No one has studied obesity's effect on physical activity though researchers have studied physical activity and its relation to obesity for decades.
So BYU exercise science professor Larry Tucker decided to look at the other side of the equation to determine if obesity leads to less activity. The findings, no surprise, confirmed what everyone has assumed for years.
"Most people talk about it as if it's a cycle," senior-author Tucker said.
"Half of the cycle has been studied almost without limit. This is the first study of its kind, in many ways, looking at obesity leading to decreases in physical activity over time," he said.
To study this reciprocal effect objectively, the researchers attached an accelerometer to more than 250 participants.
Accelerometers measure actual movement and intensity of activity. Previous studies have relied on less-dependable self-reported data.
"Roughly 35 percent of the population reports that they're regularly active," Tucker said.
"When you actually put an accelerometer on adults and follow them for many days, only about 5 to 7 percent are actually regularly active. We used an objective measure so we could determine genuine movement, not just wishful thinking," he said
The 254 female participants - 124 of which were considered obese - were instructed to wear the accelerometer for seven consecutive days at the beginning of the study, and then again for an additional week 20 months later, at the end of the study.
On average, physical activity in obese participants dropped by 8 percent over the course of 20 months.
This is equivalent to decreasing moderate to vigorous physical activity by 28 minutes per week.
In contrast, non-obese women demonstrated essentially no change in the amount of physical activity they were participating in weekly.
The study is published online in the journal Obesity.