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Original Research

The Effects of Two Stretching Protocols on the Reactive Strength Index in Female Soccer and Rugby Players

Werstein, Kira M.; Lund, Robin J.

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Journal of Strength and Conditioning Research: June 2012 - Volume 26 - Issue 6 - p 1564-1567
doi: 10.1519/JSC.0b013e318231ac09
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Abstract

Introduction

Static stretching (SS) before a performance is widely practiced in the sports arena. Static stretching is commonly thought by the general population to have health-related benefits, although several studies have refuted these claims (7,13). For many years, coaches and athletes have practiced SS before competition to prevent injury, prevent delayed onset muscle soreness (DOMS), and improve the range of motion (ROM). Although SS is widely believed to prevent the occurrence of acute injury before an activity, studies have revealed that SS is irrelevant to injury prevention. Pope et al. (12) revealed “that an average of 260 hours of stretching would be required to prevent one injury.” Likewise, results do not support the use of preexercise stretching to prevent DOMS (3). High et al. (9) found that neither SS nor a conventional warm-up prevented DOMS resulting from exhaustive exercise. It is also believed that SS performed before an event will improve the permanent ROM of an athlete (14). Although this maybe true, it is more effective to perform SS with the purpose of increasing ROM at the end of the athletic event because of greater body temperatures (5). Because SS preexercise has not shown to be an effective means of injury or DOMS prevention, the question that remains is, ‘Does preexercise stretching have an effect on athletic performance?’.

Research has shown that SS may decrease performance because of the effect it has on the stretch-shortening cycle (SSC). The SS has been observed to decrease musculotendinous stiffness and reduce neural activation, which are 2 important factors in the SCC (5). The SSC relies on both of these mechanisms for its observed improvement of performance when compared with concentric-only movements; thus, SSC movements are particularly vulnerable to SS.

The SSC movements include actions such as jumping and sprinting, which are major components of field sports such as soccer and rugby. One method of measuring the SSC ability in athletes is the test of reactive strength index (RSI; [11]), which is a drop jump onto a force plate where upon contact, the subject attempts to minimize ground contact time while maximizing flight time (18). To date, there has been limited research examining the effects of different stretching protocols on SCC movements such as RSI in female athletes.

Research has shown a better alternative to an SS warm-up may be an active warm-up such as dynamic stretching (DS; [15–17]). Studies have compared DS and SS protocols on power output in recreationally active male populations and found DS to be the superior warm-up method (16,17). These studies suggest that further research is needed to investigate the effects of DS in competitive athletes in addition to the need for data on female subjects. It is important to have data on female subjects in addition to the existing body of research on male subjects, so the results can be generalized in practice to appropriate populations.

The SSC ability has been demonstrated to be a significant predictor of sprint (10) and jumping (10) performance, and SS may negatively affect SSC. Thus, alternative warm-up methods, such as DS, may be more appropriate for athletes engaged in sprinting and jumping sports. Consequently, it is important to investigate the effects of various stretching protocols before SSC performance in a sample of athletes who rely on this mechanism for success. Furthermore, there is a lack of this type of data on female subjects. Therefore, the purpose of this study was to assess the effects of SS and DS on SSC performance in competitive female field sport athletes, who routinely engage in sprinting and jumping. It was hypothesized that SS would have a deleterious effect and that DS would have an improved effect on SSC performance.

Methods

Experimental Approach to the Problem

Soccer and rugby are both field sports that rely heavily on the SSC. Explosive movements such as sprinting, jumping, kicking, and throwing the ball are significant parts of each game. To assess the effects of SS and DS on SSC ability, the subjects participated in 3 trials of the RSI, each preceded by 3 different randomized counterbalanced warm-up protocols. The RSI is a measure of SSC ability or reactive strength. One warm-up was a control, the second emphasized SS, and the third emphasized DS.

Subjects

Fifteen female Division I soccer players and female club rugby players at the University of Northern Iowa volunteered to participate in this study. The subjects were competitive athletes, who followed the same strength and conditioning program coached by the accredited university staff. Additionally, both teams were in the off-season of their training programs. Their age, height, and body mass were 20.1 ± 5.9 years, 170.5 ± 14.22 cm, 70.4 ± 22.3 kg, respectively. Approval from the Institutional Review Board at the University of Northern Iowa was obtained before data collection. All the subjects were informed of any risks associated with the participation in this study and the voluntary nature of the study. Written informed consent was obtained before any testing.

Procedures

All testing took place over a 3-week period. Three test sessions were separated by 1 week but occurred on the same weekday and time. Each test session consisted of a 10-minute general warm-up on a cycle ergometer, followed by 1 of 3 randomized counterbalanced treatment protocols, which were warm-up only (WO), SS, and DS. The WO session involved 10 minutes of pedaling on a cycle ergometer at a minimum of 70 rpm with no resistance. The SS session began with 10 minutes of cycling followed by 4 static stretches that targeted the gluteus maximus, hamstrings, quadriceps, and gastrocnemeus. Each stretch was performed 3 times, with a duration of a 30- and 10-second rest intervals between sets. The participants were instructed to stretch to a point of mild discomfort. The DS session involved 10 minutes of pedaling followed by 4 dynamic stretches that targeted the same muscle groups. The dynamic stretches included walking knee hugs, walking single leg toe touches, walking lunges, and walking single leg calf raises. Three sets of each dynamic stretch were performed with 10 repetitions per set and 10-second rest intervals between sets. The relatively equal length of time for the SS and DS protocols was aimed at controlling for the duration of the stretching protocols. The participants had a visual demonstration of the SS and DS exercises before performing the movements and were verbally instructed on how to achieve the movement with proper form.

After the assigned treatment was completed, each subject performed drop jumps from a height of 45 cm onto a force plate. This height was chosen because it is the optimal height to determine the RSI (11). Upon contact, the subject attempted to minimize ground CT (milliseconds) while maximizing FT (milliseconds). This strategy has been demonstrated to elicit the greatest RSI values (18). The participants were given 2 unrecorded practice attempts, followed by 1 recorded trial, which was used to calculate RSI as the ratio between FT and CT. Single trial RSI tests have been observed to be consistent measures of jumping ability (6).

Statistical Analyses

Descriptive statistics (mean ± SD) of all performance variables for each treatment were calculated. Basic assumptions for linear models were checked. A multivariate repeated measures analysis of variance (ANOVA) was used to determine any treatment effect for the 3 dependent variables (RSI, FT, and CT). If rejected, 3 separate univariate repeated measures ANOVAs were conducted to determine which dependent variable had a significant treatment effect. If a significant effect was detected for any dependent variable, paired samples t-tests of all possible comparisons were conducted to determine which treatment resulted in the greatest effect. The Bonferroni technique was used to control for familywise error associated with such tests (α = 0.05/3 = 0.0167).

Results

The multivariate repeated measures ANOVA indicated that a significant treatment effect existed for at least 1 of the 3 dependent variables (WO, DS, and SS), F(6, 9) = 5.75, p = 0.010. The separate univariate repeated measures ANOVA demonstrated significant treatment effects for RSI, F(2, 42) = 7.95, p = 0.002 (Figure 1A) and FT F(2, 42) = 7.43, p = 0.003 (Figure 1B) but no significant effect for CT, F(2, 42) = 1.53, p = 0.235 (Figure 1C). Basic assumptions for linear models were met. The sphericity assumption was tenable in all 3 tests. Post hoc analysis indicated that DS resulted in a significantly (p < 0.01) greater RSI and FT compared with SS and WO.

Figure 1
Figure 1:
A) reactive strength index (RSI) and B) flight time (FT) for treatment protocols warm-up only (WO), static stretching (SS), and dynamic stretching (DS; mean ± SD). The asterisk indicates a significant difference (p < 0.0167) from WO and SS. C) The contact time (CT) for treatment protocols WO, SS, and DS (mean ± SD). No significant differences between treatments (p > 0.0167).

Discussion

The purpose of this study was to investigate the effects of SS and DS on SSC performance in female Division I soccer and club rugby players. Of the 3 warm-ups assessed, DS was the best at improving RSI. Based on previous research, it was expected that SS would have a deleterious effect on SSC performance. This hypothesis was not supported because SS did not differ from the WO condition. On the other hand, DS appeared to enhance performance. More specifically, FT improved significantly, which in turn improved RSI even though there was no improvement in CT.

A possible explanation for the improvement in RSI via FT could be the more active nature of DS compared with SS that imitates the SSC. Similar to this study, Stewart et al. (15) demonstrated increased power output during a lower body squat jump exercise after an active warm-up. The authors concluded that the greater muscle temperatures enhanced the muscle conduction velocity resulting in faster activation of the muscles and greater power output. The nature of the DS, or an “active warm-up,” may have resulted in greater body temperatures compared with WO and SS resulting in increased RSI performance. The improvement in RSI performance because of increased FT and an unchanged CT is consistent with this hypothesis.

Another possible explanation for the improvement in FT may be related to the fact that one of the movements in the DS protocol mimicked the RSI test to an extent. This “practice” effect has been demonstrated to enhance performance (17). In this study, the researchers concluded that “practicing” the test had a positive effect on drop jump performance. As was seen here, Young and Behm found no significant differences in CT among the trials. Future studies should attempt to compare DS to “practice” trials and determine whether an additive effect exists with DS and “practice” trials. However, it could simply be that the sport-specific nature of DS in and of itself better prepares the muscles for movements requiring the SSC.

Although the active nature of DS is more sport specific than SS is, the DS in this study was slow and controlled and not specific to the timing and intensity of an SSC movement. This could explain why CT, which relies heavily on a quick eccentric phase, in the RSI was not significantly different between stretching protocols. Future research concerning the mechanisms underlying the enhanced SSC performance after DS should examine a DS protocol that has fast contact SSC movements, which could help clarify the possibility of a force-timing effect. Furthermore, future research should examine separately a practice effect from more general SSC specific DS warm-up protocols to distinguish effects, if any, between a mere practice effect and a general timing-intensity specific warm-up.

Yamaguchi and Ishii (16) assessed the effects of SS and DS on leg extension power and found that 30 seconds of SS neither improved nor reduced muscular performance. In the study by Yamaguchi and Ishii and the present study, DS enhanced the performance by 13.3 and 12.4%, respectively. Collectively, these studies suggest that DS is an effective technique for optimizing muscular performance. Although the mechanism by which DS improves leg extension power could not be determined from the results of either study, muscular performance may have been improved by increases in muscular temperature (2) or postactivation potentiation (1) caused by voluntary contractions of the target muscles.

Furthermore, this study used similar stretching protocols with similar results to that of the study by Yamaguchi and Ishii (16). Their sample population included recreationally active male participants, and they suggested that future studies needed to investigate the effect of DS on SSC of competitive athletes and female participants. Therefore, the results of this study are valuable to the literature pertaining to stretching protocols in that they provide data on competitive female athletes.

For several reasons, it was hypothesized that SS would negatively affect RSI. Previous research has suggested that SS impairs maximal force production by altering the length-tension relationship and weakening the central nervous system mechanism (5,17). This is independent of SS time, because the duration of the SS does not influence the degree of force loss (3). However, a study examining SS volume and intensity on explosive force production determined that stretching for a longer duration caused impairment to fast SSC muscle performance (18). Thus, the duration of the SS employed here may not have been enough to evoke deleterious effects. Another potential mechanism may be related to the aggressiveness of the stretch. Although the subjects were instructed to stretch to mild discomfort, the subjectivity of this perception may have resulted in an ineffective stretch by some of the subjects. The greater impairment from the longest stretching condition supports a volume effect (18).

In summary, SS had neither a positive nor a negative effect on RSI performance compared with that of WO. However, DS resulted in an enhancement of RSI, mainly because of greater FT. Based on the results of previous studies, we speculate that an increase in active firing of the muscles and increased body temperature was responsible for this effect. Regardless of the mechanism by which the differences were made, DS was superior to WO and SS before an SSC movement.

Practical Applications

Based on the results of this study, DS is the preferred warm-up for athletic events involving sprinting and jumping. We suggest that coaches implement DS as a part of the warm-up before practice and competition for optimal performance in power activities. Dynamic stretching should take muscles through the full ROM actively. Examples may include but are not limited to the DS treatment in this study. The SS for flexibility is more appropriate for after practice and competition because of the passive nature of this type of stretch.

Acknowledgments

We would like to fully recognize and thank Dr. Warren D. Franke for his contribution in the revision of the article.

References

1. Baudry A, Duchateau J. Postactivation potentiation in a human muscle: Effect on the rate of torque development of tentanic and voluntary isometric contractions. J Appl Physiol 102: 1394–1401, 2007.
2. Bergh U, Ekblom B. Influence of muscle temperature on maximal muscle strength and power output in human skeletal muscles. Acta Physiol Scand 107: 33–37, 2008.
3. Buroker KC, Schwane JA. Does postexercise static stretching alleviate delayed muscle soreness? Phys Sportsmed 17: 65–83, 1989.
4. Cosgray NA, Lawrance SE, Mestrich JD, Martin SE, Whalen RL. Effect of heat modalities on hamstring length: A comparison of pneumatherm, moist heat pack and a control. J Orthop Sports Phys Ther 34: 377–384, 2004.
5. Cramer JT, Housch TJ, Johnson GO, Miller JM, Coburn JW, Beck TW. Acute effects of static stretching on peak torque in women. J Strength Cond Res 18: 236–241, 2004.
6. Flanagan EP, Ebben WP, Jensen RL. Reliability of reactive strength index and time to stabilization during depth jumps. J Strength Cond Res 22: 1677–1682, 2008.
7. Gleim GW, McHugh MP. Flexibility and its effects on sports injury and performance. Sports Med 24: 289–299, 1997.
8. Harrison AJ, Keane SP, Coglan J. Force-velocity relationship and stretch-shortening cycle function in sprint and endurance athletes. J Strength Cond Res 18: 473–479, 2004.
9. High DM, Howley ET, Franks BD. The effects of static stretching and warm-up on prevention of delayed-onset muscle soreness. Res Q Exerc Sport 60: 357–361, 1989.
10. Kraemer WJ, Newton RU. Training for the improved vertical jump. Sport Sci Exchange 7: 1–12, 1994.
11. Newton RU, Dugan E. Application of strength diagnosis. Strength Cond J 24: 50–59, 2002.
12. Pope RP, Herbert RD, Kirkwan JD, Graham BJ. A randomized trial of pre-exercise stretching for prevention of lower-limb injury. Med Sci Sport Exerc 32: 271–277, 2000.
13. Power K, Behm D, Cahilli F, Carroll M, Yong W. An acute bout of static stretching: Effects on force and jumping performance. Med Sci Sport Exerc 36: 1389–1396, 2004.
14. Roberts JM, Wilson K. Effect of stretching duration on active and passive range of motion in the lower extremity. Br J Sport Med 33: 259–263, 1999.
15. Stewart D, Macaluso A, De Vito G. The effect of an active warm-up on surface EMG and muscle performance in healthy humans. Eur J Appl Physiol 89: 509–513.
16. Yamaguchi T, Ishii K. Effects of static stretching for 30 seconds and dynamic stretching on leg extension power. J Strength Cond Res 19: 677–683, 2005.
17. Yamaguchi T, Ishii K, Yamanaka M, Yasuda K. Acute effect of static stretching on power output during concentric dynamic constant external resistance leg extension. J Strength Cond Res 20: 804–810, 2006.
18. Young WB, Behm DG. Effects of running, static stretching and practice jumps on explosive force production and jumping performance. J Sports Med Phys Fitness 43: 21–27, 2003.
Keywords:

dynamic; static; force

Copyright © 2012 by the National Strength & Conditioning Association.