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Muscle pain frequently occurs after strenuous activity. This study examines two forms of muscle pain: pain which begins during exercise, usually referred to as muscle strain; and pain which begins hours or days after strenuous exercise, commonly called delayed-onset muscle soreness. Although there has been a great deal of research dealing with muscle pain, not much is known about its pathology. This is due in part to the inadequacies of current methods for evaluating (by clinical examinations, blood enzyme tests, and imaging procedures) the precise muscle abnormality and the severity and duration of the muscle damage. Researchers used magnetic resonance (MR) imaging to assess damage to muscle tissue in sedentary subjects and marathon runners who experienced muscle strain and delayed-onset muscle soreness. MR imaging provided information not available at clinical examination. Areas of abnormality were identified in individual muscles. The presence of large collections of blood (focal hematoma) and muscle herniation were ruled out; both conditions require surgical repair. MR images also confirmed the belief that exercise-induced muscle pain does not cause long-term muscle damage. MR images demonstrated abnormalities in muscle tissue beyond the time that was indicated by other methods of evaluation. MR images revealed abnormalities up to three weeks longer than the duration of patients' symptoms. This means that MR images provide more accurate information about muscle recovery. In comparing the two groups of subjects it was observed that muscle abnormalities usually involved smaller areas in the marathon runners than in the sedentary group.