Background Physical activity (PA), is an effective means of protecting against cardiovascular disease (CVD) development. PA refers to any skeletal muscle bodily movement that requires energy expenditure. Research shows that low- to moderate-intensity muscle endurance exercise, such as walking or brisk walking for about 15-20 minutes per day, is associated with a significantly lower CVD risk regardless of body mass index. Mobile technologies such as smartphone physical exercise apps offer a potentially cost-effective platform for facilitating regular exercise, allowing individuals to set goals, receiving feedback on achievements and health information for facilitating regular exercise. "Zero-time Exercise" (ZTE) is a new concept for physical activities (PA) and exercise promotion and can easily attract attention especially when most people believe that extra time is needed to do exercise. ZTE can be done easily during most time of day, while sitting, standing, walking or waiting. These include simple movements, such as stretching and resistance (endurance) movements of the head, neck and shoulders, chest and abdominal muscles and the upper and lower limbs. When ZTE is integrated into daily life and sustained with increasing intensity, the effects could be substantial, especially for those who are sedentary and have difficulties to meet the minimal requirements of 150 minutes of at least moderate PA per week. The term is also intended to motivate people to start with simple exercises (the 'Foot-in-the-door' approach), change the mindset (that exercises need much time, money and sweats) and overcome the inertia from a sedentary lifestyle to become more active. It could also be a way to promote mental health (such as increasing happiness) through PA or exercises and enjoying the fun and satisfaction from the rapid improvements in fitness performances. Having brought some easily observable benefits quickly to those who are willing to try for a few days, ZTE might lead to more intensive or vigorous exercises requiring extra time for greater benefits.

Objective:

To test the feasibility [recruitment, dropout, and adherence to PA] of using whatsapp as an inertia reminder to promote the adherence of Zero-time Exercises (ZTE) in patients with CHD.

Methods:

A 12-week single-blind randomised control design pilot study. 100 participants aged 18-69 years or above with proven stabilized acute coronary heart disease, physical inactive and having own smartphone with adequate internet technology literacy to operate the smartphone app and able to read and understand Chinese were recruited from the outpatient Cardiac Clinic in 3 hospitals.

The intervention group will be asked to practise ZTE either using the smartphone app or a zerotime exercise booklet with a focus on exercise adherence to at least 20-30 minutes per day, and on most and preferably all days of the week. The control group (CON) were asked to do deep breathing exercise at least 20 minutes daily. The exercise adherence of 25 participants in each EXP and CON groups are measured by activity trackers.

A mobile electronic whatsapp message was used as an anti- inertia reminder (AIR) to promote ZTE adherence. We used pictures with simple exercises to remind the participants in intervention group (EXP) to do ZTE regularly. Whatsapp reminders on deep breathing exercises are sent to CON. The frequency, format, whatsapp feedback and response to enquiry are the same as for EXP.

Inclusion Criteria:

• Aged 18-69 years;
• Stable CHD;
• Able to walk at normal speed for at least 15 min with no breathlessness;
• Have own smartphone and can operate a smartphone app;
• Have not downloaded and used the ZTE mobile app;
• Able to read and understand Chinese;
• Willing to come back at 3-month FU (the second face-to-face session, S2) for the measurements and/or intervention

Exclusion Criteria:

• Presence of medical conditions that might limit ability to perform moderate-intensity PA;
• Currently engaging in more than 150 minutes/week of moderate/vigorous PA
• Had revascularisation performed within past 3 months;
• Currently participating in another clinical trial and/or cardiac rehabilitation programme;
• Experience wearing an activity tracker;
• Experience using a smartphone app for exercises;
• Not a Hong Kong resident, or cannot communicate in Cantonese or Putonghua.

No plan to make IPD available to other researchers due to confidentiality.