Regular exercise helps to bulk up our brains and improve thinking skills, studies show. But physically demanding jobs might have an opposite effect, said a provocative new study of almost 100 older people and their brains and work histories.

It finds that men and women who considered their work to be physically draining tended to have smaller memory centers in their brains and lower scores on memory tests than other people whose jobs felt less physically taxing. Few of these workers were laborers. Most had office jobs. But their brains looked different if they felt that their jobs were physically hard than if they did not.

Outside of work, though, moving was a plus. Those people who reported regular physical activity on their own time generally had greater hippocampal volume and better memories than inactive people. But physical activity at work did not amplify those benefits; it dampened them.

The study does not prove that physical demands at work shrink people’s brains. But it does raise questions. It is possible that exercise affects the brain one way and “occupational physical activity has a different” and perhaps less-desirable effect, said Aga Burzynska, an assistant professor at Colorado State University.

Most of us probably expect that physical activity is physical activity and its benefits should be about the same, no matter what conditions surrounding the activity. But a growing body of science suggests that context matters.

In studies with lab rodents, for instance, similar amounts of exercise can lead to contrasting health outcomes, depending on whether the animals run voluntarily on wheels, meaning they control their own workouts, or are placed on little treadmills, with researchers manipulating the pace and duration of their exertions. In general, wheel workouts produce healthier rodents than treadmill training.

Some studies with people have identified a related dynamic. For most of us, under most conditions, being active reduces our risk of dying young. But in multiple epidemiological studies, people — and in particular, men — whose jobs require physical labor face heightened risks of premature death compared to men working in relatively sedentary professions.