Background Many physical activity (PA) guidelines now include recommendations on strength and balance training, reflecting growing recognition of the importance of maintaining physical capability for healthy ageing. However, these recommendations are not widely communicated and are often targeted at older adults where most research evidence has been generated. Less well understood is whether there are benefits of greater levels of participation in physical activity and of reducing sedentary time for strength and balance earlier in adulthood. We therefore aimed to examine the associations of sitting time and time spent physically active with grip strength and standing balance performance in middle-age.

Methods A total of 4,726 men and women from the 1970 British Cohort study, with data on free-living sitting and activity (ascertained over 7 days using a thigh-mounted accelerometer (activPAL3 micro)), grip strength and standing balance performance at age 46 years were included in analyses. Linear and multinomial logistic regression models were used to test associations of sitting time, time spent in moderate-vigorous physical activity (MVPA) and total PA time with maximum grip strength and standing balance performance, respectively. Models were adjusted for wear time, sex, body mass index, height, disability, self-rated health, malaise, smoking and education.

Results Greater time spent sitting was associated with weaker grip strength in both sexes and this was maintained after adjustment for potential confounders and MVPA time (fully-adjusted regression coefficient: -0.51 kg (95% CI: -0.63, -0.39) per 1 hour of sitting per day). Positive associations between total PA time and grip strength were observed in confounder-adjusted models but were fully attenuated after adjustment for sitting time. There was only weak evidence of an association between sitting time and balance performance. However, there was evidence to suggest that participants who spent more time in MVPA and total PA had higher relative risks of successfully balancing for 30 seconds with their eyes closed (vs poor balance performance). However, these associations were not maintained after adjustment for potential confounders.

Conclusion In a national sample of middle-aged adults there was a robust association between greater time spent sitting, measured using a gold-standard assessment, and muscle weakness. Associations of PA time with grip strength and balance performance were attenuated in fully-adjusted models. This highlights the potential importance of promoting less sitting and activities that specifically benefit muscle strength and balance performance in midlife to ensure that people maintain all important aspects of their physical capability as they age.