The ‘muscle-bone unit’ from childhood into early adulthood: A SITAR nonlinear growth model

Baxter-Jones Adam1, Elhakeem Ahmed2, Jackowlski Stefan3,1, Barbour-Tuck Erin1, Erlandson Marta1 1College of Kinesiology, University of Saskatchewan, Saskatoon, Canada; 2MRC Integrative EpidemiologyUnit, University Bristol, Bristol, United Kingdom of Great Britain and Northern Ireland; 3Children's Hospital of Eastern Ontario Research Institute, Ottawa, Canada

One approach to modelling nonlinear longitudinal data is to fit Super Imposition by Translation and Rotation (SITAR) models. Using data from the paediatric bone mineral accrual study (PBMAS, 1991-2017) we have shown that during adolescence, bone development is driven by muscle development. We have not explored if this relationship continues into early adulthood. The aim of this study was to create bone and lean mass development curves for individuals from 8 to 40 years of age. Methods: PBMAS, initiated in 1991, recruited 251 children aged 8-15 years. Participants underwent up to fifteen annual DXA scans (Hologic QDR-2000) until 2017 when values for total body Bone Mineral Density (BMD) and lean mass (LM) were recorded. 112 males and 127 females with ≥1 bone and lean mass measure are included in this analysis. SITAR models were fitted with manually selected knots with number and position informed by the cohort-specific model. All analyses were performed in R version 4.0.2 (R Project for Statistical Computing) and RStudio integrated development environment version 1.3.1 (RStudio Team). Results: Models showed rapid increases in both LM and BMD in adolescence, plateauing in adulthood. Age at peak LM and BMD accrual in girls was 11.7 yr. (4.9 kg/yr.) and 13.0 yr. (0.06 g/m2/yr.) respectively, compared to boys whose were 13.7 yr. (9.6 kg/yr.) and 14.4 yrs. (0.07 g/m2/yr.). From peak bone mass and 35 yr. age LM increased by 9% in girls and 1% in boys compared to an increase in BMD of 9% in girls and 8% in boys. Discussion: These models graphically illustrate the nonlinear development of bone and lean mass across the life- course and show sex and tissue differences in both velocity trajectories and magnitude of change. Peak adolescent accrual of lean mass precedes BMD. From peak to 35 yrs. percentage increases in tissues are similar in girls but not boys. These results continue to illustrate the strong relationship between muscle and bone from adolescence through to early adulthood.