Body Composition and Muscle Performance and Muscle-Specific Strength in Adults: Age-Sex Related Findings and Rehabilitation Considerations

Objective: To compare body composition, muscle performance, and muscle quality across age and sex groups. Methods: ), DXA-derived body composition (ALM, kg; body-fat, %), handgrip-dynamometry, and isokinetic knee-extensor testing concentric/eccentric low/high angular velocities. MQIs were calculated as force normalized to LM. Group comparisons, effect sizes, and correlation analyses were performed. Statistical significance was set at p < 0.05. Results: A total of 152 adults were analyzed. Body composition showed significant age-related differences, with increased body-fat percentage from young to middle adulthood and reduced ALM in older adults, while BMI showed limited discriminatory capacity. Handgrip-strength declined significantly only in the oldest group and showed small effect sizes after normalization by body-weight. In contrast, isokinetic knee extensor strength (particularly high-velocity concentric-torque) demonstrated pronounced age-related declines with moderate-to-large effect sizes. Eccentric/concentric torque ratios increased with age, indicating relative preservation of eccentric strength. Handgrip-based MQIs showed no age-group discrimination, whereas isokinetic-derived MQIs did. Conclusion: Muscle aging is characterized by early adiposity increase and later deterioration of lower-limb, velocity-dependent muscle performance, better captured by isokinetic-based measures than by BMI or handgrip-derived indices.