With the intensification of population aging in China, the issue of frailty among the elderly is becoming increasingly prominent, making research on its prevention and intervention particularly important. Currently, most studies lack discussion on the dynamic relationship between changes in physical activity and frailty.

This study is based on the five waves of data from the China Health and Retirement Longitudinal Study (CHARLS) from 2011 to 2020, aiming to explore the association between physical activity changes trajectories and frailty in older adults, and to provide a scientific basis for the prevention and intervention of frailty in the elderly.

Group-based trajectory modeling (GBTM) was used to identify the potential subgroups and trajectory characteristics of physical activity over time among the survey participants during the follow-up period. Multivariate unconditional logistic regression models were employed to analyze the association between different physical activity trajectory types and frailty, as well as subgroup analyses.

The physical activity trajectories of the survey participants were divided into four groups: persistent low group (262 individuals, 13.87%), low-to-increasing group (993 individuals, 52.57%), high-to-decreasing group (122 individuals, 6.46%), and persistent high group (512 individuals, 27.10%). There were significant differences in frailty among the four groups (χ²=20.867, P<0.001). After adjusting for confounding factors such as age and gender, multivariate unconditional Logistic regression indicated that compared with the persistent low group, the low-to-increasing group (OR=0.581, 95%CI=0.414-0.815, P=0.002) and the persistent high group (OR=0.546, 95%CI=0.373-0.799, P=0.002) had significantly lower risks of frailty. Subgroup analysis revealed that, compared with the consistently low group, the initially low then rising group demonstrated significant reductions in frailty risk among the following elderly subgroups: age≥65 years (OR=0.502, 95%CI=0.345-0.730), males (OR=0.539, 95%CI=0.326-0.891), urban residents (OR=0.441, 95%CI=0.211-0.922), those without a partner (OR=0.312, 95%CI=0.160-0.606) (P<0.05). Similarly, the consistently high group exhibited protective effects against frailty in elderly individuals aged≥65 years (OR=0.425, 95%CI=0.274-0.658), females (OR=0.539, 95%CI=0.328-0.886), urban residents (OR=0.280, 95%CI=0.101-0.780), and those without a partner (OR=0.347, 95%CI=0.164-0.737) (P<0.05).

Different trajectory groups are associated with the risk of frailty. Physical activity trajectories characterized by a low-to-increasing pattern and persistent high levels can significantly reduce the incidence of frailty in older adults.