I stole this from a website. The link is posted below.
Summary: Stretching while pumped can increase muscle growth.
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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.
But your fascia may also be holding back your muscle growth. Think for a moment about your muscles. You train them and feed them properly. They want to grow and will grow but something is holding them back. They have no room to grow!
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 solution: stretching.
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.
Check out
http://www.theallineed.com/ad-health-6/health-044.htm to learn more.