When the weather turns cold and the sidewalks become covered with snow, more exercisers turn to indoor cardio machines as an alternative to outdoor exercise.

Cardio machines can be a great alternative to outdoor winter exercise or a fantastic cross-training tool. Everyone has his or her preferred cardio machine, but what happens if your favorite treadmill is taken? What machine do you use? Which one is the best?

As with everything in the fitness industry, it's not that simple. Each piece has its pros and cons, and each is ideal for achieving a slightly different fitness goal. Choose the cardio piece that most closely matches your goals, and you will be delighted with the results.

STAIRMASTER: Hands down, this is the most popular piece of equipment in the gym. Most people, however, don't use the Stairmaster properly and therefore don't receive the greatest benefit. Hanging on the rails and taking mini steps can reduce the efficiency of a Stairmaster by as much as 75 percent. To get your best workout, slow down and take larger steps. When used properly, it can be a great calorie spender.

ELLIPTICAL TRAINERS: This relative newcomer to the gym is a kinder, gentler piece. The pedals move in an elliptical motion, hence the name. Your feet never leave the pedals and the motion is very smooth. Because of its design, the elliptical trainer is a great option for individuals who do extremely long cardio workouts and those with orthopedic concerns like back and knee problems. (As always, if you are injured, check with your doctor first.) Calorie for calorie, an elliptical trainer is just about as effective as a Stairmaster.

STATIONARY CYCLE: For non-weight-bearing calorie burning, try the stationary bike. You can't beat its ease of operation. Most of us have been bicycling since we were young. You don't have to worry about balance, or about how close you are to the end of a moving belt. On a stationary bike, you just sit and pedal. The benefits include a fair amount of calories burned per minute, and you can read or study while you are cycling. For increased calorie burning, try incorporating one-minute sprints throughout your ride.

RECUMBENT BICYCLE: This machine is a combination stationary bicycle and La-Z-Boy recliner. It has a chair-like seat to support your back, which can also be easier on your hind parts. As for getting a tough workout, recumbent bikes are as effective as upright models. This is a great piece to use occasionally as a change of pace. Very few other activities use the same muscles as a recumbent bike.

TREADMILL: The line for treadmills is almost always long for two reasons. First, the price tag for a treadmill (around $5,000 to $7,000) prevents most gyms from having many of these machines. Second, people love treadmills. Walkers and runners can continue their training into the winter months thanks to the invention of the treadmill. But don't be fooled into thinking that putting in time on a treadmill is all you have to do. Make sure the speed is fast enough to challenge you. And if your objective is to burn calories, put it on an incline.

STEPMILL: Make no mistake -- this machine, formerly called the Gauntlet, is the toughest piece of cardio equipment in the gym. This is the machine that looks like actual steps coming at you in a treadmill fashion. Watching someone step slowly on a stepmill might look easy, but it is not. For those of you who want maximum sweat for your cardio time, check out this machine.

No single cardio piece is "better" than another. Each offers a unique set of benefits. It is best to rotate different pieces in and out of your workout schedule. For example, spend 20 minutes each on the treadmill, Stairmaster and elliptical trainer. The next day, substitute the stepmill for one of those pieces.

The last hint to navigating the cardio room is to become familiar with the Cardio Theatre system, bring a book or MP3 player, or work out next to a friend. The more enjoyable you can make your workout, the faster the time will pass, and the more likely you are to stick with it.