Table 2.

Overview of themes and sub-themes

Theme	Sub-themes/codes	Brief description
Commitment	Prioritising physical activity Value of physical activity Perceiving external barriers	Commitment evidenced through prioritising and valuing PA was the foundational dimension that influenced PA engagement and behaviour change amongst participants. Those that assigned a higher value and priority to PA were more likely to establish a routine or schedule PA and be successful in PA behaviour change.
Accountability and monitoring	Accountability to the trial team Accountability to others General monitoring and support	Participants that reported a reliance on external accountability for motivation tended to be less successful in PA behaviour change. Most participants valued reviews/check ins but most of those that increased and maintained PA were not so focused on external accountability for motivation.
Routine	Incidental PA Scheduled PA Influence of retirement on routine.	Participants that established a PA routine by scheduling or planning exercise appeared more likely to engage in adequate MVPA and maintain PA. There was recognition of the importance of a routine for PA engagement and yet many that were retired referred to a lack of structure to their day and some were resistant to planning and structure since retirement.
Fitbit as health coach	Instilling awareness Prompts Self-monitoring and feedback Goal setting and review Unreliability of device Fitbit as a demotivator Technical challenges	The theme of ‘Fitbit as health coach’ summarises the finding that the Fitbit™ was perceived as the primary active ingredient to increase PA for most participants. The Fitbit was viewed as helpful in raising awareness of PA level and sedentary behaviour through objective feedback and as a self-monitoring tool to achieve goals and targets.