Nothing's been proven yet. All we have are theories right now. Some researchers think stretching decreases joint stability. This makes joint movement less efficient. Tendons and muscle tissue normally absorb energy to prevent injury. An unstable joint could decrease the soft tissues' ability to do this.

Stretching too far can also change the position of the joint. Dangerous loading effects along with decreased strength occur while the muscle is still recovering from the stretch. The changes may lead to tissue damage.

The President's Council isn't saying not to stretch. From the results of research thus far, it seems best to warm-up first before stretching. Conditioning and strength training are also important.