Physical Activity Throughout Adolescence and Peak Hip Strength in Young Adults.

Importance: Peak bone strength, which occurs in early adulthood, is an important marker of the future risk of osteoporosis. It is therefore important to identify modifiable early life factors that are associated with the attainment of peak hip strength. Objective: To investigate the association of time spent in moderate to vigorous-intensity and light-intensity physical activity throughout adolescence with peak hip strength in adulthood. Design, Setting, and Participants: The Avon Longitudinal Study of Parents and Children is a prospective birth cohort study that initially recruited all pregnant women residing within the catchment area of 3 health authorities in southwest England who had an expected delivery date between April 1, 1991, and December 31, 1992. In total, 15 454 eligible pregnant women were enrolled, and 15 589 infants were delivered. Of those, 14 901 infants were alive at age 1 year. The present analysis examined 2569 healthy offspring who had valid physical activity measurements obtained during a clinical assessment for at least 1 age (12, 14, 16, and/or 25 years), with up to 4 repeated accelerometer assessments performed (1 per age-associated clinical visit). Data were analyzed from June 2019 to June 2020. Exposures: Trajectories of accelerometer-assessed time spent in moderate to vigorous-intensity and light-intensity physical activity at ages 12, 14, 16, and 25 years (measured in minutes per day) were identified using latent trajectory modeling. Moderate to vigorous-intensity and light-intensity physical activity were determined using established thresholds of acceleration counts per minute. Main Outcomes and Measures: Femur neck bone mineral density (BMD; measured in g/cm2) at age 25 years assessed by dual-energy radiography absorptiometry scans of the hip. Results: A total of 2569 participants (1588 female participants [62%]) were included in the analysis. Male participants spent more time in moderate to vigorous-intensity activity at each age and had greater adult femur neck BMD than female participants. For each sex, 3 moderate to vigorous-intensity trajectory subgroups and 3 light-intensity trajectory subgroups were identified. With regard to the moderate to vigorous-intensity trajectories, most male participants (85%) were in the low adolescent subgroup, with only 6% and 9% in the high early-adolescent and high mid-adolescent subgroups, respectively. Moderate to vigorous-intensity trajectories in female participants were divided into low adolescent-low adult (73%), low adolescent-high adult (8%), and high adolescent (19%) subgroups. Light-intensity physical activity trajectories were classified into low nonlinear, moderate decreasing, and high decreasing subgroups for both sexes. Femur neck BMD in male participants was greater in the high early-adolescent subgroup (0.38 g/cm2; 95% CI, 0.11-0.66 g/cm2) and the high mid-adolescent subgroup (0.33 g/cm2; 95% CI, 0.07-0.60 g/cm2) compared with the low adolescent (reference) subgroup. Femur neck BMD in female participants was greater in the high adolescent subgroup (0.28 g/cm2; 95% CI, 0.15-0.41 g/cm2) but not in the low adolescent-high adult subgroup (-0.12 g/cm2; 95% CI, -0.44 to 0.20 g/cm2) compared with the low adolescent-low adult (reference) subgroup. A sensitivity analysis using a negative-outcome control variable to explore unmeasured confounding supported these findings. The light-intensity trajectories were not associated with femur neck BMD; for example, differences in femur neck BMD between the high decreasing and low nonlinear subgroups were 0.16 g/cm2 (95% CI, -0.08 to 0.40 g/cm2) in male participants and 0.20 g/cm2 (95% CI, -0.05 to 0.44 g/cm2) in female participants. Conclusions and Relevance: Supporting high-intensity physical activity throughout early life may help to maximize peak hip strength and prevent osteoporosis in later life. Replication of our findings in independent studies will be important.