To Stretch or Not to Stretch: Research into the Current Controversy
Stretching recommendations passed down through the years have until recently been generally accepted by physical education teachers, coaches, and professionals in the fitness industry (1,2). These recommendations stayed in place despite conflicting evidence in regards to the benefits of stretching in the academic literature (1,2). Ingraham (2) states that many current studies that have shown positive effects from stretching have not measured the effects of stretching alone,
without some cardiovascular intervention. There have been recent reports suggesting that not only is stretching ineffective method of preventing injuries, it may actually cause injury (2,3). A variety of stretching recommendations have come out of the literature and current research is challenging concepts regarding common stretching recommendations, particularly the role of flexibility and injury prevention (1,2). Due to these factors there are many misconceptions held by many people.
The proposed physiological mechanisms of stretching are thought to be one of two mechanisms proposed by Shrier and Gossal (1). Either there is a direct decrease in muscle stiffness defined by Shrier and Gossal (1) as "the force required to produce a given change in length by passive viscoelastic changes" or indirectly by causing a decrease due to reflex inhibition, which will change viscoelasticity because of a decrease in actin-mysoin cross-bridging. Therefore, having less muscle stiffness would allow an increased range of motion (1). Does this new evidence suggest that the age old concept of stretching prior to exercise actually does not prevent injuries? Pope et al (3) conducted one of the most cited studies on questioning the effects of stretching on injury prevention. The researchers used a randomized trial involving 1,538 Army recruits and looked at the effect of muscle stretching on the risk of exercise-related injury. In regards to stretching, the researchers found that a "typical" stretching protocol did not significantly reduce exercise-related injury. Pope et al (3) did find a trend suggesting the greater the cardiovascular fitness of the recruit the less likely the individual was to have an exercise-related injury. Bell et al (4) also found that female Army recruits with a lower level of general physical conditioning were more likely to suffer from exercise-related injury. The question then arises, are we looking down the wrong avenue? Could general fitness be the most important factor we can modify?
Fitness level and the rate of injury among professional baseball players is a great example of this new trend. Ingraham (2) uses baseball training and conditioning as an example because it is restricted mainly to actions for their specific performance. Ingraham (2) suggests that a majority of these players would not be able to perform the minimum cardio respiratory requirement of 20 minutes of continuous activity maintaining their heart rate in their training zone. Ingraham (2) goes on to state that even though this group of athletes warm-up with sprints and long stretching sessions they still experience frequent hamstring injuries. This may not be the only explanation to why baseball players experience hamstring injuries, however it does raise questions of the effectiveness of stretching prior to exercise.
Does stretching actually cause more harm than good? Shrier (5) looked at stretching before exercise and its relation to injury prevention. He looked at research done on immobilization and heat-induced increases in muscle compliance, the effects of stretching during eccentric activity, the effects of stretching in activities not requiring extensive muscle length, damage induced by stretching at the cytoskeletal level, and the analgesic effect of stretching. His findings suggest that stretching prior to exercise did not reduce the risk of injury. Furthermore, Black et al (6) and Sorichter et al (7) found through analyses of the contractile properties of muscle during stretching that stretching prior to a high intensity activity could potentially contribute to major muscle injury.
Another aspect that should be looked at is the function of the joint capsule and ligaments in respect to injury and performance. Ingraham (2) indicates, that the contribution of joint laxity is difficult to assess. Nicholas (8) found that an increase in hamstring flexibility and joint laxity is associated with greater incidence of anterior cruciate ligament (ACL) injuries in football players. Boden et al (9) found that tight hamstrings are more protective of the ACL rather than a more relaxed hamstring muscle. Arendt et al (10) and Arendt et al (11) studies have indicated that women are five to ten times more likely to tear their ACL than are men in similar sports, such as basketball and soccer. Ingraham (2) argues that given the fact that research shows an association between joint laxity and injury and that women have greater joint laxity than men, the practice of increasing range of motion in females should be questioned.
Does stretching do anything to help alleviate muscle soreness? Many anecdotal stories have led to the belief that stretching will relieve muscle soreness. Ingraham (2) suggests that it is the perceived effect of stretching that has an immediate analgesic effect, but there is no evidence of lasting effects of overall muscle soreness. Shrier and Gossal (1) state that this effect comes from an increase in stretch tolerance. That is, a person will feel less pain for the same amount of force applied to the muscle. Therefore, the muscle will feel less stiff and will therefore be perceived as being less sore. Herbert and Michael (12) found that stretching had no effect on delayed onset muscle soreness or on the risk of injury.
Does stretching help improve performance? Recently, researchers have conducted trials on the effects of increased range of motion and its relationship to measurable performance rather than on the athletes perception. One study by Gleim and McHugh (13) found that decreased flexibility has actually been associated with increased economy in running and walking. Cornwell et al (14) found that an acute bout of stretching can have a negative effect on performance of a single-joint countermovement jump. Gosselin and Burton (15) also found similar conclusions in their study.
So, what about flexibility training in general? In general, Ingraham (2) argues that flexibility is increased beyond what is required to perform any motion due to stretching. Ingraham (2) also argues that people who are looking to maintain flexibility for activities of daily living can achieve this by simply using the specific joints. As we age, individuals tend to become inflexible due to lack of use and activity rather than age related muscle shortening. Ingraham (2) states that this thought of flexibility can be applied to sport training. By performing the sport itself it provides enough "stretching" to maintain the range of motion needed for that specific activity (2). Researchers such as Alway (16), Goldspink et al (17), and Yang et al (18) have demonstrated with the animal model that stretching 24 hours per day over several days compared with stretching only minutes per day outside of exercise could produce muscle hypertrophy, which the authors suggest could in theory reduce the risk of muscle injury, however this has not been extrapolated to human studies. Clinical trials of non-continuous stretching have shown to be inconclusive. More research needs to be done before any solid conclusions can be made on this issue (1,2,5).
In conclusion, stretching is a routine found in a majority of fitness and sports training programs. The question now remains that if stretching does not result in a decrease in injury, muscle soreness, or help improve performance then should our time spent stretching be invested in more cardiovascular and strength training (2)?
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