Association Between Participation in a School-Based Structured Physical Activity Program and Standing-Height Trajectory in Thai Schoolchildren: A Quasi-Experimental Study

Background/Objectives: The relationship between structured physical activity and standing-height development in normally growing children remains insufficiently understood, as previous pediatric exercise research has predominantly focused on skeletal and fitness outcomes. This study examined the association between participation in a 24-week school-based structured physical activity program and standing-height trajectory in Thai schoolchildren. Methods: The study used a prospective non-randomized quasi-experimental design with a usual-activity comparison group and included 36 Grade 5 children aged 11–12 years. Eighteen children voluntarily participated in a structured physical activity program conducted three times per week for 24 weeks, while 18 frequency-matched children continued their usual school and daily activities. Standing height was assessed at baseline and every four weeks through Week 24. Standing-height trajectory was examined using a linear mixed-effects model adjusted for baseline standing height, age in months, and sex. Total standing-height gain and average standing-height velocity were also evaluated. Results: Standing height increased progressively in both groups. The standing-height trajectory differed significantly between the program-participation and usual-activity comparison groups. Mean total standing-height gain was 4.57 ± 1.37 cm in the program-participation group and 3.32 ± 0.92 cm in the comparison group, corresponding to a mean between-group difference of 1.26 cm (95% CI: 0.46–2.05; Hedges’ g = 1.05). Mean program adherence was 88.5%, and all participants in the program group met the prespecified ≥80% adherence criterion. Conclusions: Participation in the 24-week structured physical activity program was associated with a different short-term standing-height trajectory and greater total standing-height gain compared with usual activity. Because allocation was non-randomized and biological maturation was not directly assessed, these findings should be interpreted as preliminary associative evidence rather than as evidence of a causal effect on longitudinal growth.