By training, he didn’t mean simply his recent exercises in breath holding, although there had been plenty of them during the previous year. Each morning he’d do a series of ordinary breath-holds (starting with regular air instead of pure oxygen) separated by short intervals, gradually increasing the duration and the pain. Over the course of an hour, he’d end up holding his breath for forty-eight minutes, and then he’d have a pounding headache for the rest of the day. Those daily breathing drills got his body used to the pain of carbon dioxide buildup.
