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 (1RMknee extensors), isokinetic knee extension peak torque at 60 degrees per second (knee extensionPT), 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, 1RMknee extensors and knee extensionPT. 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, 1RMknee extensors and knee extensionPT 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); 1RMknee extensors (n=24) and knee extensionPT (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), 1RMknee extensors/triceps skinfold (4.22 kg/mm, AUC=0.81), and knee extensionPT/body mass*height^0.43 (13.0 Nm/kg*m0.43, AUC=0.94); and for women: HGS/forearm circumference (1.04 kg/cm, AUC=0.70), 1RMknee extensors/body mass (0.54 kg/kg, AUC=0.76); and knee extensionPT/body mass^0.72 (3.14 Nm/kg0.72; 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.