The dual action motion of an elliptical trainer ensures that more calories can be burned than with a more traditional ‘linear’ exercise method such as running or cycling. The elliptical's handles - attached to the pedals by a series of gears that enable it to utilise their momentum without being rigidly constrained in terms of speed - allow for an upper as well as lower body workout. Since the upper body is moving against resistance provided by the pushing and pulling of the handles, the result is a more effective calorific burn, a more efficient exercise motion that allows all the muscles to work in unison, and much less impact than other forms of exercise.
The elliptical's handles can usually (certainly always on the higher-end machines) be set to provide a level of resistance independent of the elliptical pedals. This makes for the ability to create a unique and benefiting training experience every time one steps onto the trainer.
Cross trainers, or elliptical trainers, are gaining momentum and popularity; this is for very good reason. Will it ever trump the treadmill? Who knows. Scientific sports research shows that elliptical trainers provide just as much or more of an effective cardiovascular-based exercise than that of a treadmill and does so in a safer manner. Treadmills, by their very nature, force the user to pick up and put down their feet in rapid succession many thousands of times during an exercise session, the fragile tissues, muscles and ligaments located in the lower body (especially the knees and ankles) may be irreparably damaged.
The fluid motion of the elliptical means that the exerciser is not forced to pick up and hit back down with his feet and legs: there is no jarring, shocking motion to transfer force into the legs. This gives an all-round safer and much more comfortable exercise. This also means that the energy used before to jar and resist jarring can now be used to work the muscles - which is probably what we all look for when exercising, not damaged legs!
More resources detailing the advantages of the dual action of elliptical trainers , along with other elliptical trainer related articles on subjects like elliptcal trainer muscle groups aspect of elliptical trainers are available at my site about elliptical trainers .