Your muscles should feel sore on some of the 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 just start to burn, you wont 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. Scientists 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 was caused by a buildup of lactic acid in muscles, but now we know that lactic acid has nothing to do with 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.

Nobody really knows how these hard bouts make muscles stronger, but the most likely theory depends on the fact that hard exercise damages muscle fibers. Then other cells release chemicals called cytokines that cause inflammation characterized by soreness (pain), increased blood flow to the injured fibers (redness), and increased flow of fluid into the damaged area (swelling). The damaged muscle cells release tissue growth factors to heal the damaged muscle fibers, and if the athlete allows the muscle soreness to disappear before exercising intensely again, muscle fibers become larger and increase in number by splitting to form new fibers. If the athlete does not wait until the soreness goes away before exercising intensely again, the fibers can be torn, the athlete becomes injured, and the muscles weaken.

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 another intense workout until the soreness has gone away completely. Most competitive athletes exercise at low intensity during recovery, rather than taking days off; this makes their muscles more fibrous and resistant to injury. If youre not interested in competing, you can take one or more days off until your muscles feel fresh again.