What do you really need to get in shape? A personal trainer? A gym membership? It turns out that the answer may be a friend. A very special type of friend — a dog.

They'll be ready to get up and out the door before you will. They don't oversleep and they don't make excuses.

Walking is good exercise. Whether it's done on the spur of the moment or as part of an exercise program, it's an activity nearly everyone can do. People who do follow an exercise program are more likely to stick with it when they have a companion. Dogs make great walking companions. They've been described as natural exercise machines on a leash. They'll be ready to get up and out the door before you will. They don't oversleep and they don't make excuses. And they certainly don't suggest that you stop for ice cream in the middle of a walk, like some people might.

You'll walk more with a dog. This isn't just a guess; it's backed by research.

Robert Kushner is a human obesity expert and professor of medicine at Northwestern University's Feinberg School of Medicine. In 2006, he co-authored a study that tested whether pets and people could help each other lose weight. They could and did. One fact that emerged from the study was thatdog owners walked nearly twice as much as non-dog owners. The dog owners reported walking 300 minutes per week; the non-dog owners 168 minutes.

People have an entirely different relationship with a dog than they do with a human personal trainer. When personal trainers spur their charges on, people sometimes shout four-letter words back at them. How could you possibly do that to your dog? He or she would be terribly insulted.

When people start out on an exercise program that involves lots of walking, they're often confused about how fast to walk. Dawn Marcus, a professor at the University of Pittsburgh Medical Center and author of a book about dogs and human health, has a simple test. "If you're so winded you can't talk with someone, then you're probably walking at too hard a pace. If you're walking so slowly that you can easily sing, you're probably not walking fast enough."

Dogs can improve your life in many ways that have nothing to do with exercise. For one thing, you end up meeting more people walking with a dog than without one. You could ask one of them out for ice cream. Or better yet, something a bit more low calorie.

The results of the study on dogs, exercise and weight loss were published in the October 2006 issue of Obesity.