Background Several quantitative studies have found a decline in physical activity in response to COVID-19 pandemic restrictions. The aim of the present study was to use large-scale free text survey data to qualitatively gain a more in-depth understanding of the impact of the COVID-19 pandemic on physical activity, then map barriers and facilitators to the Capability, Opportunity, Motivation, and Behaviour (COM-B) Model of Behaviour to aid future intervention development. Methods 17,082 participants provided a response to the free text module, and data from those who mentioned physical activity in any context were included. Data were analysed using thematic analysis and key themes identified. Results 5396 participants provided 7490 quotes related to physical activity. The sample were predominately female (84%), white (97%) and aged <60 years (57%). Seven key themes were identified: the importance of outdoor space, changes in daily routine, impact of COVID-19 restrictions, perceived risks or threats to participation, the importance of physical health, the importance of physical activity for mental health and the use of technology. Conclusion Future physical activity interventions could encourage people to walk outdoors, which is low cost, flexible, and accessible to many. Developing online resources to promote and support physical activity provides a flexible way to deliver quality content to a large audience.