This randomized controlled trial (RCT) will investigate the effects of a 6-month, individualized, moderate-intensity cycling intervention on cognition and hippocampal volume in AD with three aims:

• Determine the immediate effect of the cycling intervention on cognition in AD.
• Examine if the cycling intervention slows cognitive decline in AD from baseline to 12 months.
• Assess the effect of aerobic exercise on hippocampal volume in AD over 12 months using MRI.

The investigators will randomize 90 participants to the 6-month cycling or stretching/range of motion exercise (20-50-minute, 3 times a week). Participants will then be followed for another 6 months. Cognition will be assessed at baseline, 3, 6, 9, and 12 months and hippocampal volume will be measured using magnetic resonance imaging (MRI) at baseline, 6, and 12 months.

Aerobic exercise holds great promise for treating cognitive impairment, the hallmark symptom of Alzheimer's disease (AD), because it may attenuate brain atrophy. AD currently affects more than 5 million Americans, costing $203 billion in 2013 and causing poor outcomes such as loss of independence, low quality of life, and nursing home placement. Available drugs have only modest short-term effects on reducing or slowing cognitive impairment in AD. Hence, there is a pressing need to develop and test aerobic exercise interventions for AD. This randomized controlled trial (RCT) will investigate the effects of a 6-month, individualized, moderate-intensity cycling intervention on cognition and hippocampal volume in AD with three aims:

• Determine the immediate effect of the cycling intervention on cognition in AD.
• Examine if the cycling intervention slows cognitive decline in AD from baseline to 12 months.
• Assess the effect of aerobic exercise on hippocampal volume in AD over 12 months using MRI.

The investigators will randomize 90 participants to the 6-month cycling or stretching/range of motion exercise (20-50-minute, 3 times a week). Participants will then be followed for another 6 months. Cognition will be assessed at baseline, 3, 6, 9, and 12 months and hippocampal volume will be measured using magnetic resonance imaging (MRI) at baseline, 6, and 12 months. Transportation will be provided to participants for all study-related activities including exercises. Participants who could not undergo MRI can still participate.

Inclusion Criteria:

• Diagnosis of probable or possible Alzheimer's disease (AD);
• 15≤ Mini-Mental State Examination (MMSE) score ≤26;
• Clinical Dementia Rating score 0.5-2;
• Community-dwelling, e.g., homes and assisted living;
• Age 66 years and older;
• English-speaking;
• Verified exercise safety by the primary care providers and the cardiologists if subjects have significant cardiac history;
• Stable on AD drugs >1 month if AD drugs are prescribed.

Exclusion Criteria:

• Resting heart rate (HR) ≤50 or ≥100 beats/min;
• Neurological disorders in the past 5 years;
• Psychiatric disorders in the past 5 years;
• Alcohol or chemical dependency in the past 5 years;
• Contraindications to exercise;
• New symptoms or diseases that have not been evaluated by a health care provider, e.g., hip fracture, ongoing and unplanned weight loss, severe shortness of breath, deep vein thrombosis, hernia, unhealed sores, joint swelling, pain or trouble walking;
• Cardiac ischemia or serious arrhythmia on the electrocardiograph during the screening exercise test.