Athletes typically perform a warm-up to prepare to engage in practice or competition. Traditionally, these warm-ups have included some form of static stretching. However, there is some evidence that static stretching can be detrimental to the power component of athletic performance. Stretching has been shown to inhibit drop-jump performance (16,17), vertical jump performance (2,5,6,7), power output as measured by maximum voluntary contraction force (3), and leg extension power (15). The mechanisms by which static stretching impairs performance are unknown, but it has been hypothesized to be related to lower levels of musculotendinous stiffness (14), a reduced ability to recruit motor units (4), or the inhibition of the acute response of muscle proprioceptors, such as the Golgi tendon organs (8).
The observation of performance decrements from static stretching has led to the investigation of alternative options for sport preparation. One such alternative is a dynamic warm-up, which incorporates movements similar to those performed in the sport (1). Dynamic warm-ups have been shown to increase power output when cycling (12). Another option that has been suggested for warm-up is dynamic stretching, which is performed to increase dynamic flexibility. Dynamic flexibility stretching consists of performing movements that take the limb through the range of motion by contracting the agonist muscle, allowing the antagonist muscle to relax and elongate (9,10). A study by Yamaguchi and Ishii (15) compared dynamic stretching to static stretching and found that leg extension power was greater after dynamic stretching. These methods of warm-up may differ in the way they affect the non-contractile components of the muscle and may not inhibit power performance in the same manner as static stretching.
The purpose of the present study was to compare the impact of these different warm-up protocols on power output in Division I male collegiate football players. Based on the results of previous research, it was hypothesized that a static stretching warm-up would not benefit power output as measured by VJ performance when recovery time for the elastic component of the muscle was not permitted. Also, a dynamic warm-up and a dynamic flexibility stretching routine were expected to lead to better VJ performance than static stretching because the same power decrements have not been found with these warm-up methods. This is important because an appropriate warm-up is critical to successful performance in sports such as football. It is also important to determine whether static stretching immediately prior to power performance is detrimental, because power output is a key component in athletic performance (13).
Experimental Approach to the Problem
This study was a between-subjects control-group design intended to examine the effects of 4 different warm-ups on VJ performance. Sixty-four participants completed a general warm-up and were then randomly assigned to a warm-up group. One group completed no further activity before completing the posttest to examine the effects of a general warm-up and to provide a control for comparison. The 3 other groups completed 1 of the following types of warm-up: a dynamic warm-up, dynamic stretching, or static stretching. The impact of these warm-ups was assessed via VJ performance and the scores were analyzed using a repeated measures analysis of covariance (ANCOVA) with pretest scores as a covariate.
Prior to conducting this study, a pilot test with 4 athletes was performed to refine and clarify the instructions given to the participants. The results of this test were used to determine a sample size with adequate power to detect differences between the groups with a power level of 0.80. Therefore, a total of 64 players from a National Collegiate Athletics Association Division I football team were recruited to participate in this study. The participants ranged in age from 18 to 25 years (mean ± SD data were 20.7 ± 1.8), and had a mean weight of 99.69 ± 21.41 kg. The participants were familiar with the VJ procedure.
This study was approved by the University's Institutional Review Board, which required the participant to sign an informed consent form prior to taking part in the study.
Each participant completed a pretest countermovement vertical jump followed by a 5-minute general cardiovascular warm-up. The participants were then randomly assigned to 1 of 4 groups using a block randomization procedure to create groups of equal number. One group completed the posttest VJ with no further warm-up. The participants in the 3 other groups completed the posttest VJ after participating in a static stretching condition, a dynamic warm-up, or a dynamic stretching warm-up. Participants in all conditions were allowed to get a drink of water immediately after completing the general warm-up. The posttest vertical jump was performed immediately after the completion of the warm up in each group.
The participants in each condition were tested on countermovement VJ height using the Vertec Vertical Jump Tester (Sports Imports, Hilliard, OH). The countermovement jump was calibrated based on the height of each participant's standing one-arm reach. The participants jumped from both feet with no step in an attempt to touch the highest vane possible. Jump height was calculated by adding the amount of vanes reached by the participant to the reference line. Participants continued to jump as long as the jump height continued to increase, until 2 jumps in a row did not result in touching a higher vane. The highest jump height was recorded and used in the analyses.
Cardiovascular activity for the duration of 5 to 10 minutes is considered an effective general warm-up due to an increased temperature in the muscles, increased heart rate, and increased blood flow (7). Increased temperature in the muscles allows for a greater amount of flexibility, which prepares the athlete for the movement demands of the activity, and an increase in heart rate and blood flow delivers oxygen and other necessary nutrients to the muscles to use during the activity (1).
In this study, the general warm-up consisted of 5 minutes of treadmill running. The participants began by running 1 minute at 4 miles per hour, and increased the speed by 1 mile per hour each minute for 4 more minutes. Once the general warm-up was completed, the participants in the warm-up only group completed the posttest.
Static Stretch Warm-Up
The participants who were assigned to the static stretch condition completed 5 passive stretches with assistance from one of the investigators. The stretches were similar to those used in previous research (15) and were aimed at targeting the muscles involved in performing the countermovement VJ; which include the hamstrings, the gluteals, the lower back, the quadriceps, and the hip flexors. The stretches were held 3 times to the point of slightly painful yet tolerable muscle discomfort for the duration of 5 seconds with rest intervals of 1 second. It is generally recommended that stretches are held longer than this, however, this was the amount of time that the participants typically held stretches prior to activity and this time interval was selected to mimic those conditions.
To stretch the hamstrings, the participant lay on his back and lifted one leg, keeping it straight as the investigator moved the leg toward the head. The gluteal stretch was also done in a supine position, with one leg flexed at the knee and crossed over the front of the body. The investigator assisted this stretch by placing his hands on the participant's knee and ankle and pushing the leg toward the participant's head. The quadriceps and hip flexors were stretched with the participant lying on his side with the top leg flexed at the knee, and the investigator moved the heel toward the gluteals. The lower back was stretched with the participant lying in a supine position with the experimenter holding both of the participant's ankles and lifting the legs toward the ceiling. Also, while lying on his back, the participant lifted both legs toward the ceiling with a slight bend in the knee and flexion in the ankle. The investigator assisted this stretch by placing his body on the participant's feet and leaning forward to apply his body weight.
The dynamic warm-up involved performing 10 walking lunges, 10 reverse lunges, 10 single-leg Romanian dead lifts, and 10 straight leg kicks with each leg. The participants in this group also completed high knees and reverse high knees over a distance of 10 yards. These movements are similar to the movements used in sport, and were selected because the participants were accustomed to them as they were a part of their normal warm-up routine.
The dynamic flexibility routine involved performing 10 repetitions on each leg of 8 different movements: standing front leg swings, standing lateral leg swings, leg scissors to the front and to the side (inverted supine position, resting on shoulders and elbows), eagles (supine position), scorpions (prone position), donkey kicks (from the knees), and lateral leg swings (from the knees). The movements selected were intended to warm up the muscles used to perform a vertical jump by engaging the agonist and antagonist muscles.
An intraclass reliability coefficient (ICC) for the vertical jump scores was calculated to examine the test reliability of the vertical jump scores. A 1-way analysis of covariance (ANCOVA) was used to determine if significant differences existed on the posttest scores across conditions after the groups were equated on the pretest scores. Therefore, the pretest scores were used as the covariate in the analysis. Fisher's least significant difference post-hoc tests were used to investigate significant differences between the groups. A result was considered statistically significant if P ≤ .05. Statistical analyses were performed using SPSS 11.0 (SPSS, Inc., Chicago, IL).
The initial sample consisted of 64 participants, but was reduced to 63 after the exclusion of one outlier. The group means for the pretest and the posttest can be found in Table 1. The overall intraclass correlation coefficient (R) for this measure was 0.984 with a lower bound of 0.974 and an upper bound of 0.990.
Pretest vertical jump scores were not equivalent between the groups; therefore, an ANCOVA was selected to perform the analysis to adjust the posttest scores for initial pretest differences. A Cohen's d measure of effect size was calculated for each group using the group's posttest score, the pretest score for the entire sample, and a pooled standard deviation for the groups in the form of mean square error. These measures of effect size can also be found in Table 1.
The assumptions associated with ANCOVA with regard to linearity, normality, and homogeneity of regression slopes were met. However, the assumption of homogeneity of variance across groups was violated.
The posttest VJ means of all 4 groups were higher than the pretest means; however, the VJ height gain in the static stretching group was significantly less than the gain in the other 3 groups. This can be seen in Figure 1.
Using these adjusted marginal means, the ANCOVA was statistically significant, (ANCOVA P = .001). The warm-up condition accounted for 23.6% of the variance in the posttest when holding constant for the pretest (ANCOVA η2 = .236). The coefficient of determination for this model was .955 (ANCOVA R2 =.955), thus the rate of error was approximately 4.5%.
The Fishers least significant difference post-hoc test was used to evaluate pairwise differences among the adjusted means. There were significant differences in the adjusted means between the static stretching group and the other groups. However, there was no statistically significant difference between the general warm-up, dynamic warm-up, and dynamic stretching groups.
The purpose of this study was to determine the effects of 4 different warm-ups on power output as measured by vertical jump test performance. The effects of static stretching were of particular interest, because past research has indicated that static stretching may have detrimental effects on power output. The main finding was a significant improvement in vertical jump performance following a general warm-up-only condition, a general warm-up plus dynamic warm-up condition, and a general warm-up plus dynamic flexibility condition. However, no statistically significant improvement was found in the general warm-up plus static stretching condition. These results are consistent with the growing body of evidence that static stretching can inhibit a muscle's maximal power output (6,14,16), and were in support of the hypothesis that static stretching would not benefit vertical jump performance.
There was also support for the hypothesis that the dynamic warm-up and dynamic stretching conditions would have positive effects on performance. There were no statistically significant differences between the general warm-up only group, the general warm-up plus dynamic warm-up group, and a general warm-up plus dynamic flexibility group; however, the measures of effect size for each group revealed that the dynamic warm-up and dynamic flexibility warm-up led to better performance than the general warm-up alone.
Although the exact mechanisms by which static stretching elicits decrements in power performance are not known, several possibilities have been suggested. Researchers have posited that stretching may reduce musculotendinous stiffness, which inhibits the production of force in the contractile component of the muscle (14). Decreases in force production may also be the result of a reduced ability to recruit motor units as a function of inhibited neural mechanisms such as myoelectric potentiation (4). Yet another explanation for decreases in force production is the inhibition of the acute response of muscle proprioceptors, such as the Golgi tendon organs or the low threshold pain receptors (8).
One factor that was not examined in the present study is the effects of time on recovery after static stretching. This study examined the effects of static stretching immediately proceeding power performance. With enough time, the elastic components of the muscle may recover but the amount of time necessary for recovery deserves further examination.
Although conclusions about the mechanisms responsible for decreases in power output cannot be drawn from the present data, the findings from this study have important practical applications. The results of this experiment showed that warm-up had an impact on vertical jump performance. However, static stretching appeared to negate the benefits of this warm-up when performed immediately before a vertical jump test. If static stretching can negatively affect performance on the vertical jump, a skill that demands maximal power output, it follows that the performance of similar power skills might be negatively affected if static stretching is undertaken prior to the activity. At the present time, it is recommended that the hamstrings, gluteals, quadriceps, hip flexors, and lower back are not statically stretched immediately prior to performing a vertical jump if the intent is to maximize jump performance. If increasing flexibility is a concern, it is recommended that it be developed in a warm-up using dynamic stretching. If static stretching is used, it may be best to delay it until after sport performance.
With regard to the dynamic warm-up and the dynamic flexibility approaches, it seems that either has a beneficial effect on vertical jump performance. However, these approaches may have the additional benefits such as increasing flexibility in dynamic movements. Future research might be aimed at the flexibility-enhancing and injury-preventative aspects of these types of warm-up.
The authors would like to thank Dr. Heath, Dr. DeBerard, Dr. Kras, and Dr. Fargo for their advice on this project.
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