Physical activity (PA) benefits bone strength in children but little is known of the effects of PA on bone strength in adolescents. In this thesis, my primary aim was to determine the effect of a secondary school based PA intervention on bone strength, structure and density in adolescents.

This 8-month cluster, randomized-controlled, whole school-based intervention study had four intervention and five control schools. Participants were 210 Grade 10 students who were 15.3 years old, on average, at baseline. The Health Promoting Secondary Schools (HPSS) intervention was a choice- based model based on self-determination theory that aimed to increase PA, promote healthy eating and reduce screen time in adolescents. I used peripheral quantitative computed tomography (pQCT) to assess bone strength, structure and density at the distal and shaft sites of the tibia and radius. I assessed PA using a validated PA self-report questionnaire and I measured a sub-set of participants’ PA objectively using accelerometry.

Part I is a systematic review and narrative synthesis of PA and pediatric bone literature. High- quality randomized-controlled trials (RCTs) with weight-bearing PA increased bone strength in children. Bone structure adaptations in response to PA were more common than adaptations in bone density (RCTs and observational studies). Only one RCT involved adolescents (average age 13.8 years) and studies often overlooked the influence of muscle on bone responses to PA.

In Part II, moderate-to-vigorous PA (MVPA), vigorous PA (VPA) and grip strength positively influenced bone strength in boys and girls after controlling for ethnicity, maturity, limb length and muscle mass. Sedentary time (SED) negated the positive influence of MVPA, but not VPA, on bone strength in girls.

In Part III, the HPSS intervention did not lead to significant gains in bone strength, structure or density in adolescents. The external factor of a province-wide teacher job action possibly hindered the execution of the HPSS intervention.

In summary, MVPA and VPA benefit bone strength in adolescents but further investigations are warranted to determine the effects of SED on bone strength. It remains to be determined the effects of a choice-based intervention on bone strength adaptations in adolescent boys and girls.