All subjects will perform baseline assessments including cognitive and behavioral performance, brain imaging, fitness, motor and balance testing, and selected blood and genetic testing. Thereafter, subjects will be randomized to either a waitlist control, or one of two exercise conditions: 1) 30 minutes at a fast pace, moderate-intensity (rapidresistive exercise; RET); 2) 30 minutes at higher-intensity (aerobic exercise; AET). Both exercise groups will perform the exercise on an elliptical trainer 3 times a week, for a session duration of 45 minutes including warm-up and cool-down. The RET group will focus on rapid reciprocal motion with minimal resistance, while the AET group will exercise at an elevated intensity known to produce an aerobic effect. After 12 weeks, all groups will repeat the baseline assessments (3 month follow-up). Following this assessment, the waitlist control group will be randomized to either RET or AET and the exercise groups will cease formal supervised exercise sessions. A third assessment visit will be performed after an additional 12 weeks (6 month follow-up).