Static stretching (SS) is often recommended as part of warm-up routines in preparation for athletic activity. The purported benefits of SS include increased joint range of motion (13) and a decreased risk of activity-induced muscle or joint injury (14). However, several authors have recently demonstrated performance decrements after an acute bout of stretching (3,5,7,9,15,16,18-23,25-27). If these data are to be believed, coaches might now consider eliminating stretching as part of a precompetition warm-up.
Typically, stretching is performed immediately before physical activity by both elite athletes and recreational participants. While long-term or chronic changes in flexibility have yet to be linked with declines in physical performance, multiple authors have linked short-term flexibility, because of an acute bout of stretching, with decreases in a number of physical variables such as vertical jump (4,16,26,27), 1-repetition maximum strength (15,20), maximal isometric strength (18,22,23), isokinetic torque production (5,7,9,19), peak power output (25) and measures of balance, reaction time, and movement time (3). Thus, the negative impact of stretching may potentially impact numerous athletic events.
Several researchers have also sought to determine if the impact of stretching is velocity specific. Thus, high-velocity activities, such a jumping, may be more or less impacted than slow-velocity activities such as weightlifting. Nelson et al. (18) demonstrated decreases in peak torque in 2 of 5 contraction speeds after a bout of SS. In their study, torque decreased at the 2 slowest velocities (60 and 90°·s−1), whereas torque remained unchanged at 150, 210, or 270°·s−1. Other studies (5-8,11) have also examined whether the effects of acute SS are velocity specific. Only 2 of these studies (5,7) found a reduction in peak torque poststretching, in which torque was reduced for all contraction speeds. In the remaining studies investigating a potential velocity-specific effect (6,8,11), torque was not reduced after stretching in any velocity. Thus, these data do not support a velocity-specific effect of SS in the knee extensors.
To date, most investigations into the influence of acute SS have focused on the knee extensors (quadriceps) (3,5-9,18,19,23-25,27). A few studies have examined other muscle groups including the plantar flexors (calf muscles) (1,2,12,26) and the knee flexors (hamstrings) (10,21,22). Additionally, most investigations have examined concentric (CON) or isometric muscle actions. Only 3 studies to date (6,8,10) have measured stretching-induced changes in eccentric (ECC) muscle actions in which tension is developed while the muscle lengthens. Maximal ECC strength exceeds both CON and isometric efforts. Additionally, neural control from the central nervous system may be different during ECC efforts where fast-twitch motor units are recruited instead of slow-twitch motor units (17). Thus, stretching may have different effects on ECC muscle performance when compared with CON efforts. The purpose of the present study was to examine the effects of an acute bout of moderate SS on peak torque production of the knee flexors. Furthermore, peak torque was measured during both CON and ECC muscle actions and during one slow and one fast contraction velocity.
Experimental Approach to the Problem
The current study was designed to investigate the effects of a moderate-length stretching program on peak torque production of the knee flexors at 2 velocities. Each subject completed 2 days of isokinetic testing, a control and stretching conditions, to assess changes in isokinetic strength after a 3-minute SS protocol. The order of testing days was randomized and balanced. Upon arrival to the laboratory, lower-body flexibility of the subjects was assessed by the sit and reach test. Isokinetic strength of the right knee flexors was evaluated for both CON and ECC muscle actions and at 2 velocities, 60 and 210°·sec−1. Contraction testing order was randomized and balanced. After isokinetic pretesting, the subject completed either the SS protocol or the control condition. Posttesting consisted of another sit and reach test, to determine if flexibility had changed, and isokinetic posttesting of the knee flexors (Figure 1).
Twenty-nine subjects (16 men and 13 women) volunteered to participate in the study. The mean (±SD) age, height, and weight of subjects were 27.2 ± 5.2 years, 175.3 ± 7.7 cm, and 77.7 ± 18.8 kg, respectively. All subjects were recreationally active and free of injury that would prevent maximal effort during testing. Subjects were informed of the experimental risks and signed an informed consent document before the investigation. The study was approved by the University of Kentucky Institutional Review Board for use of human subjects. All data were collected in the spring 2005 semester.
Each subject completed 2 days of isokinetic testing within 1 week. Both control and stretching conditions, to assess changes in isokinetic strength because of the SS protocol, were completed by each subject. The order of testing days was randomized and balanced. On both days, subjects arrived at the laboratory and completed a light intensity, 5-minute warm-up on a cycle ergometer. Lower-body flexibility was then assessed by the sit and reach test. For this assessment, subjects were asked to sit on the floor, straighten their legs, and place both feet flat against the sit and reach box (Acuflex I; Novel Prod, Inc., Rockton, IL, USA). With a straightened back, and hands flat on top of one another, subjects were instructed to reach toward their toes as far as possible without bending their knees or bouncing. An investigator held both knees to discourage bending. The best of 3 trials was used for data analysis.
Isokinetic strength of the right knee flexors was assessed for both CON and ECC muscle actions at 2 velocities, 60 and 210°·sec−1 using an isokinetic dynamometer (Biodex Medical Systems, Shirley, NY, USA). To determine peak torque, the subject performed 3 maximal efforts for each muscle action (CON 60, CON 210, ECC 60, and ECC 210) with a 1-minute rest interval between contractions. Contraction testing order was randomized and balanced between subjects. The highest torque value for each muscle action was used for data analysis. Intraclass coefficient (R) values for peak torque production were 0.95. 0.93, 0.71, and 0.81 for the CON 60, CON 210, ECC 60, and ECC 210 contraction types, respectively.
After isokinetic pretesting, the subject completed either the SS protocol or the control condition, which consisted of 10 minutes of sitting. Posttesting consisted of another sit and reach test, to determine if flexibility had changed, followed by isokinetic posttesting of the knee flexors.
Static Stretching Protocol
The first 5 minutes of the SS protocol included a brief rest for the subjects and time to relocate from the isokinetic device to the stretching area. The SS protocol consisted of 2 stretches, an unassisted modified hurdler stretch and an assisted supine hamstring stretch. The modified hurdler stretch was achieved with the subject sitting on the floor with one leg bent so that the sole of foot was next to the thigh of the opposite leg. The extended leg was then stretched by having the subject reach for their toes (Figure 2). Once the subject reached a stretched position, the stretch was held for 30 seconds. Each stretch was taken to the point of discomfort as measured by intensity 15 of a maximum 20 on the Borg Rating of Perceived Exertion Scale. After a 15-second rest, the stretch was repeated for a total of 3 repetitions.
The assisted hamstring stretch was performed with the subject on their back in a supine position. The leg to be stretched was lifted by an investigator and stretched toward the subject's head while the unstretched leg remained on the floor. Resistance for the stretched leg was provided by the investigator grasping the foot and holding the knee in an extended position. Stretch intensity was maintained at a rating of 15 on the Borg Rating of Perceived Exertion Scale. The stretch was held for 30 seconds. After a 15-second rest, the stretch was repeated for a total of 3 repetitions. Both legs were stretched during the SS protocol, with the right tested leg always stretched before the left. Three minutes of stretching occurred per leg for a total of 6 minutes for the SS protocol.
All data are presented as mean ± SEM. The change in sit and reach score and isokinetic strength values from pretest to posttest were analyzed using a 2-way repeated measures analysis of variance (ANOVA). Statistical significance was set at p ≤ 0.05. Post hoc tests were performed using paired t-tests. Power analyses indicated that our study had 80% power to detect changes in isokinetic torque production as small as 3.9 N·m or a difference of 4.5% for the CON muscle actions and 8.9 N·m or a difference 6.5% for the ECC muscle actions.
The results of the current study indicate that the SS protocol did significantly increase lower-body flexibility when compared with the control condition (mean change in sit and reach score, 2.6 vs. 0.5 cm; p = 0.000). For all conditions and muscle actions, posttesting peak torque values were lower than pretesting values. Results of the repeated measures ANOVA indicate that there was no significant main effect of SS on peak torque production (Table 1 and Figure 3). Mean change in torque for the control condition (mean decrease = 6.3 N·m) was not significantly different from the stretch condition (mean decrease = 10.2 N·m). Additionally, the change in torque was not significantly different between CON and ECC contraction types or the slow and fast velocities. Only a nonsignificant trend (p = 0.059) toward greater diminished torque compared with the control was detected for the ECC 210 contraction type (decrease of 14.2 vs. 6.6 N·m).
As a result of the moderate SS protocol, lower-body flexibility, as measured by the sit and reach test, increased 10.5% (mean increase = 2.6 cm) when compared with the control. This improvement in sit and reach score is similar to that found in previous studies (15,20,22,23). Lower-body flexibility did not change as a result of the control condition. Thus, acute SS of 6 minutes can significantly impact lower-body flexibility.
When compared with the control condition, moderate SS did not significantly reduce isokinetic torque of the knee flexors. In only 1 of 4 contraction types (ECC 210) did stretching induce a reduction in torque that approached statistical significance. This finding suggests that high-velocity ECC muscle actions may be more susceptible to impairment from SS when compared with other muscle actions. In the only study to measure ECC torque production of the knee flexors, Davies et al. (10) also found a trend toward reduced ECC torque production after 1 minute of SS. However, Cramer et al. (6,8) did not find a significant decrement in ECC peak torque production of the knee extensors at either 60 or 180°·sec−1. Thus, only a trend toward reduced torque production was identified in 1 of 4 contractions tested in the current study.
Previous studies on a velocity-specific effect of SS have been mixed. Nelson et al. (19) found that slower velocity movements were more impaired than faster speeds, whereas multiple studies did not identify a velocity-specific effect of SS (5-8,11). In agreement with previous data, in which a velocity-specific effect of SS was not found for the knee extensors, the current study suggests only a trend toward greater impairment in torque production at high-velocity contraction speeds and only for ECC muscle actions of the knee flexors.
The results of the current study do not support the findings of previous studies in which SS significantly reduced muscular performance (2,4,5,12,15,18-23,25-27). It is important to note that several studies used a pretest-posttest design without a control condition (5,11,18,19). In the current study, peak torque production decreased from pre- to posttesting in both the control and stretching conditions indicating that the testing protocol itself contributed to the decrement in force production. Thus, any investigator using a pretest performance measure as the control will be unable to attribute any loss in performance to the stretching intervention alone.
Perhaps one explanation for the discrepancy in results is the difference in stretching protocol durations. For example, the current study stretched each leg for 3 minutes, whereas other studies have stretched the experimental leg for as little as 30 seconds (22) and a maximum of one hour (2). Thus, the effects of stretching on subsequent performance could certainly be impacted by the duration of stretching. Two studies have sought to investigate the effect of stretching volume on muscular force production. Ogura et al. (22) compared isometric knee flexion production after 30 seconds, 60 seconds, and no SS. These authors found a significant decrease in force after 60 seconds of stretching but not after 30 seconds when compared with the no stretch condition. Similarly, Young et al. (26) examined the effects of 1, 2, or 4 minutes of SS of the plantar flexors on drop jump performance when compared with a run warm-up. These authors found that both the 2- and 4-minute stretch conditions significantly reduced drop jump performance when compared with the run condition. The 4-minute condition was also significantly lower than the 1-minute stretch condition. Results from these 2 studies strongly support a volume effect of SS in which muscular performance will be further reduced with increasingly longer stretching protocols.
The impact of SS on muscular performance appears to be highly dependent upon stretching protocol duration and study design. The results of the current study indicate that moderate-length stretching, 3 minutes per leg, does increase lower-body flexibility, although is unlikely to reduce torque production of the hamstrings. It is important to note that the SS protocol did not improve torque production in any muscular contraction tested, further questioning the use of SS as part of a precompetition warm-up. If SS is to be included in a training program, it should be performed after practice or competition to avoid potential decreases in muscular performance.
We would like to thank our volunteers for their participation in the study.
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Keywords:© 2010 National Strength and Conditioning Association
maximal strength; flexibility; isokinetic strength; eccentric; concentric; range of motion