Here are the calorie expenditures for five popular machines (based on a 180-pound person working for 30 minutes at a perceived exertion between "hard" and "somewhat hard"). We got the pros and cons from John F. Graham, MS, CSCS, secretary-treasurer of the National Strength and Conditioning Association and vice president of Orthopedic Associates of Allentown (Pennsylvania), a fully equipped human performance center. Choose wisely.

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1) Upright Bike

Calories Burned: 350-400

Pro: It's fairly easy to get the hang of, so most people can feel comfortable right away. It's also less stressful on the knee joints than weight-bearing equipment.

Con: No matter how ergonomic the seat is, low-back stress from spinal compression can be a problem.

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2) Treadmill

Calories Burned: 450-475

Pro: Walking and running are natural activities, so most people can do them for a long period right away.

Con: Rapid increases in speed and/or incline can change gait patterns and cause injury. Use very gradual speed increments, don't go above a 3%-4% incline and use speed as the primary intensity adjustment.

3) Recumbent Bike

Calories Burned: 350-375

Pro: A wider seat with a backrest makes this a better option than the stationary bike for anyone with low-back problems.

Con: Sitting for long periods can still be uncomfortable for some people. Also, people with foot control problems such as foot drop (a weakness in the ankle and toe muscles) may have trouble because the feet can slip out of the pedal straps.

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4) Elliptical Machine

Calories Burned: 420-429

Pro: It's less weight-bearing than a treadmill, and it's fairly easy to use. Plus, many come with upper-extremity attachments so you get a better overall workout.

Con: Depending on your size, the movement arc on the machine can be either too large or too small. A small person may be placed in an awkward position, throwing off the natural gait; a large person may not get enough of a stride for a good workout.

5) Stair-Stepper

Calories Burned: 430

Pro: It's challenging because essentially you're climbing uphill the entire workout. It's also less weight-bearing, so it can be an excellent rehab for most knee injuries.

Con: There can be a long learning curve. People often hold on way too much with their arms, and that can take away 30% of the effort. Also, a lot of people lean too far forward, which puts them in a poor postural position that can lead to injury.