Here are four example articles that support the claims that yoga helps in injury prevention, increased flexibility and strength.

[1] Knapik et al. 1991. Preseason strength and flexibility imbalances associated with athletic injuries in female collegiate athletes. Am J. Sports Med.[2] Thacker et al. 2004. The Impact of Stretching on Sports Injury Risk: A Systematic Review of the Literature. Official Journal of the American College of Sports Medicine[3] Madanmohan et al. 1992. Effects of yoga training on reaction time, respiratory endurance and muscle strength. Indian J Psysiol Pharmacol.[4] Mahadevan et al. 2008. Effect of six weeks yoga training on weight loss following step test, respiratory pressures, handgrip strength and handgrip endurance in young healthy subjects. Indian Journal of Physiology and Pharmacology