The participants were divided into five groups, from least fit to most fit. Fitness levels were determined by how well participants did on a treadmill test. This test measures exercise capacity, the highest amount of physical exertion a person can sustain. For people who are middle-aged and older, the highest level of fitness can be achieved by walking briskly most days of the week, for two and a half hours or more per week.

When researchers adjusted for other factors that could affect risk of Alzheimer’s disease, they found that the people in the most fit group were 33% less likely to develop Alzheimer’s disease than those in the least fit group. The second most fit group was 26% less likely to develop the disease, while the middle group was 20% less likely and those in the second least fit group were 13% less likely to develop the disease than those in the least fit group.