Are treadmills just as good as running outdoors?

NY Times:

The most obvious difference with indoor exercise is a lack of wind resistance, Dr. Cavanagh said.

“The important variable here is speed relative to the air,” he said. For example, if you are running at 8 miles per hour into a 10-m.p.h. headwind, your speed relative to the air is 18 m.p.h. Dr. Cavanagh explained in an e-mail message: “Work done against air resistance can be extremely costly because the ‘drag force’ (force caused by air resistance) is proportional to the square of speed and the power required to overcome drag force is proportional to the cube of speed.”

One recent study, in fact, found that people can run 11.5 percent faster on treadmills than outdoors.

Many runners, including Dr. Thompson, set their treadmills at a 1 percent incline to make up for the lack of wind resistance. But that is not a complete solution because there are other aspects of outdoor running that a treadmill can’t mimic. For instance, the treadmill surface is just too smooth.

“If you run all the time on a smooth surface you are not training all muscles in your legs and feet that you need to run on the road,” Dr. Cavanagh said. “If you are going to race under certain conditions, you might as well train there.”

Safety may be the single best reason to opt for more treadmill training, particularly in the winter

Mr. Sell, an elite marathoner who ran in the 2008 Beijing Olympics, trains in Rochester, Mich. He bought a treadmill four years ago, after he had fallen a few times on icy roads, injuring himself so badly he could not train at all while he healed.

“I probably fall at least once a year here in Michigan,” he said. “My injuries ranged from a bruised hip to a pulled groin. That time it took three weeks to get back. I said, ‘If I was doing this on the treadmill, I wouldn’t have missed three weeks of training.’ ”

Mr. Sell continued, “If it’s really icy out or if it’s negative 10 degrees and you are doing an easy six-miler, it probably makes a lot more sense to do it on a treadmill than to risk hurting anything.”

Often I have worked out on treadmills when staying in neighborhoods characterized by pollution and traffic-congested streets.