Tyler, Texas

On a recent Saturday morning, Bob Pfohl raced up six flights of stairs, hauled a fully charged hose 75 feet and dragged a 175-pound body another 100 feet. The 54-year-old career firefighter from San Diego wasn't responding to a call, but competing in the Firefighter Combat Challenge, a nationwide competition that tests the physical fitness and stamina of thousands of firefighters every year.

Often...