The onset of severe muscle cramps in the course of road racing led a champion to undergo a muscle biopsy. Fuscinophilic deposits were detected around muscle cell fibers together with a few PAS positive deposits. These findings were related to the accumulation of abnormal mitochondria and glycogen granules. The subsequent study of the mitochondrial respiratory chain did not reveal any modification and so ruled out a genuine mitochondrial myopathy. Such mitochondrial modifications are probably related to the onset of muscle cramps, but no metabolic disturbance could be found to account for these two phenomena.