Studies have been showing over the past decade or so that resistance training (RT) improves flexibility. Furthermore, it reduces your risk of lengthening injuries – like a hamstring tear during running, or back strain from bending – by not only increasing the ability of the muscle to pull more load but also resisting a load that’s pulling it. In other words, when a force is greater than the tension of the muscle, the joint will move as the muscle lengthens, rather than shortens. Think of bending over to tie your shoes – your hamstrings, glutes, and low back muscles lengthen to allow gravity to lower your upper body down. When they contract to raise you back to vertical, they shorten.
Why the science lesson? If you do RT through a large range of motion, muscle will get stronger even at the extremes of it’s range. muscle gets stronger by three mechanisms: first, neurological; then collagen deposition which makes it tougher; finally, hypertrophy, or protein accretion. When you stretch, you get some of the first two but little of the third. When you do RT you get them all. Therefore, when you stretch to tie your shoe, your muscles have learned to go through that range but have the collagenous strength to withstand the load at that angle, and the strength to pull you back up safely.
Bottom line – strength train throughout a joint’s range of motion.