Stretching? Slow down!

Have you ever been to a doctor and had your reflexes checked? You're lying quietly on the bed, the doctor approaches with what looks like a a very large pin, a patella hammer, and proceeds to tap your arms at your elbow and your legs at your knees. Involuntarily, to your surprise, your arm flexes and your knee moves.  This is a reflex.

What just happened? The gentle tap by the hammer stretched your muscle in a fast and furious way. The muscle protects itself by a quick sudden contraction so that it won't be torn. 

Consider stretching before a run. If you stretch the muscle too fast you will cause a similar reaction to the the hammer - the muscle will contract. That's why you need to stretch in a slow careful manner giving the muscle time to lengthen. Don't force it.  When you feel that you have reached the end of your range, wait, stop, breath, take your time and you'll find the stretch will be much more effective