Goal: Maintain Weight

Type of exercise: Aerobic exercise.

Frequency: One study from Duke University found that just 30 minutes of moderate exercise a day was sufficient to prevent weight gain. However, other studies have found that most people who successfully lose weight and keep it off for a year or more do 60 minutes of cardiovascular exercise (usually walking) no fewer than five days a week.

Intensity: Mild, moderate or vigorous.

How it works: Maintaining weight requires the same thing as losing weight: burning more calories than you take in. “Study after study shows that most weight-loss winners keep their calories in check through consistent, steady aerobic exercise – and the majority of the time, the activity of choice is walking,” says Olson. Still, she notes, those who add resistance training do themselves a favor because the extra muscle they obtain increases their calorie burning, making it that much easier to keep the pounds off.  

The evidence: Strong. Ninety percent of participants in the National Weight Control Registry, a group of more than 3,000 people who have successfully maintained at least a 30-pound weight loss for a minimum of one year, consider exercise to be crucial to their long-term weight maintenance. They report, on average, burning 2,687 calories a week, the equivalent of walking four miles each day.

Goal: Prevent Osteoporosis or Reduce Bone Loss

Type of exercise: Any weight-bearing exercise (that is, exercise that requires your bones to support your body weight). Resistance training is most effective, but many forms of aerobic exercise, including walking, will also work.   

Frequency: At a minimum, 15 minutes of resistance training two to three times a week, and 20 minutes of aerobic exercise three times a week, says Vijay Vad, MD, a sports medicine specialist and arthritis researcher at the Hospital for Special Surgery in New York City.

Intensity: Mild to moderate.

How it works: “Weight-bearing exercise puts stress on your bones. Your bones’ cells react by creating additional bone mass,” explains Michael Gloth, MD, an associate professor of medicine at Johns Hopkins University School of Medicine in Baltimore who helped develop the National Osteoporosis Foundation guidelines for osteoporosis. Dr. Gloth notes that people who’ve already developed the condition will need to supplement their workouts with a calcium and vitamin D in supplement and/or dietary form in order to effectively rebuild density and prevent further loss.

The evidence: One long-term study by researchers in Germany found that a mixed program of running, jumping (such as skipping rope), weight-training and stretching performed four times a week helped a group of 86 premenopausal women with osteopenia preserve their bone density over two years, compared to a less-active control group. While the control group lost 2.3 percent of the bone mass in their spines in 26 months, the exercising group saw no change. The study was published in the Archives of Internal Medicine.

Exercise Intensity Guidelines

To determine how hard you are working out, follow these guidelines.

Mild: You should be able to hold a detailed conversation with another person. If you're using a heart rate monitor, aim for 40 to 50 percent of your maximum heart rate.

Moderate: You can say a few words (“I’m doing good”) but would find it difficult to hold a conversation. Aim for 50 to 70 percent of your maximum heart rate.

Vigorous: You’re edging out of your comfort zone and would find it hard to hold a conversation. You’ll be walking four miles an hour or faster. Aim for 70 to 80 percent of your maximum heart rate.

Note: Always contact your physician before starting any exercise routine.