When you're running up stairs or out on a jog, your muscles eventually start to feel heavy and weak. That's fatigue setting in, a sign that the muscles’ energy reserves are becoming depleted. But a team of researchers led by Rensselaer Polytechnic Institute (RPI) biology professor Doug Swank, Ph.D., have discovered something surprising: certain muscle fibers have a built-in backup system that fights back against fatigue, potentially helping us keep going when we'd otherwise have to stop.