People always say there's a difference between running on the ground vs. running on a treadmill vs. running on an elliptical. There is a different load on your body during each exercise, and each exercise has a benefit the other doesn't have.
However, in American Fitness, "Treadmills vs. Elliptical Trainers," Jul/Aug2002, Vol. 20 Issue 4, p9, Thomas Altena, a professor of nutritional and exercise physiologist of the University of Missouri-Columbia measured oxygen retention, lactic acid build-up, heart rate, and perceived rate of exertion to compare treadmills and elliptical trainers. He writes ""In this study, the physiological responses associated with elliptical exercise were nearly identical to treadmill exercise." ((and if this is the case, why do so many people I know with bad knees insist on running on a treadmill because it burns more calories?))
Okay... so riddle me this: why does my body crap out on me (mainly a tightness in my chest and a lactic acid buildup) when I run 10 minutes on a treadmill, but I can run the same pace on an elliptical for 60 minutes? I expect the elliptical to be a little bit easier, but I didn't expect there to be such a dramatic difference in my ability. Who knows, maybe I have some sort of mental block/prejudice against treadmills.