The old presumption that holding a stretch for 20 to 30 seconds — known as static stretching — primes muscles for a workout is dead wrong. It actually weakens them. In a recent study conducted at the University of Nevada, Las Vegas, athletes generated less force from their leg muscles after static stretching than they did after not stretching at all. Other studies have found that this stretching decreases muscle strength by as much as 30 percent.
When I was young, my athletic instructors prided themselves on promoting slow, long stretches, not the stupid bouncy ones that were common in the 1970's. It turns out we were wrong too, just in a different direction.