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 a physical activity related word 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 (British/Irish/Other) (97%) and aged <60 years (57%). Seven key themes were identified: the importance of outdoor space, changes in daily routine, COVID-19 restrictions prevented participation, 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.