Explosive power output of the lower limbs is one of the key determinants of performance in many elite sports (10) and is required to perform jumping, sprinting, and many weightlifting activities (15). Consequently, warm-up protocols are often used in an attempt to optimize power production during explosive movements (18) for both competition and weight training purposes. One common test of the peak power output of the lower limbs is the countermovement jump (CMJ). Warm-ups incorporating both high (4,25) and low load (20) exercises, usually involving the squat exercise, have both been reported to be effective at improving CMJ performance. It is possible that, if they could be shown to improve explosive power characteristics in athletes, low load warm-up exercises may be of value to coaches because they are less likely to induce athlete fatigue before competition or weight training sessions.
It has been suggested that performing exercise with low loads may activate the central nervous system to create a favorable environment for the performance of explosive movements (24). The gluteal muscle group is a key contributor to explosive movements of the lower limb including sprinting and jumping (13). During a CMJ, the gluteus maximus, medius, and minimus are all highly activated (10,14). Modeling suggests that although the gluteus maximus generates large force and work output in the sagittal plane, the gluteus medius and minimus also play substantial roles in the generation of a jumping motion by stabilizing the movement of the hip joint (15). Low load isometric exercises of the transversus abdominus have been demonstrated to acutely improve the activation of this muscle during functional movement (23). It is currently unknown whether this acute carryover effect might be transferrable to muscles involved in lower limb power production such as the gluteal muscle group.
It has also been suggested that the exposure to mechanical vibrations, often referred to as whole-body vibration (WBV) training, may be of use when preparing an athlete for explosive athletic events (11). Research into the acute effects of WBV on the explosive characteristics of elite athletes, however, shows mixed results. Cochrane and Stannard (5) reported improved vertical jump height after a WBV warm-up protocol in elite athletes when compared with that in control conditions (p < 0.001). However, another study found that WBV had no acute effect on any CMJ parameters, including peak velocity and acceleration in elite athletes, and that after WBV, the subjects had a decreased total vertical distance traveled in an unloaded squat jump when compared with a control condition (p = 0.006) (3).
The intention of the study was to devise and compare practical protocols that could readily be applied as a warm-up immediately before competition or weight training sessions with consideration given to the minimization of fatigue. The specific purpose of this study was to investigate and compare the acute effect of a warm-up protocol incorporating low load exercises targeting the gluteal muscle group (GM-P), a WBV warm-up protocol (WBV-P), and a control group (CON) on peak power production during CMJ testing. Consequently, the study attempted to address the research question: “Which is the most effective warm-up protocol in terms of its effect on peak power production during CMJ testing: GM-P, WBV-P, or CON?”
Experimental Approach to the Problem
Two warm-up protocols were devised that could be easily applied as a warm-up either precompetition or before weight training sessions with a minimal risk of fatigue. The first used low load exercises targeting the gluteal muscle group (GM-P), whereas the second incorporated the use of WBV (WBV-P). A third protocol designed to act as a control was also incorporated (CON). To address the research question, and establish which of the 3 warm-up protocols was the most effective, the subjects were randomly allocated into 3 separate groups with each completing all 3 warm-up protocols: WBV-P, GM-P, and CON. Testing was conducted over 3 different sessions in a randomized order. This randomized, counterbalanced, repeated measures design was employed to analyze differences in peak power production during CMJ testing (dependent variable) among the 3 different warm-up protocols (independent variables). The CMJ testing was performed using a Smith machine so that peak power could be measured using a linear encoder.
Thirty elite Australian Rules Football players were recruited in the study. All the players were healthy at the commencement of the study. All the players provided informed consent to participate in the study, and institutional ethics approval was granted to retrieve and analyze deidentified data after the completion of the testing.
Three testing sessions were held on training days over a 10-day period during the team's preseason training phase. No 2 sessions were conducted on consecutive days to avoid the effects of fatigue from training or testing. All the training sessions were of a similar intensity and volume. The testing sessions were performed in the late morning 1 hour after the completion of a field training session. Given this consistent time of testing and the professional environment, it could be expected that players' arousal levels would have remained consistent for the duration of the study. The WBV-P consisted of subjects standing with 10–30° knee flexion in an unloaded static squat stance with feet shoulder width apart for 45 seconds on a side-alternating platform vibrating at 30 Hz and an amplitude of 6.4 mm. The protocol was conducted on a commercially available Galileo Sport machine (Novotec, Pforzheim, Germany). Enhanced electromyographic (EMG) activity has been reported with increasing frequencies on side-alternating platforms up to 30 Hz (17).
The GM-P consisted of 7 exercises performed in sequence. One set of 10 repetitions was performed for each exercise. The subjects were familiar with all the exercises before the study and were encouraged to maintain a neutral spinal posture. The use of a verbal cue to engage the gluteal muscles before movement may be of benefit in improving gluteal muscle activation during exercise (12). The subjects were consequently instructed to engage their gluteal muscle group before and during the exercise movements. The exercise protocol was performed under supervision and took approximately 5–7 minutes to complete.
The 7 exercises in the GM-P included a double leg bridge, side lying hip abduction exercise, and quadruped lower extremity lift that were performed as described by Ekstrom et al. (9) except in the case of the quadruped lower extremity lift where, in this study, no arm movement was involved. A side lying gluteal clam in 60° hip flexion was performed as described by Distefano et al (7). A prone single leg hip extension exercise was performed using verbal cues according to a protocol described by Lewis and Sahrmann (12). The remaining 2 exercises, stability ball wall squats and hip abduction in quadruped or “dirty dog,” were performed according to the instructions in the American Council on Exercise online library of exercises (2). Where available, previous data on EMG recordings of gluteus maximus and gluteus medius muscle activation during these exercises are listed in Table 1. The CON group performed no specific preparation before CMJ peak power testing.
Testing of peak power was undertaken within 5 minutes of the performance of each warm-up protocol. The subjects performed 5 consecutive CMJs using a Smith machine with a bar mass of 20 kg, and no additional weight was added. The players jumped off and landed on a thin foam mat. Peak power was measured using a Gymaware linear encoder (Kinetic, Mitchell, Australia), which was secured to the floor directly below the Smith machine bar at one end. An infrared receiver enabled data captured by the encoder to be saved on a palm top and subsequently downloaded to a computer. The use of a linear encoder has been shown to be valid and reliable (6,8). Jump technique was closely monitored to ensure consistency of testing. In particular, the subjects were not allowed any upward movement of the bar before commencing their countermovement. The highest peak power achieved during the 5 jumps was used for analysis.
A Shapiro-Wilk's (<50 subjects) test was found to have an alpha level of p > 0.05 indicating that all data were normally distributed. A repeated measures analysis of variance (ANOVA) was used to investigate any differences between conditions. After the ANOVA, a pairwise comparison post hoc test was performed with a Bonferroni adjustment. Baseline comparability was established using a Student's t-test to assess any differences in age, body mass, height, or previous vertical jumping performance between groups. Statistical analysis was conducted using SPSS (SPSS Inc., Chicago, IL, USA). In all cases, the alpha level was set at 0.05.
Twenty-two of the subjects completed all the 3 testing conditions. Eight subjects each missed 1 testing session because of training-related soreness (n = 7) or absence because of illness (n = 1) and were consequently excluded from the analysis. No subjects were injured during the testing. No differences were found between the baseline characteristics of the 3 testing group's height, body mass, age, and previous best vertical jumping height (p > 0.05). Baseline characteristics are reported in Table 2.
Table 3 shows the differences in peak power production during CMJ testing between the 3 warm-up protocols. Repeated measures ANOVA showed a significant difference in peak power production between warm-up protocols during CMJ testing (F = 6.376, effect size = 0.233, p < 0.01). Pairwise comparisons found that the subjects performed 4.2% better after the GM-P than after the CON condition (p < 0.05), and 6.6% better than in the WBV-P condition (p < 0.01). A 2.4% decreased performance for the WBV-P compared with that of the CON was found to be nonsignificant (p > 0.05).
The results demonstrate improved peak power production during CMJ testing after the GM-P when compared with that of both CON and WBV-P and suggest that the low load warm-up protocol described in this article may be a useful tool for coaches who desire to enhance the explosive power of their athletes before either competition or training. This protocol would be most applicable for athletes competing in sports with high demands for explosive power output from the lower limbs and has the obvious advantage, because of the low loads involved in the protocol, of minimizing any risks of fatigue or injury before performance. The other advantage is that no equipment is required and makes the protocol suitable for a large portion of athletes at various levels of competition.
The results of this study are consistent with those of other research in healthy active men, which demonstrated that improved CMJ height and mean power output can be achieved with a warm-up incorporating light loads (25–35% 1RM) (20). Interestingly however another study in highly trained subjects reported that a warm-up involving low loads (30% 1RM) was inferior to one involving high loads (80–95% 1RM) at improving CMJ performance (18). Neither of these 2 other studies however specifically targeted the gluteal muscle group and instead used 5 minutes of generalized cardiovascular warm-up followed by loaded half squats. It is possible that a series of low load exercises targeting the gluteal muscle group may influence explosive characteristics differently to the protocols undertaken in the 2 previous studies.
Postactivation potentiation has been proposed as a mechanism for improved power production following a voluntary contraction of a muscle (21). However, this contraction is typically performed at maximal or near-maximal intensity and many of the proposed mechanisms, which include phosphorylation of myosin regulatory light chains and a possible change in pennation angle (21), are unlikely to be applicable after low-intensity exercise (19). The enhancement in power production after low-intensity exercise that was observed in this study is likely to be because of other mechanisms. It has been reported that low-level isometric contractions of the transversus abdominus muscle have the acute effect of increasing activation levels of this muscle during functional tasks (23). These increases are accompanied by altered motor cortical activity measured with transcranial magnetic stimulation (22). It is possible that exercises eliciting low to moderate levels of activation of the gluteal muscle group such as those contained in the GM-P may also lead to a transfer of increased muscle activation within the gluteal muscle group to functional movements such as countermovement jumping. Future research could validate the findings of this study with EMG recordings of the gluteal muscle group during CMJ testing and other measures of explosive power output both before and after the performance of the GM-P.
The results also demonstrate that the WBV-P used in this study was not effective at improving peak power production during CMJ testing when compared with control. This result is consistent with the findings of Bullock et al. (3) who reported no changes in any CMJ performance parameters after 3 × 1 minutes of vibration exposure when compared with a control protocol. One other study that reported improved vertical jump heights postvibration in elite athletes applied a vibration exposure that lasted for 5 minutes and included dynamic exercises (5). It is possible that the differences in vibration exposure times and the inclusion of dynamic exercises on the WBV machine might explain the difference in results. The WBV-P in this study was dictated by a number of practical considerations: It was intended as a warm-up immediately before matches and at half-time, and given the limitations of the number of WBV machines available and warm-up time required for a team of 22 players, the exposure time was set at 45 seconds and WBV dynamic exercises were avoided to minimize fatigue. Another reason, which may have contributed to the WBV-P not being effective, is the knee flexion range used by the players. Pilot work revealed that WBV with the knees extended frequently resulted in discomfort to the lower back and head regions. The players were instructed to bend their knees slightly and hold the self-selected position, which ranged from 10 to 30°. It is now acknowledged that having a 10–30° range of knee flexion can influence the level of transmissibility, but it is unclear as to what the difference in the magnitude of the transmissibility is to the gluteal region. It has been reported that head acceleration decreased by about 50% by increasing the knee flexion angle from 10 to 30° (1). Whether transmissibility to the gluteal muscles is influenced by the same magnitude is unknown given that gluteal EMG data were not reported and the gluteal region is significantly closer in proximity to the knees than to the head. It is not only the distance that varies but also the number of joints and other structures capable of dissipation such as the intervertebral joints and discs. The WBV can be performed with the feet flat or with the heels raised. Enhanced activation of the knee extensors (rectus femoris and vastus medialis) and the hip extensor (biceps femoris) was found for normal stance with the feet on the platform compared with raising the heels off the platform (16). Although the gluteals were not directly monitored, it could be expected that the normal stance position would be more favorable for not only the biceps femoris but also the other hip extensors such as the gluteal maximus.
Despite some players dropping out because of nonexperimental condition factors, training-related soreness (n = 7) and absence because of illness (n = 1), an even spread of subjects across testing groups was present, and a sufficient number of subjects were included in the analysis to ascertain any differences between warm-up protocols. No control for hydration status was undertaken in the study, and it is possible that mild variations in hydration levels may have existed between the groups. However, it was considered unlikely that, except in cases of severe dehydration, this variation would have affected explosive output during a single set of jumps. There was no indication that any of the athletes were dehydrated during any of the testing.
This study has investigated the specific research question, “Which is the most effective warm-up protocol in terms of its effect on peak power production during CMJ testing: WBV-P, GM-P, or CON?” It has demonstrated that improved peak power production during CMJ testing is possible by undertaking a low load warm-up protocol that targets the gluteal muscle group. It has also found that a short-duration WBV protocol was no better than control at improving peak power production in highly trained elite Australian Rules Football players.
A warm-up protocol involving low load exercises targeting the gluteal muscles is effective at acutely enhancing explosive power output in the lower limbs. Coaches may consider this protocol when preparing athletes for competition or training in sports involving explosive lower limb movements such as jumping, sprinting, and some weightlifting movements. Low load exercises of this nature are likely to be more acceptable to the athlete and coach than are protocols incorporating heavier loads because of a lower risk of athlete fatigue and no equipment requirement. Coaches of athletes in sports involving explosive lower limb movements should also exercise some caution when deciding how best to use WBV in the lead up to competition or training sessions because the WBV-P used in this study did not acutely enhance jumping performance in elite athletes.
The authors appreciate the players of the Collingwood Football Club, who participated in this study. The results of this study do not constitute endorsement by the authors or the National Strength and Conditioning Association of any products referred to within.
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