Exercising too much doesn't affect only your muscles. It can have an affect on your kidneys, too. "Rhabdomyolysis is a little-known and rarely discussed condition in which the energy supplies in the body are depleted and then, through a series of cellular processes, muscle breakdown occurs and leads to potential kidney damage," said Phillip J. Adler, MA, AT, ATC, and manager of Spectrum Health Orthopedic & Sports Medicine Programs. "If you continue to damage the muscles to a point that is detrimental and you don't allow them to heal, you have the potential to build up CK levels in your blood that can lead to kidney problems. So if you frequently over-exercise and push your body beyond recoverable limits, you run the risk of rhabdo."
Adler said warning signs for rhabdo include darkened urine (like dark soda), muscle pain, weakness, stiffness, or cramping. He explained that the condition is often confused with heat stroke because of its similar symptoms.
He lists CrossFit and similar high-intensity workouts as a good example of exercise that may make you susceptible to rhabdo. "This is intense physical activity that pushes the body to its physical limits on a regular basis. The activity itself may not be harmful, but what is harmful is not allowing your body to recover because you feel like you need to continue to do more. Just like dosing limitations on over-the-counter medications, this is a situation where more is not always better. Some may be helpful, but too much could kill you."