If we had to pick only one cross-training activity, this would be it. It builds a solid aerobic base and taxes your anaerobic system, prepping you for intense efforts such as tight bumps and jump turns on steeps. Climbs strengthen your lower body, while downhills (if you don't sit) teach your legs to serve as shock absorbers. Both build muscle endurance. Mountain biking also employs ski techniques such as shifting your weight, maintaining a quiet upper body, adjusting your balance in response to changing terrain, keeping your eyes ahead to select your line and staying relaxed while hurtling downhill at high speeds. Another bonus: Once you've flipped over your handlebars, trashing your body and scraping your face over rocks and dirt, wiping out on snow will seem like child's play.
What It Works quadriceps, hamstrings, calves, hip flexors and extensors, lower back, abdominals
Insider's Tip The hamstrings are used in every phase of the pedaling stroke, but the greatest effort usually comes from the quads. Because skiing also emphasizes the quads, your hamstrings and quads may already be out of balance. (How to tell? If the back of your legs burn or cramp during climbs, this likely applies to you.) To reduce your chance of injury and to keep from getting farther out of whack, try hamstring bridges. Lie on your back with your knees bent and your feet flat on the floor. Keep your abs tight, and raise your pelvis up toward the ceiling until your body forms a straight line from your knees to your shoulders. Slowly lower your pelvis to about an inch above the ground. Do three sets of 12 reps.