This study investigates the longitudinal changes in muscle strength, quality, and adipose tissue infiltration in older adults over a 5-year period. The Health, Aging, and Body Composition (Health ABC) study cohort, consisting of 1678 well-functioning men and women aged 70-79, was analyzed. Key findings include: 1. **Muscle Strength and Quality**: Both men and women experienced significant losses in knee extensor torque (MT), with men losing 16.1% and women losing 13.4%. The loss of MT was 2-5 times greater than the loss of muscle cross-sectional area (CSA), indicating a decrease in muscle quality (MQ). 2. **Adipose Tissue Infiltration**: There was a significant increase in intermuscular fat (IMF) area in both men and women, regardless of weight changes or subcutaneous fat (SF) changes. IMF increased in all weight-change groups, but the increase was more pronounced in men who gained weight. 3. **Weight Changes and Muscle Changes**: Weight gain did not prevent the loss of MT, despite a small increase in muscle CSA. Only those who gained weight had an increase in SF, while those who lost weight also lost SF. 4. **Race Differences**: No significant race differences were observed in the changes in muscle composition or strength with age. 5. **Diabetes and IMF**: Higher IMF was significantly associated with diabetes in men but not in women. The study concludes that aging is associated with a progressive loss of muscle strength and quality, even in weight-stable individuals, and that muscle fat infiltration worsens with age. These findings highlight the importance of addressing muscle health in aging populations to prevent sarcopenia and its associated health risks.This study investigates the longitudinal changes in muscle strength, quality, and adipose tissue infiltration in older adults over a 5-year period. The Health, Aging, and Body Composition (Health ABC) study cohort, consisting of 1678 well-functioning men and women aged 70-79, was analyzed. Key findings include: 1. **Muscle Strength and Quality**: Both men and women experienced significant losses in knee extensor torque (MT), with men losing 16.1% and women losing 13.4%. The loss of MT was 2-5 times greater than the loss of muscle cross-sectional area (CSA), indicating a decrease in muscle quality (MQ). 2. **Adipose Tissue Infiltration**: There was a significant increase in intermuscular fat (IMF) area in both men and women, regardless of weight changes or subcutaneous fat (SF) changes. IMF increased in all weight-change groups, but the increase was more pronounced in men who gained weight. 3. **Weight Changes and Muscle Changes**: Weight gain did not prevent the loss of MT, despite a small increase in muscle CSA. Only those who gained weight had an increase in SF, while those who lost weight also lost SF. 4. **Race Differences**: No significant race differences were observed in the changes in muscle composition or strength with age. 5. **Diabetes and IMF**: Higher IMF was significantly associated with diabetes in men but not in women. The study concludes that aging is associated with a progressive loss of muscle strength and quality, even in weight-stable individuals, and that muscle fat infiltration worsens with age. These findings highlight the importance of addressing muscle health in aging populations to prevent sarcopenia and its associated health risks.