Boxing has surged in popularity in recent years as a mainstream fitness activity, with many gym chains and health clubs offering classes and equipment for those interested in taking up the sport.

People at the gym explain how getting inside the ring turned their lives around. Andres Garcia is 18 but started at the gym when he was 12. “I always liked to fight; I was that troublemaker," he said.

The hardest part about boxing isn't always being punched in the face or body. For many fighters, the weeks and months leading ...