Exercise is all about moving your body. But some body parts move more than you want them to.

For many overweight exercisers, every step of a workout comes with an unintended cascade of motion -- breasts bounce, belly fat shakes and thighs rub. The added jiggle and friction of moving body fat is more than just bothersome. It can alter people's gait and make them more prone to injuries and joint problems. The discomfort prevents many overweight people from exercising altogether.

"Almost all of my clients end up expressing this, how uncomfortable the bouncing around feels," said Kelly Bliss, a fitness instructor and author in Lansdowne, Pa., who works with overweight people. "They say, 'I turn right, and part of me is still going left.'"

But the jiggle factor, familiar to the overweight and the large-breasted, has been largely ignored by exercise researchers and most sports-gear makers. Only a handful of studies have tried to document the challenges and strain endured by large bodies in motion.

"There's very little research on the biomechanics and locomotion of obesity," said Ray Browning, research instructor at the University of Colorado Health Sciences Center, who has conducted several exercise studies of the overweight and obese.
 

While brisk exercise clearly offers cardiovascular benefits, less is known about the risks and benefits of exercise to other parts of an overweight body. In his own research, Browning has found that when the foot of an overweight person strikes the ground, the knees and hips endure far greater forces than the joints of a slim person. Exercising at a faster pace increases joint loads even further, raising the risk for arthritis and injury among the overweight.

In September, the journal Medicine and Science in Sports and Exercise published a study by Browning that compared the walking biomechanics of 20 adults, half of them obese and half of normal weight. Although both groups had similar strides, the heavier exercisers landed with their feet farther apart then slim exercisers. The wider steps may be the way overweight people stay balanced. Or the wider step may simply be a function of leg thickness or large thighs rubbing together.

This small adjustment in stride can take a physical toll, shifting the forces of exercise to the inside knee, the part most vulnerable to arthritis.

"When people become overweight or obese, we tell them to exercise more, but we may want to be careful with that prescription," Browning said. "The last thing we want is someone with obesity to develop arthritis in a large joint, because then their options for physical activity are much reduced."

Bliss, the fitness instructor, said large people often want to wear light, baggy clothes while exercising, but wearing tight, stretchy garments underneath is a better way to support large stomachs, thighs and buttocks and reduce unwanted movement. Some specialty clothing firms such as Junonia now make large-size Lycra tights to minimize exercise bounce. The sports-bra maker Enell even offers a $75 support vest for men with excess upper body fat.

"Any soft tissue that is unsupported should be supported," said Bliss, whose Plus-Size Yellow Pages Web site (plussizeyellowpages.com) includes links for large-size exercise gear.

Another way to reduce bounce is to switch to water-based exercise or use recumbent equipment such as step machines and bikes, which are operated in a reclining position.

Browning said he hopes more exercise researchers will begin to focus on the discomfort overweight people face when they work out.