There is strong evidence that regular physical activity (PA) is associated with a range of physical health benefits for school-age youth, including improvements in body composition, physical capacity, and overall health-related indicators (e.g., blood pressure, insulin resistance, lipid profile). Emerging evidence also suggests that PA and physical fitness have a positive effect on mental health, cognitive function, and academic performance. Executive functions are generally defined as "high-level cognitive processes" that manage other basic cognitive functions. They consist of functions such as planning, self-regulation, initiation and inhibition, and cognitive flexibility. These functions are believed to be important prerequisites for successful learning in preadolescent children, predict better health and wealth, and have been associated with a reduced likelihood of being convicted of a criminal offense. Several recent meta-analyses have suggested that PA may positively affect cognition and executive function in children. Despite the known benefits of an active lifestyle, more than half of Hong Kong's children and young people fail to follow the current physical activity recommendations, and trends show a decline in health-related physical fitness. Executive functions develop from early childhood and through adolescence into adulthood, with large developmental changes occurring during the elementary school years. Accordingly, effective interventions implemented early in life when the higher cortex is still developing, have the potential to elicit significant long-term improvements. Therefore, it is important to find effective strategies to promote the PA of children, so as to improve their executive function during their childhood.

Attention-deficit/hyperactivity disorder (ADHD) is a commonly diagnosed developmental disorder, with a 5.3% worldwide prevalence in children and adolescents. The most common symptoms include inattention, impulsivity, impaired inhibition, and hyperactivity. Also, young people with ADHD are often characterized by dysfunction in high-level cognitive functions such as executive function. This executive dysfunction may play an important role in the commonly observed behavioral problems in children with ADHD. There are a number of ways to treat ADHD, e.g., medication, psychotherapy, psychoeducation, neuro-feedback, behavior management, etc. Of these treatments, PA and exercise have emerged as effective strategies to manage ADHD given that neither are associated with negative side effects (see recent reviews. To summarize, although not always consistently, PA, especially moderate- to high-intensity aerobic exercise, may improve the emotion/mood, behavior, executive function, and some physical measures of children with ADHD. Acute aerobic exercise may have a positive effect on a variety of measures with a large effect size up to 1.26 in children with ADHD. Two studies have reported medium-to-large effects of acute exercise on executive function. Long-term exercise interventions have also been shown to benefit children with ADHD, with improvements in some measures showing a large effect size up to 0.96. However, these benefits have been mainly described as an improvement of behavioral and emotional problems. Several studies have reported that long-term 'mixed exercise programs' may have moderate to significant effects on several aspects of executive function (e.g., inhibition) and attention in children and adolescents with ADHD, but other studies did not report the same findings. Therefore, so far evidence-based suggestions regarding the optimal exercise for children with ADHD remain somewhat inconsistent.

Research into the relationship between PA and ADHD has generally involved mixed exercise programs at a low- to moderate-intensity; running and stationary cycling have been the most common exercise modes. Whether structured PA is effective for young people is yet to be examined. Children's habitual PA patterns are characterized by participation in games or "unpredictable" sports activities (e.g., football, basketball). Given that children's intrinsic motivation, or level of enjoyment, is also a strong predictor of PA participation, any intervention program should be designed to optimize their enjoyment of PA and thus enhance the likelihood of long-term adherence. When compared to structured exercises such as running or cycling, games-based PA arguably provides a more attractive, acceptable, sustainable, and enjoyable exercise model for young children. To date, only a few studies have examined the effect of an acute bout of team game-based activity on cognitive function in healthy children and adolescents. Research has found that different aspects of cognitive function, such as free recall memory, attention, executive function, and working memory are improved following an acute bout of game-based team exercise (e.g., basketball, tennis). For children with ADHD, mixed exercise protocols have generally been used in intervention programs, and only a few studies have used team sport-based games as part of their exercise intervention. However, given the design of these studies, it is not possible to distinguish the sole effect of game-based PA from those of the mixed exercise protocols in these studies. A recent study has reported that a 12-week table tennis exercise has positive effects on the gross motor skills and some of the executive function performances (mainly inhibition) in the ADHD training group, compared with the ADHD non-training group and a control group. It should be noted that the game-based PA in these aforementioned studies was generally 'aerobic' in nature and completed at low- to moderate intensities.

Recently high-intensity interval training (HIIT) has emerged as a feasible and efficacious strategy for improving the physical health of young people. The HIIT can be completed in a short period of time while resulting in equivalent physiological adaptations to longer sessions of traditional aerobic training. Recent research has suggested that traditional HIIT intervention programs, including running and cycling, may improve executive function in healthy children and adolescents. It has been suggested that a very brief HIIT intervention over two weeks reduced off-task behavior and enhanced selective attention in primary school children. For children with ADHD, to the best of our knowledge, only one recent study was conducted to investigate the effect of a traditional HIIT program on physical fitness, motor skills, social behavior, and quality of life. In this randomized controlled trial, 28 boys with ADHD were assigned to either a traditional HIIT group or a standard multimodal therapy (TRAD) group. After the three-week intervention, the authors reported that HIIT was more effective in improving motor skills, self-esteem, relations with friends, competence, and subjective ratings of attention, compared with TRAD. However, despite this encouraging preliminary evidence, it remains unclear whether HIIT can be adapted to treat children with ADHD to improve their executive function, a key aspect in many facets of life.

While the limited findings regarding the effects of HIIT on executive function are encouraging, and HIIT has emerged as an enjoyable and effective exercise for children, previous studies have tended to prescribe HIIT interventions with a focus on running and jumping. The effect of game-based HIIT interventions on the executive function of children has yet to be investigated. Research of this nature is needed, particularly to determine whether game-based HIIT can improve outcomes (e.g., executive function, social behavior, sports skills, etc.) for children with ADHD. Therefore, the aim of the proposed study is to investigate the effect of two different kinds of exercise programs, i.e., an 8-week game-based HIIT (GameHIIT) program and an 8-week game-based structured aerobic exercise (GameSAE) program, on the executive function of children with ADHD. The hypothesis of the proposed study is that both GameHIIT and GameSAE programs will significantly improve the executive function of children with ADHD, compared with those in the control group. A secondary hypothesis is that the GameHIIT group may confer additional benefits when compared with the GameSAE group.