Pre-exercise warm-up routines involving stretching exercises performed after light aerobic activity have been advocated to reduce injury and enhance athletic performance (28). Stretching exercises have been proposed to increase the range of motion about a joint (2), reduce the risk of injury (27), and rehabilitate a muscle after injury (16). Recently, however, it has been shown that static stretching (SS) before athletic performance may actually decrease performance in a number of explosive activities (4,18,30,33,35). This has led researchers to question the use of SS performed before athletic events requiring high power outputs (22).
Although there is a body of research that has identified deleterious effects of SS protocols on subsequent muscular performance, there are a number of studies that have failed to report significant reductions. For example, Behm et al. (3) reported that SS performed on the hip, knee, and ankle extensors did not compromise isometric strength of these muscles in a group of recreationally active men. Similarly, Egan et al. (10) failed to show any negative effects of SS on isokinetic strength in trained female athletes. The absence of a significant reduction in jumping performance after SS has also been reported (7,17,23,31). In a recent review of the effects of SS on subsequent strength performance, Rubini et al. (26) noted that many of the stretching protocols used in the extant literature are excessive in terms of the number of exercises performed and the duration of the exercises. Indeed, the authors summarized that the acute negative effects observed after a period of SS tended to diminish when published recommendations for stretching protocols were followed (i.e., 3 sets of stretching exercises with each lasting no more than 30 s) (5,20,24). Therefore, the specific stretching protocol used has a significant effect on subsequent athletic performance and should reflect published recommendations, particularly if findings from research are to be used by practitioners.
An understanding of the effect of SS is crucial in such sports as basketball and volleyball where increased jumping capabilities are vital. Specifically, a high vertical jump in volleyball is a critical component in hitting and blocking. The most effective spike is likely to be dependent upon body position at take-off (e.g., the height of the center of mass above the ground), vertical jump height (JH; the maximum vertical displacement achieved by the athlete's center of mass during the flight phase of the jump), and the body position adopted before ball contact (e.g., reaching with 1 or both arms). Indeed, the vertical jump is a common tool used to assess explosive strength in volleyball athletes (12,13,29). Recently, Thompsen et al. (30) reported a significant decrease in vertical JH performed 2 minutes after 3 repetitions of SS for the hip, knee, and ankle extensors held for 20 seconds in a group of female athletes. Interestingly, the athletes comprised those who could be considered as trained in the vertical jump (basketball players) and those who may not be (ice hockey players). In contrast, Unick et al. (31) reported no deleterious effects when SS were performed 4 minutes before vertical jump performance in a group of female basketball players. Therefore, skill level and familiarity with the performance test may be a significant mediator in the acute effects of SS on subsequent athletic performance (34). However, the differences in time that elapse between completing the stretches and performing the athletic movement should also be considered, and a time-series approach may be worthwhile in future research.
Despite the inconsistencies in the extant literature, some researchers conclude that a dynamic stretching (DS) routine may be a safer and more efficient alternative to SS before athletic performance and should be implemented into warm-up routines (21). Indeed, McMillian et al. (22) found that agility and horizontal jump performance were significantly improved after a series of DS compared with static stretches in a mixed group of army cadets. Similarly, Thompsen et al. (30) reported that vertical JH was significantly greater after a series of dynamic warm-up exercises compared with jump performance after SS in a group of female athletes from different sports. Elsewhere, however, Unick et al. (31) reported that vertical jump performance of female basketball players was unaffected by either DS or SS. Again, however, the time course of the responses is not well understood.
Clearly, there is a need for more conclusive research to determine the effects of different stretching programs on athletic performance in trained athletes using protocols reflecting published recommendations. This will aid practitioners in their decision-making with regard to specific exercises to adopt in the warm-up protocols. Therefore, the purpose of the present study was to determine the effect of stretching on peak JH during vertical jump performance, specifically focusing on static stretching (SS), dynamic stretching (DS), and no stretching (NS) performed immediately before countermovement vertical jumps (CMJ) in female collegiate volleyball athletes.
Experimental Approach to the Problem
This study used a randomized balanced design to investigate the effects of performing different stretching protocols (SS, DS, NS) on vertical JH in competitive female volleyball players. Five CMJ, with 1 minute rest between each trial, were performed on a force platform 1 minute after completing each stretching condition (Figure 1). Jump heights were calculated for each of the jump trials. This design allowed for the determination of the effects of the different stretching protocols on jump performance across a series of jumps.
Twelve female collegiate NCAA Division II volleyball players (mean ± SD; age 19.5 ± 1.1 yr; height 1.71 ± 0.06 m; mass 71.3 ± 8.5 kg) participated in this study, which was approved by the Institutional Review Board for the Protection of Human Subjects of East Stroudsburg University. All subjects were free from injury at the time of testing. Testing was performed before the subject's off-season training. None of the subjects were involved in a structured resistance or other training program during the time of testing. Subjects were familiar with all stretching techniques and vertical jumping and completed an orientation session on how to use the force platform before the first testing session. All subjects were informed of the methods and risks of the study and signed an informed consent. They were advised not to eat for 4 hours or engage in any resistance training 48 hours before the testing sessions.
The subjects participated in 3 CMJ testing sessions across a 3-week period. During each testing session, the subjects performed a light jog of low intensity for 5 minutes around an indoor track. Then, whichever stretching protocol they were randomly assigned to (SS, DS, NS) was performed 2 minutes after the jog. Stretching time lasted for a total 8 minutes. All subjects were supervised during the jog and stretching protocol to ensure the correct method was used. One minute after completing the specific protocol, the athletes performed a series of 5 CMJ with each jump separated by 1 minute of passive recovery (Figure 1).
The second testing session was held 1 week later so subjects had adequate time to rest in between sessions. This same method was used for the third testing session. During the second and third testing sessions, the subjects completed the other stretching protocols they had not already performed.
Static Stretching Protocol
The SS protocol incorporated active stretches of 4 muscle groups in the lower body (Table 1). The muscles stretched were the plantar flexors, quadriceps, hamstrings, and hip extensors. Each subject performed 3 sets of each stretch, holding the stretch for 15 seconds. If both legs were stretched simultaneously, there was a 20-second rest period between sets. If each leg was being stretched individually, there was a 5-second rest period in between to ensure the same 20-second rest period in between the sets. This protocol was designed to mimic an athlete's stretching regimen and reflect published recommendations (5,20,24). The stretches were chosen to actively stretch the large muscle groups of the lower body, and the total duration of the SS was 180 seconds.
Dynamic Stretching Protocol
The DS protocol incorporated 4 stretches of the same muscles groups as the SS (Table 2). Each dynamic stretch was completed over an 18-m long course to mimic that of a volleyball court. Subjects performed 2 sets of each dynamic stretch, and 2 sets of stretches were used to ensure the same total time as the SS protocol. There was a 20-second rest interval between sets of each stretch. The stretches included calf raises, butt-kicks, leg swings, and knee tucks. These stretches were chosen to mimic the SS protocol, and some were replicated from previous research (14).
No Stretching Protocol
During the NS treatment, the subjects performed a 5-minute jog followed by 8 minutes of rest, not performing any physical activity before testing.
Countermovement Jump Performance
One minute after each of the protocols was completed, the subjects performed 5 maximal vertical jumps on the force platform (Kistler type 9286AA, Winterthur, Switzerland) using a CMJ and an arm swing. Subjects rested passively for 1 minute in between jumps. Data from all 5 jumps were used in the subsequent analyses. Vertical JH was calculated using the vertical force trace. Four seconds of data were sampled at 500 Hz. A Butterworth Filter was used to smooth the vertical force trace before analysis, and residual analysis was used to select the cut-off frequency of 20 Hz.
Jump height was calculated from the vertical take-off velocity (TOV) of the center of mass. The subject's body weight was calculated by averaging the vertical force trace over the first 1 second of data collection (500 data points) when the subject was stationary on the platform. The start of the movement was identified by calculating the peak residual from the vertical force trace during the 1 second of quiet stance. This was used to identify the start of the movement, with the peak residual during quiet stance being added to or subtracted from the subject's body weight, depending upon whether the initial movement by the subject caused an increase or decrease of vertical force. From the start of the movement, the vertical force trace was taken back until a value within 1 Newton of the subject's body weight was identified, and this point was used as the start of the jump. Take-off was identified by calculating the peak residual across a 0.3-second period (150 data points) during the flight phase of the jump with the force platform unloaded. The vertical force trace was then integrated using the trapezoid rule, beginning at the start of the jump and ending at take-off. The TOV was then used in the following equation of uniform acceleration to calculate JH:
JH = TOV2/2g,
where g = 9.81 m·s−2.
The intraclass correlation coefficient for JH measured this way was 0.95, whereas the coefficient of variation was 3.9%.
All statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS for Windows, version 15.0, SPSS Inc., Chicago, IL, USA). Measures of central tendency and spread of the data were represented as means and SDs. A general linear model with repeated measures on 2 factors (warm-up condition [SS, DS, NS], and jump [1 min, 2 min, 3 min, 4 min, 5 min after stretching conditions]) was used to assess the differences in JH caused by the different stretching conditions. Pair-wise comparisons with Bonferroni corrections for multiple comparisons were used identify the differences in JH. Alpha was set at p ≤ 0.05 for all analyses.
Table 3 shows the group averages for the 5 jumps after each stretching condition. The results of the analysis of variance revealed that there were no significant differences between the peak JH achieved after the SS, DS, and NS conditions for any of the 5 jumps (p > 0.05).
Despite the lack of significant differences for the group results, there were notable changes is JH caused by the different warm-up conditions in individual subjects. Figure 2 shows the individual percent changes in peak JH achieved during the 5 jumps after the DS and SS protocols relative to the NS condition. From this graph, it can be seen that 7 subjects produced greater increases in peak JH after the DS protocol (range of increase 3-8%), whereas only 1 subject produced a greater peak hump height after the SS protocol when compared with the DS protocol. Four subjects demonstrated no difference between the stretching conditions.
In the present study, peak JH during CMJ performance was not significantly different for up to 5 minutes after both SS and DS protocols in a group of female volleyball players. Unick et al. (31) reported that vertical jump performance of female basketball players was unaffected by DS and SS when the jumps were performed 4 minutes after the stretching protocols. Others have reported that a series of dynamic exercises performed 2 minutes before a vertical jump resulted in significantly greater heights compared with SS in a group of female athletes comprising primarily basketball and ice hockey players (30). The duration of the SS protocol has been highlighted as influencing the effect on subsequent athletic performance, with longer total stretch durations tending to cause a greater decrement in performance (26). Both the present study and that of Unick et al. (31) used stretch protocols lasting a total of 180 seconds, whereas the protocol of Thompsen et al. (30) lasted 240 seconds. It is also noteworthy that women athletes that may be considered trained in the vertical jump (i.e., basketball and volleyball players) were used in Unick et al. (31) and the present study, whereas Thompsen et al. (30) used a mixture of athletes that could be considered trained (basketball players) and untrained (ice hockey players) in the vertical jump. It is probable that the familiarity and skill of the subjects in the athletic performance test, as well as the duration of the stretching protocol, influences the effects of SS.
It is interesting to note that very few studies involving women have found a reduction in jumping performance after a series of SS (e.g. 7,31). This is in contrast with studies investigating the effects of SS on vertical jump performance in men, which have invariably reported reductions in JH (8,9,36). Kubo et al. (19) investigated sex differences in the viscoelastic properties of tendon structures and found that women had decreased tendon stiffness in their medial gastrocnemius muscle as compared with the males within their study. An increase in muscle compliance has been forwarded as a hypothesis to explain the loss of muscular performance after SS (9,25). It is possible that women are less affected by SS because of their already reduced stiffness of the musculotendinous units of the targeted muscles. However, this assumes that structural as opposed to neurologic mechanisms are responsible for the reductions in muscular performance observed after a bout of SS (1,4,11). Further investigation is required to determine the source of the sex differences in response to different warm-up protocols.
Previous investigators have reported an improvement in muscular performance after a bout of DS compared with NS (6,32). The total stretching time in the present study was almost identical to that of Yamaguchi and Ishii (32), although the protocol for the DS was different. Specifically, the subjects of Yamaguchi and Ishii (32) performed 5 DS of the lower-body muscle groups, and each subject contracted the antagonist of the target muscle group intentionally, while standing, once every 2 seconds so the target muscle was stretched. This was performed 5 times slowly and then 10 times as quickly as possible without bouncing. This protocol was performed on each leg with a 20-second rest interval in between legs. The DS protocol used in the present study was slightly different in that each subject performed 4 different stretches of the lower body, also contracting the antagonist muscle so the target muscle was being stretching, but while walking. This contraction of the antagonist muscles happened in between steps of the walking phase, so the target muscle was being stretched in between steps. Each subject walked for a total of 18 m for each set, and they performed 2 sets of each stretch. The total stretching time was equivalent to that of the SS protocol used within this study. This would suggest the way the DS protocol was performed might have had an impact on the results of the study. The methods of Yamaguchi and Ishii (32) may have elicited a greater increase in muscle temperature, which may have increased performance and caused the increase in leg extension power.
Of particular interest in the present study were the individual responses of the athletes to the different warm-up protocols (Figure 2). Although not statistically significant, the majority of subjects produced their greatest JHs after DS, with the magnitudes of the improvements in some cases being greater than the typical error associated with the measurement (15). However, 4 others showed no difference in peak JH after any of the conditions used in the present study, whereas 1 subject actually improved to a greater extent after the SS protocol. Interestingly, 1 subject who produced a greater peak JH after the DS protocol relative to the SS protocol actually produced her greatest jump after no warm-up. Such individual responses are particularly relevant for practitioners but are likely to be lost in studies where only group responses are analyzed. Recently, Young (34) highlighted the possible psychological impact of changing an athlete's warm-up routine given their previous use of certain exercises, including SS. It is possible that such a response influenced the athlete's performances in the present study. Practitioners should be aware of this issue in conjunction with the likely individual responses of their athletes when they prescribe warm-up routines.
The use of SS or DS performed for 8 minutes does not significantly affect CMJ performance in female collegiate volleyball players. These findings suggest that both SS and DS exercises could be incorporated into the warm-up routines of similar athletes. However, given the individual responses noted in the present study, practitioners should consider the individual athlete and their previous use of specific exercises in their warm-up routines before eliminating or prescribing preperformance exercises.
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