The investigators are studying whether metabolic abnormalities in calf (leg) muscle in older people with and others without HIV are associated with decreased abilities to exercise.

This research is being done to better understand why some older people living with HIV experience more fatigue and difficulty exercising than older people without HIV. This is important because fatigue and problems exercising are predictors of disability and hospitalization in older individuals. We will use non-invasive magnetic resonance imaging (MRI) and spectroscopy (MRS) of the leg muscle to measure muscle metabolites and fat content, as well as blood samples measures levels of inflammation.

Inclusion Criteria:

• Participants who are >= 60 years of age
• HIV serostatus positive participants on stable antiretroviral therapy (ART) (no regimen change in last month) with undetectable HIV viral load (<100 copies).
• HIV serostatus negative participants

Exclusion Criteria:

• Unable to understand the risks, benefits, and alternatives of participation and give meaningful consent
• Contraindications to MRI
• Significant cardiovascular, pulmonary or musculoskeletal or orthopedic disease that significantly limits exercise capacity
• Significant lower extremity peripheral vascular disease.
• Weight greater than 350 lbs
• Cognitive or speech impairments that would limit completion of questionnaires or fatigue reporting.

People with and without HIV, ages 60 and above will be enrolled