Static stretching is when you lengthen the muscle and hold it. Some people believe that by holding the muscle in a static stretch it causes the muscle to seize. You also place a fair amount of stress on tendons and ligaments. If you are training and are fatigued then the soft tissue is weakened and damaged already and your risk of injury is increased.
Possibly a safer way to loosen up is Ballistic stretching done GENTLY. Ballistic stretching is using momentum to elongate the muscle and is more sport specific. Below is a simple way to loosen up before a run. This stretch is also very good for strengthening the supporting leg. It has to stabilize your body and this causes all the little muscles in your lower leg to fight the momentum of the swinging leg. To find out how much it has to fight... do this exercise for 1 minute straight and you'll feel the supporting leg start to fatigue.