Gabe Mirkin, M.D.
Your muscles should feel sore on some days after you exercise. If you go out and jog the same two miles at the same pace, day after day, you will never become faster, stronger or have greater endurance. If you stop lifting weights when your muscles start to burn, you won't feel sore on the next day and you will not become stronger. All improvement in any muscle function comes from stressing and recovering. On one day, you go out and exercise hard enough to make your muscles burn during exercise. The burning is a sign that you are damaging your muscles. On the next day, your muscles feel sore because they are damaged and need time to recover. Scientist call this DOMS, delayed onset muscle soreness.
It takes at least eight hours to feel this type of soreness. You finish a workout and feel great; then you get up the next morning and your exercised muscles feel sore. We used to think that next-day muscle soreness is caused by a buildup of lactic acid in muscles, but now we know that lactic acid has nothing to do it. Next-day muscle soreness is caused by damage to the muscle fibers themselves. Muscle biopsies taken on the day after hard exercise show bleeding and disruption of the z-band filaments that hold muscle fibers together as they slide over each other during a contraction.
Scientists can tell how much muscle damage has occurred by measuring blood levels of a muscle enzyme called CPK. CPK is normally found in muscles and is released into the bloodstream when muscles are damaged. Those exercisers who have the highest post-exercise blood levels of CPK often have the most muscle soreness. Using blood CPK levels as a measure of muscle damage, researchers have shown that people who continue to exercise when their muscles feel sore are the ones most likely to feel sore on the next day.
Many people think that cooling down by exercising at a very slow pace after exercising more vigorously, helps to prevent muscle soreness. It doesn't. Cooling down speeds up the removal of lactic acid from muscles, but a buildup of lactic acid does not cause muscle soreness, so cooling down will not help to prevent muscle soreness. Stretching does not prevent soreness either, since post-exercise soreness is not due to contracted muscle fibers.
Next-day muscle soreness should be used as a guide to training, whatever your sport. On one day, go out and exercise right up to the burn, back off when your muscles really start to burn, then pick up the pace again and exercise to the burn. Do this exercise-to-the-burn and recover until your muscles start to feel stiff, and then stop the workout. Depending on how sore your muscles feel, take the next day off or go at a very slow pace. Do not attempt to train for muscle burning again until the soreness has gone away completely. Most athletes take a very hard workout on one day, go easy for one to seven days afterward, and then take a hard workout again. World-class marathon runners run very fast only twice a week. The best weightlifters lift very heavy only once every two weeks. High jumpers jump for height only once a week. Shot putters throw for distance only once a week. Exercise training is done by stressing and recovering.