compared with Post-Immediate (p = 0.002).

Discussion

To date and knowledge, this is the first study to examine the effects of PAP on short sprint performance after warming up with heavy back squats accompanied with accommodating resistance through elastic bands. The results from this study indicate a significant mean improvement of 0.08 seconds (3.5%) and that 83% of the variance was accounted for by this decrease in sprint times when preceded 4 minutes with a heavy back squat with accommodating resistance protocol.

The results from this study are in accordance with previous data reporting improvements in a power movement after a heavy resistance exercise in complex training. The improvement in performance has been attributed to PAP (34). However, the exact mechanisms responsible for the decrease in sprint time after heavy resistance exercise, more specifically with accommodating resistance, are unclear. In this study, no tests were conducted to examine the level of neuromuscular activation, such as electromyography, H-reflex, or twitch response; therefore, the responsible reasons could not be assessed and explained. However, previous researchers have attempted to explain PAP as being caused by an increase in the rate of phosphorylation of myosin light chain (14,40) or by an increase in Ca2+ kinetics and rate of cross-bridge formation (30). In addition, it was also reported that PAP could possibly be accounted for by an increase in muscle stiffness (34) or by an increase neural drive, which leads to higher recruitment of bigger and faster motor units (23,34,40). Also, in high-intense plyometric activity, such as jumping and in this case, sprinting, Burgess et al. (9) demonstrated muscle stiffness to be correlated to plyometric performance.

The current results revealed no significant changes in sprint time in either of the experimental protocols (STND, BAND) within 3 minutes after heavy lifting. A possible explanation for this lack of change could be attributed to the coexistence of neuromuscular fatigue and PAP after the heavy back squats in either protocol (32). The results from this study support the notion that the first 3 minutes after heavy resistance exercise, the neuromuscular fatigue overrides the PAP effect. This is supported by previous research relating to jumping performance that used similar resting periods (12,19,25). On the contrary, the results from this study indicate a positive PAP effect 4 minutes after the heavy resistance conditions. It may be at this point that the PAP effect is now more dominant than the fatigue, as fatigue now is reduced. This observation is in agreement with previous studies demonstrating that a period of 4–5 minutes is required to restore creatine phosphate levels and that the PAP effect could last 4–20 minutes after heavy resistance exercise (5,22,25).

In this study, one of the conditions was to perform heavy squats with standard weights with no elastic bands. One reason for using this condition was due to previous research demonstrating the positive effects of heavy standard weight squats on sprint time (7,10,11,27,45). It would have been expected to see a similar change in sprinting performance in the STND protocol as to the BAND condition. Over longer distances, such as 30 and 40 m, sprinting performance increased after heavy squats (11,27,45), but those same studies also reported no significant sprint performance in shorter sprint distances (0–10 m) following the same heavy squat protocol. Yetter and Moir (45) rationalized that heavy back squats may not provide the proper stimulation in the specific activation pattern that will transfer to proper 0–10-m sprint technique. Therefore, the data presented in this study are in agreement with previous data where heavy back squat with standard load did not enhance short sprint performance. But what separates the current results from previous PAP/sprint data is the use of accommodating resistance during the squats.

Lifting with elastic bands, compared with a standard isoinertial load, removes the sticking point of the lift, allowing the athlete to accelerate the load through the full range of motion. Muscle contractions during the BAND protocol could have kept the muscles working closer to a maximal force capacity through the full range of motion, while reducing the biomechanical advantage, with then may alter the muscle recruitment patterns (1). If muscles and their contractions have less of a sticking point, this may lead to greater type IIx fiber recruitment, therefore, greater adaptations to these powerful fibers. During each repetition, all subjects were reminded to squat up as fast as they could during the concentric phase. This type of lifting could translate better to sprinting, as athletes need to produce maximal force rapidly through the full range of motion, possibly explaining the positive changes in sprint performance.

During the eccentric phase of the squat, subjects were reminded to control the weight until they reached the bottom of the squat. While no specific eccentric cadence was prescribed, the researchers wanted the subjects to control the eccentric, since the elastic bands are actively pulling the load greater than gravity. Anderson et al. (1) stated that the lowering of the weight with accommodating resistance may require more fiber recruitment and impose effort at a greater percentage of capacity than an isoinertial load. Cronin et al. (13) reported that lowering with accommodating resistance caused significant greater EMG activity compared with lowering with a load without accommodating resistance. Therefore, greater fiber stimulation and activation during the eccentric portion of the squat with accommodating resistance, compared with the standard load condition, may elicit greater adaptations and type IIx recruitment, possibly explaining the increase in sprint performance.

One aspect of improving sprint speed is enhancing their rate of force development (RFD). Previous research has demonstrated the efficacy of accommodating resistance on improving RFD (17,32,35,39,42). Wallace et al. (43) suggested that lifting with accommodating resistance allows the athlete to achieve longer peak velocity phase during the lift. As the resistance increases as the mechanical advantage increases, the athlete can then generate higher forces during the concentric phase of the lift longer, when the muscle are at or near their optimal force-length relationship (19).

Another explanation of the possible increase in RFD, as demonstrated by an increase in sprint performance following the BAND protocol, is the exploitation of the stretch-shortening cycle. During the eccentric portion of the back squat, the muscles are able to store elastic potential energy, due to the bands actively pulling the load greater than gravity's effect. This potential energy is then released as kinetic energy during the concentric phase, thus increasing force production.

Across all 3 conditions, there was a wide variance in sprint times. Gullich and Schmidtbleicher (22) noted that PAP effects were only reported for well-trained subjects. Hamada et al. (23) reported that the PAP effects are greatest in those subjects demonstrating the greatest percentage of type II fibers in the stimulated muscles. Recently, data have suggested that stronger individuals seem to be able to express PAP earlier and better, compared with weaker individuals (35,46). Seitz et al. (35) suggested that “strong” is defined by an athlete being able to maximally squat at least twice their body mass. Strength-trained individuals were selected for this study for this very reason. But a variation existed in the present subjects' utilization of resistance training protocols, which may account for some of the variation in the treatment response. Their sprint-type training activities could also be a variance explanation. Therefore, future direction for a study of this type should attempt to first identify specific subjects who are predisposed to a PAP effect in response to the specific types of treatments implemented in this study. This then could provide coaches with greater confidence in the efficacy of specific treatments with their speed athletes.

Another limitation of this study is the lack of measures to equate the loads between the STND and BAND group. The optimal method of equating the loads would be utilization of a force platform. For future directions, it is recommended that subjects perform the squats on a force platform. While the types of load would be different (standard, isoinertial vs. accommodating resistance), the force exerted into the ground should theoretically be equal. A better comparison between the efficacies of treatments on sprint speed would be possible. But the reason why ground reaction force was not measured in this study is the practical application, as most coaches and athletes would not have access to a force platform either in their facility, or before competition on the track, field, or court, as well as the fact that this current laboratory where the data collected did not possess a force platform to use.

A possibility exists for using the acute effects reported from this study to produce training adaptations. Previous research has provided evidence of the long-term benefit of using complex training to improve performance (3,15,18). A long-term training adaptation could have significant implication for coaches and athletes. Based on the current reports, long-term improvements in 9.1-m sprint performance may be increased from performing heavy back squats with accommodating resistance as a warm-up 4 minutes before sprint trials. This could lead to longer benefits after sprint training by allowing the athlete's neuromuscular system to perform at a higher level during each successive training session. While the possible effects sound appealing, more research is warranted to specifically investigate the long-term effects of complex training with accommodating on sprint performance. Currently, it is unknown how the neuromuscular system adapts to the long-term use of this specific type of complex training.

In summary, the data from this study reflect positive changes in short sprint performance 4 minutes after sets of heavy back squats with elastic bands. However, the exact mechanisms for improved performance in the BAND condition compared with the CTRL and STND protocols remain unclear. Further research is suggested to delve into the specific mechanisms of change, as well as fine-tuning this warm-up condition for consistent performance changes across a variety of athletic populations and abilities.

Practical Applications

The use of a heavy back squat protocol with the addition of accommodating resistance (85% of 1RM at the top of the lift) performed 4 minutes before sprinting trials can significantly lower the time taken to cover the first 9.1 m in recreationally trained males. These results suggest that coaches and athletes could incorporate such warm-up to increase sprinting performance. It is important to consider the variation in the responses to the treatments in this specific subject pool. Therefore, coaches should be cautious in prescribing this activation warm-up with all athletes, especially the rest interval between the squatting exercise and the explosive movement. If performed too soon, neuromuscular fatigue may outweigh the PAP affect, thereby reducing performance. But, it could be suggested that this type of complex training warm-up could be a reasonable alternative to lower-intensity activities, such as jogging and stretching. This concept could be applied to any activity requiring a short burst of sprint performance, between 0 and 10 m.

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    Keywords:

    sprints; postactivation potentiation; heavy resistance exercise

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