Ahh you get going, your feeling good and suddenly you can't move your legs seize up and you get a funny taste in your mouth, and you have to limp back beaten by your lazy body, not to mention the soreness that you're going to get in a few hours.
It’s the excessive mechanical force which results in damage to the muscle or connective tissue, which is really the muscle pain you feel after excercise. The body responds by repairing it and this stimulates sensory nerve endings which results in pain. This is why it usually occurs when you first start an exercise program, or when you train at an unusually high intensity.