This is from another site by someone who's advice I always adhere to.
Stretching. Not only is stretching important for flexibility, it is CRITICAL for muscle growth.
When you think about gaining lean muscle, stretching is probably not the first thing that springs to mind. Every muscle in your body is enclosed in a bag of tough connective tissue known as fascia. Fascia is important for holding your muscles in their proper place in your body.
Because fascia is so tough, it doesn't allow the muscle room to expand. It is like stuffing a large pillow into a small pillowcase. The size of the muscle won't change regardless of how hard you train or how well you eat because the connective tissue around your muscles is constricting the muscles within.
The best example of this is the calf muscle. The lower leg is riddled with fascia because of its tremendous weight-bearing duties in the body. It is because of this fascia that many people have great difficulty developing their calves. The solution: stretching!
Using the pillowcase example from above, imagine you can expand the size of the pillowcase by stretching it. Suddenly, the pillow within has more room and will expand to fill that new space.
By stretching your muscles under specific conditions, you can actually stretch your fascia and give your muscles more room to grow.
The key to effective fascial stretching is the pump. The best time to stretch to expand the bags that are holding in your muscles is when your muscles are pumped up full of blood. When your muscles are fully pumped up, they are pressing against the fascia. By stretching hard at that time, you increase that pressure on the fascia greatly, which can lead to expansion of the fascia.
Be sure you do not stretch so hard that you cause the muscle to tear or cause injury to yourself. You will rapidly learn to distinguish the difference between a good stretch and a bad stretch. You should not feel any sharp pain, just a steady pull.
Hold each stretch for at least 20 to 30 seconds as you must give your fascia time to be affected by the stretch. Stretch hard like this only when you have a fully pumped muscle as you must give your fascia a reason to expand.