Extreme endurance races may damage red blood cells, making them less flexible and accelerating their breakdown. The longer the distance, the greater the signs of cellular stress. Credit: Shutterstock Running extreme distances may do more than exhaust muscles. A study published in the American Society of Hematology's journal Blood Red Cells & Iron reports that ultra-endurance events can injure red blood cells in ways that may interfere with how they function. Researchers do not yet know how long