Ditch the archaic warm-ups and static stretching sessions in favor of movement-based preparation. Research shows that not only are dynamic warm-ups preferable for performance, the simple act of lifting develops flexibility and builds muscle at the same time.
A recent study used 240 untrained women in their mid-thirties and split them into four groups: a control group, a strength training group, a strength training and flexibility, and a flexibility group.
All three training groups demonstrated gains in flexibility and interestingly, the strength training group alone increased flexibility by six centimeters, a significant amount. The strength training and flexibility group improved by 12 cm, while the flexibility group improved by 13 cm.
Reasons for the dramatic flexibility gains in the strength training only group are not completely understood, but possibilities include neuromuscular adaptations resulting in muscular and connective tissue plasticity, and a reflex activity in the Golgi Tendon Organs (mechanisms that when activated lead to a relaxing of the muscle).
Additionally, it’s possible that strength exercises increase tension in tendons and ligaments, improving muscle contractility, which may lead to a greater range of motion.
Always do a low-load warm-up and then spend your training time using exercises with a full range of motion to get the most bang for your buck. If you have specific flexibility goals, add stretching exercises to your program post-workout for optimal strength and mobility.