IF YOU NEVER MISS A SPIN CLASS: HIT THE BIKE LANE

WHY SWITCH: There's no wind resistance in a gym. You'll get an overall better cardiovascular workout when the two wheels actually take you somewhere.

THE PERKS: "People who ride outside tend to go farther because they aren't confined to a 45-minute class. And you can expect your legs to fatigue very fast because you'll be working your hamstrings more intensely," says Jade Alexis, who teaches Spin classes at Reebok Sports Club/NY.

THE CAVEAT: In class, you often get up off the seat to let your legs rest; outside, it's rare. The only time you're off your seat is to push yourself up a hill.

HOW TO GET STARTED: Check out USA Cycling's state-by-state database for group rides. (usacycling.org/clubs)

IF YOU HOG THE TREADMILL: GO FOR A RUN
WHY SWITCH: Treadmills are self-propelled, so you won't work your glutes and quads as hard, says Jose Gomez, assistant director of personal training at NYC's Peak Performance.

THE PERKS: You'll burn up to 5 percent more calories and get flatter abs faster. "You're no longer moving linearly—adjusting your body to the terrain forces you to work the abdominals," says Lacey Stone, a personal trainer at Equinox Fitness Clubs in New York.

THE CAVEAT: For those with creaky joints, beware: Pounding the pavement increases the risk of stress fractures by more than 50 percent. Try running on grass or astroturf, which is actually easier on your knees than the treadmill's rubber belt, says Stone.

HOW TO GET STARTED: Niketown organizes free running clubs for all levels, from walkers to marathoners. (nikerunning.com/runningclub)

IF YOU LIKE THE ROWING MACHINE: RENT A KAYAK
WHY SWITCH: When you're on the water, there's no sliding seat, so you get a more intense upper-body workout.

THE PERKS: "You're twisting and turning to move the oars, and that will activate your core, which means a much larger caloric expenditure," says Dave Canter, national director of fitness for Sports Club/LA.

THE CAVEAT: The rowing machine is one of the most challenging total-body exercises. Outside, you'll lose the leg workout.

HOW TO GET STARTED: Act like a tourist and rent one for a Saturday-afternoon workout. Burns more calories than sunbathing in the park.

IF YOU CLOCK AN HOUR ON THE ELLIPTICAL:SWIM LAPS AT THE POOL
WHY SWITCH: "We call the elliptical trainer the dry pool," says Stone, citing the similar range of motion between cross-training on the machine and swimming. "There's absolutely no impact, so you won't feel anything in your joints. And swimming's a good place to start for those who are overweight or have recurring joint issues," she explains.

THE PERKS: Again, you're not limited to linear movements, which means you're working a wider range of muscles. And it's a full-body workout, so the toned arms are an added bonus.

THE CAVEAT: It'll take you 33 laps in a typical pool to swim the equivalent of one mile.

HOW TO GET STARTED: Squeeze in a few laps at the hotel pool while on vacation or take a dip at your local Y.

IF YOU'RE HOOKED ON THE STAIRMASTER: TAKE A HIKE
WHY SWITCH: There are a thousand ways to cheat on a stair-climber. Simply leaning on the machine (and who doesn't?) skimps on your core workout.

THE PERKS: You're working toward a goal—it's no longer 30 minutes at level four. The unpredictable terrain means your muscles will continue to be surprised and work harder.

THE CAVEAT: There's no calling it quits in the middle of this uphill battle.

HOW TO GET STARTED: Grab a friend and find a local trail that fits your fitness level. (trails.com)

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