Moderate to vigorous physical activity, leucine, and protein intake contributions to muscle health in middle age.

Objective: Identify contributors to differences in the muscle size and strength of sedentary and active young and middle-aged adults.

Methods: This cross-sectional study included 98 participants aged 20-65 years. Participants were categorized based on age and self-reported physical activity (PA) habits. Participants completed a strength assessment of knee extensors (KEPT), knee flexors (KFPT), plantar flexors (PFPT), and dorsiflexors (DFPT), a 3-day dietary intake log, 7-day accelerometry, and a magnetic resonance imaging (MRI) scan for muscle cross-sectional area analysis of the right quadriceps (CSAq).

Results: There were significant age and activity-related group effects for relative protein intake (p<0.001), relative energy intake (p=0.04), KEPT (p=0.01), CSAq (p=0.002), PFPT (p=0.004) and DFPT (p=0.003). Moderate, moderate-to-vigorous, and vigorous PA were positively associated with CSAq (R²=0.69- 0.71; p<0.05), KEPT (R²=0.61-0.63; p<0.05), and PFPT (R²=0.31-0.36; p<0.05). Relative protein intake and daily leucine intake were significantly and positively associated with CSAq (R²=0.70 and 0.67 respectively; p<0.05), KEPT (R²=0.62 and 0.65 respectively; p<0.05), and PFPT (R²=0.29 and 0.28 respectively; p<0.05).

Conclusion: Muscle size and strength were lower in middle age relative to younger age, but increased PA, protein intake, and leucine intake was associated with the preservation of muscle size and strength in larger muscle groups of the lower body.