Stretching prior to physical activity is a common practice believed to reduce the risk of injury and enhance performance. While there are many types of stretching exercises used during the warm-up, static stretching is the easiest and most frequently used stretching method. The intent of static stretching during the warm-up is to improve the range of motion (ROM) of a joint to allow maximal force production, increase performance in the activities that follow, and reduce the possibility of muscle tears during activity. A number of studies have concluded that stretching enhances performance (21) and prevents injuries (18). In recent years, however, the beneficial effects of stretching have undergone considerable scrutiny. Although numerous studies have shown that stretching can induce muscle strength and force deficits (3,6-8,12,23) that may last for up to 2 hours post-stretch (16), others (2) find no detrimental or beneficial effect on performance.
Many of the discrepancies between reports on the effects of stretching may be due in part to the varying volume of stretching performed in each experiment. Much of the research conducted on force production following stretching has included stretching protocols that last 20-30 minutes in duration but only focus on 2 or 3 muscle groups in the lower limbs (3,8,12,16). Although these stretching protocols produce a decrease in strength or force, they are not representative of the action that a muscle typically undergoes before a bout of exercise. As a result, the significance of these studies to understanding the effects of a typical static stretching protocol may be limited by the extreme duration of the treatment procedures that may never be done in a practical setting.
Very little research has been done describing the effects of a stretching protocol of a moderate duration on force production. The few studies (14,22) that used 3 sets of static stretches held for 15 or 30 seconds and found a decrease in jumping performance allowed the use of a countermovement as opposed to a pure concentric contraction when assessing vertical jump performance. A countermovement allows the muscle to be prestretched before it is allowed to shorten, a process known as the stretch-shortening cycle (SSC). Muscles undergoing the SSC generally exhibit a higher contractile performance than it does without prestretch (19). When a pure concentric type jump was used, no significant decrease in jumping performance was found (11,22).
In addition to the variation in stretching volumes, differences in testing protocols may cause some of the disparity in the results of the effects of stretching on force production. Most testing procedures immediately follow the stretching treatment and require the individual to perform an isometric maximal voluntary contraction (MVC) (2,8). Surface integrated electromyographic activity and muscle inactivation have also been used (3,16). However, Power et al. (16) concluded that static stretching may impair isometric force production for up to 120 minutes, which could negatively affect the results of the testing procedures using MVC, interpolated twitch technique (ITT), or isometric procedures. Other researchers administered isokinetic torque production protocols at various velocities (6,15). As a form of functional testing, 1 repetition maximal (1RM) protocols have been used (12), while others used vertical jump performance (5,11,16,22,23) to examine the effects of stretching on force production.
There have been significant advances made in the past decade to uncover the effects of acute static stretching on skeletal muscle function, especially force production. Competitive and recreational athletes rarely complete the prolonged duration of stretching protocols typically used in the research setting, and therefore little of this knowledge can be applied in a functional setting. In order for this knowledge to translate to a functional setting, it needs to be analyzed with a practical volume of stretching. Therefore, the purpose of this study was to examine the relationship between varying amounts of acute static stretching on vertical jumping performance. By systematically increasing the amount of stretching, possible differences in jump height may be discovered, defining a line where acute static stretching becomes detrimental to performance. It was hypothesized that increasing the amount of stretching will lead to a progressive decrease in vertical jump height.
Experimental Approach to the Problem
A within-treatment study was designed to determine what effects varying amounts of acute static stretching has on vertical jump performance. All participants completed 3 different stretching treatments and 1 control treatment, performed on different days 2-4 days apart in a counterbalanced order. Treatment was done on the quadriceps, hamstrings, and plantar flexors. Each session consisted of a 5-minute warm-up on a commercial upright cycle followed by a 4-minute rest period, which involved slow walking to minimize any reduction in body temperature (22). Participants then performed 3 trials of a vertical jump test before performing one of the treatment protocols. After another rest period, a second set of vertical jump trials was performed as a measure of vertical jumping performance (Figure 1).
Ten healthy male collegiate athletes and 10 healthy male recreational athletes (age, 20.25 ± 1.29 years; height, 183.52 ± 7.94 cm; weight, 88.30 ± 11.10 kg) participated in the study. All subjects were recruited as volunteers from the football team and student population at the University of Toledo. Permission for participation was obtained from the subjects and the University of Toledo Human Subjects Research Review Committee.
Participants performed a 5-minute submaximal warm-up on a Precor C846 Commercial Upright Cycle (Precor USA, Woodinville, WA) and pedaled at 70 rpm at the first resistance setting to increase muscle temperature, as adapted by Power et al. (16). This warm-up protocol has been shown to cause a slight increase in muscle temperature, which has been recommended for optimal performance (4). Four minutes of rest was used because it has been shown that that time interval is more than sufficient to allow muscle recovery from the dynamic warm-up (10,13,17) while minimizing any reduction in muscle temperature.
The stretching protocol used an active stretch of the primary muscle groups involved in the vertical jump, which include the quadriceps, hamstrings, and plantar flexors. Each stretch was held for 15 seconds with a 15-second rest period between each stretch. The order in which the muscle groups were stretched was randomized. All stretches were held to the onset of tension, which was described to the subjects as stretching the muscle to the greatest voluntary length beyond which the subjects would feel pain might occur. The sit-and-reach stretch was used to stretch the hamstrings. This was done by having the subjects sit on the floor with their legs extended and then having them reach out with theirs hands to their feet while lowering their head to their knees.
The quadriceps stretch was performed by having the subjects stand upright with 1 hand against a wall surface for balance. The subjects then flexed their knee to a 90° position and grabbed the ankle of the flexed leg. The subjects then raised the foot so that the heel was as close as possible to their buttocks while extending the hip of the flexed leg.
In order to stretch the plantar flexors, the standing straight knee stretch was used. The subjects faced a wall and leaned against it with outstretched arms while bending the front leg at the knee at approximately 90° and keeping the other leg fully extended behind the body. The heel of the back leg remained in contact with the floor at all times while the subjects dorsiflexed the ankle of the extended back leg.
Although the change in flexibility or ROM was not assessed, previous studies determined that static stretching regimens similar to that used in the current protocol was sufficient to increase flexibility and ROM (12,16).
In the control condition, participants were given a 15-minute rest period, comparable to the time it took to complete the longest stretching treatment. Subjects performed the 3 jump trials following the resting period.
Vertical Jumping Test
All squat jumps were performed with both legs placed shoulder width apart on the Just Jump contact mat system (Probotics, Huntsville, AL). The apparatus is a 27-inch square mat attached to a handheld computer. Microswitches embedded in the control mat time the interval from when the feet first lifted off to their return to the mat. This information is automatically entered into the computer as the equation 1/8(g·t2), where g (the acceleration due to gravity [9.81 m·sec−2]) and t (the air time) determines the jump height. Following the vertical jump, the computer instantaneously displays both the air time (0.01 second) and the height of the jump to the nearest half inch.
Participants held a static squat position at approximately a 100° knee angle for 2 seconds. The subjects jumped for maximum height as fast as possible while extending their legs and were allowed the full use of their arms (Figure 2). Three trials were performed, and the average was retained as a representative of the result. A 1-minute rest period was given between each jumping trial.
All subjects were familiar with the vertical jump technique because it was used as an assessment tool in their sport or were given the opportunity to practice, thus reducing any learning effect. The squat jump eliminates an active prestretch of the musculature and therefore only used concentric contraction.
A 1-way analysis of variance (ANOVA) with 1 repeated measure (pre versus post) was performed to determine whether significant differences in vertical jump performance existed between the different treatment protocols (Condition) and time (Pre/Post). A significant main effect was analyzed using the Student-Newman-Keuls post hoc multiple comparison test. In a separate group of subjects (n = 10), the coefficient of variation and the intraclass coefficient was determined for 10 repeated vertical jumps. The level of significance was set a priori to be p ≤ 0.05.
All subjects' average data were analyzed together, with the means of each group shown in Table 1. Figure 3 is a graphic representation of the data. The results of the ANOVA and subsequent multiple comparison test showed that Post-6 sets, vertical jump height was significantly lower than Pre-6 sets (p ≤ 0.05). In fact, Post-6 sets vertical jump height was significantly lower than Pre-4 sets, Pre-2 sets, and Pre-Control (p ≤ 0.05). No other conditions were significantly different.
For reliability purposes, all the Pre- conditions were analyzed for each volume of stretching to ensure that those averages were all within 1 SD of each other. If a subject's data fell outside the SD, the subject was retested for that volume of stretching. Only 3 conditions, all in different subjects, had to be retested. In addition to the retesting, an additional 10 subjects with physical characteristics of those included in the original study were recruited to verify the reliability of the timing mat and jumping approach used in this study. After allowing sufficient opportunity for learning and practicing the vertical jump procedure, each subject performed 10 vertical jumps with no less than 2 minutes of rest allowed between attempts on a separate occasion. The mean coefficient of variation for the group was 1.8% (range, 1.5-2.2%) and the ICC was 0.98 indicating good reliability.
The major finding of this study was that there was a significant difference between the Pre-6 sets and Post-6 sets of stretching, indicating that 6 sets, or 90 seconds, of stretching significantly decreases vertical jump height. This result is also consistent with previous studies (6,12,23) that reported decreases in explosive power production or vertical jump performance following static stretching of similar volumes. Young and Behm (23), who held each stretch for 30 seconds, concluded that 2 minutes of static stretch per muscle group appeared to elicit a negative influence on concentric and SSC explosive force measures and jumping performance. Kokkonen et al. (12) found a significant decrease in 1RM performance for both knee flexion and knee extension following an acute stretching treatment of 6 sets held for 15 seconds.
The nonsignificant decrease found with 2 sets of stretches was also similar to other findings, although no other studies tested a low volume of stretching similar to the protocol used in the current study (30 seconds per muscle group). Unick et al. (20) found nonsignificant decreases in the drop jump and countermovement jump as a result of a stretching treatment lasting 45 seconds per muscle (3 sets of 15 seconds). Young and Elliot (22) also found a nonsignificant decrease in the squat jump with 45 seconds of stretching per muscle group (3 sets of 15 seconds); however, the authors speculated that the magnitude of the reductions was diluted by the positive influence of the warm-up since they were able to find a significant reduction in drop jump performance with 45 seconds of static stretching. Despite this, no other studies found a significant decrease in vertical jump performance until each muscle group was stretched for 90 seconds, comparable to the 6 sets of stretches for 15 seconds that was done in the current study.
An interesting observation in the results was how much the resting period affected the control response. Although it was not a significant decrease, the control group exhibited nearly the same decrement in vertical jump performance as the group performing 4 sets of stretches. This may have been caused by a decrease in muscle temperature that occurred in the subjects during the resting period. The subjects had to rest for 15 minutes, and doing so may have allowed the body temperature, which was raised due to the warm-up and prestretch jumping, to cool. This cooling effect alters the performance of the muscle, which is enhanced at elevated temperatures, causing the decrease in the control group following the resting period.
Several authors have cited a decrease in motor neuron excitability as a possible reason for the decrease in jumping performance (3,5,8,22). The Hoffman reflex (H reflex) is a common measure used to indicate changes in motor neuron excitability. Avela et al. (1) examined the effect of prolonged and repeated stretching on the H reflex and found a depression in the H reflex after stretching. However, the effect of the stretching on the reflex was almost completely recovered 4 minutes after stretching. Guissard et al. (9) also found that the H reflex recovered quickly after stretching. The design of this study allowed for a 4-minute rest period after the stretching was completed and before the vertical jump assessment began. Therefore, a decrease in motor neuron excitability is unlikely to be the cause of any decrease in vertical jumping ability.
Unick et al. (20) allowed for 4 minutes of walking prior to the first countermovement jump performed by each subject and found no decrease in countermovement or drop jump performance immediately after stretching and up to 30 minutes post-stretch. They cited that the rest period between the stretching phase and jumping phase could have allowed a return to motor neuron excitability, causing any alterations that had occurred to return to the prestretching or near-prestretching status. This rest period may be one of the reasons for the varying results between studies, such as with Young and Behm (23), who found significant decreases in drop jump and squat jumps but only used a 2-minute rest period between stretching and jumping activities.
The answer to determining which mechanism is the primary cause of the decreased performance may lie in the amount of time passed between stretching and the vertical jumps. Studies that allowed ≥4 minutes of rest before performance testing (1,9,12,20) cited that muscle stiffness or musculotendinous compliance was likely the cause for the decreased performance. On the other hand, studies that reported neuromuscular inhibition as the primary mechanism (11,23) allowed only ≥2 minutes of rest. Therefore, neuromuscular inhibition may initially cause the decrease in performance, but after approximately 4 minutes of rest, muscle stiffness appears to be the primary cause for decreased performance.
The purpose of this study was to examine the relationship between varying amounts of acute static stretching on explosive force production and jumping performance. The underlying goal of the study was to determine how much stretching could be performed before performance would suffer. The results showed that up to 4 sets of stretches for a total duration of 1 minute per muscle group can be completed without significantly risking performance. Despite the significant strides research has made in understanding the muscular response to stretching, the effect of stretching for 60-75 seconds per muscle group on performance is still widely unknown. Future studies should examine the effect of a varying rest period duration on performance after an acute static stretching protocol. The results of such studies could lead to finding the optimal amount of static stretching and rest period duration that should be completed during the warm-up in preparation for athletic competition.
The results of this study demonstrate that lower volumes of acute static stretching can be performed without significantly compromising maximal force production. Strength and conditioning professionals can safely administer up to 1 minute of static stretching per muscle group to athletes in sports that require high levels of force production as long as the athlete has at least 4 minutes of rest after the stretch. The results also apply to recreational athletes and coaches because of the various groups tested in the study.
The authors thank each of the individuals who participated in this study.
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Keywords:© 2008 National Strength and Conditioning Association
warm-up; musculotendinous unit compliance; squat jump; rest interval