An adaptive model of achilles tendon mechanical properties during adolescence: effect of sex

Abstract

Background. This longitudinal study aimed to quantify the relative effects of body mass, peak muscle force, maturation and sex on Achilles tendon (AT) mechanical properties and to examine the external factors that trigger mechanical changes, or intrinsic tendon adaptations during adolescence. Methods. We measured AT mechanical properties and dimensions during pre-pubertal and adolescent growth in 41 participants (20 boys and 21 girls). Participants were tested over 18 months; longitudinal changes were examined through linear mixedmodelling. Results. Sex and maturation were found to be the major factors influencing AT mechanical changes. Their effects were largely exerted through increases in muscle force, which imposed greater stress on AT and strongly predicted changes in stiffness and Young’s modulus in boys and girls, while strain was consistent. Conclusions. The more rapid increase in stiffness before the age of peak height velocity in boys was associated with an increase in force at that time, which may have evoked the molecular signaling required for adaptations to internal tendon structure leading a different adaptive response between sexes. The present data are suggestive of an adaptive model in which increases in muscle force production impose greater mechanical loading, on the tendon to trigger increases in stiffness as children mature through adolescence.