The investigators are examining the effects of exercise and cardiovascular fitness on cognitive processes, brain function, and the amount of several proteins in the blood. These proteins include a hormone called cortisol, also known as the "stress hormone," and a growth factor called "brain-derived neurotrophic factor" (BDNF). The "stress hormone" cortisol is produced by the adrenal glands. Stress, exercise, obesity, and other factors may influence cortisol levels. BDNF is a protein that promotes the health of nerve cells in the brain and in the body. It plays a role in the growth, maturation, and maintenance of these cells. The amount of this protein in blood samples is therefore an indicator of healthy nerve cell function. Here, the investigators are investigating if exercise improves brain function by changing BDNF levels. Participation in this research study will take approximately 4 months. During this time, participants will make four initial study visits. The first visit is for informed consent and screening, the second and third visits are for baseline fitness testing, and the fourth visit is for a blood draw, cognitive testing, and a functional Magnetic Resonance Imaging (fMRI) exam. Functional MRI is a brain imaging technique that uses a magnetic field to "take pictures" of the brain while a person performs a given task. It will take up to approximately three weeks to complete these initial four study visits. Following the four initial study visits, the exercise-training program will begin. Participants will be randomized to one of two training programs: an aerobic exercise program and a non-aerobic exercise program consisting of strengthening, balance and stretching exercises. The exercise training program will last 12 weeks. There will be three one-hour exercise sessions per week. After completion of the exercise-training program, participants will attend three follow-up study visits. The first two follow-up visits are for fitness testing. The third and final follow-up visit is for a blood draw, cognitive testing, and an MRI exam.

Inclusion Criteria:

• Non-smoking
• Age between 18-35 years or between 55-85 years
• Sedentary status (physically inactive)
• Generally healthy
• Native-English speaker

Exclusion Criteria:

• Diagnosis of severe anemia

• Past or present condition that affect cognitive functioning:

  1. learning disability
  2. neurological disorders or conditions (movement disorder, history of head trauma or stroke, epilepsy, etc.)
  3. psychiatric disorders or conditions (depression, anxiety disorder, etc.)

• Poor vision that cannot be corrected with glasses or contact lenses

• Presence of an infection

• Diagnosis of kidney failure

• Diagnosis of liver disease

• Diagnosis of thyrotoxicosis/hyperthyroidism

• Diagnosis of cancer

• Past or present conditions that are counter indicators for participation in cardio-respiratory fitness assessment and physical exercise:

  1. heart (e.g. heart attack, arrhythmias, etc.)
  2. circulatory (e.g. uncontrolled hypertension, high cholesterol, etc.)
  3. respiratory conditions (e.g., asthma or lung conditions, such as chronic obstructive pulmonary disease, acute bronchitis, acute common cold, lung cancer, pneumonia, etc.)
  4. current musculoskeletal impairments (e.g. leg claudication, fractures, hemiplegia, chronic joint pain, arthritis, osteoporosis, not able to walk comfortably without assistance, etc.)
  5. diagnosis of electrolyte disorder or abnormality
  6. presence of diabetes mellitus

• Claustrophobia (fear of small, encloses spaces)

• Female subjects: breast-feeding or potentially pregnant

• Obesity

• Eating disorder

• Compulsive exercising

• Ferro-magnetic metal in or on the body that cannot be removed (e.g. pacemaker, defibrillator, permanent tattoo or eyeliner with metallic specks)

• Prescription medication or other drugs that are cardio-active

• Prescription medication or other drugs that are psycho-active

• Drug abuse or alcohol misuse

• Regularly exercising

• Unavailable for the approximately 4-month duration of the study