December 20, 2012

The effects of stretching prior to exercise on running performance. A systematic review of randomized controlled trials

Michael Rosenblat

PLAIN LANGUAGE SUMMARY

Stretching is a type of exercise that places a muscle in a lengthened position. There are two types of stretches that are commonly discussed in the literature including static stretching, a lengthening of a muscle for a long duration, and dynamic stretching, a repetitive controlled movement at the end range of motion. The purpose of this review is to systematically evaluate the available evidence on the effects evidence of stretching on markers of running performance in trained and highly-trained runners.
There is moderate quality evidence that demonstrates a decline in running performance following a static stretching program in trained, long-distance male runners. A second study of similar design, showed no difference in running performance in trained female runner.
There is moderate quality evidence from two that demonstrates a decrease in running economy shown by a change in caloric expenditure during running following a DS program in trained middle- and long-distance male runners.
There is moderate level evidence from three that suggests that stretching prior to running has no effect on perceived exertion during a 30-minute run in trained middle- and long-distance male and female runners.
Studies of greater methodological design are required to provide more conclusive results on the effects of stretching on running performance.

INTRODUCTION

Stretching is a type of exercise that places a muscle in a lengthened position. There are two types of stretches that are commonly discussed in the literature including static stretching (SS), a lengthening of a muscle for a long duration, and dynamic stretching (DS), a repetitive controlled movement at the end range of motion. Static stretching prior to exercise has been shown to decrease the risk of musculotendinous injuries (6), however, has a detrimental effect on muscular strength (4) and no effect on preventing the severity of delayed muscle soreness following exercise (3).
            Stretching prior to exercise is a controversial topic, especially in endurance sports, because there is limited evidence that supports its use. Currently there are no systematic reviews of randomized controlled trials that analyze the effects of a pre-stretching program on running. The purpose of this review is to systematically evaluate the available evidence on the effects evidence of stretching on markers of running performance in trained and highly-trained runners.

Outcomes Measures
The outcome measures included in the studies consisted of those related to running performance (distance completed), running economy (caloric expenditure, submaximal oxygen consumption), neuromuscular performance (isometric strength, countermovement jump), flexibility (sit-and-reach), and perception (rating of perceived exertion (RPE)).

Objectives
1.     Does stretching prior to exercise effect running performance?
2.     Does stretching prior to exercise effect running economy?
3.     Does stretching prior to exercise effect neuromuscular performance during running?
4.     Does stretching prior to exercise effect RPE during running?

METHODS

Search Strategy
Embase, Medline and Pubmed were the databases used to perform the literature search. The search terms included: stretching and running. Limitations included: humans, English, randomized controlled trials.

Selection Criteria
Articles were selected if they included subjects who were considered middle- or long-distance runners and between the 18 and 65 years of age. Articles were excluded if they included resistance exercise, sprinting or chronic stretching (stretching not performed directly prior to exercise).

RESULTS

Search
The literature search was conducted on December 11, 2012. Embase, Medline and Pubmed yielded a total of 24, 32 and 35 articles respectively. Five articles were selected following the removal of articles that did not meet the selection criteria.

Study Quality
The PEDro scale (score out of 10) was used to rate the quality of the articles included in this review. Four of the studies scored of 6/10 and one study scored 5/10 (Table 1).


Study Characteristics
One study investigated the effects of DS on running, three on the effects of SS on running, and one on the effects of both SS and DS on running (Table 2). The stretching exercises were performed on muscle groups including knee flexors and extensors, hip flexors and extensors and plantar flexors (Table 2). The stretching protocols are described in detail in Table 2.




DISCUSSION

Does stretching prior to exercise effect running performance?
Three of the five studies assessed the effects of stretching on running performance (5, 7, 9). Zourdos et al 2012 included a DS program, and Mojock et al 2011 and Wilson et al 2010 both included a SS program. The study by Wilson et al 2010 was the only study that demonstrated a significant difference in running performance following stretching exercises (7).
Wilson et al 2010 included ten trained (VO2max 63.8 ± 2.8 ml/kg/min) male, middle- and long-distance runners (7). The subjects completed two trials; one that included SS, the other included 15 minutes of quiet sitting (control) prior to completing a 30-minute treadmill run at 65%VO2max and a 30-minute maximal distance run (7). The quiet sitting group demonstrated a significantly (p < 0.05) greater distance covered in the maximal distance run compared to the SS group (6.0 ± 1.1 km versus 5.8 ± 1.0 km) (7). The study by Mojock et al 2011 included 14 trained female runners and had a very similar protocol, however found no significant differences between the stretching group and the control group (5).
The study by Wilson et al has a number of strengths and limitations associated with its design. One of its strengths is that the stretching program was described in great detail and can easily be reproduced. The study was of a counterbalanced design (subjects acted as their own controls), which can be seen as a strength, however, there was no blinding of subjects or assessors, which significantly increases the risk of bias. Also, due to the lack of subject blinding, it is possible that the performance results are due to the placebo effect. Finally, the study does not mention if all subjects completed each trial or how missing data was handled. Without adequate follow-up the main purpose for randomization (controlling for confounding variables) is eliminated, decreasing the validity of the results.

Does stretching prior to exercise effect running economy?
All five of the studies analyzed the effects of stretching on running economy (1, 2, 5, 7, 9). Zourdos et al 2012 and Wilson et al 2010 were the only two studies that demonstrated a significant difference in running economy following a stretching program (7, 9).
Zourdos et al 2012 analyzed the effects of a DS program on caloric expenditure during 30 minutes of running at 65%VO2max (9). They included fourteen trained (VO2max 63.1 ± 8.3 ml/kg/min), male runners who completed a similar protocol to the Wilson et al 2010 study, however performed DS prior to exercise in place of SS. The results demonstrated a significant (p < 0.05) increase in caloric expenditure in the DS group (416.6 ± 44.9 kcal) compared to the control group (399.3 ± 50.4 kcal) (9). The results of the Wilson et al 2010 study (study design and protocol described above) also demonstrated a significant (p < 0.05) increase in caloric expenditure in the intervention group (SS) (425 ± 55 kcal) compared to the control group (405 ± 53 kcal) (7).
It is possible that the increase in caloric expenditure seen in the two studies is due to a decline in running economy as a result of the pre-stretching program. Another possible explanation is that the stretching protocol altered the subjects’ resting metabolism prior to exercise, thus increasing their caloric demand. The subjects in the study by Zourdos et al 2012 demonstrated a significant (p < 0.05) increase in resting VO2 from pre-stretch values (6.2 ± 1.7 ml/kg) to post-stretch values (8.4 ± 2.1 ml/kg), while no change seen in the control group (9). These elevated values may have an affect on the total caloric expenditure during exercise, resulting in the elevated values noted in the two studies.
The results from these two studies may appear to be meaningful however, it is important to note that there was no blinding of subjects or assessors, and the authors did not mention if all subjects completed the study. As previously mentioned, this affects the validity of results and increases the risk of bias.

Does stretching prior to exercise effect neuromuscular performance during running?
Only one study, by Allison et al 2008, analyzed the effects of stretching on on neuromuscular performance. In this study, ten healthy male runners (VO2peak 60.1 ± 7.3 ml/kg/min) (1) completed a SS program prior to exercise (Table 2) (1). The subjects completed three experimental trials, two of which consisted of a 10-minute run at 70%VO2peak following either the SS program or quiet sitting. The third trial consisted of the SS program followed by a series of neuromuscular tests including sit-and-reach, isometric strength and a countermovement jump.
            The results of the study demonstrated a significant decline in neuromuscular performance in both isometric strength (p < 0.001) and in the countermovement jump (p < 0.001) (1). These results are quite significant, however, they do not assess the effects on neuromuscular function associated with running. Jumping and strength exercises requires the use fast twitch motor units, whereas running recruits an entirely different set of motor units. The authors did, however, analyze the effects of stretching on running stride and found no significant difference between the SS group and the control group (1). Running stride (length, cadence) may be a more appropriate method to functionally measure neuromuscular performance during running.

Does stretching prior to exercise effect RPE during running?
Three of the five studies analyzed the effects of stretching on RPE during running (5, 7, 9). None of the studies found a significant difference in RPE between their respective stretching groups and control groups (5, 7, 9).

Limitations
The current review systematically analyzed randomized controlled trials on the effects of stretching on running performance. One limitation to the study design is that only three databases (Embase, Medline, PubMed) were used to conduct the literature search. This may limit the number of studies included in the study and increases the risk of selection bias. The review only discusses the findings of the articles that demonstrated a significant difference in one of the four outcomes (running performance, running economy, neuromuscular function, perceptual) described above. This was done in order to generate a more condensed review, and can be considered as a form of selection bias. The main tool used to critically appraise the articles included in the study (PEDro scale) only considers 10 categories. This may decrease the quality of the appraisal, however, utilizes a tool that can provide an objective measure.
Recommendations for Future Research
Stretching prior to sport and exercise has been a controversial topic for a number of years. Over the last five years there has been an increase in the number of higher quality studies (RCTs) that analyze the effects of stretching on running performance. More research is necessary to accurately assess the effects of stretching on neuromuscular function associated with running. Finally, studies of greater methodological design are required to provide more conclusive results.  

CONCLUSION

·      There is moderate quality evidence (one RCT, PEDro 6/10) that demonstrates a decline in running performance following SS compared with a control group in trained, long-distance male runners.
·      There is moderate quality evidence (two RCTs, both PEDro 6/10) that demonstrates a decrease in running economy shown by a change in caloric expenditure during running following a DS program in trained middle- and long-distance male runners.
·      There is moderate level evidence (three RCTs, all PEDro 6/10) that suggests that stretching prior to running has no effect on perceived exertion during a 30-minute run in trained middle- and long-distance male and female runners.

REFERENCES

1.     Allison S, Bailey D, Folland J. Prolonged static stretching does not influence running economy despite changes in neuromuscular function. J Sports Sci 2008;26(14):1489–1495.

2.     Hayes P, Walker A. Pre-exercise stretching does not impact upon running economy. J Strength Cond Res 2007;21(4):1227–1232.

3.     Herbert R, de Noronha M, Kamper S. Stretching to prevent or reduce muscle soreness after exercise. Cochrane Database Syst Rev 2011;7:1–49.

4.     Kay A, Blazevich A. Effect of acute static stretch on maximal muscle performance: a systematic review. Med Sci Sports Exerc 2012;44(1):154–164.

5.     Mojock C, Kim J, Eccles D, Panton L. The effects of static stretching on running economy and endurance performance in female distance runners during treadmill running. J Strength Cond Res 2011;25(8):2170–2176.

6.     Small K, Mc Naughton L, Matthews M. A systematic review into the efficacy of static stretching as part of a warm-up for the prevention of exercise-related injury. Res in Sports Med 2008;16(3):213–31.

7.     Wilson J, Hornbuckle L, Kim J, Ugrinowitsch C, Lee S, Zourdos M, et al. Effects of static stretching on energy cost and running endurance performance. J Strength Cond Res 2010;24(9):2274–2279.

8.     Wilson J, Marin P, Rhea M, Wilson S, Loenneke J, Anderson J. Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises. J Strength Cond Res 2012;26(8):2293–2307.

9.     Zourdos M, Wilson J, Sommer B, Lee S, Park Y, Henning P, et al. Effects of dynamic stretching on energy cost and running endurance performance in trained male runners. J Strength Cond Res 2012;26(2):335–341.