The "Muscle-Bone Unit" in Children and Adolescents: A 2000 Overview

In former views hormones, calcium, vitamin D and other humoral and nonmechanical agents dominated control of postnatal bone strength (and "mass") in children and adolescents. However later evidence that led to the Utah paradigm of skeletal physiology revealed that this control depends strongly on the largest mechanical loads on bones. Trauma excepted, muscles cause the largest loads and the largest bone strains, and these strains help to control the biological mechanisms that determine whole-bone strength. That makes the strength of children's load-bearing bones depend strongly on growing muscle strength and how bones respond to it. Most hormones and other nonmechanical agents that affect bone strength can help or hinder that "bone strength-muscle strength" relationship but cannot replace it. In addition some agents long thought to exert bone effects by acting directly on bone cells, affect muscle strength too. In that way they could affect bone strength indirectly. Such agents include growth hormone, adrenalcorticosteroid analogs, androgens, calcium, genes, vitamin D and its metabolites, etc. Thus bone and muscle do form a kind of operational unit. It is part of the Utah paradigm that supplements earlier views with later evidence and concepts. The paradigm explains how the "bone strength-muscle strength" relationship works. This article provides an overview of that physiology, and some of its implications for pediatric endocrinologists.