Identification of muscle weakness in older adults from normalized lower and upper limbs strength

ABSTRACT Introduction Weakness is a natural age-related condition meaning the loss of muscle strength that impairs older adults’ mobility and quality of life. Because the relationship between muscle strength and body-size variables is non-linear, weakness is misclassified in older adults with extreme body size (e.g., light, short, heavy, or tall). This misclassification can be overcome using the allometric approach. Objectives To propose cut-off points for older adults’ weakness for upper and lower limbs muscle strength normalized by body size with the ratio standard and allometric scaling. Methods Ninety-four community-dwelling older adults (69.1% women) were assessed for 49 body-size variables (anthropometry, body composition and body indexes), handgrip strength (HGS), one maximum repetition measurement for knee extensors (1RM knee extensors ), isokinetic knee extension peak torque at 60°/s ( knee extension PT 60°/s ), and six-minute walk test (6MWT). Ratio standard (muscle strength/body size) and allometric scaling (muscle strength/body size b ; when b is the allometric exponent) were applied for body-size variables that significantly were correlated with HGS, 1RM knee extensors and knee extension PT 60°/s . Cut-off points were computed based on ROC curve and Youden index. When there was mobility limitation (6MWT<400m) cut-off were computed according to sex. Results Absolute HGS, 1RM knee extensors and knee extension PT 60°/s cut-off points were not adequate because they were associated with body size (r>0.30). But it was corrected with muscle strength normalization according to body size-variables: HGS (n=1); 1RM knee extensors (n=24) and knee extension PT 60°/s (n=24). The best cut-off points, with the highest area under the curve (AUC), were found after normalization for men: HGS/forearm circumference (1.33 kg/cm, AUC=0.74), 1RM knee extensors /triceps skinfold (4.22 kg/mm, AUC=0.81), and knee extension PT 60°/s /body mass*height 0.43 (13.0 Nm/kg*m 0.43 , AUC=0.94); and for women: HGS/forearm circumference (1.04 kg/cm, AUC=0.70), 1RM knee extensors /body mass (0.54 kg/kg, AUC=0.76); and knee extension PT 60°/s /body mass 0.72 (3.14 Nm/kg 0.72 ; AUC=0.82). Conclusion Normalization removes the effect of extreme body size on muscle strength and improves the accuracy to identify weakness at population level, reducing the risk of false-positive cases. Highlights This study proposed a new approach to identify muscle weakness in older adults based on upper and lower limbs muscle strength normalized by body size (ratio standard and allometry). The identification of functional limitation is more precise when procedures of muscle strength normalization is applied.