ObjectivesQuantitative data show that physical activity (PA) reduced during the COVID-19 pandemic, with differential impacts across demographic groups. Qualitative research is limited, so reasons for this have not been explored in-depth. This study aimed to understand barriers and facilitators to PA during the pandemic, focusing on groups more likely to have been affected by restrictions, and to map these onto the Capability, Opportunity, Motivation Model of Behaviour (COM-B).DesignSemi-structured qualitative interview study.MethodsOne-to-one telephone/videocall interviews were conducted with younger (aged 18-24) and older adults (aged 70+), those with long-term physical health conditions or mental health conditions, and parents of young children, probing about their experiences of PA. Barriers and facilitators were identified using reflexive thematic analysis, and themes were mapped onto COM-B dimensions.Results116 participants were included (18-93 years old, 61% female, 71% White British). Key themes were the importance of the outdoor environment, impact of COVID-19 restrictions, fear of contracting COVID-19, and level of engagement with home exercise. Caring responsibilities and conflicting priorities were a barrier. PA as a method of socialising, establishing new routines, and the importance of PA for protecting mental health were motivators. Most themes mapped onto the physical opportunity (environmental factors) and reflective motivation (evaluations and plans) COM-B domains.ConclusionsFuture interventions should increase physical opportunity and reflective motivation for PA during pandemics, to avoid further negative health outcomes following periods of lockdown. Strategies could include tailoring PA guidance depending on location and giving education on the health benefits of PA.Statement of ContributionWhat is already known on this subject?Physical activity (PA) levels reduced during the COVID-19 pandemic.The extent of this reduction varied across demographic groups.Very few qualitative studies have explored reasons for these changes.What does this study add?Novel interview data, giving context to existing quantitative data.Insight into which themes were important for different demographic groups.Suggestions for increasing PA in future pandemics, by mapping findings to a theoretical framework.