The influence on sarcopenia of muscle quality and quantity derived from magnetic resonance imaging and neuromuscular properties

The relative contributions of intrinsic and extrinsic neuromuscular factors on sarcopenia are poorly understood. The associations among age-related declines of strength, muscle mass, and muscle quality in response to motor unit (MU) loss have not been systematically investigated in the same groups of subjects. The purpose was to assess MU loss, MRI-derived muscle cross-sectional area (CSA), muscle protein quantity (MPQ), and normalized strength of the dorsiflexors in one group of young (similar to 25 years) adult males compared with two groups of healthy men aged 60-85 years. Muscle strength was assessed on a dynamometer and was similar to 25% lower in both older groups, but CSA was less only in the older (>75 years) men, with no differences between the young and old (60-73 years). Normalized strength tended to be lower in both groups of aged men compared to young. For MPQ, only the older men showed similar to 8 % lower values than the young and old men. Older men had fewer functioning MUs than old, and both groups of aged men had fewer MUs than young men. Muscle quality appears to be maintained in the old likely due to compensatory MU remodeling, but in the older group (>75 years), MU loss was higher and MPQ was lower.