Kristen Beavers of Wake Forest Baptist Medical Center looked at data on how changes in muscle and fat in the thighs affected the walking speed of about 2,300 people in their 70s.
"Increasing levels of fat in and around the thigh muscle itself were associated with slowed walking speed in otherwise healthy older adults," Beavers said in a statement.
Walking speed was only part of a larger picture, Beavers said.
Other studies indicated slower walking speed can indicate other physical problems and so can be a sign of disability developing, Beavers said.
The study was published in the American Journal of Clinical Nutrition.