This is an exploratory study designed to investigate the effects of aerobic dual belt treadmill walking versus traditional treadmill exercise on gait performance and functional capacity in older adults at risk for mobility disability. Identifying the relationship between aerobic treadmill exercise (dual belt or traditional) and the recovery of walking abilities will serve to optimize current rehabilitation approaches.

Participants will be forty-eight older adults with pre-clinical mobility disability who will be randomized into one of three intervention groups: dual-belt treadmill exercise, traditional treadmill exercise, or non-contact control.

Prior to and following 16 weeks of therapy, we will observe the participants' performing several functional movements (walking, standing from sitting, sitting from standing, walking up and down stairs) and measure their muscular activity. These performance-based measures will be obtained to determine whether their movement function improves as a result of therapeutic intervention. The participants' absolute performance on any of these tasks does not matter as much as that they make the very best effort on each of the tests. They are welcome to take a break at any point in the testing or training.

Inclusion Criteria:

• Age 65 and older
• Gait Speed of ≤ 1m/s
• Summary score 7-10 on the Short Physical Performance Battery, indicating mild to moderate impairment
• Sedentary as defined by reporting <20 min/wk of performing regular physical activity in the past month and <125 min/wk of moderate physical activity on the CHAMPS-18 questionnaire
• Willingness to be randomized to any of the treatment groups

Exclusion Criteria:

• Failure to provide informed consent
• Planned surgery in next year
• Hospitalization within the past 6 months
• Smoker > 1 pack per day
• Significant cognitive impairment, defined as a known diagnosis of dementia or a Modified Mini-Mental State score <24
• Significant cognitive executive impairment, defined as a Montreal Cognitive Assessment (MoCA) score of<25
• Progressive, degenerative neurologic disease, e.g., Parkinson's Disease, multiple sclerosis, Amyotrophic lateral sclerosis (ALS)
• Severe rheumatologic or orthopedic diseases, e.g., awaiting joint replacement, active inflammatory disease
• Terminal illness with life expectancy less than 12 mos, as determined by a physician
• Severe pulmonary disease, requiring either steroid pills or injections or the use of supplemental oxygen
• Severe cardiac disease, including New York Heart Association Class III or IV congestive heart failure, clinically significant aortic stenosis, history of cardiac arrest, use of a cardiac defibrillator, or uncontrolled angina;
• Other significant co-morbid disease that would impair ability to participate in the exercise-based intervention, e.g. renal failure on hemodialysis, severe psychiatric disorder (e.g. bipolar, schizophrenia), excessive alcohol use (>14 drinks per wk); persons with depression will not be excluded
• Develops chest pain or severe shortness of breath during exercise test
• Unable to communicate because of severe hearing loss or speech disorder or language barrier
• Lives outside of the study site or is planning to move out of the area in next year or leave the area for >1 month during the next year
• Severe diabetes, requiring use of insulin

Older adults with sedentary lifestyle willing to be be randomized to and treatment groups.