Rhabdomyolysis or "rhabdo" occurs when a muscle is so broken down that it releases a protein called myoglobin into the blood stream. The myoglobin is then filtered out of the body by the kidneys and breaks down into substances that can damage kidney cells. This can cause kidney failure. Rhabdo has been talked about a lot in the popular media – especially in conjunction with CrossFit – but it is extremely rare. Kaeding says that people most at risk for developing rhabdo are very serious athletes who are working out a part of their body they usually don't.