There is currently limited knowledge regarding the basic mechanical properties of musculoskeletal structures in youth and the daily forces/stresses these structures incur. There is also limited knowledge of how youth mechanical properties may vary as a result of growth rate, physical activity, sex, and time. Our long-term objectives are to develop biomechanically-based physical activity guidelines for youth that, if followed, will minimize their risk for overuse injuries. Key to establishing appropriate youth biomechanically-based physical activity guidelines is understanding the factors that affect the structural properties and loading of youth muscles, tendons, ligaments, and bones. A single structure, the Achilles tendon (AT), was considered in this dissertation as a starting point for achieving the long term objectives of biomechanically-based physical activity guidelines for youth that, if followed, will minimize their risk for overuse injuries. The primary objectives of this dissertation research were:​ (1) test our hypotheses that sex, growth rate, time, and/or physical activity level affect AT mechanical properties and AT forces during locomotion of youth as they grow, and (2) empirically model the average frequency and magnitude of AT forces during daily physical activity.

AT mechanical properties for 20 boys (12-14 years) and 22 girls (10-12 years) before and after six months of growth were determined. Additionally the relationship between peak AT stress, peak AT strain, and AT stiffness with sex, physical activity (PAQ-C score), time, and growth rate were examined with repeated measures multiple regressions. Peak AT strain and AT stiffness did not differ between sexes or between 0 and 6 months. Physical activity, sex, and growth rate had no relationship with either peak AT strain or AT stiffness. Peak AT stress increased at 6 months compared to 0 months and was smaller in subjects with greater growth rates.

The peak forces in the AT (FAT) during walking and running before and after 6 months of growth were quantified and the relationship between peak FAT and peak normalized FAT (nFAT) with PAQ-C, growth rate, time, and sex evaluated. Physical activity level had positive relationship with peak FAT and peak nFAT during walking (both self-selected and target speeds) and during running at self-selected speeds. Target peak FAT increased at 6 months compared with 0 months. No other significant relationships were determined.

Lastly, exploratory models of physical activity were developed for six subjects using novel methods that estimate peak FAT during walking and running from hip acceleration using an activity monitor. Hip acceleration was quantified for multiple weeks (6 weeks over 3 months) during daily physical activities. Models of tolerated physical activity (i.e. no overuse injury was reported) for all 6 subjects were developed. Using the models, the average tolerated minutes of walking and running per week were determined to illustrate the application of similar models that could be developed through future studies with a larger number of subjects.