Stretching Part 2: Can it decrease risk of injury and/or improve performance?

In the May edition of my post, we talked about stretching and its effects on the muscle tissue itself. This edition will focus on the question: does stretching improve performance and thereby reduce the risk of injury?

Let’s begin by clarifying the role that stretching could play in reducing the risk of injury. The argument behind this proposed benefit is that it allows for increased muscle extensibility and decreased muscle stiffness via changes in the visco-elastic properties of muscle tissue. For purposes of a review, visco-elasticity of biological tissue implies that upon holding tissue under tension for a prolonged period of time, the force required to hold that tissue at that length is reduced over time. This implies that a tissue can therefore withstand more tensile force at a specific length without experiencing damage. This has obvious implications for athletic events in terms of reduction in injury and improvement in performance. If an athlete can achieve increased ROM without injury and by using less energy to achieve that range, then theoretically, that athlete’s performance will improve and her/his risk of injury will be reduced. However, as mentioned in May’s post, increases in ROM are achieved via increased stretch tolerance on the part of the subject in the long-term, rather than a change in the mechanical or visco-elastic properties of the muscle (Hollie et al, 2006, Magnusson et al., 1996). Also, in regards to improved performance, it is almost impossible to objectively measure performance during an actual athletic event due to the confounding variables inherently present. However, specific performance related outcome measures have been used in the literature to determine the effect of stretching on performance. Shrier in 2004 performed a review of the literature and found that of the 23 articles that looked at the effect of pre-exercise stretching on isometric force, isokinetic torque, or jumping height, 22 of them showed no benefit of stretching on the specific performance tests analyzed. So, what this means is that stretching technically has little to no effect on performance. But does it have an effect on other aspects of athletics, such as running ability?
Shrier (2004) also reviewed 5 studies on the effect of stretching on running performance. One study specifically looked at running economy and found that stretching had a positive effect improving the efficiency of running. However, the study only included runners with short and/or stiff hip flexors or extensor muscles. Therefore, whether or not stretching is beneficial for those who are fairly flexible in the above mentioned muscle groups remain unanswered. With respect to running speed, four studies were of acceptable quality to qualify for the review, and of those four, two had equivocal results, while the remaining two reported contradictory findings. One study reported beneficial results with dynamic stretching, while the other reported detrimental outcomes to running speed with static stretching. This may imply therefore, that dynamic pre-exercise stretching is beneficial when attempting to improve running speed and that static stretching may limit the ability to achieve optimal running speed.

Results from Shrier (2004) were the exact opposite when the literature on regular stretching was reviewed. What qualifies as “regular” varied between the seven studies that found benefit to stretching. Generally, regular stretching can be considered stretching that is performed at regular frequencies throughout a week period and for a predetermined length of time. This is in contrast to the stretching described above as “acute/pre-exercise” stretching where stretching is only performed before an athletic event or a specific bout of exercise. The evidence suggests that with regular stretching, force production and velocity of contraction both increase, while it had no effect on the economy of motion. As mentioned above, this is in stark contrast to the effects of acute/pre-exercise stretching on performance in which no effect was reported on isometric force production and velocity of contraction, but a positive effect was seen with running economy.

So, what does this all mean? Well, the benefit that you experience from stretching will depend on your specific goals. If your goal is to improve the efficiency of your runs, then acute/pre-exercise stretching performed dynamically may be of more benefit. But if your goal is to improve the amount of force and speed that your muscles can produce at a given length as in most athletic events, then regular bouts of stretching may be of benefit, and you should avoid acute/pre-exercise stretching all together. Now, in terms of injury prevention, the theory remains that with improvement in specific measures of performance, one would ultimately reduce the risk of injury. However, it is important to emphasize that you have to know when to use stretching and what type of stretching to use at that specific instance. This will be topic of Part 3 of this three part series that can be found in the next edition of my blog posts.

Here's to your Saúde!

Dr. Henry

References

Hollie, F., Simon, D., Harvey, L. A., and Gwinn, T. (2006). Can apparent increases in muscle extensibility with regular stretch be explained by changes in tolerance to stretch? Australian Journal of Physiotherapy 52: 45-50.

Magnusson, S. P., Simonsen, E. B., Aagaard, P., Sorensen, H., and Michael Kjaer, M. (1996). A mechanism for altered flexibility in human skeletal muscle. Journal of Physiology 497(1): 291-298.

Shrier, I. (2004). Does Stretching Improve Performance? A Systematic and Critical Review of the Literature. Clinical Journal of Sports Medicine 14: 267–273.