COVID-19 forces exceptional changes in work and school cultures across the world. The school suspension has led teachers to look for innovative ways of engaging students online. The remarkable benefits of doing exercise are relieving stress, maintaining our sanity, staying healthy and preventing muscle atrophy due to spending much more time at home and adopting a more sedentary lifestyle. 

Teachers have been playing a significant role in encouraging their students to be physically active, while the reduction in student-teacher interaction during the closure of schools makes this less achievable. The safest place to exercise during the pandemic is at home where students and their guardians are likely to spend most of their time. However, motivating children to exercise can be challenging. Technology has connected our community with a brave new, impeccably choreographed channel. Physical education teachers can upload daily workout for students to complete at home using crude equipment. However, without having any theory to guide implementation strategy, the effectiveness is in doubt. Research on exercise motivation from the standpoint of the Self-determination Theory (SDT) has been growing substantially recently. SDT advocates a motivational approach that prompts favourable outcomes by satisfying the three innate, essential, and universal psychological needs, namely autonomy, competence, and relatedness. Taken together, research grounded in SDT that encourages guardians to promote children’s PA level and motivation is limited. Therefore, this research aims to i) investigate the effectiveness of online daily workout for improving physical activity (PA) level at home among primary school-age children and ii) examine the underlying mechanisms behind the change of PA associated with SDT. A randomised controlled trial will be conducted. 290 guardian-children pairs will be recruited and allocated to four groups: i) given exercise choices, ii) coaching cues, iii) both choices and cues, and iv) nothing (waiting-list-control). Students’ PA level, sleep quality, physical fitness, motivation and basic psychological needs as well as guardians’ behaviour will be assessed before, immediately after, and eight weeks after the eight-week intervention so as to evaluate the immediate effect and sustainability of the intervention. The findings will provide evidence on the mechanism for online daily workout relating to basic psychological needs, which may ultimately improve PA level at home. The information will be valuable to schools’ emergency response to bad weather or any disruption in society.