Many think that stretching before a workout can help prevent injuries. However, very little data exists to support this opinion. Stretching can, in certain cases, increase injury risk and will more than likely impair performance when done before a workout. Keep your warm-ups to dynamic movements like plyometrics, jogging, or any other form of warm-up that gets a light sweat going. If you still want to stretch, do so after your workout. Check out article, “Stretching and Performance,” here for more info.
Reference: Thacker, S. B., Gilchrist, J., Stroup, D. F., & Kimsey Jr, C. D. (2004). The impact of stretching on sports injury risk: a systematic review of the literature. Medicine & Science in Sports & Exercise, 36(3), 371-378.