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Dear Doctor: Is walking or running on a treadmill as good as walking or running on concrete streets or sidewalks?

Dear Reader: This is an interesting question, because the definition of "as good as" is quite ambiguous. Good can be defined as having cardiovascular, or other, health benefits, but it can also be defined as having mental or social health benefits, such as those that come from running or walking with another person. For the purposes of this question, let's look at the cardiovascular aspects.

A 1985 study in the journal Medicine & Science in Sports & Exercise measured the amount of oxygen consumed during both treadmill running and outdoor running. The level of oxygen consumption, termed VO2, is a measure of energy expenditure with exercise. The subjects ran 950 meters on a treadmill and outdoors. The subjects also ran at varying speeds and at different elevation grades. The investigators found no difference in the amounts of oxygen consumed with either the outdoor or treadmill running. This was consistently seen, even when there were changes in speed or elevation grade.

A 1996 study in the Journal of Sports Sciences similarly looked at the differences in VO2 in nine athletes who ran both outside and on treadmills at varying speeds and elevation grades. This study showed some significant differences in the VO2 between outside and treadmill running. Overall, those who exercised outside consumed more oxygen than those on a treadmill. This may be partially related to the wind resistance seen with outdoor exercise.

The authors concluded that running outside on flat terrain was similar to treadmill running at a 1 percent elevation grade. So, if you want to equate your treadmill running to what you might experience outside, place your treadmill at a 1 percent grade.

But treadmill running is quite dissimilar from outdoor running in one respect: Your muscles respond differently. Because a treadmill is powered by a machine, not your body, the hamstring muscles are not used as much -- something you may notice if you normally run on a treadmill and then decide to go running outside. In that case, to prevent an injury, slow your pace.

A significant benefit from running on a treadmill is that the belt of the treadmill is much more forgiving than the concrete of the sidewalk. While there has been no long-term study regarding the benefit of treadmill running on the joints, it stands to reason that exercising on a treadmill could decrease the incidence of knee and back injuries.

Alternatively, one definite positive from outdoor running is that it's less monotonous than treadmill running. The changes of scenery and terrain keep things interesting. When I run on a trail, I am constantly looking at the rocks I have to avoid, and I often have to move laterally to follow the trail. When I get to the top of the mountain, looking down at the expanse of earth below me, I get a perspective that is much different than running on a treadmill. This cannot be measured, but it motivates me to run again.

If you like running or walking on a treadmill, by all means continue doing this. The cardiovascular benefits are similar to outdoor exercise, and it may be less hard on your joints. The most important thing is to take the time and exercise, whether it be inside or outside.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095.)

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