1. Stationary bikes
Long the workhorses of exercise programs, stationary bikes have many advantages: they are compact, durable, quiet and relatively inexpensive. Upright bikes are more compact, but some people find recumbent bikes more comfortable. Most will exercise only your leg muscles, but dual-action upright bikes have handlebars that permit simultaneous arm exercise. Standard exercise bikes use flywheels for resistance; a heavy wheel will give you smoother action. Other models use air paddles for resistance; you'll get a cooling breeze that may encourage you to keep pedalling.

Look for a sturdy bike with a comfortable, well-padded seat. One-piece pedals with stirrups are best. Pick a machine with smooth pedalling action and precise controls that allow you to vary the resistance level.

2. Treadmills
Walking is the most natural form of exercise. A treadmill will allow you to walk or run at home; you'll miss the scenery, but you'll also miss the summer's smog and the winter's worst. Treadmills allow you to select and vary your speed; better models also allow you to vary the incline as you walk or jog.

Most people find it easier to use a treadmill than other exercise machines. Their drawbacks are size and cost. If you have the floor space, look for a midpriced model with a sturdy frame and a strong, quiet motor. Your treadmill should have simple controls that allow you to vary the belt speed easily. Models that permit you to vary the incline while you are walking are also desirable.

3. Cross-country ski machines
Nordic skiing is among the most demanding of aerobic sports; ski machines can bring the challenge home -- without the snow. Cross-country ski machines have the advantage of working your arms and back as well as your legs. Because they produce little impact, they cause few injuries. People who are athletically inclined may find skiers more stimulating than some other home machines. But they do require more balance and coordination, and they tend to take up more space than stationary bikes or climbers.

Machines that have skis that glide independently, cables for your arms and flywheels for resistance provide better action than models with linked ski motions, poles and pistons. Look for a sturdy frame and for controls that allow you to set your arm and leg resistance separately.

4. Rowing machines
Like ski machines, rowers have the advantage of exercising more than just your legs; they also exercise your arms and torso. You can get a fine low-impact workout, but you'll need some floor space, and you'll need good technique to avoid back injuries; instruction can help. Models with cables for "oars" and flywheels for resistance give a more authentic feel than rowers with handles and pistons. Look for a stable frame and a comfortable seat that glides smoothly.

5. Stair-climbers
Stair-climbing is an excellent, strenuous way to condition your heart and your legs. That's why competitive athletes run stadium steps and dancers do step aerobics. Climbing machines have soared in popularity since their introduction in 1985. They offer moderate or high-intensity exercise with low impact. Most models exercise legs only, but dual-action climbers have "ladders" for arm exercise.

Because of their expense, motorized climbers are more suitable for clubs than for home use. But piston climbers are affordable and compact. Look for a sturdy frame, large pedals, and a comfortable handrail. Above all, your climber should have smooth action and easiy adjustable resistance settings.

6. Exercise riders
Spurred by televised promotionals, riders were the rage of the 1990s. As on a rower, you pull with your arms and push with your legs -- but on a rider, you lift your body up instead of sliding it forward as on a rower. Riders have the advantage of exercising your arms and legs without impact. Look for a stable machine with a comfortable seat, smooth action, and variable resistance settings.

7. Elliptical trainers
What do you get if you cross a bike, a stair-climber and a cross-country skier? With a good designer and a little luck, you'll come up with the latest word in exercise machines, an elliptical trainer. To use an elliptical, you stand on footplates while grasping handlebars. As you walk or run, your feet move in an elliptical fashion; as one moves forward and up, the other moves down and back -- it's halfway between the circles your feet make on a bike and the vertical motions they make on a climber. At the same time, you can move your arms against resistance, as you would on a ski machine.

Elliptical trainers provide moderate to intense low-impact workouts for your legs and, to a lesser degree, your arms. You can vary the intensity by adjusting the resistance and incline; better models also allow you to pedal backward to give your buttock muscles some extra work.

Because they are new and sexy, ellipticals tend to be expensive, and they do take some getting used to. They are more suitable for moderate to intense exercisers than beginners. Look for broad, nonslip pedals, comfortable handlebars, and a smooth, easy-to-adjust mechanism.

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Excerpted from The No Sweat Exercise Plan: Lose Weight, Get Healthy, and Live Longer by Harvey B. Simon, M.D. Copyright 2006 by the President and Fellows of Harvard College. Excerpted with permission from McGraw-Hill. All rights reserved. No part of this excerpt may be reproduced except with permission in writing from the publisher.