The terms stretching and flexibility are often used interchangeably but this can lead to confusion with respect to providing practical advice. Stretching is something that you do, while flexibility is something that you have. It is true that we stretch to improve flexibility, but the reality is that our flexibility is mostly inherited. Habitual activity will alter our general flexibility. Baseball pitchers have greater external rotation motion in their throwing shoulders compared to the non-throwing shoulder. But the non-throwing shoulder of a baseball pitcher has more external rotation motion than a non pitcher. So, there is a combination of acquired motion and inherited motion.^1,2 

There is a general belief that more flexible people are at decreased risk of injury in sports and physical activities and that stretching will decrease the risk in less flexible individuals. However, flexibility is not a strong predictor of injury and stretching does not universally reduce injury risk.^1,2,3 The risk factors for injury are sports specific and injury specific. For example, hamstring stretching prior to a soccer match will likely decrease the risk of a hamstring strain but the same stretching will be of no benefit for a marathon runner.^2,3

When considering the role of stretching versus flexibility it is also important to understand the difference between muscle injury and normal exercise-induced muscle damage (sometimes referred to delayed onset muscle soreness – DOMS). Stretching prior to exercise can decrease risk of muscle injury^2,3 but does not affect exercise-induced muscle damage.⁴ The opposite is the case for flexibility. Less flexible individuals have stiffer muscles⁵ and people with stiffer muscles are more susceptible to exercise-induced muscle damage.⁶ However, less flexible individuals are generally not at increased risk for injury.¹ More importantly decreased flexibility is beneficial in some instances. There is a reason why most long-distance runners are very inflexible. Less flexible individuals expend less energy while running (have better running economy) and running economy is a good predictor of race performance in trained runners (better than a measure of aerobic fitness).¹ 

One final area of confusion that needs to be considered is the effect of stretching on muscle strength and performance.³ A lot of good research has clearly demonstrated that immediately after a bout of stretching maximal strength is decreased. In some studies it was shown that the muscles are not activated fully after stretching. This led to a lot of recommendations to no longer employ stretching as part of the warm-up routine before sports. However, it has since been shown that such recommendations belied a misinterpretation of the research. Any weakness due to inhibitory effects of stretching is transient and when followed by active warm-up is inconsequential. Furthermore, stretching changes how muscles produce force; after stretching we are weaker at short muscle lengths but stronger at long muscle lengths. This is likely important for preventing muscle injuries.³

The role of stretching and flexibility in sport and physical activity is more complicated than is generally appreciated. There are lots of good questions but few simple answers.

Sources:

McHugh MP, Cosgrave CH. To stretch or not to stretch: the role of stretching in injury prevention and performance. Scandinavian Journal of Medicine and Science in Sports. 2010 Apr;20(2):169-81.

Gleim GW, McHugh MP: Flexibility and the effects on performance and sports injuries. Sports Medicine. 24: 289-299, 1997.

Behm DG, Blazevich AJ, Kay AD, McHugh M. Acute effects of muscle stretching on physical performance, range of motion, and injury incidence in healthy active individuals: a systematic review. Appl Physiol Nutr Metab. 2016 Jan;41(1):1-11.

Nesse M, McHugh MP. The effect of static stretching on strength loss and pain following eccentric exercise. Medicine and Science in Sports and Exercise. 38:S387, 2006.

McHugh MP, Kremenic IJ, Fox MB, Gleim GW: The role of mechanical and neural restraints to joint range of motion during passive stretch. Medicine and Science in Sports and Exercise. 30: 928-932, 1998.

McHugh MP, Connolly DAJ, Eston RG, Kremenic IJ, Gleim GW: The role of passive muscle stiffness in symptoms of exercise-induced muscle damage. American Journal of Sports Medicine. 27 (5):594-599, 1999.

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