Mark Newcomb, who's chalked up first ski descents of Otterbody and Black Ice couloirs on the Grand Teton, hammers up local slopes like Snow King (an hour-long, 1,571-vertical-foot cardio blast) and Jackson Hole Mountain Resort (4,139 vertical) in the summer. "I just really enjoy covering ground," he says.
Body Benefit: Quad/Calf Strength + Eccentric Control
Trails build quadriceps and calf endurance, as well as strength, by improving eccentric control-a muscle's capacity to stretch while contracted-on downhills and uneven ground. "A lot of people have muscles that burn out quickly when they're skiing, because they haven't been worked eccentrically," says Keller.
Says Newcomb: "Sudden changes in snow quality and terrain require immediate reactions. If you're a runner and you work on improving your reaction time, your skiing will benefit."