Obese children may have a hard time exercising because of the effort needed to move their larger bodies -- not because their hearts aren't up to the challenge, according to a new study.

The findings, published in the journal Pediatrics, suggest that significantly overweight children should stick with low-intensity activities -- such as walking at an easy pace -- so that they don't fade quickly or ultimately give up on exercise.

The study included 129 obese and 34 normal-weight teenagers who underwent fitness tests on a stationary bike.

During higher-intensity cycling tests, both groups of teenagers showed similar oxygen use relative to their weight -- a measure of aerobic fitness. However, the obese teens breathed harder on the easiest cycling test, where there was no "uphill" challenge. They also covered less distance when asked to walk or run on a flat surface for 12 minutes.

All of this suggests that obese kids' lower capacity for exercise stems from the demands of simply moving a larger body, according to the study authors.

"Carrying that amount of extra weight means that exercises that might seem very mild to someone of normal weight can quickly exhaust (obese children's) ability to supply oxygen to their muscles," explained study co-author Dr. Jack A. Yanovski, a researcher at the National Institute on Child Health and Human Development in Bethesda, Maryland.

On the other hand, the findings indicate that the problem is not one of the cardiovascular system.

"For most obese adolescents, even if they weigh more than 300 pounds, their hearts and lungs are still healthy enough to allow them to exercise vigorously," Yanovski said.

According to the researcher, understanding the roots of obese children's exercise limitations is important because it gives insight into why these kids may not like to exercise -- which, in turn, can help doctors design better exercise plans for them.

For obese children in this study, simply pedaling a bike at the lowest setting for 4 minutes brought them close to the "lactate threshold," a point where working muscles aren't receiving enough oxygen and exercise can quickly become unsustainable.

"For such children," Yanovski said, "very low intensities of exercise are needed so that movement can be sustained."

SOURCE: Pediatrics, June 2005.