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.

By Sarmax.

In many practical situations such as the treatment of hypertension, it is important to determine whether an improvement of condition following exercise prescription is due to an increase in aerobic fitness, or whether it simply reflects a reduction in body fat content (1) A previous review of 61 studies of training-induced changes in resting blood pressure (2) concluded that any reduction in resting pressures could not be attributed to concomitant weight loss, since the changes in systolic and diastolic readings showed very small and statistically non-significant correlations with changes in body mass.

In their recent paper, Barrone et al. (1) wished to test whether the same was true of exercise hypertension, and in support of such a conclusion they claim to have demonstrated independent correlations of delta fitness and delta fat with changes of pressure through the use of generalized estimating equations. A variety of valid measures of body fatness were obtained on their subjects, but unfortunately an inappropriate measure of aerobic fitness was chosen for the analysis. The outcome is reported as a change in peak oxygen transport, expressed in ml/kg/min. This is dimensionally incorrect, but let us assume that the authors intended to indicate a relative change of oxygen transport, expressed in ml/[kg.min]. Aerobic fitness is in fact the overall ability of the cardio-respiratory system to transport litres of oxygen to the working tissues. Any accumulation of body fat reduces the utility of this transport in terms of daily activities (including treadmill running). To take a practical example, a man with a body mass of 70 kg and an aerobic fitness of 3.5 l/min has a relative VO2max of 50ml/[kg.min]. If that same person accumulates an extra 14 kg of body fat, the aerobic fitness may remain at 3.5 l/min. but the relative VO2max decreases to 41.7 ml/[kg.min]. Plainly, the relative units of oxygen transport confound the influence of fitness and fatness, and cannot be used to distinguish the importance of changes in fitness relative to changes in fatness.

The authors must have the data to make a more convincing independent analysis of the two variables, and I would encourage them to do so.

REFERENCES.

1. Barone BB, Wong N-Y, Bacher AC et al. Decreased exercise blood presure in older adults after exercise training: contributions of
increased fitness and decreased fatness. Br J Sports Med 2009; 43: 52-56.

2. Hagberg JM, Park J-J, Brown MD. The role of exercie training in the treatment of hypertension. An Update. Sports Med 2000; 30: 193-206.

By Adriana Suely de Oliveira Melo, MD, MSc et al.

Barakat et al. 1 have presented us with a paper of excellent methodological quality, following all the steps recommended in the Consolidated Standards of Reporting Trials (CONSORT) and dealing with a question that never fails to generate controversy with respect to the practice of physical activity during pregnancy: prematurity. nother strong point of the paper is the fact that the physical exercise was systematized and monitored, guaranteeing that the pregnant woman indeed followed the prescribed program.

Various controversies continue to surround the topic of physical exercise and pregnancy and the real effects of exercise on the conceptus remain to be clarified. The spectrum of these effects ranges from fetal growth to the duration of the pregnancy, with some studies associating prematurity and growth restriction with the practice of physical exercise 2-4. Despite these speculations, until recently no randomized clinical trials (RCT) with adequate sample sizes had been identified in which pregnant women were systematically followed up for a period encompassing the second and third trimesters.

The excellent quality of this paper prompted us to examine it in detail in an attempt to understand some points that we would now like to put to the authors. Since the objective of the RCT was to evaluate the risk of premature labor, would it not have been better to have excluded all the pregnant women with a history of premature labor in view of the fact that the results show that one of the cases of prematurity in the intervention group was precisely due to a prior history of prematurity?

Another point that drew our attention concerns the exclusions in both groups, which were the result of various situations that may have affected the outcome “gestational age”, such as bleeding, pregnancy-induced hypertension and threatened preterm labor. In our opinion, these women should have continued in the study and an intent-to-treat analysis should have been carried out. We were also intrigued by the fact that one patient was excluded because her pregnancy was a twin pregnancy. Was a single pregnancy not one of the inclusion criteria?

It may perhaps have been interesting NOT to have included women with a history of premature delivery. Although the inclusion criteria accepted the possibility of the participants having had at the most one previous premature delivery, this may have had an effect on the mean gestational age reported in the present study.

We were unable to identify in the paper any description of the parameters used to calculate sample size to determine whether the final number of participants included was sufficient to demonstrate any
differences between the groups. Could a type II statistical error have occurred?

Another minor question we would like to pose is whether the intensity of the prescribed exercise was light-to-moderate or moderate, since it is described in different ways in the various sections of the manuscript and it is known that some outcomes are dependent on the intensity of exercise.

Finally, we would like to know whether the authors have data on other gestational or perinatal outcomes, since such a well-conducted RCT as this one should have generated interesting results that deserve to be published.

1. Barakat R, Stirling JR, Lucia A. Does exercise training during pregnancy affect gestational age? A randomised controlled trial. Br J Sports Med 2008; 42(8):674-8.

2. De Ver Dye T, Fernandez ID, Rains A, Fershteyn Z. Recent studies in the epidemiologic assessment of physical activity, fetal growth, and preterm delivery: a narrative review. Clin Obstet Gynecol 2003; 46(2):415-22.

3. Grisso JA, Main DM, Chiu G, Synder ES, Holmes JH. Effects of physical activity and life-style factors on uterine contraction frequency. Am J Perinatol 1992; 9(5-6):489-92.

4. Misra DP, Strobino DM, Stashinko EE, Nagey DA, Nanda J. Effects of physical activity on preterm birth. Am J Epidemiol 1998; 147(7):628-35.