**Background:** Physical inactivity is a global health problem, with women less active than men and the gender gap widening from late adolescence. Young women are attracted to using physical activity apps but disengage more often than men, suggesting apps may not meet their behaviour change needs. While content analyses show commercial apps use 5–9 behaviour change techniques (e.g., self-monitoring, goal setting, social comparison), little qualitative research has focused specifically on how young women experience these techniques. This study aimed to explore young women's lived experiences of behaviour change techniques in popular physical activity apps and identify factors impacting their behaviour not addressed by apps.
**Methods:** Participants were recruited via Facebook advertising in Australia (June–July 2020) from 15,600 total views. Of 124 respondents, 68 were eligible and consented; 39 scheduled an information session (7 withdrew), and 32 completed the study. Eligible participants: lived in Australia, identified as female, aged 18–24, owned a smartphone, did not meet physical activity guidelines, and had no medical contraindications. Participants were randomly assigned (1:1) to FitOn (n=17) or Map My Fitness Workout Trainer (n=15) and instructed to use the app for two weeks to achieve a personal goal. Using photovoice methodology, participants documented experiences through photographs (minimum 4) with captions, then attended a 40–45 minute semi-structured interview using the SHOWeD technique. Interviews were transcribed verbatim. Inductive thematic analysis was conducted in NVivo 12 Plus by three researchers, with an initial coding framework independently applied to three participants' data, then refined and applied to all data. Member-checking was performed by sending conceptual diagrams to participants. Participants provided 134 photos (mean 4.2 per participant; range 3–8). Most participants were university students (25/32, 78%), had prior app experience (22/32, 69%), and reported moderate to extreme confidence in physical activity (29/32, 91%).
**Key Results:** Two central organising themes emerged: mobile app techniques and external factors. App techniques clustered into four themes: (1) Logging and monitoring physical activity—fostered a sense of achievement and supported goal progress, but motivation depended on whether indicators were personalised and meaningful. Appearance-based indicators (e.g., calories) were described as 'triggering' by some. Inability to track activity was demotivating, with participants feeling their exercise 'didn't count'. Some users found tracking had no effect on motivation. (2) Reminders and prompts—created a sense of external accountability and prompted exercise when language was encouraging (e.g., emphasising stress-relief). Ineffective reminders led some to feel guilt and disengage (e.g., turning off notifications). When reminders were absent or non-functional, participants forgot to exercise and disengaged from the app. (3) Workout videos and written instructions—improved knowledge and confidence, especially for beginners, and helped overcome environmental barriers (e.g., bad weather, lack of space during COVID-19 restrictions). However, high-intensity exercises led to disengagement when participants felt unable to complete them. Trainer relatability (body type, ethnicity, gender) affected motivation, though responses varied even among similar participants. Both choice and guidance were important; lack of structured programming led to disengagement. (4) Social features—few participants engaged; those who did reported enhanced motivation. However, privacy concerns (e.g., GPS route sharing, unwanted contact) led some to delete the app. Most participants were uninterested in sharing workouts with friends, citing social norms and fear of negative evaluation. External social support from friends and family was a key motivator, with group exercise described as a 'safe space' and outdoor activity enhancing enjoyment and mental health.
**Clinical Implications:** The findings suggest that behaviour change techniques in physical activity apps can positively influence determinants like motivation and competence in young women, consistent with social cognitive models and self-determination theory. However, operationalisation is critical: (a) self-monitoring should allow logging of diverse activities common among young women and use opt-in, behaviour-focused indicators rather than appearance-based ones (e.g., calories/weight) to avoid triggering unhealthy fixations; (b) workout videos should include low-intensity options for mastery experiences and feature diverse, relatable trainers; (c) reminders should use language promoting identified or intrinsic motivation (e.g., stress-relief, enjoyment) rather than guilt-inducing messages; (d) social features should be opt-in with strong privacy protections, especially given young women's increased risk of online harassment. External social support from existing networks (friends, family) was more positively received than in-app social features, suggesting apps should facilitate rather than replace real-world social connections. Future research should examine individual-level traits (e.g., competitiveness, body image concerns) that may moderate responses to techniques, and explore how to safely incorporate social support from existing networks into app design.