Isolated Cyclic Loading during Adolescence Improves Tibial Bone Microstructure and Strength at Adulthood

Abstract

Bone is a unique living tissue, which responds to the mechanical stimuli regularly imposed on it. Adolescence facilitates a favorable condition for the skeleton which enables the exercise to positively influence bone architecture and overall strength. However, it is still dubious for how long the skeletal benefits gained in the adolescence preserved at adulthood. The current study aims to use a rat model to investigate the effects of in vivo low (LI), medium (MI) and high (HI) intensitycyclic loadings applied during puberty on longitudinal bone development, morphometry and biomechanics during adolescence as well as at the adulthood. Forty-two young (4-week-old) male rats were randomized into control, sham, LI, MI, and HI groups. After a 5 days/week for 8 weeks cyclic loading regime applied on the right tibiae, loaded rats underwent a subsequent 41-week of normal cage activity period. Right tibiae were removed at 52-week of age and a comprehensive assessment was performed using micro-computed tomography, mechanical

testing, and finite element analysis. HI and MI groups exhibited reduced body weight and food intake at the end of the loading period compared to shams, but these effects disappeared afterwards. HI cyclic loading increased bone mineral density, bone volume fraction, trabecular thickness, trabecular number and decreased trabecular spacing after loading. All loading induced benefits, except bone mineral density, persisted until the end of normal cage activity period. Moreover, HI loading induced enhanced bone area, periosteal perimeter and moment of inertia, which remained up to the 52nd week. After the normal cage activity at adulthood, HI group showed an increased ultimate force and stress, stiffness, post-yield displacement and energy, and

toughness compared to the sham group. Overall, our findings suggest that even though both trabecular and cortical bone drifted through age-related changes during aging, high intensity cyclic loading performed during adolescence can render lifelong benefits in bone microstructure and biomechanics.

Key words

mechanical loading, biomechanics, detraining, growth and development, bone adaptation