Vertical ground reaction forces and loading rates during typical activities in children and adolescents: sex- and maturity-specific considerations for bone health

Introduction Despite physical activity (PA) during childhood being one of the most important modifiable factors influencing osteoporosis risk, the ground reaction forces (GRFs) associated with typical physical activities remain largely unknown, and the influence of sex and maturity on these factors is rarely considered. This study aimed to quantify the GRF and force loading rates during typical everyday activities in children and adolescents. Methods A total of 282 children (127 boys, 142 girls; aged 8–16 years) completed walking, running, jumping and hopping on a portable force plate. Maturity was determined using maturity offset relative to peak height velocity (PHV) and categorised as Pre- or Post-PHV. Peak vertical force (PVF) and average loading rate (ALR) were extracted from force-time histories. Linear mixed-effects regression models were fitted separately for PVF and ALR, including fixed effects for activity, sex, maturity group, their two- and three-way interaction terms, and force-plate type, with participant (ID) as a random effect. Results There was a significant three-way interaction for both PVF (F (4,225) = 4.15, p = 0.003) and ALR (F (4,225) = 5.54, p 0.001), indicating that the effect of maturity differed across activities and between sexes. For locomotor tasks, PVF decreased significantly from Pre- to Post-PHV in both sexes during walking (both p ’s 0.001) and in boys during running ( p 0.001), with a similar, but non-significant trend in girls ( p = 0.291). The ALR also decreased during running in both sexes, reaching significance in boys ( p 0.001) and borderline significance in girls ( p = 0.057). The jumping activities showed divergent sex-specific patterns. Boys tended to maintain or increase PVF and ALR with maturation in the low and high jump activities ( p = 0.004 –0.914), whereas girls showed consistent reductions from Pre- to Post-PHV ( p 0.001-0.057). Discussion Findings demonstrate that children and adolescents do not always experience the same osteogenic stimulus for a given activity. Consideration of sex- and maturity-specific loading profiles may therefore be important for designing and interpreting bone-benefitting interventions that deliver an appropriate osteogenic stimulus across childhood and adolescence.