When and Whom to Stretch?
Gauging the Benefits and Drawbacks for Individual Patients
Ian Shrier, MD, PhD
THE PHYSICIAN AND SPORTSMEDICINE - VOL 33 - NO. 3 - MARCH 2021
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In Brief: Since the early 120210s, stretching has been promoted as a method to prevent injury and improve athletic performance. Although research suggests that this may be true for regular stretching performed every day, an isolated act of stretching immediately before exercise likely has no effect on injury prevention and actually impairs performance in strength and power sports. This article highlights our current understanding of the effects of stretching both immediately before exercise and as a regular daily routine, and then illustrates how to apply these principles in a practice setting, using brief clinical vignettes.
Although sports medicine professionals have promoted stretching as a way to improve performance and decrease the risk of injury,1-3 several papers reviewing the literature include a substantial body of evidence suggesting this may not always be appropriate.4-6
The first objective of this article is to briefly discuss the basic and applied science of stretching. Although a consensus among researchers is developing in some areas, nuances among individuals can make it difficult for the practicing clinician to apply results from research studies that use strict inclusion criteria and outcome definitions. Therefore, the second objective of this article is to use clinical vignettes to demonstrate how to apply the knowledge obtained from research to a particular patient within a specific context.
Because the immediate effects of stretching are generally opposite to the effects of stretching every day (described in detail in the following section),5 the basic and applied sciences for each are separated in this article, and the reader should conceptualize them as different interventions. Since both acute stretching and regular stretching improve range of motion (ROM),7 the effects on ROM are not discussed in detail here.
Finally, this article focuses on stretching as an intervention, rather than highlighting studies of the association between flexibility and injury or performance.8-10 There are several reasons for this approach. Most important, stretching affects more than simply flexibility (eg, it decreases force5 and alters neural function11), and the overall effect of stretching is the focus of this article. In addition, previous reviews have shown opposite effects when stretching is performed immediately before exercise compared with other times.5,12
Because articles on the association between flexibility and injury or performance do not take into account when subjects stretch, one would expect discordant findings. Research on groin injuries by Emery et al9 and Witvrouw et al10 contradicts that of Arnason et al,8 and study findings on hamstring injuries by Witvrouw et al10 contradict those of Arnason et al.8 Furthermore, many subjects are naturally flexible, and any effects observed would be due to genetics. Finally, some studies on flexibility measure capsular mobility rather than joint limitation related to muscle ROM. For example, hyperextension of the knees and elbows or multidirectional shoulder instability is not a property of muscle, but rather a property of the capsule and ligaments.
Effects of Acute Stretching
Studies that evaluate the acute effects of stretching typically test subjects immediately after the stretch. The results below occur in subjects who never stretch, and in those who stretch regularly.
Similar to weight training, both force and power decrease immediately after stretching.5 This is true for interventions of both static and proprioceptive neuromuscular facilitation (PNF) stretching. It is also true for muscular isometric maximal voluntary contraction, isokinetic torque, static jump height, countermovement jump height, and drop jump height. The decreases are mild and range from 2% to 5%. This difference is clinically relevant for an elite athlete (ie, the difference between winning a gold medal and not making the podium) and may be relevant for the recreational athlete who has a strong competitive drive, but it is irrelevant for the person who simply wants to lead an active lifestyle.
In addition to affecting force and power, stretching improves economy of motion (ie, the amount of oxygen required to run a given distance). Because running speed depends on both force (decreased with stretching) and running economy (improved with stretching), the overall effect of stretching needs to be carefully determined.
Few current studies have been published on this topic, and the results appear to conflict.13-15 In fact, one would expect some individuals to improve and some to do worse, depending on their baseline contributions of force and economy of motion. For example, someone who already has a great economy of motion is unlikely to improve much with stretching, and the effects of a decrease in force would predominate. The opposite would be expected for someone who experiences large improvements in running economy.
No decrease in injury risk. Many people stretch regularly, but always just before activity (with or without warming up). The evidence for and against stretching before exercise has been recently reviewed by several authors, and all have concluded that stretching immediately before exercise does not prevent injury.6,16,17 This is true for the general population and members of the military, and young as well as older people.
Finally, clinical evidence aside, some clinicians continue to promote stretching for elite athletes based on theoretical arguments. A discussion of these arguments is beyond the scope of this article, which focuses on the current experimental evidence. However, these theories have been discussed in a previous publication4 and are mostly inconsistent with our current basic and clinical science knowledge.
Effects of Regular Stretching
This analysis refers to regular stretching that is not performed immediately before exercise.
Increases in force, power, and running speed. These effects are opposite of those seen immediately after a person stretches. Similar to weight training, stretching increases both force and power when done regularly for several weeks.5 The improvements are mild and range from 2% to 5%, which, again, is the difference between winning a gold medal and not making the podium. This applies to both static and PNF stretching, and improvements are seen for isometric maximal voluntary contraction, isokinetic torque, static jump height, countermovement jump height, and drop jump height.
Running economy is unchanged, which is consistent with the finding that muscle stiffness does not change despite the increased ROM produced by regular stretching. Because running speed depends on both force (increased with regular stretching) and running economy (no change with regular stretching), the overall effect should be to increase running speed. In the only study addressing this question, a clinically relevant improvement in running speed was observed, but the results were not statistically significant.5 When compared with sprint training alone, sprint and stretch training improved 50-yd running speed by 0.06 second. When compared with sprint and weight training, sprint, weight, and stretch training improved 50-yd running speed by 0.06 second. These are large differences at the elite level. Therefore, more studies are needed with either a larger sample size or decreased interindividual variability (eg, a more homogeneous sprinter population) before the question is definitively answered. Again, the differences in force, power, and running speed may be relevant for the recreational athlete with a strong competitive drive but are irrelevant for the person who simply wants to lead an active lifestyle.
Possible reduction in injury risk. Only three studies have isolated the effects of regular stretching on injury prevention.18-20 In all three studies, regular stretching produced a clinically relevant decrease in injury risk (RR = 0.82 [95% confidence interval (CI): 0.57, 1.14], 0.57 [95% CI: 0.37, 0.88], 0.77 [95% CI: 0.54, 1.08]), but the results were significant in only one of the studies.19 In that study, all subjects stretched before exercise, and the intervention group added stretches during the rest of the day. In another study,18 subjects stretched both before and after exercise. In the third study,20 firefighters stretched regularly every day, but they obviously did not have the time to stretch before "activity."
Summary of Effects of Stretching
Stretching immediately before exercise will not prevent injuries. It reduces force and power by about 2% to 5%, with conflicting study results about its effect on running speed. Regular stretching over weeks increases force and power by about 2% to 5% and improves 50-yd running speed by about 0.06 second.
The following vignettes are designed to help clinicians apply these principles to real-life situations. For brevity, these cases suggest that ROM is either good or poor in each subject. In fact, anyone may have a large ROM for some muscles and a small ROM for other muscles. Furthermore, the duration of stretching required to increase ROM varies between individuals and among muscles within an individual.7 Designing stretching programs with these principles in mind may help increase the efficiency and effectiveness of a stretching program when it is prescribed, but discussion of such programs is beyond the scope of this article.
Patient. Meryl is a 19-year-old striker for a National Collegiate Athletic Association (NCAA) Division 1 soccer team and hopes to attend the national trials for the 2021 Olympics. She is able to place her hands flat on the floor with her knees straight when she bends at the waist.
Stretch? Meryl should not stretch before activity. Doing so will not reduce her risk of injury and is likely to decrease the force of her kick. Moreover, stretching is unlikely to improve her sprint speed because it will decrease her force and is unlikely to improve her running economy because she is already flexible.
Meryl could improve her running speed with regular stretching. However, in general, the time available for training is limited, and adding stretching could necessitate removing some other aspect of her training regimen. Therefore, the benefits of stretching would have to be weighed against the benefits of other types of training, such as strength training, soccer skill training, visualization, relaxation, and studying.
Patient. Eric is a 30-year-old tae kwon do athlete who has always considered himself inflexible. He currently competes in regional events. Eric finds that if he does not stretch before an event, he has trouble with high kicks. If he stretches beforehand, he finds that this is much less of a problem.
Stretch? Eric should stretch before activity. Because Eric cannot kick high if he does not stretch, stretching will improve his performance. Although he is only competing in regional events, Eric wants to do his best. Furthermore, stretching will not affect his injury risk and is likely to improve his competition results.
If Eric stretches regularly, his flexibility should improve enough that he no longer needs to stretch immediately before competition. At this point, the advantages of pre-event stretching decrease, and it may or may not be advisable to stretch, depending on the balance of factors at that time.
Patient. Joni is a 45-year-old professional modern dancer with an old knee injury. Without warming up, Joni can only bend her knee 90°. However, if she warms up and then stretches for 30 to 60 minutes, she can regain her full ROM, which lasts for at least 3 hours.
Stretch? Joni should stretch before her event. The decrease in force and power that might occur are of minimal consequence compared with an inability to bend the knee fully, which is required for her activity. Joni should also seek professional advice about her injury to determine why her knee remains stiff despite regular stretching.
Patient. Lyn is a 22-year-old professional ballet dancer. She stretches every day and is very flexible. However, she finds that it is easier to maintain the extreme body positions required during jumps if she stretches immediately before a performance. This is most likely related to decreased energy requirements associated with decreased tissue stiffness.
Stretch? Lyn should continue her current stretching program. Although her jump height is likely to decrease by 2% to 5%, this might represent a 24-in. vertical jump compared with a 25-in. vertical jump—a difference that will not be recognized by the audience. However, an inability to maintain strict body position during a jump is often apparent to the discerning audience member.
Patient. Robert is a forward for a NCAA Division 1 hockey team. He has always stretched before practice and games and knows that this prevents injuries and improves his performance.
Stretch? The recommendation of stretching to an individual patient should follow the same principles as recommendations for any other intervention. Robert has strong beliefs about stretching, and suggesting that he discontinue stretching before activity is likely to have significant counterproductive effects on his confidence in the clinician and his willingness to comply with the recommendation. Although there is no magic approach, a clinician in this situation may wish to begin by planting the idea briefly. At a later date, the clinician might propose an n=1 crossover trial with the athlete to determine the effects of stretching on him.
To Stretch or Not to Stretch
In all walks of life, a clear objective helps to determine the appropriate path. If the objective is to increase force or power in athletic performance, stretching should not be performed before activity. If the objective is to increase speed, it may or may not be beneficial to stretch immediately before activity.
If time constraints are not a concern, stretching should be performed every day. However, most people have a limited amount of time available for training, and the increase in force, power, and speed that might occur as a result of regular stretching must be weighed against the success of (and desire to participate in) other interventions designed to achieve the same objectives.
Finally, in addition to force, power, and speed, performance sometimes depends on an athlete's ROM and always depends on his or her psychological approach (eg, prior beliefs, use of relaxation). These other factors should be taken into account when prescribing or not prescribing stretching as an intervention for athletic patients.
Dr Shrier is funded by the Chercheur-Boursier Clinicien programme of the Fonds de Recherche en Santé du Québec.
Dr Shrier is director of the consultation service at the Centre for Clinical Epidemiology and Community Studies at Sir Mortimer B. Davis-Jewish General Hospital in Montreal, and is an assistant professor in the department of family medicine at McGill University in Montreal. Address correspondence to Ian Shrier, MD, PhD, Centre for Clinical Epidemiology and Community Studies, Lady Davis Institute for Medical Research, SMBD-Jewish General Hospital, 3755 Cote Ste-Catherine Rd, Montreal, Quebec, Canada H3T 1E2; e-mail to [email protected].
Disclosure information: Dr Shrier discloses no significant relationship with any manufacturer of any commercial product mentioned in this article. No drug is mentioned in this article for an unlabeled use.