Background Physical activity declines with age, despite its vital role in preventing and managing disease and disability in older adults. Interventions promoting co-engagement in physical activity are potentially more effective than individually-focused solutions, due to processes of behavioural normalisation and social diffusion, but little is known about the kinds of relationships that are important. This study sought to address this gap.

Methods Data from older adults, aged 55-75 years, from two communities in Scotland was used (3679 social contacts reported by 140 participants). Participants completed a social network survey, which collected demographic and health data, as well as detailed information about their social contacts. Multilevel modelling was used to identify the characteristics of participants and social contacts that were predictive of co-engagement in physical activity.

Results The characteristics of social contacts and relationships were more important for co-engagement than the attributes of participants or their overall network composition. On average, participants were co-active with 21% of their social contacts. These tended to be people in higher age groups (odds ratio (OR) = 1.13, 95% confidence interval (CI) = 1.04, 1.24), in the same age group as the participant (OR = 1.30, 95% CI = 1.01, 1.67), perceived as having good health (OR = 1.69, 95% CI = 1.47, 1.94), and who had more connections to other social contacts (OR = 1.03, 95% CI = 1.01, 1.06). They were more likely to be active with partners (reference category) and friends (OR = 1.39, 95% CI = 0.71, 2.75) than family members (OR = 0.27, 95% CI = 0.14, 0.53) and neighbours (OR = 0.11, 95% CI = 0.04, 0.28). Co-engagement was more likely with social contacts who lived geographically closer (OR = 1.10, 95% CI = 1.10, 1.29) and with whom they interacted more frequently (OR = 1.68, 95% CI = 1.53, 1.86), but less likely with people they had known for longer (OR = 0.66, 95% CI = 0.58, 0.75). Relationship quality did not affect the probability of co-engagement with social contacts (OR = 1.03, 95% CI = 0.94, 1.12).

Conclusion Older adults’ physical activity may benefit from connections to people to do physical activity with. The convenience and accessibility of relationships, as well as age, health, and connectedness of social contacts, were more important than factors often identified in previous studies, such as the quality of relationships or number social connections. These findings may inform the development of appropriate interventions.