For Americans seeking to get fit on their own time, online personal training is increasingly popular. Eight percent of 75,000 personal trainers in the United States are now coaching clients online.

Mr. Cotton’s clients pay $60 a year, compared with hourly rates of as much as $100 to $150 that some trainers charge for a session at a club or a client’s home. And working with a trainer via the Internet does not require a trip to the gym, so it appeals to those who are pressed for time.

Whether online training is effective is another question. Many exercise physiologists and other fitness experts doubt that customized workouts offered online can get many sedentary Americans doing arm curls at home, no matter how many encouraging e-mail messages are sent.

Others caution that without working face to face with a client, a trainer cannot correct improper form or properly goad a person to finish a demanding set. But some people say that being accountable to a coach, even one they have never met, inspires them to get moving.

Mr. Cotton admits that his 1,000 virtual clients are not buying a lot of personal attention. In fact he urges his charges to post questions on his Web site’s forum, where his staff members may respond.

Other trainers, with far fewer clients, offer a somewhat more hands-on approach. Kelli Calabrese, an exercise physiologist who was named online trainer of the year by Personal Fitness Professional magazine, trains 50 people online, in addition to 26 she coaches near her home in Long Valley, N.J. She has the time to respond promptly to her clients’ e-mail messages and also tailor her recommendations to their needs each week. She charges $49 a month.

One of her Internet clients is Jim Cameron, 43, a software consultant who travels constantly. “When I’d say, ‘I’m going to be in Benton Harbor, Mich., at a Courtyard Marriott, and they have a universal gym and a treadmill,’ ” Mr. Cameron said, “Kelli worked out new ideas with what I’d have available.” And when he would sign on to to print his workout, it would come with tips.

For Mr. Cameron, knowing that Ms. Calabrese was charting his progress helped him to stop making excuses. “The thing that got me hooked was the accountability,” said Mr. Cameron, who has lost 70 pounds through the program.

While some active people turn to online coaching to ramp up their training for a race, for many others it is a tentative first step toward basic fitness. Valerie Midkiff, 39, is a mother of three from Hillsboro, Ore., who thought the trainers at her gym miscalculated her skills. “They were living in a very fit world, and I wasn’t,” she said.