A novel muscle property score and its sex-specific associations with fatigue, physical activity, and nutrition in middle-aged adults

A single measure cannot adequately represent overall muscle health. Although muscle properties include structural, functional, and mechanical dimensions, an integrated metric combining muscle mass, strength, and mechanical properties is lacking. This study developed a composite muscle health index, the Total Score of Muscle Properties (TSMP), and examined its associations with fatigue, physical activity, and nutrient intake in middle-aged Japanese adults. A total of 122 participants (59% women; aged 40–64 years) were assessed for appendicular muscle properties, including muscle mass, handgrip strength, knee extensor strength, muscle tone, and stiffness. Principal component analysis was performed separately by sex, with the first principal component defined as TSMP. Fatigue, physical activity, and nutrient intake were assessed using validated questionnaires. The first principal component explained 43.4% of the variance in men and 40.4% in women. In women, higher TSMP was associated with lower upper-body fatigue, instability, uneasiness, and local pain or dullness. In men, higher TSMP was associated with lower risks of physical inactivity and insufficient total physical activity after adjustment, and with higher intake of dietary fiber, calcium, magnesium, iron, B-complex vitamins, and vitamin D. These findings provide initial support for TSMP as an index of musculoskeletal health.