Muscle-Specific Strength Better Predicts Physical Performance Decline Than Conventional Metrics: The I-Lan Longitudinal Aging Study.

Muscle-specific strength (MSS), defined as grip strength normalized to arm muscle mass, may better reflect muscle efficiency than grip strength. Despite its potential utility in diagnosing sarcopenia, longitudinal studies evaluating MSS as a predictor of sarcopenia-related outcomes remain limited.

This study investigated the associations of MSS with physical performance deterioration and biomarker profiles in community-dwelling older adults, compared it with conventional measures, determined whether MSS-defined sarcopenia offers superior predictive utility versus traditional diagnostic criteria and examined associations with key cardiometabolic and inflammatory biomarkers. This prospective cohort study included 1609 participants (mean age 64.5 ± 6.7 years; 50.5% male) from the I-Lan Longitudinal Aging Study.

MSS was calculated by dividing handgrip strength by dominant arm muscle mass. Participants were categorized into low and normal MSS groups using age-sex-specific cutoffs based on quintiles.

Physical function was assessed using the five-time chair stand test, with impaired physical performance (PPI) defined as taking ≥ 12 s. Logistic regression models examined the associations between MSS and PPI, adjusted for demographics, comorbidities, cognitive function and skeletal muscle index.

Biomarker profiles, including metabolic, inflammatory and hormonal parameters, were compared across MSS groups. Low MSS was found in 19.9% of participants.

Those with low MSS had significantly higher muscle mass (skeletal muscle index, 7.7 ± 1.2 vs. 7.1 ± 1.2 kg/m 2, p < 0.001; dominant hand muscle mass, 2.7 ± 0.8 vs. 2.3 ± 0.6 kg, p < 0.001) but weaker grip strength (26.1 ± 8.1 vs. 32.0 ± 8.8 kg, p < 0.001), indicating disproportionately low strength relative to muscle size. PPI was more common in the low MSS group (47.8% vs. 29.0%, p < 0.001).

Low MSS was significantly associated with higher odds of PPI (adjusted OR = 1.49, 95% CI: 1.11-1.99, p = 0.008), particularly among participants aged ≥ 65 years (OR = 1.80, 95% CI: 1.18-2.74, p = 0.006) and males (OR = 1.64, 95% CI: 1.09-2.47, p = 0.018). MSS-defined sarcopenia showed a stronger association with PPI (OR = 3.31, 95% CI: 1.26-8.74, p = 0.015) than conventional sarcopenia definitions.

Individuals with low MSS demonstrated adverse metabolic profiles, including higher fasting glucose (101.0 ± 27.3 vs. 95.0 ± 18.7 mg/dL, p < 0.001), HbA1c (6.0% ± 0.9% vs. 5.8% ± 0.7%, p < 0.001) and HOMA-IR (2.6 ± 2.2 vs. 1.8 ± 1.4, p < 0.001). In adjusted models, high HOMA-IR (OR = 2.71, 95% CI: 1.94-3.79, p < 0.001) and elevated hsCRP (OR = 1.66, 95% CI: 1.18-2.33, p = 0.001) were strongly associated with low MSS.

Low MSS independently predicts physical performance deterioration and associates with adverse metabolic and inflammatory biomarker profiles in older adults. MSS may better indicate muscle health than traditional metrics, supporting its inclusion in sarcopenia assessment frameworks as recommended by recent consensus guidelines.