Static stretching is alleged to have several benefits including injury prevention and enhancement of athletic performance (10,20). However, several recent studies have questioned these widely held beliefs, producing results that not only contradict the perceived advantages of static stretching but also indicate that stretching can have a detrimental effect on athletic or muscle performance.
The effect of static stretching on various aspects of performance, such as maximal strength and explosive force production, has been investigated (2,3,5,6,8,9,22-26). However, the relationship between preactivity static stretching and sprint performance has been sparsely explored. Static stretching has been found to negatively impact 20-m sprint performance in collegiate track athletes (15). Sprint performance was inhibited after static stretching, and stretching the muscles in one leg had the same negative effect as stretching the muscles in both legs, suggesting a central nervous system (CNS) mechanism. The investigators also conjectured that reduced stiffness of the musculotendinous unit contributed to the decrease in performance (15). A similar relationship between static stretching and sprint performance has been observed in male rugby union players (7). While also postulating that the detriment in performance could be attributed to reduced stiffness of the musculotendinous unit, the investigators noted that static stretching changes tendon structure, increasing the compliance of the tendon and, therefore, reducing force production and delaying muscle activation.
The effect of static stretching on high-speed motor capacities (including acceleration and maximal velocity) in professional soccer players has also been investigated (14). Contrary to previous findings, static stretching was found to have no effect on sprint performance. However, in this study, acceleration and maximal velocity were measured not in one continuous sprint but as two separate components.
Although the aforementioned studies address the effect of static stretching on overall sprint performance, there is no existing research on the effect of static stretching on the different phases of a continuous sprint. The acceleration phase of a sprint can be measured for a distance of 10 m from a stationary start, and maximum velocity can be recorded for a distance of 20 m from a flying start (13). Therefore, a 30-m sprint can be divided into two phases: the acceleration phase (0-10 m) and the maximal-velocity phase (10-30 m).
The potential impact of static stretching on sprinting is important to soccer players because sprint running comprises a relatively large portion of a player's activity pattern during a game. In the game of soccer, sprint bouts occur approximately every 90 seconds, with sprinting constituting up to 11% of total distance covered (20). Additionally, 96% of sprints during a soccer game are for distances of 30 m or less. No previous studies have been performed on the effects of static stretching for a 30-m distance. Given the paucity of data on stretching and athletic performance in soccer players, and the importance of sprinting to the game of soccer, research on this topic is warranted.
Therefore, the purpose of the study was to determine which phase of a 30-m sprint is impacted by preperformance static stretching in elite female soccer players. It was hypothesized that static stretching before a 30-m sprint would result in an increase in overall sprint time compared with the no-stretch condition. Additionally, it was hypothesized that static stretching before a 30-m sprint would result in an increase in the time spent in the acceleration phase and the maximal-velocity phase of the sprint compared with the no-stretch condition.
Approach to the Problem
Twenty subjects were randomly assigned to either a stretch or no-stretch condition. For the no-stretch condition, each participant completed a standard warm-up protocol and then performed three 30-m sprints. The first sprint time was not recorded, and the best time of the next two sprints was used for analysis. When in the stretch condition, each participant completed the standard warm-up protocol before undertaking the stretching intervention and then performing three 30-m sprints. The 30-m sprint was analyzed in two phases. Acceleration was measured for the first 10 m, and maximal velocity was measured for the final 20 m.
Participants included elite female soccer players (N = 20) from a professional female team that participates in the Women's Professional Soccer League, ranging in age from 18 to 29 years. The subjects were asked to complete a questionnaire detailing playing history and experience. Participation was voluntary. Players with any form of physical injury at the time of data collection were excluded from the study. Injuries were identified through a self-reporting process. Approval was granted from the university institutional review board, and the staff of the soccer team also approved player participation in this study. Testing occurred during the third week of a 12-week season. During the season, the subjects practiced three times per week and played one game, with no additional weight training being undertaken. Each day of testing occurred after a rest day. The subjects were asked to refrain from caffeine intake on each testing day and to avoid food consumption in the 2 hours before testing.
A common limitation among studies investigating the effects of static stretching on various dependent performance variables is the lack of quantification of the stretching intervention. The vast majority of previous work describes the participant stretching or being stretched to the onset of discomfort or pain. In the present study, an attempt to quantify this intervention was incorporated by finding each participant's maximum stretch capacity of two out of the three muscles being stretched. Each participant then performed the stretches in the protocol to 85% of her individual maximum capacity.
Maximal hamstring flexibility was measured using the backsaver sit-and-reach assessment with an Acuflex 1 modified flexibility sit-and-reach test box. One leg was measured at a time. Participants sat with shoes removed and one leg extended, with the foot against the box. The leg not being measured was bent at the knee, with the sole of the foot flat on the ground. Participants then reached along the measuring line as far as possible with the hands on top of each other, palms down, and extended leg held flat. The farthest point of three attempts represented maximal hamstring stretch. The procedure was then repeated with the other leg.
Calf flexibility was measured using an adjustable tilt board. Participants were asked to stand with their back against a wall, remove their shoes, and place one foot on the tilt board, with the heel still touching the ground. The other leg was bent at the knee, with the sole of the foot flat on the wall. The angle of the tilt board was increased until the heel left the ground. At this point, the angle of the tilt board was measured with a protractor. The specific angle represented maximal calf flexibility. The procedure was then repeated with the other leg.
The third and final stretch involved the quadriceps and hip flexors and was adapted from Alter's no. 92 stretch (1). The participants began in a supine position. They flexed the knee of one leg and brought the heel back toward the buttocks. They then grasped the ankle with the hand and pulled the heel toward the buttocks. The stretch was increased throughout the range of motion until the participant acknowledged the onset of discomfort.
Each participant was required to complete three 30-m sprints from a standing start. Sprints were timed using a Speedtrap 2 automated timing device (Brower Timing Systems, Utah), using a pressure pad and two electronic timing gates. The pressure pad was placed at the start line, with the participant placing the lead foot on the pad. When the participant began the sprint, the foot released the pressure on the pad and the timer started. The electronic timing gates were set at 2 m high and 1 m apart and were placed at 10 and 30 m from the start line. The timing gate at 10 m recorded the time for the acceleration phase of the sprint (0-10 m). The timing gate at 30 m stopped the timer and recorded the total time taken to sprint 30 m. Time spent in the maximal-velocity phase was calculated by subtracting the acceleration time from the overall time.
Data collection was completed on three nonconsecutive days. On the first day, the participants completed the informed consent form in addition to the playing-history questionnaire. Baseline data were collected on this day, along with maximal stretch capacity of the hamstrings and calf muscles. On the second day, the participants were randomly assigned to one of the two conditions: stretch or no-stretch. All testing took place on an indoor, artificial playing surface, with the participants wearing their preferred indoor soccer shoes, because this type of shoe is similar to soccer cleats worn on natural playing surfaces. The participants performed the following light mobility exercises before stretching or sprinting as used by Nelson et al. (15): jog 800 m, forward skips 4 × 30 m, side shuffles 4 × 30 m, and backward skips 4 × 30 m. The no-stretch condition group then performed three 30-m sprints. The first sprint was not timed, because this was used as a familiarization exercise. The next two sprints were timed, with a rest period of 2 minutes between each sprint, as used by Fletcher and Jones (7). The best overall time of the two sprints was used for analysis. Figure 1 depicts the timeline for the procedures followed by the no-stretch condition group.
After completing the warm-up protocol, the stretch condition group performed the stretching intervention under the supervision of qualified strength and conditioning coaches.
The stretching intervention consisted of statically stretching the hamstrings, calf muscles, and quadriceps. With regard to the hamstring and calf stretches, the participants stretched to 85% of their maximal capacity for each leg and held the position for 30 seconds. The quadriceps stretch was increased throughout the range of motion until the participant acknowledged the onset of discomfort similar to that felt during normal stretching activities. The stretch was held for 30 seconds, at which point the participant stretched the other leg. The stretches were completed in a randomized order, with a 10- to 20-second rest between each stretch. Each stretch was completed once, constituting one complete stretch cycle. The cycle of stretches was completed three times. Participants in this group then performed a trial run, followed by two timed sprints, with rest periods consistent with the no-stretch condition group. Figure 2 depicts the timeline for the procedures followed by the stretch condition group. The final day of testing consisted of an identical procedure, with participants performing the opposite condition to eliminate any order-of-testing effect.
Acceleration was measured for a distance of 10 m, with the players beginning in a stationary position. Maximal velocity was recorded for the last 20 m of the 30-m sprint. Reliability of the overall sprint, acceleration, and maximal-velocity measures was assessed using an interclass correlation coefficient on the second and third sprint times recorded by each participant when in the no-stretch condition. Means and standard deviations of study variables were calculated. One-way repeated-measures analyses of variance were used to compare acceleration times between the stretch and no-stretch conditions, maximal-velocity times between the stretch and no-stretch conditions, and overall times between the stretch and no-stretch conditions. An alpha level of p < 0.0167 (i.e., 0.05/3) was used to maintain the total at 0.05 for the three tests. See Howell (10) for a description of the Bonferroni procedure.
Description of Participants
Descriptive statistics for the sample are presented in Table 1, along with means and standard deviations from each variable in both conditions. The average length of playing experience at the elite level was 3 years. Three participants were excluded from the study because of injury. The interclass correlation coefficients for 30-m sprint time, acceleration, and maximal velocity were 0.999, 0.993, and 0.991, respectively.
One-way repeated-measures analyses of variance were used to analyze the difference in sprint performance between the stretch and no-stretch conditions. The result of the F test for the overall 30-m sprint time, using Wilks' lambda, was F(1,19) = 22.18, p < 0.0167. Statistical power for this analysis was 0.99, and partial eta squared was 0.54. Static stretching before the 30-m sprint resulted in an increase in overall sprint time when compared with the no-stretch condition.
Static stretching before the 30-m sprint resulted in an increase in the time spent in the acceleration phase of the sprint compared with the no-stretch condition (F(1, 19) = 6.65, p < 0.0167). Statistical power for this analysis was 0.69, and partial eta squared was 0.26. Likewise, static stretching before the 30-m sprint resulted in an increase in the time spent in the maximal-velocity phase of the sprint compared with the no-stretch condition (F(1, 19) = 16.42, p < 0.0167). Statistical power for this analysis was 0.97, and partial eta squared was 0.46.
Static stretching before sprinting negatively affected performance. A bout of static stretching before performing a 30-m sprint resulted in a significant increase in time to complete the sprint among elite female soccer players, when compared with the time to sprint this distance without stretching. This finding supports the work of Fletcher and Jones (7) and Nelson et al. (15), who documented a similar detriment for a distance of 20 m in rugby players and track athletes, respectively. A significant difference was found in the acceleration phase of the sprint between the stretch and no-stretch condition. Static stretching had a negative affect on acceleration. Additionally, a significant difference was found in the maximal-velocity phase of the sprint, providing evidence that static stretching before performance diminishes the maximum speed of an elite female soccer player.
Mechanical (peripheral) and neurological (central) theories have been postulated as potential mechanisms behind the decrease in performance caused by static stretching. Fletcher and Jones (7) compared the detriment in running performance after a bout of stretching with the performance decreases in drop jump height found by Young and Elliott (25). Sprinting requires a rapid transition from eccentric to concentric muscle action, as does performing a drop jump, albeit with the latter using a single stretch-shortening cycle, and the former using several in continuous fashion. Fletcher and Jones (7) suggest that static stretching primarily affects the eccentric phase of the stretch-shortening cycle. During this phase, the series elastic component (SEC) lengthens, storing elastic energy to be reused in the concentric phase of the stretch-shortening cycle, when the SEC springs back to its original configuration (17). However, after a bout of static stretching, the SEC may already be lengthened, thereby impeding the preactivation of the musculotendinous unit and decreasing its ability to store and reuse as much elastic energy during the stretch-shortening cycle.
Additionally, Shorten (19) reports that the amount of elastic energy that can be stored in the musculotendinous unit is a function of stiffness. Therefore, because static stretching reduces the stiffness of the musculotendinous unit, less elastic energy can be retained and used after a bout of static stretching. This peripheral mechanism may have contributed to the decrease in overall sprint performance experienced by participants in this study.
Neurologically, it has been suggested that stretching may cause a decrease in neural transmission from the CNS to the muscle, a phenomenon known as neural inhibition (12,15,18). Specifically, stretching inhibits the myoelectric potentiation initiated during the eccentric phase of the stretch-shortening cycle, which is responsible for initiating muscle activation during the concentric phase. Hence, a decrease in performance during the concentric phase of each stretch-shortening cycle of the sprint would result. The results of the present study support the notion that a mechanical mechanism, a neurological mechanism, or a combination of both could be responsible for the decrease in 30-m sprint performance.
Given the repetitive use of several muscles during sprinting, the potential effect of stretching on muscle strength endurance cannot be ignored. Nelson et al. (16) found that static stretching resulted in decreased muscle strength endurance performance. They postulate that the observed performance decrement was a result of static stretching placing some of the available motor units in a fatigue-like state, therefore depleting the amount of motor units available for recruitment during the performance activity. This led to the onset of fatigue earlier in the activity and, therefore, to a decrease in performance. This concept can be applied to sprinting, in that overall sprint time could be negatively affected by a stretching-induced decrease in the number of motor units available for recruitment, thereby hastening fatigue and reducing sprint performance. However, whether a 30-m sprint is long enough for this particular mechanism to be considered is unknown.
Static stretching had a negative effect on the acceleration phase of the sprint and the maximal-velocity phase of the sprint. There are mechanical differences between these sprint phases. During acceleration, maximum stride length has not yet been reached. As such, in this phase of the sprint, the stretch-shortening cycle time is of shorter duration. Maximum stride length only occurs when the runner has reached peak speed-that is, during the maximal-velocity phase. Additionally, ground-contact time is longer during the acceleration phase.
From a neurological standpoint, a decrease in neural drive from the CNS to the muscle would occur throughout the course of the sprint, regardless of the speed at which the sprinter is traveling. Theoretically, during the acceleration phase, the myoelectric potentiation, a stretch reflex that increases muscle activation during the concentric phase, initiated during the eccentric phase of the stretch-shortening cycle, may not be sufficient to produce a maximal response during the concentric phase. Similarly, during the maximal-velocity phase, when both stride length and the stretch-shortening cycle duration are at their respective greatest, the stretch-induced inhibition of neural transmission may also result in an insufficient stretch reflex during the concentric phase, therefore causing a decrease in performance.
Mechanically, a muscle that has been statically stretched has more slack than an unstretched muscle (4). During the acceleration phase of a sprint, muscle contractions occur at a slower rate than during the maximal-velocity phase. Therefore, there is more time for the force generated in the muscle to be transferred to the bone to cause joint movement. However, even given the slower contraction rate, the force generated by a stretched muscle still does not match that of an unstretched muscle because of the excess slack. Because muscle contraction speed is at its greatest during the maximal-velocity phase, the detriment in performance caused by the excess slack in the muscle may hamper the maximum contraction rate to a greater extent, resulting in slower maximum muscle contraction after an acute bout of static stretching. In addition, the stretch-induced slack in the muscle may prevent maximal storage and reuse of elastic energy during the stretch-shortening cycle in both phases of the sprint. Research to identify which of these mechanisms (central or peripheral) is responsible for the deleterious effect of stretching would offer further insight into this topic. It is possible that a combination of both mechanisms could exist; further research might be needed to identify the extent to which each mechanism has an influence on performance.
Static stretching can reduce maximal running speed during a 30-m sprint. It is important that coaches, athletes, physical educators, and other persons involved in either undertaking or administering activities during which success can be dependent on maximal performance are aware of the potential negative affects of static stretching before performance or competition. The largest difference in overall sprint performance between the stretch and no-stretch condition was 0.39 seconds, and the mean difference was 0.1 seconds. In a sprint that takes 4-5 seconds to complete, this difference can be critical. The stretching-induced detriment exhibited here could mean beating an opponent to the ball to create a goal-scoring opportunity, or recovering defensively to prevent an opportunity for the opposing team. In elite soccer, the ability of each player to perform to his or her maximal potential is especially important, because even the smallest detail can mean the difference between winning and losing. Given this, gaining every possible advantage over one's opponent is vital.
Additionally, at the elite level, accelerating and sprinting at one's maximal velocity are considered to be two different aspects of speed and, as such, are trained separately. It can now be said that static stretching before training either of these aspects of speed negatively affects performance.
The results of this study, coupled with supporting research in this area, suggest that static stretching should not be included in a warm-up routine before participating in any activity that requires maximal sprinting. Previous investigators have suggested that a warm-up including dynamic exercises that replicate movements to be performed during the activity are beneficial to performance, although further investigation is needed in this area (7).
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