Losing weight is easy — you eat a lot less and the weight comes off. It’s maintaining that loss — keeping the weight off — that’s the hard part. If the scale starts creeping back up, a new study suggests, it’s time to examine your diet and activity habits. You may be spending too much time on your tush.

Successful weight-loss is generally described as a loss of about 10 percent of a person’s weight that is maintained for at least one year. At any given time about 17 percent of Americans are on a diet. Only about 20 percent of these people are successful, according to this definition.

Over 4,300 people who had maintained a roughly 54-pound weight-loss for about three years were compared to a group of people who were obese, but whose weight was stable.

In 2006, the National Weight Control Registry, a study of more than 10,000 people who have lost at least 30 pounds and kept if off for at least a year, found that people who maintained their weight watched much less television than those who didn’t; but other activities performed while sitting were not examined in that study. In the past 15 years, the use and availability of computers, video games and other portable electronic devices has increased substantially and may be a new part of the weight maintenance equation.

California Polytechnic State University researchers took a broader view of sedentary time. They compared the duration and types of sitting activities among a group of people who had maintained their weight long term. Over 4,300 people from WW (formerly known as Weight Watchers) who had maintained an approximate 54-pound weight-loss for about three years (weight-loss maintainers) were compared to a group of people who were obese but whose weight was stable (weight-stable individuals).

Weight-loss maintainers sat less — three hours less each day during the week and on weekends — than those who were not able to maintain their weight-loss. They also spent one less hour per day in a sitting activity unrelated to work. Maintainers used over twice the calories during the week in physical activity.

The weight-loss maintainers and weight-stable individuals did not differ that much in time they spent sitting in more mentally active types of sedentary activities such as reading, studying, traveling, talking, texting or socializing. The main difference was time spent in non-work-related time on a computer or playing video games.

If you want to keep weight off, it’s not simply a matter of not sitting, but being mobile when you are not couch surfing. The message is simple: You need to sit less and move more.

This study is published in Obesity.