THE DANGER of overtraining syndrome for professional long-distance runners:

The earliest known scientific reference to OTS was made by a researcher and athlete named Robert Tait McKenzie, who noted in his 1909 book, Exercise in Education and Medicine, an acute exhaustion and “slow poisoning of the nervous system which could last weeks or even months.” Decades later, renowned South African exercise-science professor Timothy Noakes wrote in detail about the condition in The Lore of Running. Published in 1985, it’s one of the few books athletes with OTS have as a reference. The runners Noakes examined had pushed themselves to a point at which their bodies—and, more perplexingly, their minds—had simply stopped responding. As a result, they suffered everything from “generalized fatigue” and “recurrent headaches” to “an inability to relax, listlessness,” and “the swelling of lymph glands.” . . .

Through some combination of excessive exercise and inadequate recovery, athletes experience a severe shock to the parasympathetic nervous system, which controls the body’s inflammatory pathways. Under normal circumstances, when your body is stressed, your sympathetic nervous system kicks in to help you respond. Your heart races. Your pupils dilate. The blood rushes from your digestive system to organs needed for immediate survival. The parasympathetic system is the counterbalance, bringing the body back to a state of equilibrium. After a hard outing, your heart rate calms and blood returns to your extremities, restoring your body’s normal functions. For athletes with OTS, those balancing responses no longer occur. The parasympathetic system effectively goes haywire.