Strenuous exercise can result in muscle injury that may persist for 2 weeks. Our purpose was to determine if muscle injury can be detected with 31-P magnetic resonance spectroscopy. Normal subjects performed repeated lengthening contractions with either arms or legs designed to result in mild muscle injury. One hour after the arm exercise, there was a significant increase in the inorganic phosphate to phosphocreatine ratio (Pi/PCr), with the maximum increase in Pi/PCr occurring 1 day postexercise (0.12 +/- 0.01 to 0.21 +/- 0.05). Pi/PCr remained elevated for 3-10 days. Similar results were seen following the leg exercise protocol. ATP/(Pi + PCr) decreased in all the arm exercised subjects. Exercise protocols that did not contain lengthening contractions did not result in changes of Pi/PCr or ATP/(Pi + PCr). Patients with various neuromuscular diseases with evidence of muscle damage (elevated CK, muscle soreness, and histopathological findings) also showed increased Pi/PCr at rest. We conclude that elevated Pi/PCr at rest can reflect nonspecific muscle damage in normal and diseased subjects.