For ballistic stretching, subjects performed the same stretches as previously described. However, instead of holding the stretch, subjects were instructed to get into the specific stretch position until a strong stretch sensation was felt. Within 2 seconds of feeling a stretch sensation, subjects bounced through the movement at the end of range of motion at a rate of one bounce per second for a total of 30 seconds. To perform the ballistic stretching, a metronome was set at 60 bpm, and subjects bounced to the beat of the metronome. When stretching the quadriceps, subjects flexed the knee until a strong stretch sensation was felt, and then each subject extended the knee to the point at which the stretch sensation was no longer felt. Subjects flexed and extended the knee rhythmically to the metronome within the identified range. When stretching the hamstrings, subjects flexed at the hip while reaching toward their toes until a strong stretch sensation was felt, and then extended at the hip to the point where the stretch sensation was no longer felt. Subjects bounced forward with hip flexion and backward by hip extension rhythmically to the metronome within the identified range.
For the no-stretch control condition, subjects only completed the warm-up by walking for 5 minutes on the treadmill, and then subjects immediately began VJ testing.
Vertical Jump and Power Testing
Vertical jump height was assessed by the force plate and the Vertec VJ system, and lower-extremity power was assessed on the force plate. Subjects performed three CMJs on each day following their stretching condition. Subjects were instructed to stand on both feet on the force plate and lower their body toward the ground by moving into flexion at the knee, hip, and trunk while extending both shoulders (Figure 3). When subjects comfortably reached this point of flexion, they instantly jumped up as high as possible while reaching for the Vertec VJ system with their dominant hand (Figure 4). Subjects jumped up and hit the highest marker possible on the Vertec. The VJ height was determined from the highest moved marker. Ground-reaction forces (GRF) were recorded for each jump with the force plate. The highest VJ determined by the Vertec VJ system from each testing session was recorded. The height of this jump was also calculated using the GRF recorded by the force plate through the following method. The sum of all forces produced during the CMJ was calculated from time 1 to time 2, to determine the take-off velocity through the following equation:
In the equation, F is the sum of all forces, t is time, m is mass, and v is vertical velocity at take-off. Time 1 was identified as the point after the jump was initiated and the point at which GRF equaled body weight. Time 2 was identified as the point at which GRF decreased to equal body weight just before take-off. Take-off was identified as the point at which GRF fell to zero. Take-off velocity was used to determine the VJ height through the following equation:
In the equation, g is acceleration attributable to gravity, and h is VJ height. Power produced during this jump was determined by calculating work, which is the product of the sum of all forces and vertical velocity at take-off (Fv).
Torque Output Testing
Each subject was positioned in the Biodex chair so that the axis of rotation of the dynamometer lined up with the joint line of the right knee. The lower leg was strapped to the dynamometer lever arm approximately two finger-widths above the medial malleolus. To ensure reliable measurements, the dynamometer was calibrated, all stabilization straps were used to prevent unwanted movement, subject's hands were required to remain free, and no visual feedback was provided during testing. Subjects performed three maximal isokinetic concentric muscle actions for knee extension and flexion at 60°·s−1 through a 10-105° range of movement (0° = full knee extension) (Figure 5). The highest peak torque from the three maximal repetitions was recorded for each muscle and was used in the analysis.
For each dependent measure, the largest recorded value from the three repetitions was used for the analysis. For example, the largest peak torque produced by the quadriceps and hamstrings under each condition was used for analysis. Data normalized for body weight were analyzed using five separate, 3 (stretch condition) × 2 (gender) analysis-of-variance procedures with repeated measures on the factor of stretch condition. An alpha level of p ≤ 0.05 was used as the level of significance.
The statistical analyses yielded very similar results across measures. These measures and their respective results are detailed below.
VJ-calculated: The main effect for stretch condition on VJ as measured by the force plate was not significant, F 1,22 = 0.660, p = 0.425. The main effect for gender was significant, F 1,22 = 67.645, p < 0.001. The gender × stretch interaction was not significant, F = 0.023, p = 0.881.
VJ-Vertec: The main effect for stretch condition on VJ as measured by the Vertec VJ system was not significant, F 1,22 = 1.201, p = 0.285. The main effect for gender was significant, F 1,22 = 68.168, p < 0.001. The gender × stretch interaction was not significant, F = 0.030, p = 0.864 (Figure 6).
Quadriceps torque: The main effect for stretch condition on quadriceps torque was not significant, F 1,22 = 0.427, p = 0.520. The main effect for gender was significant, F 1,22 = 26.230, p < 0.001. The gender × stretch interaction was not significant, F = 0.050, p = 0.825 (Figure 7).
Hamstring torque: The main effect for stretch condition on hamstring torque was not significant, F 1,22 = 0.275, p = 0.605. The main effect for gender was significant, F 1,22 = 6.692, p = 0.017. The gender × stretch interaction was not significant, F = 0.008, p = 0.931 (Figure 6).
These results suggest that static and ballistic stretching did not affect VJ, hamstring torque, or quadriceps torque. The gender × stretch interaction was not significant for any of the four measures above, suggesting that the stretching conditions did not affect men and women differently. However, the significant main effect for gender, even when the variable was normalized for body weight, demonstrates that men produced more torque and VJ than women.
Power: The main effect for stretch condition on power was significant, F 1,22 = 7.124, p = 0.014. The main effect for gender was significant, F 1,22 = 76.260, p < 0.001. Once again, the gender × stretch interaction was not significant, F = 0.779, p = 0.387. These results indicate that the mean value for the control group was significantly greater than the two stretching conditions. Normalized means for power were 48.4, 48.9, and 50.1 for static, ballistic, and control, respectively (Figure 8). These results show that static and ballistic stretching had an adverse effect on power. The significant main effect for gender, even when the variable was normalized for body weight, demonstrates that men produced more power than women.
The purpose of this study was to determine whether acute static and ballistic stretching had any effect on measures of strength and power, and to compare the effects of stretching between genders. Static and ballistic stretching did not cause any changes to three out of the four measures when compared with the control condition. Vertical jump, quadriceps torque, and hamstring torque values were no different with the three stretch conditions. Vertical jump was assessed with two different measures, a force plate and the Vertec VJ System. A correlation of 0.99 was found between these two measures, so the results will be discussed as one. The time lapse between stretching and performance would likely affect performance; therefore, it should be reported. The time from the end of stretching until the assessment of VJ and power was approximately 30 seconds. The assessment of torque required a more time-consuming setup, so the elapsed time was approximately 1 minute. However, the elapsed time was the same for each stretch condition; thus, it did not affect our comparison of the three types of stretching.
A number of studies have reported that static stretching had a detrimental effect on VJ (5,32,37,38) and torque output (1,7,10,15,20,21,22,24). The exact mechanism for the stretch-induced decline in performance is not known; however, authors have speculated that a decrease in muscle activation and MTU stiffness are the cause (1,5,10,15,18,22,23,33). Power et al. (24) found that static stretching of the plantar flexors, hamstrings, and quadriceps for 4.5 minutes each resulted in a 5.4% decrease in muscle activation; however, the authors were not able to determine how long this decrease lasted. Studies that found muscle activation and MTU stiffness to decline for prolonged periods involved intense stretching that is not comparable with sports stretching because of their excessive length and concentration (1,10). The results of this study conflict with these studies that found stretching to cause decrements in VJ and torque.
One reason for this discrepancy may be the difference in the design of the present study when compared with others. The present study was designed to test a practically relevant stretch duration similar to the routine used by athletes. This is the reason that subjects only stretched each muscle for 90 seconds total, in one set of three repetitions, each held for 30 seconds. Most athletes do not spend prolonged periods of time stretching before activity. The previous mentioned studies (1,7,10,15,20,21,22,24) all used study designs that required the muscle to be stretched for extended periods ranging from 8 to 30 minutes. Also, several of these studies only focused on one muscle; for example, Cramer et al. (6) had subjects perform four different static stretches, all focusing on the quadriceps; each stretch was held for 30 seconds each and was repeated four times. This stretching routine resulted in a total of 8 minutes spent stretching one muscle. The present study focused on two muscles, the quadriceps and the hamstrings, which were chosen because they could both be tested for torque output and because both function as major muscles used in the VJ. The difference in 8 minutes devoted to a single muscle vs. 3 minutes devoted to two muscles could certainly account for the differing results.
The results that static and ballistic stretching did not affect VJ or quadriceps and hamstring torque support the findings of other studies (2,3,14,24,31). Burkett et al. (2) found that a static stretching routine of the lower body, consisting of 14 stretches, each held for 20 seconds, did not cause any detrimental effects to VJ. Unick et al. (31) found that static and ballistic stretching of the lower body for 3 minutes did not decrease VJ. The results of these studies and the present study indicate that a practical stretch duration of 90 seconds, similar to the stretch duration used by athletes, can be used preactivity without adversely affecting VJ and torque output.
Wallmann et al. (32) have suggested that the present VJ results might be attributable to the specific muscles that are stretched. The present study used the same stretching duration, 1.5 minutes, as the Wallmann et al. study (32), but it did not stretch the same muscles. In the Wallmann et al. study, subjects only stretched the gastrocnemius after resting for 15 minutes after prestretch measurements, and the subjects' VJ was significantly reduced after static stretching. Similar to the present study, Church et al. (3) used static stretching of the quadriceps and hamstrings, and it was determined that VJ was not affected by the static stretching routine. Further research should be completed to compare the effects of acute static stretching on different muscle groups in regards to VJ.
One measure that was significantly affected by stretching was power. The results of the present study show that both static and ballistic stretching caused significant declines in power production when compared with the control condition that involved no stretching. Power was calculated with the GRFs that were collected from the force plate while subjects performed the VJ test. Vertical jump is commonly accepted as a predictor of power because VJ is related to leg power, which means that to perform a VJ, a person needs to effectively generate force with his or her legs at a rapid speed. Surprisingly, the VJ test was not affected by the stretching routine as power was.
It was hypothesized that static and ballistic stretching would affect VJ and power equally. This, however, was not the case in the present study. One potential explanation for this is that the VJ requires a certain amount of technique. Therefore, the VJ used in this study comprised three factors: force, speed, and technique. It is speculated that stretching does not have any effect on VJ technique. In comparison, raw power involves only force and speed. Therefore, the main difference in VJ and power is thought to be technique, and it is proposed that the stretching routine did not affect VJ because of the technique involved with the actual movement. For example, a person could adequately produce enough power to jump 30 in, but if the person's technique were poor, he or she would not effectively utilize the power to maximize jump height. In the present study, power was significantly reduced, but it did not cause any changes to VJ. It should be mentioned that the decrements, although significant, were relatively small. Static stretching only caused a 3.4% decline, and ballistic stretching only caused a 2.4% decline. Power et al. (24) also found surprising results when subjects performed a static stretching routine that lasted 4.5 minutes per muscle. The results show that the static stretching routine adversely affected torque output and muscle activation of the quadriceps but did not, however, affect VJ. Power et al. (24) conclude that the VJ was not affected because it was performed unilaterally and that these types of jumps possibly benefit from compliant MTU.
Although this finding was unexpected, another recent study found a similar result with a different type of preactivity warm-up. Cormie et al. (4) tested the effect of whole-body vibration on several performance variables and found that vibration caused a significant increase in VJ yet no change in peak power. Unfortunately, the authors offer no explanation for these findings.
Another significant finding from the study was the main effect for gender. These results show that even when all values were normalized for body weight, men produced significantly higher results than did women for all four variables. Unick et al. (31) conducted a study using only women and suggested that further research was needed to determine whether stretching affects the genders differently. Through the design of the present study, it was possible to assess this. Despite the obvious performance differences between genders, it was determined that the static and ballistic stretching routine did not affect the genders differently. The performance of men was consistently and significantly higher than the performance of women for all three stretching conditions.
Results of the present study reveal that acute preactivity stretching does not affect genders differently when a practical stretch duration of 90 seconds is used. The results also reveal that acute static and ballistic stretching performed preactivity does not affect VJ and torque output of the quadriceps and hamstrings. Coaches that use stretching as a part of the warm-up can continue to do so by limiting the duration of stretching to 1.5 minutes per muscle. However, because power was adversely affected, sports that require maximal power output should not be preceded with acute stretching. Instead, it is suggested that athletes perform a whole-body continuous activity followed by dynamic stretching that involves rehearsal of sport-specific movements. Dynamic stretching can function to properly prepare the athlete's body for dynamic movements without the stretch-induced decrements that have been seen with preactivity static and ballistic stretching by improving performance (8,9,17,35). If static stretching is used as a part of a training program, it should be performed at the end of activity to increase range of motion (7,15,24) and to improve performance (12,23,28,34).
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Keywords:© 2008 National Strength and Conditioning Association
vertical jump; torque; flexibility; performance