Muscle soreness explained

There are two types of muscle soreness:
muscle soreness that develops immediately following intense activity and delayed muscle soreness.
Muscle soreness that develops immediately following intense workout is caused by poor blood flow and lack of oxygen. With consistent training, your muscles will grow additional capillaries to increase blood flow and hence oxygen and nutrient delivery.
Delayed muscle soreness (one or two days after intense workout session) is more likely due to damage to the muscles and/or connective tissues.

If you get muscle soreness after 12-36 hours, than you know that you really had a hard workout and really destroyed your muscles. Injured tissue produces chemicals called prostaglandins that co-ordinate the inflammation process. This creates swelling, that causes pain in the muscle. The most bodybuilders or serious lifters get sore second day after workout and consider the soreness a good pain. Most people think that if they stretch after workout, soreness will be prevented. Stretching does not prevent soreness, since post-exercise soreness is not due to contracted muscle fibers.