IT SEEMS ABSURD. How could two humans -- seated for hours, exerting themselves in no greater manner than intermittently extending their arms a foot at a time -- face physical demands? Still, the evidence overwhelms. The 1984 World Chess Championship was called off after five months and 48 games because defending champion Anatoly Karpov had lost 22 pounds. "He looked like death," grandmaster and commentator Maurice Ashley recalls. In 2004, winner Rustam Kasimdzhanov walked away from the six-game world championship having lost 17 pounds. In October 2018, Polar, a U.S.-based company that tracks heart rates, monitored chess players during a...