It is estimated that approximately 30% of child and adolescents manifest subthreshold depression (including other specified depressive disorder and unspecified depressive disorder), which can further develop into major depression with as high as 25%-50% within one year. The cognitive development of adolescents is a critical area of research, given its significant implications for academic performance, mental health, and overall well-being. During adolescence, the brain undergoes substantial structural and functional changes, particularly in regions associated with executive function, memory, and processing speed. These changes provide a unique opportunity to explore interventions that can support and enhance cognitive development. One such promising intervention is physical exercise. Adolescence is characterized by rapid cognitive growth, including improvements in executive functions such as planning, decision-making, and inhibitory control. These cognitive abilities are essential for academic success and social interactions. However, this period also presents risks for cognitive and emotional disturbances, making it vital to identify effective strategies to promote healthy cognitive development. Despite the promising evidence, there is a need for more rigorous research, particularly long-term randomized controlled trials (RCTs), to establish the causal relationship between exercise and cognitive function in adolescents, including those with sub-threshold depression. Most existing studies have focused on short-term interventions or specific cognitive tasks, leaving a gap in our understanding of the sustained effects of exercise over an extended period. Additionally, the transition from supervised to unsupervised exercise and its impact on adherence and cognitive outcomes is underexplored. There is also a need to evaluate the potential of exercise as a preventive strategy against the development of major depressive disorder in this vulnerable population. Additionally, evidence suggests that participants with lower baseline levels of physical activity may experience more significant cognitive improvements from exercise interventions than those with higher baseline activity levels. This highlights the importance of considering baseline physical activity levels when assessing the effectiveness of exercise on cognitive outcomes.

This study will particularly focus on the cognitive domains of attention, memory, and processing speed. These domains are critical for academic success and daily functioning and are often impacted in adolescents with sub-threshold depression. Previous research has shown that these domains are particularly responsive to physical exercise interventions.

This is multi-center, psycho-education randomized controlled trial, consisting of two periods with each of 6 months (i.e supervised and maintenance period). In the exercise intervention arm, during the first 6 months, participants will be asked to do moderate-intensity aerobic exercise 3-4 times a week, which will be supervised in person by physical educators/professionals. In the next 6-month maintenance period, participants will be asked to continue doing aerobic exercise at the same intensity and frequency, which will not be supervised in person by educators/professionals, but they will be sent reminders and report adherence monthly. All of the physical activity will be recorded by accelerometer at scheduled time points (e.g. the first week, 6 week, 3 month, 6 month, 9 month, 12 month). Psycho-education group will be given 6 sections of general psycho-education onsite and/or virtually dependent on the surroundings with regards to the impact of the COVID-19 pandemic. The main aim of this trial is to investigate the effect of long-term aerobic exercise on cognition and prevention depression in young people with or without subthreshold depression.

Inclusion Criteria:

• PHQ-9 ≥5
• 12~17 years old.

Exclusion Criteria:

• Current or past DSM-5 criteria for major depressive disorder,Bipolar disorder, Schizophrenia.
• Musculoskeletal problems such as arthritis, gout, osteoporosis, or back, hip or knee pain that may interfere with exercising
• A history of organic brain disease or brain trauma
• Alcohol, drug or other psychoactive substance abuse or dependence
• Currently enrolled in another exercise study
• Any conditions that would make exercise unsafe or unwise
• Taking medication that interferes with heart rate response to exercise such as beta blockers