In the second visit, participants are first given a series of cognitive tests designed to quantify such cognitive faculties as visual short-term memory, selective attention, processing speed, and verbal fluency. These tests consist of seeing simple images on a computer screen, such as a series of colored shapes, and responding using a keypad. A blood sample is also collected from the participants to serve as a baseline. The subject is then randomized to one of three groups: the first group rests for 30 minutes; the second group walks on the treadmill at a mild intensity, defined as maintaining 60% of the heart rate observed during the max VO2 from the first visit; the last group walks on the treadmill at a hard intensity, defined as maintaining 80% of the heart rate observed during the max VO2 from the first visit. The subjects then undergo a second blood draw after cooling down, followed immediately by another round of the same cognitive tests. An hour after finishing the exercise intervention, the same procedure of blood draw followed by cognitive tests is repeated, and an hour after that it is repeated again.