Treadmill desks, which are growing more popular with fitness buffs, appear to have a significant downside. A new study finds workers who use them score more poorly on mental tasks than those who remain sitting.

The findings, published in the Public Library of Science journal PLOS One by Brigham Young University researchers, are based on a study of 75 healthy young men and women randomly assigned to workspaces outfitted with a computer and either a chair or a treadmill desk, The New York Times reports.

The participants underwent a battery of cognitive tests designed to measure working memory and delayed recall, and the ability to concentrate. The results, when the researchers compared the treadmill walkers with the people sitting at their desks, substantially favored sitting.

The people who had walked during the testing performed worse on almost all aspects of thinking, including the ability to concentrate and remember, compared with those who had been seated.