Skeletal Muscle Function Deficit: An Operational Definition. Results from the InCHIANTI Longitudinal Study

BackgroundAge-related muscle dysfunction is a major contributor to disability, frailty, and poor clinical outcomes in older adults. Muscle mass and strength provide limited insight into the multifactorial nature of muscular decline. Skeletal Muscle Function Deficit (SMFD) framework integrates multiple domains muscle mass, quality, strength, and power to capture a broader spectrum of age-related muscle dysfunction. ObjectiveTo develop and validate a composite SMFD score and evaluate its association with key geriatric outcomes in older adults. MethodsThis study used data from the InCHIANTI longitudinal cohort (1998-2018), including 1,035 participants and 3,196 total assessments. The SMFD score (range 0-20) was computed by assigning quintile-based values of muscle area, density, strength, and lower limb power. Associations with disability in basic and instrumental activities of daily living (BADL/IADL), frailty phenotype, poor physical performance (SPPB <7), hospitalization, falls number, and major chronic diseases were analyzed using mixed-effects models, adjusting for age, sex, fat area, and multimorbidity. ResultsThe SMFD score declined significantly over time and was independently associated with lower risk of BADL (OR 0.57), IADL (OR 0.70), frailty (OR 0.72), poor performance (OR 0.68), hospitalization (OR 0.96), and falls number (OR 0.96). Higher SMFD scores were also inversely associated with the prevalence and incidence of Parkinsons disease, stroke, and hip osteoarthritis. ConclusionsThe SMFD score is a valid, multidimensional measure that predicts adverse outcomes in older adults more effectively than traditional sarcopenia, dynapenia, and powerpenia. It holds promise for use in clinical assessment, risk stratification, and targeted interventions.