Occupational Activities: Factors That Tip the Balance From Bone Accrual to Bone Loss

Abstract

It is commonly assumed that beneficial adaptations in bone occur with vigorous exercise, yet any adaptive re/modeling in bone undergoing persistent overloading can be counteracted by superimposed inflammatory, compressive, and tensile loading-induced damage responses above thresholds of tissue fatigue failure and repair. This leads to a tenuous balance between achieving bone accrual and loss.

Figure 1

Schematic demonstrating that interactions of force, repetition, and duration of bone loading can elicit a pro-anabolic (bone accrual) response up to a point at which the combination of these factors exceeds a threshold (the transition region between the green and red zones), resulting in a net catabolic response due to tissue fatigue failure and damage (microcracks, osteocyte apoptosis). The point at which the threshold is crossed varies from one individual to another, and the factors that influence its location (e.g., age, sex, genetic background) should be considered in future studies. The figure was created in consultation with Susan Fecho, M.F.A., Barton College.

Figure 2

Image of rat performing the upper extremity reaching and grasping lever pulling task. A. Rat in an operant chamber with one limb extended into a portal located at shoulder height. B. Rat shown pulling on the lever bar, with digits four and five positioned in an ulnar position, indicative of ulnar deviation while pulling. C. Rat shown pulling in a neutral position. D. Rat retrieving a food reward.

Figure 3

A and B. MicroCT images of ulnar (U) and radius (R) of control, 18-wk LFHR task, and 18-wk HFHR task rats. A. Transaxial images of the distal metaphyseal ulna and radius. Note the reduced numbers of trabeculae in HFHR rat radius. B. Sagittal images. C. Microcracks in the radial trabeculae (after basic fuchsin staining) of 18-wk HFHR rats lead to increased osteocyte apoptosis, sclerostin, and RANKL, which decreased osteoblast and increased osteoclasts numbers, respectively, compared with controls (C). (Adapted from (51). Copyright © 2019 Elsevier. Used with permission.)