Muscle strength and the risk of Multimorbidity in Middle-aged and older Chinese adults: A prospective cohort study.

backgroundMultimorbidity, a condition impacting more than 50% of older adults worldwide and creating a mounting burden in China, is strongly associated with reduced muscle strength. However, the precise mechanistic links underpinning this relationship are poorly defined. This prospective cohort analysis employs longitudinal information collected across multiple waves of the China Health and Retirement Longitudinal Study (CHARLS) to examine the association between a composite measure of muscular fitness and the onset of multiple chronic conditions.

methodsThis prospective study employs longitudinal data collected over four survey waves from the CHARLS, specifically the 2011 baseline along with subsequent follow-ups in 2013, 2015, and 2018. Low muscular strength was operationalized using a composite definition, where participants met the threshold if their handgrip strength fell below established cutoffs (< 28 kg for male or < 18 kg for female) or if they required 12 s or more to complete the five-times chair stand test. The primary outcome, multimorbidity, was characterized by the concurrent presence of two or more self-reported, physician-diagnosed chronic diseases. After completing the selection of variables, this study used the Variance Inflation Factor (VIF) to screen out variables with multicollinearity. Using Kaplan-Meier survival analysis to investigate the occurrence of comorbidities over time in individuals with low and normal muscle strength. The link between muscle strength and new-onset multimorbidity was analyzed via multivariable-adjusted Cox proportional hazards regression. Results are presented as hazard ratios (HR) with 95% confidence intervals (CI), relative to the low-strength group. We assessed whether the observed associations were modified by other variables through comprehensive subgroup analyses and by testing for statistical interactions. To ensure a robust characterization of the exposure-outcome relationship, a restricted cubic spline regression approach was employed. This flexible method allowed us to model non-linear dose-response curves for the risk of multimorbidity in relation to absolute grip strength, relative grip strength, and time taken to finish the five-times chair stand test.

resultsThe final analytical cohort comprised 8,073 participants. Kaplan-Meier survival analysis showed that the probability of remaining free of comorbidities was significantly lower in the low muscle strength group than in the normal muscle strength group. The multivariable-adjusted Cox model indicated an inverse relationship between normal muscle strength and multimorbidity risk. The hazard ratio for the normal-strength group was 0.91 (95% CI: 0.85-0.99), signifying a protective effect compared to their low-strength counterparts. The robust link between muscle strength and multimorbidity remained consistent when examining various subgroups categorized according to age, sex, educational attainment, marital status, geographic location, and behavioral factors. Furthermore, a restricted cubic spline analysis revealed a non-linear, U-shaped correlation between grip strength and the likelihood of developing multiple chronic conditions (p_overall<0.001, p_nonlinear < 0.005). Further analysis revealed that relative grip strength also had a non-linear relationship with the threat of multimorbidity (p_overall<0.001, p_nonlinear < 0.001), while the time for Five-Times Chair Stand had a linear relationship with the threat of multimorbidity (p_overall<0.001, p_nonlinear = 0.542).

conclusionsA low level of muscle strength is significantly associated with an increased risk of comorbidities among middle-aged and older adults. Therefore, incorporating strength training into comorbidity prevention and control strategies is of great importance and should be considered a key intervention. Particular emphasis should be placed on lower limb strength exercises, which do not exhibit a plateau effect in reducing comorbidity risk and can thus yield more substantial health benefits.