As is often the case in life, the simplest things are often the best. This applies to exercise too. For example, one of the best forms of exercise, ]]>running]]> , is a direct outgrowth of one of the first things we learn to do—walk. And running, often forbidden as small children (as in "I told you kids, no running!") can reap great physical benefits as we get older.

Indeed, along with ]]>swimming]]> , running offers one of the best overall workouts. But running does have its drawbacks. Prolonged running, especially on hard surfaces, can wreak havoc with your lower body. Unless you have an indoor track available to you, extremes of weather and temperature can complicate or curb even the most ambitious running program.

Never one to overlook a potential market, the exercise machine industry has responded with all types of indoor running machines. One that continues to grow in popularity is the home treadmill. Why? Here are three key reasons:

Convenience —Most people prefer not to exercise outside when it is cold, inclement, or dark. Even if you have access to an indoor track or ]]>health club]]> , finding a regular time to run during a busy week's schedule can be difficult. Owning a treadmill solves these problems; people who own one can exercise more often. Since most people are familiar with the basics of ]]>walking]]> and running, treadmills are pretty easy to use.

Physical benefits —Exercising on a treadmill has wide-ranging benefits. As Gregory Florez, president of Chicago's First Fitness Inc., points out, "Walking or running on a treadmill...has great cardiovascular value for the heart, lungs, and circulatory system. It's a very efficient way to lose body fat, and, since it's a weight-bearing activity, it has musculoskeletal benefits as well." Because running can reduce stress, you actually feel calmer and more relaxed (albeit sweaty).

Low-Impact workouts —Despite all its benefits, years of walking or running can take its toll on feet, legs and hips, especially if you exercise on hard surfaces like asphalt or concrete. Treadmills, particularly higher-end models with built-in shock absorption properties, can reduce the stress placed on your feet, legs, and joints.

Motorized vs. Non-motorized Treadmills

The sale of treadmills has skyrocketed over the last few years no doubt due in large part to these benefits. Not surprisingly, the increase in treadmill popularity has also led to an increase in the type and number of treadmill designs available.

However, the biggest decision for prospective buyers is still whether to purchase a motorized or a non-motorized treadmill. The basic difference? With a motorized treadmill, you have to keep pace as the machine's motor moves the treadbelt (at a speed you electronically set). With a non-motorized treadmill, you push the belt with your feet, so you only go as fast as you push.

There are other differences as well. For one, non-motorized treadmills are much more affordable. Starting as low as $200, even top of the line models can be purchased for under $800. Conversely, quality motorized models start at about $1000 and can go as high as $10,000. Non-motorized treadmills also take up much less space and, theoretically, can help you burn calories more efficiently, since you supply the muscle power to move the treadbelt.

Alas, however, this last fact is not necessarily true. Because you supply the muscle power, unless you're in good shape, you tend to move slower and tire more quickly. As a result you may actually burn fewer calories. Non-motorized models also have other drawbacks. Aerodynamics dictate that to get a non-motorized treadmill going, it has to be tilted on an incline. This can make running difficult if you're out of shape or not used to running; therefore you derive less benefit. Finally, as you tire on a non-motorized treadmill, you'll tend to push harder on one leg. This can make the motion of the treadmill choppy, and thus, harder to run on.

Making a Choice

The bottom line? Unless you're in very good physical condition, it's generally better to buy a motorized rather than a non-motorized treadmill. If you do go the motorized route, here are some variables you might consider:

AC or DC motor —AC motors tend to be noisier. They also drain more power and often require a dedicated power source.

Belt thickness —Two-ply belts are stronger and tend to curl less around the edges.

Running surface length —A longer running "deck" will allow for a more natural stride and easier running motion.

Speeds —If you plan to walk on your treadmill, 0.5 mph minimum to 6 mph maximum speed should be sufficient. If you plan to use it for running, 0.5 mph minimum to 8-12 mph is preferable.

Shock absorption —Manufacturers use different techniques to absorb the shock to your feet and legs. These range from thicker running belts and thinner running decks to actual shock absorbers placed under the deck (more expensive) and "floating beds," where the treadmill floats on special springs (more expensive still). Remember that the higher the level of shock absorption built into the machine, the less wear and tear on your feet, legs, and joints.

Incline —Treadmill inclines vary from 2% maximum incline to as much as 15%-25% maximum incline. The greater the possible degree of incline, the more varied a workout the machine can offer. In addition, check the machine's incline mechanism; electronic switch inclines are preferable to manually operated inclines. Automatic inclines (which are tied directly to and change automatically during your workout) can be nice, but expensive.

Electronic feedback —Almost all motorized treadmills offer electronic speed, time, and distance displays. Most also include preset or customized workout programs. Your choices are limited primarily by your wallet. For more money you can purchase models that store personalized programs and/or records of your workouts. Many machines also offer heart rate monitors, up to and including wireless monitors and monitors that set off an alarm if you're outside your target heart rate. Note, however, that many of these monitors can fall somewhat short of accuracy, with chest strap monitors likely to be the most accurate.

In conclusion, here are three points: First, try to negotiate a month's free trial before purchasing a treadmill. Second, consider the purchase of a used treadmill; check your local want ads and local health clubs (many upgrade equipment regularly and need to sell old equipment). Finally, as always, before beginning any new exercise program, get approval from your healthcare provider.