In brief: Healthy people as well as highly trained athletes commonly experience delayed muscle soreness after severe exercise. Although many theories have been advanced, the pathophysiological basis for delayed muscle soreness is not known. The authors studied muscle biopsy samples from ten male runners immediately before and after a marathon and at one, three, five, and seven days after the marathon. Results showed muscle degeneration after training and after the marathon, and there was a significant inflammatory response throughout the week of recovery after the marathon. Therefore, the authors suggest that anyone experiencing postexercise delayed soreness is probably suffering from some degree of acute inflammation.