Over the past few years Elliptical trainers have taken the fitness industry by storm. What was once a fad relegated to late night infomercials is now perhaps the single most dominant piece of equipment at the majority of gyms and personal training centers.

Let's start with the guide by taking a look at the differences between a treadmill and an elliptical trainer. The most immediate thing you're going to notice is that the elliptical is almost entirely impact free. This means you will no longer be subjected to the lower joint pain most associated with running. Even a treadmill with great shock absorbtion and an orthopedic belt, while better than running on pavement, will still abuse your knees and ankles.

On the flip side, a treadmill is much better for targeted training for joggers and runners. You can't really train properly for a marathon on an elliptical. As close as the feel of an elliptical is to walking or running, it's still not as good as actually running.

Now, let's take a look at the "pain" of running. Recent studies have shown people to work out hard and longer on ellipticals because the perceived amount of effort seems to be much less. The more "comfortable" a work out, the longer and hard someone will go. This is one of the main reasons a lot of seniors and older exercisers have taken to the elliptical so readily.

Another glaring difference is the crosstraining arms most ellipticals have (with a few exceptions such as the majority of the Precor elliptical units). With the addition of the crosstraining bars, the elliptical allows you an additional aspect in your work out -- strength training. Almost all fitness experts agree that all proper workouts need a strength training aspect and the elliptical packages it with the cardio training quite nicely.

Let's talk about what to look for in an elliptical trainer. First, which is better, a front wheel or rear wheel elliptical. Truth be told, they both work exactly the same. Anyone who tells you differently is just trying to see you a bill of goods. They give you the exact same workout. The only real difference is if you're a larger person or if you're on a lighter weight (or less sturdy) machine. In this case, the front drives will definitely feel quite a bit sturdier. This is because when you work out on an elliptical, you are throwing all of your weight forward. Having the drive and heavy flywheel up front will help to balance you out. Larger flywheels are better as they tend to last longer and give more sturdiness to a machine.

Most elliptical trainers have magnetic brakes and you don't want to mess with anything else. The old friction based brakes tended to wear out too quickly, so stay away from any dinosaur machines that still use them.

Stride is one of the most important features on an elliptical because they have the most to do with comfort. Longer strides are better on ellipticals -- unless, of course, you're a shorter person, but for most people strides of 18" or longer are best. Personally, I've find 20-21 inch strides to be the best for anyone up to about 6'4". Anything shorter than 18" will be uncomfortable for most people.

Footpads: make sure the unit you are looking at has articulating footpads. The reason for this is because elliptical traininers with static footpads (such as the Tunturi units) can cause your feet to start to go numb on longer work outs. This is from the lack of circulation caused by the static pedals. Looks for footpads that flex with your feet. You'll thank the sensei for it.

Things like programs, heart rate control and personal fans are nice, but the above points are the main things to keep an eye on. Also, try out the feel of the way you mount an elliptical. There are two varieties of mounting -- side mount machines (such as the Precor 546 or Star Trac Rear Drive) and rear mount machines (such as the Matrix E5x or Horizon EX44). Some people have a bit of difficulty on mounting from the side and should look at rear drive options. As with the position of the flywheel, rear-mount versus side-mount ellipticals work exactly the same way.

The best way to pick out an elliptical is to head out to either a gym or local sports retailer and give them a try. Be warned, tho, once you work out on a commercial grade elliptical, it's hard to transition over to a residential grade one. The top of the line commercial ellipticals are by Star Trac, Matrix, Precor and Life Fitness. The top residential grade ellipticals include Horizon, New Balance and Endurance. Fitnex and Spirit / Sole are great mid range brands that do an exellent job of giving commercial quality work outs for not much more than a residential price.