In is Out

Personal Best - Winter Training - Faster and Safer Indoors? - NYTimes.com: "The sad answer, exercise researchers say, is that you really cannot get the same training effect with indoor substitutes. That’s not to say that indoor training is useless, but rather that it has real limitations, with differences that sometimes are subtle, but significant.

“I think most athletes know that,” said Peter R. Cavanagh, an exercise researcher at the University of Washington in Seattle. “That’s why they are out there in all seasons.”

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.”"