We agree with Ian Shrier that the finding of an effect of stretching on risk of muscle, ligament and tendon injuries should be interpreted with caution. That is why we wrote “The finding of an effect of stretching on muscle, ligament and tendon injury risk needs to be considered cautiously because muscle, ligament and tendon injury risk was a secondary outcome, and there was no evidence of an effect of stretching on the primary outcome of all-injury risk. If stretching had reduced the risk of muscle, ligament and tendon injuries without increasing the risk of other injuries, we would expect a reduction in all-injury risk.” Nonetheless, after a prolonged discussion of this issue we decided that the finding could not be totally dismissed. We believe that it was appropriate to report the observed effect on muscle, ligament and tendon injuries with an explicit acknowledgement of the uncertainty associated with this finding.

Regardless of whether one accepts the finding that stretching reduces risk of muscle, tendon and ligament injuries, the implications would appear to be the same. Even if the effect is real, it is quite small in absolute terms (even in this population, at quite a high risk of injury, only “one injury to muscle, ligament or tendon was prevented for every 20 people who stretched for 12 weeks”). For this reason the data from this study do not appear to provide support for the practice of stretching, at least in so far as the aim is to reduce injury risk. The stronger justification for stretching, though still a marginal one in our view, is provided by the clear evidence of a very small effect of stretching on soreness. For other outcomes, such as performance or range of motion our study did not provide any data.

It is not yet known whether stretching is best carried out before exercise, after exercise, or both before and after exercise. We were surprised, when planning this study, to learn that most Australian stretch before exercise but not after, and most Norwegians stretch after exercise but not before! It was for that reason we designed a trial in which participants stretched both before and after exercise. We do not agree with Ian Shrier’s suggestion to conduct an unplanned post-hoc comparison of the non-randomised subgroups that chose to stretch only before, only after, or both before and after exercise. Such an analysis would almost certainly be seriously confounded and would probably be uninterpretable; at any rate it hardly seems consistent with his disapproval of our much more disciplined pre-planned secondary comparison between randomised groups. The only truly satisfactory way to resolve the issue of whether it is better to stretch before or after exercise is to conduct a further randomised trial in which participants are randomised to those two conditions.

Conflict of Interest: None declared

Editor,

I recently read the article Jamtvedt et al on whether pre and post stretching prevents injury (1) with interest. I commend the authors for their well-conducted study and would like to comment on two particular issues.

First, the authors correctly point out that there was no difference in the primary outcome of all injuries, and that the analysis showing an absolute 22% reduction in muscle, ligament and tendon injuries with stretching should be interpreted cautiously. However, they then continue to say “Nonetheless, it is plausible that stretching reduces muscle, ligament and tendon injuries, and it may be implausible that stretching increases other injuries”. Moreover, in the conclusion, they only mention the “probable reduction in muscle, ligament and tendon injuries” and omit the absence of an effect on the primary outcome of overall injuries. This type of thinking appears to be gaining popularity. For example, Small et al (cited by the current article) emphasized the decrease in musculotendinous injuries they observed in their review of stretching and discounted the associated increase in stress fractures and “shin splints” (2).

In other areas of medicine, we have already learned the difficult lesson that “all- cause mortality” is generally a much more important outcome compared to “disease- specific mortality” because interventions can cause damage through unrecognized mechanisms. It would be a pity if the sport medicine world has to go through the same lessons. Plausible reasons why stretching would increase some types of injuries are already available from a review of basic science evidence (3). Because Jamtvedt et al do not actually detail the non muscle-tendon-ligament injuries, I will use the example from Small et al. related to stress fractures and “shin splints” (not defined, but presumably periostitis and compartment syndrome). An acute bout of stretching causes weakness, (4) which is expected to lead to 1) an increased force transmission to the bone (5), (6), which would lead to increased stress reaction and stress fractures and 2) a possible increase in compensatory muscle use, which could theoretically cause shin splints of any cause. Further, stretching-induced weakness would theoretically also decrease proprioception, although this remains to be studied. Authors who decide to report sub-group analyses need to show the same analyses for all the sub-groups created by the categorization.

Second, “stretching” as an intervention is intricately related to the timing of the stretch, and one expects different results from stretching before exercise compared to stretching at other times (7). In their conclusion, Jamtvedt et al suggest that “the results of this trial support the decision to stretch” (1), with no mention of the timing; reviews by Small et al (2), and Thacker et al (8) (cited by the current article) made the same error. In brief, the effects of “stretching” are similar to those of “weight lifting”. An acute bout of weight lifting or stretching will cause an immediate decrease in strength, power and endurance 4. However, if one weight lifts or stretches for weeks, there is an increase in strength, power and endurance (4). Based on this, one would expect that stretching before every exercise session would increase the risk of injury due the acute effects, but there would also be an expected decrease in injury risk as the body adapts and strengthens over time. If the two effects were relatively balanced, one would expect no effect on overall injury rate. However, if one stretched regularly but not before exercise, then one would expect only the benefits, with a decrease in overall injury rate. Indeed, there have been three randomized trials prior to this study and a meta-analysis of these (one study had subjects stretch before and after exercise as in the current study (9)) suggests regular stretching not before exercise reduces injury risk [OR=0.68 (95%CI: 0.52, 0.88)] (7).

Given these previous studies, it would be interesting for the authors to conduct a post-hoc analysis (with the appropriate cautious interpretation) comparing the injury risk among those who stretched only before exercise, those that stretched only after exercise, and those that stretched both before and after exercise.

In summary, there should be little controversy about 1) post-exercise stretching reducing acute muscle soreness, just as it reduces any chronic musculoskeletal pain (10), presumably due to its well-studied effects on stretch-tolerance (a form of analgesia) (11, 12), and 2) stretching not before exercise reducing injury risk given that both basic science and clinical science provide consistent evidence, although a couple more confirmatory studies could be helpful.

Future research priorities should focus on questions where there is little to no evidence such as 1) whether post-exercise stretching is as beneficial as stretching at other times, 2) what are the effects for high intensity sports, 3) the effects of stretching on rehabilitation of injuries, and 4) the effects on the performance in injured athletes (all published studies examined healthy subjects) (13).

Ian Shrier MD, PhD, Dip Sport Med, FACSM Centre for Clinical Epidemiology and Community Studies SMBD-Jewish General Hospital 3755 Cote Ste-Catherine Rd Montreal, Qc H3T 1E2 Tel: 514-340-7563

Fax: 514-340-7564

References

1. Jamtvedt G, Herbert RD, Flottorp S, et al. A pragmatic randomised trial of stretching before and after physical activity to prevent injury and soreness. Br J Sports Med. 2010;44:1002-1009.

2. Small K, McNaughton 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 Sports Med. 2008;16:213-231.

3. Shrier I. Does stretching help prevent injuries? In: MacAuley D, Best T, eds. Evidence-based sports medicine. London: BMJ Publishing Group; 2007.

4. Shrier I. Does stretching improve performance: A systematic and critical review of the literature. Clin J Sport Med. 2004;14:267-273.

5. Mizrahi J, Verbitsky O, Isakov E. Fatigue-related loading imbalance on the shank in running: a possible factor in stress fractures. Ann Biomed Eng. 2000;28:463- 469.

6. Christina KA, White SC, Gilchrist LA. Effect of localized muscle fatigue on vertical ground reaction forces and ankle joint motion during running. Hum Mov Sci. 2001;20:257-276.

7. Shrier I. Meta-analysis on preexercise stretching. Med Sci Sports Exerc. 2004;36:1832-1832.

8. Thacker SB, Gilchrist J, Stroup DF, et al. The impact of stretching on sports injury risk: a systematic review of the literature. Med Sci Sports Exerc. 2004;36:371-378.

9. Amako M, Oda T, Masuoka K, et al. Effect of static stretching on prevention of injuries for military recruits. Mil Med. 2003;168:442-446.

10. Law RY, Harvey LA, Nicholas MK, et al. Stretch exercises increase tolerance to stretch in patients with chronic musculoskeletal pain: a randomized controlled trial. Phys Ther. 2009;89:1016-1026.

11. Magnusson SP, Simonsen EB, Aagaard P, et al. Mechanical and physiological responses to stretching with and without preisometric contraction in human skeletal muscle. Arch Phys Med Rehabil. 1996;77:373-378.

12. Halbertsma JPK, Mulder I, Goeken LNH, et al. Repeated passive stretching: acute effect on the passive muscle moment and extensibility of short hamstrings. Arch Phys Med Rehabil. 1999;80:407-414.

13. Shrier I. Stretching perspectives. Curr Sports Med Rep. 2005;4:237-238.

Stretching, or not, has been a controversial issue.

This month’s BJSM helps to unravel (or add to) this puzzle.

In an entirely web-based study (that gained awareness through media coverage), 2377 physically active adults were recruited in an effort to determine whether stretching modifies injury risk and soreness.

The verdict? In the short-medium term stretching reduces the risk of soreness and does not reduce the all-injury risk.

For a link to this article and to make up your own mind, see the November  BJSM.

Click here to read the original article.

We thank Professor Thornton for raising these issues and are grateful for the opportunity to clarify details of the trial protocol and analysis.

There is, in fact, no discrepancy between the description of outcomes in the trial protocol published on the trial web site, the protocol that was registered on the Australian New Zealand Clinical Trials Registry, and the report published in the British Journal of Sports Medicine. All three sources describe the same primary outcomes (risk of soreness and time to injury) and the same secondary outcomes (severity of soreness, time to (“preventable”) muscle/ligament/tendon injuries, time to injuries for which professional care was sought, and perceptions of looseness during and after exercise). In addition, all three sources describe the same secondary analyses of the interactions between frequency of activity, age and strength of belief in affects of stretching and each of the two primary outcomes. Confusion may have arisen because in the trial report we refer to the analysis of the interactions as “outcomes”, although the analyses of interactions were conducted on the same primary and secondary outcomes as listed above. And we may have added to the confusion by inconsistently referring to the perceptions of looseness during and after exercise as either one or two outcomes. We acknowledge that the wording may not have been clear but we reiterate that there was no inconsistency between the analyses described in the protocol, register and trial report.

The protocol, register and trial report describe an analysis of the subset of injuries which could plausibly be prevented by stretching. The protocol (which is more detailed than the registry entry) indicated that the classification of injuries into those that could and could not plausibly be prevented by stretching would be classified by an independent expert. We found that the data lacked sufficient detail to inform decisions about whether individual injuries were preventable so we decided simply to classify all muscle, ligament and tendon injuries as potentially preventable. This decision was made before the allocation code was broken without reference to the data. We did not know, at the time that decision was made, that there would be a significant effect of stretching on the subset of muscle, ligament and tendon injuries.

As the protocol indicated, no adjustment was made for multiple comparisons. We interpret frequentist analyses as Bayesian analyses with vague priors[1] and, from this perspective, the decision not to make adjustments for multiple comparisons is justified. At any rate, there were only two primary outcomes so adjustment for multiple comparisons would not have changed the conclusions from the primary analysis. We agree that the finding of an effect on the secondary outcome of muscle, ligament and tendon injuries is less robust than the finding of an effect on the primary outcome risk of soreness because muscle, ligament and tendon injuries were a secondary outcome. That is why we concluded that stretching “probably reduces the risk of some injuries and does reduce the risk of bothersome soreness”.

Gro Jamtvedt
Executive Director
Norwegian Knowledge Centre for the Health Services, Oslo, Norway

Reference:

1. Barnett V (1973). Comparative Statistical Inference. London:
Wiley.