There's no more efficient cardio exercise than running. Take it off-road, and the perks for skiers multiply. A trail-running workout can be either aerobic or anaerobic, and it strengthens the entire lower body-even the smaller stabilizer muscles, which kick in to keep you upright on uneven surfaces. Rugged trails also mean more lateral movement and less chance of repetitive stress injury than with road running, in which every footstrike is the same. Uphill efforts tax your heart and lungs and build strength in your hamstrings, glutes and lower back. Downhill cruises recruit your quads and abdominals to serve as shock absorbers and stabilizers, much as they do in skiing. In fact, downhill running technique mirrors that of skiing: eyes ahead to find your line, quiet upper body, legs making quick adjustments to terrain. Fail in any of those areas, and you may be eating dirt.
What It Works quads, hamstrings, gluteals, calves, lower back, abdominals
Insider's Tip When running up steep hills, try to step with a heel-to-toe motion rather than with just your toe. That will reduce stress on your calf muscles. On downhills, keep your steps light and springy to lessen the beating your joints will take.