Frailty in Chinese older adults: the roles of sedentary behavior, relative sit-to-stand power, and their additive interaction

Frailty is a critical geriatric syndrome. While sedentary behavior (SB) and low lower-limb muscle power, which can be efficiently assessed by the 30-second sit-to-stand (STS) test, are independent associated factors for frailty, their additive interaction, especially in Chinese older adults, remain unexplored. This study aimed to investigate the individual and additive interactions of SB and relative sit-to-stand (STS) power with frailty, and to explore urban-rural disparities in these relationships. This cross-sectional analysis used data from 13,686 community-dwelling older adults (aged 60–79) from the 2024 Annual National Physical Fitness Surveillance in China. Relative STS power was assessed using the 30-second STS test and the Alcazar equation, low relative STS power was defined as a value below 2.53 W/kg in men and below 2.01 W/kg in women. SB was measured via the Global Physical Activity Questionnaire. Frailty was defined using the Chinese FRAIL scale. Logistic regression was used to estimate odds ratios (ORs) and additive interactions were assessed using RERI (Relative Excess Risk due to Interaction) and AP (Attributable Proportion). The prevalence of frailty was 20.4%. Both SB and low STS power were independently associated with higher frailty odds (per 1-hour SB increase: OR = 1.11, 95% CI: 1.09–1.14; per quintile decrease in STS power: OR = 1.08, 95% CI: 1.05–1.12). A significant additive interaction was found (RERI = 0.31, 95% CI: 0.02–0.59; AP = 0.17, 95% CI: 0.01–0.29), indicating that approximately 17% of the frailty prevalence in the study, can be attributed to the interaction between SB and low relative STS power. This interaction was significant in rural areas (AP = 0.33) but not in urban areas. Prolonged SB and low relative STS power jointly increase the prevalence of frailty in an additive manner among Chinese older adults, with a more pronounced effect in rural settings. Public health interventions should concurrently target reducing sedentary time and enhancing muscle power, with tailored strategies for urban and rural populations.