Muscle stiffness and soreness is the result of tiny tears in the muscle fibers. The body responds with fluid and white blood cells to the area to help the healing process. An inflammatory response occurs that's often more than the body really needs. All that activity in the muscle turns on the cells that pick up messages of pain and distress.

Whenever we use (or overuse) a muscle in a way it's not used to, we can end up with sore muscles. Even if you think you are doing the same thing as always, there are times when we change the routine just enough that the muscle reacts as if it's new training.

Other factors enter in, too. Fatigue and dehydration affect the muscles' ability to contract repeatedly without microscopic damage. When we contract a muscle and then length it slowly it's called an eccentric contraction. It's like putting on the brakes during an activity or movement.

Muscles aren't trained as well in eccentric motions and are more likely to get sore after activities requiring eccentric contractions. Since it doesn't hurt at the time, it's easy to over do an activity or motion. This is especially true for new exercises not done before.