When you exercise for a long time, your muscles start to burn and feel sore. This forces you to slow down, because keeping up the pace will make your muscles burn and hurt even more. You call this fatigue and tiredness, but a recent study from Japan shows that muscle fatigue is caused by damage to the muscle itself (Medicine & Science in Sports & Exercise, July 2005). This also explains why exercising long and hard enough to feel the burn for an extended period leaves your muscles sore for one or more days afterwards. Athletes call this Delayed-Onset Muscle Soreness (DOMS) and they learn that they have to have this next day soreness to improve for competition.

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