Table 1: Example message content

Message	Example Theoretical construct^+
Hi! Set a realistic goal for today. Begin with small steps (using stairs/brief walk). Increase physical activity gradually. Congratulate yourself for starting!	Suggestion Preparation for physical activity Psychological capability Perceived behavioral control Cue to action Building self-efficacy
We all like (and can manage) different things. Is there are a type of physical activity you enjoy? Text us about it! What is it and how does it make you feel?	Reflection Contemplating physical activity Social norms Motivation/Reflection Benefits of physical activity Building self-efficacy
Interesting fact: as well as helping to manage your arthritis, being physically active helps reduce the risk of osteoporosis, diabetes and some types of cancer!	Information Contemplating physical activity Motivation/Reflection Attitudes to physical activity Susceptibility/Severity Benefits/Barriers Cue to action
If you feel demotivated, write a list of all the good reasons for you to be active when you can - and remind yourself how well you’ve done so far. Keep it up!	Motivation Maintenance of physical activity Susceptibility/Severity Benefits/Barriers Perceived behavioural control Cue to action Building self-efficacy
What do you do when you get pain when being active? A) Stop completely; B) Have a rest then continue; C) Keep going; D) Other. Text back and let us know!	Reflection Preparing for physical activity Physical activity behavior Susceptibility/Severity Benefits/Barriers Cue to action Building self-efficacy

+based on Transtheoretical Models of behaviour change; Social Cognitive Theory; and informed by National Institute for Health and Care Excellence (NICE) guidelines for the treatment of knee osteoarthritis (NICE, 2014).