Runners are typically told to stretch before beginning their runs to prepare the muscles for running. To do this, they do passive or static stretches. But, doing static stretches, especially to a “cold” muscle, relaxes the muscle and can be injurious. Active stretches, however, activate and warm up the muscle in preparation for action.
Because static stretching of a cold muscle can cause injury, many runners are advised to do their stretching after the run when the muscles are warmed up. Sometimes such stretching is useful in helping to prevent soreness.
However, if the muscle is fatigued, the stretching can be dangerous because its built-in mechanisms to prevent further extension have been weakened. If you go beyond the normal limits of muscle and connective tissue extension around the joint you can easily overstretch the ligaments and tendons. Thus, stretching at this time is not recommended unless you want to increase flexibility.
Runners do not need great amounts of flexibility.Because of this, you should ask yourself what the reason is for any stretch. Merely stretching for the sake of stretching can be counterproductive. See Explosive Running or Build a Better Athlete for discussion on active and passive stretching.