Two years after breaking my leg in a freak running accident, I was logging up to 100 miles a week on the treadmill in preparation for a 36-hour adventure race. A veteran of 15 marathons and countless other athletic events, I was in peak physical shape. Or so I thought—until one Sunday morning when I could barely lift my arms. After years of lifting weights, I was too tired to lift the laundry basket. My own fitness, it seemed, had felled me. Was it overtraining? Had I pushed so far beyond my limits that my body could no longer keep up?