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 that enables the exercise to positively influence bone architecture and overall strength. However, it is still dubious for how long the skeletal benefits gained in adolescence is 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) intensity cyclic loadings applied during puberty on longitudinal bone development, morphometry, and biomechanics during adolescence as well as at adulthood. Forty‐two young (4‐week‐old) male rats were randomized into control, sham, LI, MI, and HI groups. After a 5 day/week for 8 weeks cyclic loading regime applied on the right tibia, loaded rats underwent a subsequent 41‐week, normal cage activity period. Right tibias were removed at 52 weeks of age, and a comprehensive assessment was performed using μCT, 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 with shams, but these effects disappeared afterward. HI cyclic loading increased BMD, bone volume fraction, trabecular thickness, trabecular number, and decreased trabecular spacing after loading. All loading‐induced benefits, except BMD, persisted until the end of the 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, the HI group showed increased ultimate force and stress, stiffness, postyield displacement and energy, and toughness compared with the sham group. Overall, our findings suggest that even though both trabecular and cortical bone drifted through age‐related changes during aging, HI cyclic loading performed during adolescence can render lifelong benefits in bone microstructure and biomechanics. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Part 2. Review and meta‐analysis of studies on modulation of longitudinal bone growth and growth plate activity: A micro‐scale perspective

“…46 Similarly, increases in bone mineral density, bone volume fraction, trabecular bone thickness, trabecular number, and decreased trabecular spacing were observed after 1250 με at 2 Hz was applied in vivo to rat tibiae for eight weeks. 47 However, at a nonstress-normalized intermittent cyclic load of 17 N at 2 Hz applied for 10 min/day to adolescent rat ulnae, mineralization was arrested completely, and furthermore partially suppressed at 8.5 and 4 N loads applied in a similar manner. 18 One possibly contra-indicating study by Niehoff et al 20 reported that adolescent rats allowed unlimited exercise showed increased osteonectin in the hypertrophic zone relative to limited or no exercise groups, which correlates to promotion of ossification.…”