Background Social relationships are associated with health and longevity, and lacking social relationships serves as a risk factor for mortality, cognitive decline, and frailty. However, the longitudinal effect of social relationships on physical capacity is not well understood and only a limited number of studies have examined this effect in China. This study therefore aims to investigate the influence of lacking social relationships –social isolation, loneliness, and lacking social support– on measured physical capacity –balance, chair stand speed, hand grip strength and walking speed– among older Chinese people over time.

Methods Up to 5,997 participants who were older than 50 years and attended all three waves (2011-2015) of China Health and Retirement Longitudinal Study were included. Social isolation index (consisted of not being married, living alone, lacking social contact with children and friends, and not participating in any social activities), feeling of loneliness (yes/no) and self-reported lacking social support (yes/no) were measured at baseline. Balance (fail/complete, evaluated through standing at side-by-side, semi-tandem and full-tandem positions), chair stand speed (collected during 5 rounds of sit-stand-sit cycle), hand grip strength (maximal measurement from both hands and adjusted by height), and walking speed (collected during a 2.5-meter walk) were measured at all three waves. Multilevel regression models were used to estimate the associations between each social relationship characteristic at baseline and trajectories of each physical capacity in unadjusted and adjusted models. Adjustments were made for age, sex, socioeconomic status, and urban-rural area. Missing value was handled through multiple imputation excluding outcomes.

Results Associations were most consistent for loneliness in fully adjusted models, with slightly slower chair stand speed (β = − .03, p < .001), weaker grip strength (β = − .61, p < .001) and slower walking speed (β = − .03, p < .001) in those individuals who reported loneliness at baseline. A weak association was observed for loneliness and chair stand speed over time (β = − .01, p = .062). The effect of social isolation was largely attenuated after introducing covariates and no real association for lacking social support was found.

Conclusion This study suggested that loneliness played a role in the decline in physical capacity among older Chinese people. It provides evidence that public health strategies that enrich older people’s social relationships may promote a healthier physical capacity. Future research will consider investigating whether bidirectional associations exist between social relationships and physical capacity over time.