The specific contribution of flexibility training toward increasing athletic performance has not been conclusively determined. The two goals of a flexibility program (i.e., chronic stretching) are to decrease injury (14,15) and increase performance (9,17). It has been hypothesized that if chronic stretching increases the compliance of the muscle, then the energy needed to move the limb may be reduced, causing an increase in the force and/or rate of muscle contraction (16). Although the acute effects of static stretching on force and performance have recently been found to be detrimental (16), fewer studies (4,6-8,10-13,19) have investigated how chronic stretching affects performance.
Worrell and colleagues (19) reported an increase in peak knee flexion torque after 3 weeks of static hamstring stretching in subjects with limited hamstring flexibility, even though no significant changes in flexibility were found. A study by Hortobahyi et al. (8) found an improvement in repeated isometric contractions with concurrent flexibility increases but did not find increases in knee extension maximal voluntary contraction (MVC) force. Similarly, Guissard and Dechateau (7) showed an increase in ankle flexibility and passive stiffness but no changes in MVC force, torque, or rate of force development at the ankle after 6 weeks of calf stretching. Another study (3) reported that hip flexibility increased after 4 weeks of stretching, but increases in knee extension and flexion MVC isometric force were not found. Klinge and associates (12) compared a strength training program with a strength and flexibility training program. They reported that the addition of a flexibility component did not provide an added benefit to isometric MVC or muscle stiffness. A recent review by Shrier (16) concluded that chronic stretching increased isometric force production and velocity of contraction. The results from these studies show somewhat consistent performance improvements during single joint movements, which may or may not translate to improvements in gross athletic performance measures such as sprinting or jumping.
Studies investigating the effect of chronic stretching on overall performance have utilized many different outcome measures, finding varying results. Two studies (6,13) found that running economy was not enhanced, although flexibility was increased. One of these studies (6) also found that walking economy was not improved. Similarly, Kerrigan et al. (11) found that comfortable walking speed was not changed in an elderly population. They did, however, find increases in hip external extension torque during walking and hip extensor range of motion (ROM). Sprinting and vertical jump performances, and the effects of chronic stretching on them, have been investigated in only three studies (3,4,10). One study (10) investigated the effects of a 10-week stretching program on vertical jump performance in a countermovement jump (CMJ) and drop jump (DJ). Stretches were performed for the hamstrings, quadriceps, hip extensors, hip adductors, hip abductors, and plantarflexors 4 days per week. Increases in CMJ and DJ performance were found after the stretching protocol compared with the control group; however, the authors concluded that these were not significant. Similar to this study, Behm et al (3) found that 4 weeks of static stretching (hamstrings, quadriceps, plantarflexors) 5 days per week did not increase CMJ or DJ performance. Dintiman (4) found that 8 weeks of static stretching and sprint training increased flexibility and running speed significantly compared with the control group. Significant differences in running speed were not found between the stretch and sprint training group and the sprint training-only group. These results suggest that the effects of chronic stretching on overall performance remain inconclusive.
Although it is performed regularly by both recreational athletes and athletic teams as a regular part of a training program or warm-up, chronic static stretching has only shown improvements in single joint movements. Additionally, the paucity of studies investigating the effects of chronic stretching on gross athletic performance does not allow for a conclusion to be made on its efficacy. Therefore, the purpose of this study was to examine the effects of a 6-week chronic static stretching program on hamstring ROM, vertical jump height, and sprint performance in athletes.
Experimental Approach to the Problem
The primary research hypothesis of this investigation was that a flexibility training program would increase ROM, sprint speed, and vertical jump height. ROM was recorded to measure the effect of the stretching program on individuals, whereas the sprint and jump measures were recorded because they most closely resemble track and field events. Subjects were randomly sorted into two groups (10 stretching and 11 control). The control group was given instructions to not perform any stretching (chronic or acute) of their hamstring muscles on their own, but they were allowed to stretch any other muscles they wished. All participants performed the same warm-up routine; however, the stretching group performed the designated flexibility training as part of their warm-up before participating in event-specific training. Stretching was performed after aerobic warm-up to reduce the risk of injury during stretching and increase muscle compliance. Additionally, stretching is normally performed after warm-up and before event- or sport-specific training in most practice environments. The stretching group was instructed by the investigators as to how to stretch their hamstrings. This stretching protocol consisted of four repetitions held for 45 seconds (with 45-60 seconds' rest) on each leg, slightly exceeding previous minimal recommendations (1,2) to promote flexibility in the trained study sample. Subjects were positioned standing with their heel on an elevated surface (approximately 8-12 inches), toe slightly dorsiflexed, leg fully extended, and pelvis in anterior tilt, square to their foot (see Figure 1). Subjects then forward flexed their trunk toward their foot while keeping their back straight until mild discomfort was felt in the hamstring muscle group. This protocol is similar to those used by Sullivan et al. (18), which were shown to be effective in increasing ROM. Stretching was performed on 4 consecutive days per week for 6 weeks. All stretching subjects were observed during the entire protocol by the investigators or research assistants to ensure proper technique.
Twenty-one division III women's track and field athletes (mean age ± SD 18.57 ± 0.73 y; height 140.26 ± 18.63 cm; weight 66.43 ± 2.56 kg) voluntarily participated in this study and provided written consent. Subjects were members of a program that finished fifth and third at the division III national indoor and outdoor championships, respectively, the year following this study. Additionally, four subjects qualified for one or both of the aforementioned competitions. All subjects were free of injury that resulted in athletic time loss in the last 3 months. This group of collegiate track athletes (pole vault, hurdles, jumps, sprints, throws) was chosen because of their familiarity with sprinting and jumping (no distance-trained individuals participated). These individuals were generally experienced with the tested movements and performed them frequently. Stretching and control groups were evenly matched for event. All individuals were involved in a sport-specific pre-season conditioning program, which included standardized resistance training prescription, while participating in this study. The conditioning program contained no prescribed stretching, and subjects participated in practice-like activities specific to their event(s). This study was approved by the University of Wisconsin-La Crosse institutional review board.
Each subject was tested before the first week, at 3 weeks, and after the last day of the study. No training or stretching was performed on testing days, and all subjects were tested on the same day, at the same time of day, in the same indoor track facility. Subjects participated together in a warm-up progression specific to sprinting before testing (approximately 10 minutes). After the group warm-up, subjects were allowed to perform self-selected exercises in preparation for sprinting. We assumed that each individual could determine her own “warmed-up” status most efficiently and that this would allow them to perform their maximal performance on the single sprint test because of the regularity with which this type of activity is performed. This warm-up did not include any static stretching exercises so that acute negative effects (16) would not contaminate our results.
Immediately after the warm-up, subjects' flexibility was tested using the active knee extension test (AKET). Although this test tends to underestimate flexibility (5), it was utilized instead of a passive test to reduce the chance of tester interference through excessive force. Additionally, we wanted a test measuring the active ROM to which the subjects may be exposed during practice or competition. The Pearson reliability coefficient of this method was shown by Gajdosik and Lusin (5) to be 0.99 (n = 30). During this test, subjects lay supine and were instructed to extend their lower leg while they also stabilized their thigh at 90° of hip flexion with their hands (see Figure 2). The investigator observed the subject to ensure that they kept their hip flexed to the appropriate angle. The subjects extended their knee four times, as far as possible without eliciting reflexive contraction, and the final ROM was measured during the fourth extension. Measurements were made with a handheld goniometer using the lateral malleolus, lateral knee joint line, and greater trochanter as landmarks. All AKET measurements were taken by the same investigator (M.H.G.), who was blinded to group. Although the thigh was not stabilized by a bar, as in previous research (1), the intraclass correlation coefficients (ICC) for the AKET in this study were 0.90 and 0.91 for left and right leg measures, respectively.
Subjects then performed the 55-m sprint and vertical jump tests. These tests were utilized to provide an analysis most comparable to overall performance in this population. Subjects started the sprint test in a three- or four-point position with a finger on the timing device, Speedtrap II Wireless Timing System (Brower Timing Systems, Draper, UT). Subjects were asked to simulate competition as closely as possible, including preparation (see Testing Procedures) and start position. Timing gates were placed at 55 m, and each subject's time was recorded as she ran through the gates. A single test was utilized to simulate a maximal performance much like that of a race and to eliminate any fatigue that may be incurred as a result of multiple trials. The ICC and ICCR values for the 55-m test were 0.93 and 0.46, respectively. A Kistler Quattro Jump Force Plate (Kistler Instrument Corp., Buffalo, NY) was used to test vertical jump height. Subjects stepped on the plate and were instructed to place their hands on their hips. This position was used to eliminate any effects from an added arm-swing movement. Each subject performed two countermovement vertical jumps, the average of which was used for analysis. The ICC for this test was 0.94. The test-retest values for the 0-, 3-, and 6-week CMJ tests were 0.83, 0.91, and 0.92, respectively.
A two-way repeated-measures analysis of variance design (2 × 3) was used to analyze the data. Differences between each group (treatment and control) and among testing sessions (pre, mid, post) were investigated. A t-test was performed at baseline to ensure that groups (treatment and control) were similar. An α value of P ≤ 0.05 was considered statistically significant for all comparisons. Bonferroni post hoc analysis was used for multiple comparisons when appropriate.
The four variables measured were left and right knee ROM, 55-m sprint time, and mean vertical jump height. t-tests showed no significant differences between groups in any of the four variables at baseline.
No significant differences were found with the four variables between the stretching and control groups. The stretching group showed no significant mean increases in left (1.7 ± 4.6°) and right (2.1 ± 5.4°) knee ROM over the 6-week period, whereas the control group showed no significant decreases in left (−4.6 ± 6.6°) and right (−5.0 ± 7.2°) knee ROM (F1,25 = 2.512, P = 0.129 and F1,25 = 2.176, P = 0.157; respectively). Changes in ROM for both legs, shown as an increase or decrease in degrees from the pre value (baseline measure at week 0), can be seen in Figures 3 and 4. As for the performance variables, significant changes were also not found between groups in 55-m sprint time (F1,25 = 0.214, P = 0.649) or vertical jump (F1,25 = 0.002, P = 0.961). Performance changes for sprint time and vertical jump can be found in Figures 5 and 6, shown as a percentage of the baseline measure. Performance and ROM means and standard deviations of the stretching and control groups are outlined in Table 1.
The results of the current investigation suggest that chronic static stretching has no effect on sprint or vertical jump performance. However, the 6-week flexibility program did have a small, non-significant effect on ROM. The results of this study, that flexibility training does not increase vertical jump (3,10) or sprint (4) performances, are supported by previous studies. Whereas the current study used a single muscle group, previous studies (3,4,10) did not show any added benefit of stretching performed on multiple muscle groups (i.e. quadriceps, plantarflexors). These studies (3,4,10) also showed that chronic flexibility (3, 4) or stretch tolerance (10) increases do not result in improvements in athletic performance. Additionally, compared with the current study, only Hunter and Marshall's (10) flexibility program contained a greater number of training sessions, and only the current study utilized competitive athletes as subjects. These differences create much variability between the previous and current studies; however, some comparisons and conclusions can still be made.
In line with the hypothesis that chronic stretching increases muscle compliance and force via a reduced energy requirement (16), it may be suggested that performance improvements were not found in the current study because of the lack of ROM improvements. However, compliance was not measured, and this hypothesis is in contrast to previous studies (3,6-8,10,11,13) that have found flexibility improvements without performance changes. Hunter and Marshall (10) found that a 10-week stretching protocol did not change the level of eccentric lower-limb stiffness in the CMJ, although jump height improvements were found. They speculated that increased compliance of the series elastic component (via an increase stretch tolerance), in combination with unchanged eccentric lower-limb stiffness, resulted in greater eccentric force because of an increase in the stored energy. These same results did not occur during DJs, leaving the investigators to conclude that stretching may provide performance benefits to stretch-shorten cycle tasks, but further research is needed to determine the extent of benefits, if any do exist.
Hunter and Marshall (10) hypothesized that the possible performance benefits from chronic stretching would more positively affect inflexible individuals. Although it seems that individuals with reduced flexibility would benefit the most from stretching, only one study (19) has investigated the performance effects of chronic stretching in this population. Because many of the subjects in this study had been stretching regularly before participating in the study, they may have already achieved their “optimal” ROM. However, the control group experienced a greater decrease in flexibility than the small increases seen in the stretching group with no concurrent changes in performance. Therefore, competitive athletes with normal ROM may not benefit from regular stretching, nor would they experience performance decreases from the cessation of a stretching program.
Stretching as a part of a warm-up or training program has long been accepted without scientific proof of its effectiveness. In agreement with past research, the results of this study that chronic stretching does not enhance overall performance, but much more research is required to further investigate this under-studied, often-used activity. Further studies need to investigate differing chronic stretching protocols (e.g., muscle groups, time of stretch, length of program participation) and their effects on all athletic activities (e.g., swimming, running) and populations (e.g., inflexible, recreational, and/or aging athletes).
A warm-up or training program that includes a flexibility component is often thought of as a critical aspect of training or preparation for physical activity, with the goals being decreased injury occurrence and enhanced performance; however, this notion may need to be rethought. Chronic static stretching did not elicit an added benefit to the performance of women's track and field athletes in this study; however, this may not be representative of all athletic populations. If stretching is included in practice, this study has shown that it does not negatively affect performance if performed during the warm-up. Stretching should not be performed before competition (16) so that negative acute effects reported throughout the literature (e.g., reduced force and power) are not experienced. An individual with limited ROM whose sport or event requires flexibility must also weigh these effects against the needs of their activity. If a hurdler cannot get his or her leg over the hurdle without stretching, then an acute bout of stretching might allow performance. Coaches and practitioners should consider the requirements of each individual athlete's sport or event when deciding the most appropriate stretching scheme. Individuals with injuries or restrictive flexibility may also still find that chronic stretching is beneficial.
The authors would like to thank Coach Patrick Healy for the use of his “classroom” in which to perform our research. We would also like to thank Dr. Abdulaziz Elfessi and Dr. W. Holmes Finch for their assistance with data analysis. Last, we must thank our research assistants, Deborah Bahr and Jason Bahls, for their help with this study. Without their help, this study would not be possible.
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Keywords:© 2008 National Strength and Conditioning Association
flexibility; range of motion; power; maximal force; strength