This study will explore the effectiveness of two different interventions, a Health Action Process Approach (HAPA) mobile health (mHealth) intervention and a Just In-Time Adaptive Intervention (JITAI), on reducing sedentary behaviour in office workers. One third of participants will receive the mHealth HAPA intervention, consisting of a theory-driven behavioural counselling session with personalized daily SMS text messages, and another third of participants will receive the JITAI intervention, a behaviour tracking mobile phone application that will alert participants once a set sedentary behaviour condition has been met. The last third of participants will act as a control where they will not receive an intervention or any further information from the letter of information. The study will take place over four weeks, with the first acting as a baseline and the intervention period filling the latter three weeks.

This three-arm repeated measure randomized control trial will randomize participants into two intervention groups and one control group to attempt to reduce sedentary behaviour in office workers. The study will last for four weeks, the first week of the study will be used for baseline measurement and the remaining three weeks used for the intervention.

Participants will be full time office workers primarily recruited from large businesses and corporations. The primary purpose of the study will be to decrease sedentary behaviour in office workers from pre- to post-intervention by breaking up extended periods of sedentary time. The secondary objectives of the study are to compare the effectiveness of each group against one another to assess for any significant differences in sedentary behaviour profiles, total time spent sitting, break frequency, break duration, and time not spent sedentary as well as to examine whether physical activity changed over the course of the intervention.

One intervention group will attempt to reduce sedentary behaviour by receiving push notifications from an EMA tracking mobile phone application after a pre-set sedentary behaviour condition has been met. The application will monitor and collect data from the on-board accelerometer of the participant's phone and when a threshold of 30 consecutive sedentary minutes has been reached, the application will administer a notification asking the participant to break the extended sedentary behaviour they are currently engaged in.

The other intervention group will receive a theory-based behaviour change counselling session paired with daily personalized SMS text message boosters to achieve the study's aims. The counselling sessions and SMS text messages will be framed through the Health Action Process Approach, a behaviour change model that centres around action planning and coping planning. The control group will receive no intervention or further information from the letter of information.

Sedentary behaviour will be collected in the form of duration of time spent sitting, frequency of sedentary behaviour breaks, and duration of sedentary behaviour breaks. These variables will be measured through an EMA tracking mobile phone application that will be downloaded by each participant as well as a weekly online sedentary behaviour questionnaire. Outcome measures will be compared within and between groups to detect differences.

Inclusion Criteria:

• 18 years of age or older
• full-time employee in a desk-based office job (work from home accepted)
• have access to a mobile phone with internet connection
• can read and write in English

Exclusion Criteria:

• any medical or physical limitation that would prevent standing, stretching, and/or light physical activity