Why We Stretch

Jules Mitchell in her book Yoga Biomechanics: Stretching Redefined asks this question to explore the scientific literature on stretching for a deeper understanding of biomechanics. The book  provides a unique evidence-based exploration into the complexities of human movement. The following article is an excerpt of the book, courtesy and copyright of Handspring publishing.

Surprisingly, the bulk of the research on the effects of stretching is relatively new. In the last 10–20 years, when papers on the subject were first published, the data on static stretching began to reveal that influences on performance, injury prevention, and ROM may not be as positive as we originally assumed. This discovery caused a reversal of opinion in many fitness circles and, in some cases, stretching was even vilified. Fortunately, continued research has shown that those conclusions may have also been premature. There is still much we do not understand and, for now, the conclusions lie somewhere in the middle – stretching is sometimes good, but not that good, and when good, only under certain conditions.

What the body of literature lacks in tenure, reliability, validity, and reproducibility it makes up for in volume. Literally hundreds of papers set out to determine once and for all why we should or should not stretch. It can be quite dizzying as they examine multiple types of stretching of varying dosages against different controls on diverse populations. Fortunately, a recent collaboration between the top stretching researchers produced a systematic review summarizing the results of the high-quality RCTs published to date (Behm, Blazevich et al., 2016).

This book sets out to explore how tissues behave under tension beyond the constraints of conventional stretching. We will rely on this current and high-quality review to provide us with a condensed, yet focused, summary of stretching outcomes so that we may move on to explore other biomechanical principles and how they fit into yoga asana.

Regarding sports performance, the researchers assembled the data into multiple configurations to establish various relationships. To begin, all types of stretching were evaluated for acute influences on overall performance (whichever specific performance outcome was being measure in any given paper). Acute effects of stretching refers to the results immediately (usually within 1 hour) after a stretching bout. Stretches under 60 seconds resulted in an average decrease in performance by 1.1% and stretches over 60 seconds decreased performance by 4.6%. Whereas both results had a negative impact, the evidence may only have clinical significance in highly competitive situations like training for the Olympics. The results may not be compelling enough to advise a recreational athlete who enjoys his stretching routine prior to his sport, and who feels better because of it, to forgo it.

When the same data were rearranged to divide performance into the categories strength and power, the numbers told a different story. The acute effects of stretching resulted in a 4.8% deficit in strength, but only a 1.3% deficit in power-speed. The caveat here is that the duration of the stretches in the strength data was longer and we have no way of determining if time in the stretch was a factor in the greater deficit.

The variable, strength, is measured by how much weight someone can move, power is measured by the speed at which one can move said weight. Power has a time component to it, strength does not. In the following chapter, when we discuss the mechanical behavior of connective tissue, we may gain insight into why strength and power may respond differently to stretching.

Arranged by type of stretch, static stretching diminished overall performance by 3.7% and PNF by 4.4%, but dynamic stretching improved performance by 1.3%. Anyone familiar with the literature expects dynamic stretching to improve performance as it has been the recommended pre-activity stretching method for roughly the last decade. ACSM suggests engaging in static stretching or PNF either post-activity or entirely separate from the activity or sport.

Static stretching affected tasks requiring short range performance negatively by 10.2% but positively by 2.2% in tasks requiring long range performance. It seems that specificity, as should be expected, applies again – end range training improves end range performance.

Concerning injury, the acute effects of static stretching or PNF seem to slightly reduce injury frequency in muscle injuries relating to sprinting type activities but not endurance sports. For overuse injuries and other “all-cause” injuries, stretching did not appear to have any effect. In any case, there appeared to be no adverse effects from pre-activity static stretching, rendering the intervention harmless but only somewhat beneficial, and sometimes even slightly detrimental, depending on the performance goals.

The authors examined dozens of other configurations in addition to the few I selected above. Among the many outcome variables they examined, those I have highlighted tell us enough of the story for our purposes here. I mention this for full transparency, so it does not seem I am cherry-picking data to support my own opinions. I encourage you to read the paper in its entirety if the subject matter intrigues you (Behm, et al., 2016).

As per our discussion on why we stretch, it appears the evidence does not hold up to the popular consumer belief about performance and injury prevention. One issue with measuring performance and injury prevention as primary outcomes, however, is that there are any number of internal factors and external environmental conditions that could influence outcomes. This complicates study design for the long-term effects of stretching on performance. If the subjects continue to train, any long-term benefits could be a result of the actual training. If subjects discontinue training, we would expect to see deficits and injuries, but I’m not aware of any studies looking at the effects of stretching on injury occurrences in sedentary populations as that would seem irrelevant. I’m also unaware of any studies designed to test whether stretching causes injuries because that is not the theory sports scientists are striving to validate.

The emerging yoga narrative about the stretching injuries that yoga may cause will be discussed in future chapters. We must first exhaust our investigation of conventional stretching and how the body responds and adapts to it. This is discussed in detail in  Yoga Biomechanics: Stretching Redefined .

This article is an excerpt from Yoga Biomechanics: Stretching Redefined copyright of Handspring publishing (2019).

Reference:

Behm, D.G., Blazevich, A.J., Kay, A.D. and McHugh, M., 2016. Acute effects of muscle stretching on physical performance, range of motion, and injury incidence in healthy active individuals: a systematic review. Applied physiology, nutrition, and metabolism, 41(1), pp.1-11. https://www.nrcresearchpress.com/doi/full/10.1139/apnm-2015-0235