The relationship between skeletal muscle mass index and spinal pain: a cross-sectional study comparing middle-aged and elderly individuals in China and the United States

Background Spinal pain is a leading cause of global disability, with prevalence increasing with age. While skeletal muscle mass index (SMI) is a key indicator of sarcopenia and influences spinal pain through biomechanical, metabolic, and neuromuscular mechanisms, large-scale cross-national studies examining this relationship are lacking. This study aimed to investigate the association between SMI and spinal pain in middle-aged and elderly populations from China and the United States. Methods This cross-sectional study used data from the National Health and Nutrition Examination Survey (NHANES, 1999–2004) and the China Health and Retirement Longitudinal Study (CHARLS, 2015). Baseline analysis of the population, multivariable logistic regression, restricted cubic spline (RCS) analysis, and subgroup analyses were performed to assess associations, adjusted for demographic, lifestyle, and health-related covariates, and to evaluate SMI's capacity to predict spinal pain. We employed the receiver operating characteristic (ROC) curves and the area under the curve (AUC). Results A total of 6,563 participants from NHANES and 12,221 from CHARLS were included. After full adjustment, higher SMI was significantly associated with a lower risk of spinal pain in both cohorts (NHANES: OR for trend < 1, P  < 0.01; CHARLS: OR for trend < 1, P  < 0.01). RCS analysis revealed a linear negative association between SMI and spinal pain (non-linear P  > 0.05). Subgroup analysis identified a differential effect modifier. ROC analysis compared the predictive performance of two databases. Conclusions Higher SMI is independently associated with a reduced risk of spinal pain in both Chinese and American middle-aged and elderly populations, exhibiting a linear relationship. The protective effect of SMI is influenced by specific population factors, underscoring the importance of different racial/social prevention strategies. Moreover, CHARLS has better predictive ability than NHANES. Future research should longitudinally verify the causal relationship between these findings and explore targeted interventions to enhance muscle mass to prevent or treat spinal pain.