Physiologists at Columbia University Medical Center in New York suggest the leak weakens muscle force and turns on a protein-digesting enzyme that damages muscle fibers.

The researchers' first author, Andrew Bellinger, and senior study author, Dr. Andrew Marks, said the process produces extreme fatigue.

The study, published online in the Proceedings of the National Academy of Science, shows that fatigue in both patients and athletes probably stems from the same leak.

The leak in mouse muscle was found after an intense three-week daily swimming regimen. The leak was found in human athletes after three days of daily intense cycling.

"People with chronic heart failure are subject to this same kind of muscle leak and damage constantly even without doing any exercise," Marks said in a statement. "One of these patients' most debilitating symptoms is muscle weakness and fatigue, which can be so bad they can't get out of bed, brush their teeth, or feed themselves."