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stretching exercises, stretching injuries, static stretching, stretching warm up

Stretching exercises and injuries: Does stretching during warm-up really decrease the risk of injuries?

Warming up before workouts and competitions is an almost universal practice, and athletes and coaches generally believe that good warm-ups decrease the risk of injuries and increase the ability to perform at a high level. The warm-ups performed by many athletes contain three key components:

(a) Continuous, relatively low-intensity activity designed to increase muscle temperature and activate the cardiovascular system,
(b) 'Rehearsal' of the activity which is about to be performed, including high-intensity actions, in order to prepare the neuromuscular system for the challenges it is about to face, and
(c) Stretching exercises, especially for the muscles which will play a key role in the ensuing activity. It is often believed that such stretching increases range-of-motion at joints, relaxes muscles, and decreases stiffness in muscles and tendons, thereby reducing the risk of injury during the subsequent workout or competition(1).

As you can see, stretching activities are thought to represent the key portion of a warm-up which limits the possibility of getting injured.

But does stretching during a warm-up really cut an athlete's chances of getting hurt? To find out, researchers from the Kapooka Health Centre, the University of Sydney, and Charles Sturt University in Australia recently examined the effects of pre-exercise stretching on lower-limb injury over 11 weeks of training in 1538 (!) subjects ranging in age from 17 to 35(2). This study was carried out with a particularly apt study group: army recruits undergoing basic training (contact the lead researcher – Rodney Peter Pope – at Rodney.Pope.69210450@army.defence.gov.au). Although army recruits are not necessarily élite athletes, they do undertake a rigidly controlled and strenuous programme of exercise during basic training, and they also sustain a high frequency of lower-limb injury(3). Thus, if stretching is really beneficial as an injury-preventer, one would expect to see its effects in a large group of military-service signees.

What the programme involved
The 1538 recruits were randomly divided into stretching (735 individuals) and non-stretching groups (803). The Australian researchers decided to utilise a stretching programme comparable to the type of routine employed by many athletes and thus settled on 20 seconds of stretching for each of six key lower-limb muscles or muscle groups (the gastrocnemius, soleus, hamstrings, quadriceps muscles, hip adductors, and hip flexors) during warm-up. The chosen form of muscle unkinking was static stretching, in which a limb or portion of a limb is moved to close to the limit of its range of motion and then held in this stretched position, without continuous motion or overall body movement. Static stretching carried out in 20-second dosages has been shown to be effective at increasing joint range of motion(4) and at reducing muscular resistance to applied stretch(5).

The static stretches were interspersed with jogging and side-stepping activity during the warm-ups; naturally, individuals in the control group performed only jogging and side-stepping, without a hint of static stretching. During the 11-week period, 40 actual workouts were completed by the recruits, adding up to 50 total hours of hard physical effort. The training was divided into route marching (10 hours), running (10.5 hours), obstacle-course workouts (12.5 hours), circuit training (7.5 hours), swimming (four hours), and battle training (5.5 hours).

The results
Over the course of the 11 weeks (and 60,000 total hours of training), 333 lower-limb injuries were recorded – 175 in the control group and 158 in the stretching recruits, which represented an overall injury rate of 5.5 injuries per 1000 hours of training. The three most common injuries were patellofemoral pain (67 cases), tibial stress fractures (56), and ankle sprains (46). As it turned out, stretching during warm-up had no statistically significant effect on the risk of injury, either for soft-tissue problems or bony disorders. Height and weight of the military personnel were also non-factors when it came to predicting injury.

Although pre-exercise stretching was totally unimportant from an injury standpoint, other easy-to-determine factors actually did a decent job of prognosticating who would get hurt. For example, age was a good predictor of injury (the older the athlete, the higher the injury frequency), and even a non-training factor such as date of enlistment worked better than pre-workout stretching in terms of injury prediction (recruits who enlisted later in the year were more than twice as likely to get hurt, compared with those who enlisted in January, February, or March). In addition, 20-metre shuttle-run time was an outstanding predictor (the faster the time, the lower the risk of injury), a relationship which suggested that overall fitness – not the presence or absence of pre-workout stretching – had the paramount influence on injury occurrence (a simple test like the 20-metre shuttle run is considered to be a reliable fitness assessment, since it can be used to accurately predict VO2max and running capacity). Incidentally, other studies have also found age to be positively related to injury risk in individuals who are embarking on new exercise programmes, as were the recruits involved in this Australian study(6). However, additional research has revealed – somewhat surprisingly – that age is not a good predictor of injury in experienced, well-trained athletes(7&8). Evidently, creakiness of limb can be compensated for by the injury-preventing knowledge associated with being rather long of tooth, or else sport-specific (injury-preventing) strength is sport-specific strength, whether you are 20 years old or 60.

Close-up on Honolulu
Although the Australian research suggests that stretching has little to say about injury risk, a caveat is that the subjects were not real athletes, and many of them may have been embarking on serious exercise programmes for the first times in their lives; the results might be different in a more experienced athletic population. However, in a study carried out with real-live athletes, pre-exercise stretching again failed to provide immunity to injury: in fact, stretching was actually linked with a heightened risk of getting hurt in a key subset of the athletes.

The study linking stretching to muscle problems was carried out by David Lally, PhD, at the University of Hawaii-Manoa; his study group consisted of 1543 serious runners who participated in the Honolulu Marathon(9). When Lally analysed the training habits of these marathon runners, he uncovered some of the usual relationships between training and injuries. For example, high-mileage runners and individuals who conducted unusually long workouts tended to have significantly higher rates of injury, compared with low-training-volume people (to count as an injury in Lally's research, a physical problem had to prevent usual training for at least five days).

The striking finding in Lally's survey was that 47% of all male runners who stretched regularly were injured during a one-year period, while just 33% of male runners who didn't stretch were hurt, a statistically significant difference! An adept researcher, Lally was able to control for the possibility that those individuals who had been injured before his study began had taken up stretching as a prophylactic measure, a linkage which would have strongly biased the results. After all, the strongest predictor of a future running injury is a past injury; about 50% of running injuries are simply recurrences of earlier problems. Thus, including runners who had taken up stretching after a prior injury would automatically make stretching look bad. When Lally threw the males with previous injuries out of his study, things were still bad for the stretchers, who had a 33% greater risk of injury, compared to non-stretching runners. The stretched-out runners did not run more miles per week than the non-stretched ones, so higher mileage was not a possible explanation for the stretching-and-injury phenomenon.

One problem with Lally
A fascinating aspect of Lally's investigation was that the link between stretching and injury applied only to male marathoners, not to females; lady stretchers had the same rate of injury as lady non-stretchers. The positive relationship between stretching and injury also did not apply to Oriental runners of either sex; only in white male marathoners was there a connection between stretching and injury. The mechanism underlying these differences is unknown, but bear in mind that one weakness in the Lally study was that there was no control of the stretching process. As a result, white males may have carried out larger amounts of ballistic stretching (in which muscles are stretched to their limits and then rather quickly relaxed and re-stretched in an alternating, hurried pattern), compared with the other groups of athletes. If it is difficult for you to imagine why ballistic stretching would harm muscles, remember that muscles behave a lot like Silly Putty. That is, a slow, low-force stretch can gradually elongate Silly Putty without harming or breaking it, but a fast stretch will often snap the putty into pieces; like Silly Putty, muscle tissue is OK with slow stretching but is much more resistant to fast extensions – and can break after fairly minimal elongation.

What other mechanism might have raised injury rates in stretched athletes? As it turns out, stretching can temporarily weaken muscles; up to a 20% decrease in strength has been observed following passive stretching in both animal(10) and human studies(11). Investigations concerning passive stretching in animals have revealed that the force which can damage a previously stretched muscle can be as low as 25 to 30% of the usual force required to harm the sinew(12). True, it has not been proven that the forces produced during marathon training and running are large enough to selectively snap pre-stretched muscles, but the possibility is intriguing. Certainly athletes in high-power sports (such as powerlifting or rock climbing, for example) in which muscular force production is extremely great would be wise to evaluate whether stretching should be part of their warm-up routines.

Stretching after a workout?
Lally's probings did yield one additional bit of tantalising information: those marathoners who stretched before their training sessions (ie, as part of their warm-ups) had higher rates of injury, compared with runners who did not stretch. However, those athletes who stretched after their workouts actually enjoyed a lower risk of getting hurt.

When you think about the role that stretching should actually play, this latter finding makes pretty good sense. Although it is indeed popular to position stretching before the actual beginning of a workout, there's actually very little resemblance between the act of stretching out a muscle statically and the rapid-fire shortenings (contractions) which muscles undergo during a workout or competitive event. In other words, stretching does not represent specific preparation for a real-live training session or competition. In a static stretch, a muscle is elongated and then held in a stationary position; in a workout or competition, a muscle shortens and elongates repeatedly and seldom achieves the degree of elongation characteristic of most stretching routines (unless one is a gymnast or ballet dancer). In short, sitting on your rump while trying to elongate your hamstrings does little to prepare them for what they will endure during your ensuing activity and thus may have little chance of helping your hams avoid getting hurt.

What Lally himself thinks
On the other hand, muscles often are fairly tight – and in some cases are even close to going into spasm – after a very strenuous workout ends. At that point, stretching is a fine way to transform a hypercontracted muscle into a relaxed collection of fibres which can comfortably adapt to the more passive activities which follow a training session – and thus be better prepared for the next difficult workout which is undertaken. Is the dissimilarity between static stretching and actual exercise the reason why static muscle untightening does not appear to reduce the risk of injury? 'I'm not completely sure why stretching is sometimes associated with a higher risk of injury, compared with non-stretching' said Hawaiian-researcher Lally in an interview I conducted with him. 'But there is no a priori reason why stretching should limit injury risk. I believe that most injuries in endurance athletes are caused by overuse, and stretching your muscles before your workouts begin is not going to prevent you from overusing your muscles.'

In addition, it is important to bear in mind that static stretching is definitely effective at producing a short-term increase in the range of motion at a particular joint(13). However, this increase in range of motion is usually measured during static positions, not during the dynamic movements associated with training and competition. Thus, it is not clear that there is a real carry-over from the stretching-associated upgrades in what might be called stationary range of motion to the dynamic range of movement associated with sport (remember that few competitions feature athletes sitting around trying to outstretch each other).

Making matters worse, it is not even clear that possible stretching-induced augmentations of dynamic range of motion would necessarily be beneficial to an athlete, from either the injury-prevention or performance standpoints. In fact, one could imagine situations in which excessive range of motion could be injurious and inefficient. It is clear, for example, that increased range of motion at a joint can actually increase the instability of the joint (14). Note, too, that some research has indicated that athletes who are in the highest 20% of the flexibility continuum are actually the ones with the highest injury rates(15)!

What stiffness means
Thus, it is likely that the beneficial effect of a warm-up (from an injury-prevention standpoint) may reside in a short-term reduction in muscle stiffness rather than an increase in joint range of motion(16). While this may seem intuitively obvious to you, bear in mind that stiffness as a measure of dynamic flexibility actually has a counterintuitive meaning for most people. In actual fact, the higher the stiffness of a muscle, the greater is its elasticity (resistance to stretch); naturally, less stiffness in a muscle means greater compliance or extensibility(17). Contrary to popular belief, stiffness is not the passive tension a muscle possesses at a given length; it is actually a measure of the rate of increase of passive tension as a muscle is stretched out. Athletes are generally aware of the passive tension in a particular muscle group near the end of its range of motion during stretching and refer to the degree to which muscles can be elongated during stretching as 'stiffness' or 'looseness', depending on the individual's characteristics. However, the tensions experienced during such stretching and the total magnitude of the stretch are not stiffness; once again, stiffness refers to a rate of change of muscle tension, with stiff muscles being very resistant to changes in tension – and possibly much more susceptible to injury.

It is commonly believed that reduced stiffness would be associated with a lower risk of injury, since there is probably less chance of damage when a less-stiff muscle is elongated (in effect, the muscle 'goes with the flow', instead of tautly trying to resist the change and potentially being pulled apart). Some studies have found that static stretching can reduce passive muscle-tendon stiffness for up to an hour or so(18), but other work has found no effect of static stretching on muscular stiffness at all(19).

In addition, stretching seems to have little impact on the thing which is really important – active stiffness, which is simply muscle-tendon stiffness during muscular contractions (ie, sports activity)(20). By contrast, simply running at a modest intensity for 10 minutes has been found to be effective for reducing the active stiffness of leg muscles(17). In addition, increased muscle temperature has been shown to enhance muscles' resistance to tearing, suggesting that hikes in temperature can decrease stiffness(21). Taken together, such findings suggest that increasing muscle temperature by carrying out submaximal exercise may be much more effective than stretching for decreasing the risk of muscle and tendon injuries during sports activity.

What a warm-up should include
But if a warm-up does not include stretching activities, what should it actually look like? At least 10 minutes of continuous, submaximal exercise seem to be important, since such exertion may raise muscle temperature and downplay muscle stiffness. In addition, it appears that the classic, popular stretches could be replaced by dynamic activities which bear a close resemblance to the actions which are to follow during the post-warm-up workout or competition. For example, various, active drills could be completed which focus on the body's key joints and the important movements associated with a particular sport, and these drills could be performed with gradually increasing intensity, eventually mimicking the intensity of the most red-hot moments of one's sport(22). Muscular stiffness would certainly be decreased, and an athlete's neuromuscular system would be specifically ready to carry out the challenging work ahead.

So what is the ultimate, deepest, best bottom line concerning stretching before workouts and injury prevention? To put it bluntly, there is no evidence at all to support the idea that pre-exercise stretching reduces the risk of injury; if you want to decrease your chances of getting hurt, the research indicates that upgrading your fitness and carrying out activities which decrease your muscles' active stiffness will probably do more than a flood-tide of pre-exercise stretching to keep the injury bug at bay. If you want to stretch, save it for your post-workout routine.

Owen Anderson

References
(1) 'Should Static Stretching Be Used During a Warm-Up for Strength and Power Activities?' Strength and Conditioning Journal, Vol. 24(6), pp. 33-37, 2002
(2) 'A Randomised Trial of Pre-exercise Stretching for Prevention of Lower-Limb Injury', Medicine and Science in Sports and Exercise, Vol. 32(2), pp. 271-277, 2000
(3) 'Injuries in Australian Army Recruits, Part III: The Accuracy of a Pretraining Orthopedic Screen in Predicting Ultimate Injury Outcome', Military Medicine, Vol. 162, pp. 481-483, 1997
(4) 'Effects of Static Stretching on the Maximal Length and Resistance to Passive Stretch of Short Hamstring Muscles', Journal of Orthopaedic Sports Physical Therapy, Vol. 14, pp. 250-255, 1991
(5) 'Viscoelastic Response to Repeated Static Stretching in the Human Hamstring Muscle', Scandinavian Journal of Medicine and Science in Sports, Vol. 5, pp. 342-347, 1995
(6) 'Rates and Risks for Running and Exercise Injuries: Studies in Three Populations', Research Quarterly for Exercise and Sport, Vol. 58, pp. 221-228, 1987
(7) 'Predicting Lower-Extremity Injuries among Habitual Runners', Archives of Internal Medicine, Vol. 149, pp. 2565-2568, 1989
(8) 'The Ontario Cohort Study of Running-Related Injuries', Archives of Internal Medicine, Vol. 149, pp. 2561-2564, 1989
(9) 'New Study Links Stretching with Higher Injury Rates', Running Research News, Vol. 10(3), pp. 5-6, 1994
(10) 'Muscle Damage Induced by Eccentric Contractions of 25% Strain', Journal of Applied Physiology, Vol. 70, pp. 2498-2507, 1991
(11) 'Acute Muscle Stretching Inhibits Maximal Strength Performance', Research Quarterly for Exercise and Sport, Vol. 69, pp. 411-415, 1998
(12) 'Identification of a Threshold for Skeletal Muscle Injury', American Journal of Sports Medicine, Vol. 22, pp. 257-261, 1994
(13) 'Influences of Strength, Stretching and Circulatory Exercises on Flexibility Parameters of the Human Hamstrings', International Journal of Sports Medicine, Vol. 18, pp. 340-346, 1997
(14) 'Physiology of Range of Motion in Human Joints: A Critical Review', Critical Reviews in Physical and Rehabilitative Medicine, Vol. 6, pp. 131-160, 1994
(15) 'Strength, Flexibility, and Athletic Injuries', Sports Medicine, Vol. 14, pp. 277-288, 1992
(16) 'Flexibility and Its Effects on Sports Injury and Performance', Sports Medicine, Vol. 24(5), pp. 289-299, 1997
(17) 'Effect of Passive Stretching and Jogging on the Series Elastic Muscle Stiffness and Range of Motion of the Ankle Joint', British Journal of Sports Medicine, Vol. 30, pp. 313-318, 1996
(18) 'Passive Properties of Human Skeletal Muscle during Stretch Maneuvers', Scandinavian Journal of Medicine and Science in Sports, Vol. 8, pp. 65-77, 1998
(19) 'Stretching during Warm-Up: Do We Have Enough Evidence?', Journal of Physical Education, Recreation, and Dance, Vol. 70(7), pp. 24-27, 1999
(20) 'Investigation into the Effect of Static Stretching on the Active Stiffness and Damping Characteristics of the Ankle Joint Plantar Flexors', Physical Ther. Sport, Vol. 2, pp. 15-22, 2001
(21) 'Thermal Effects of Skeletal Muscle Tensile Behavior', American Journal of Sports Medicine, Vol. 21(4), pp. 517-522, 1993
(22) 'Dynamic Warm-Ups', Sports Coach, Vol. 24(1), pp. 20-22, 2001

stretching exercises, stretching injuries, static stretching, stretching warm up