Importance of the mechanical environment in the biological determination of bone structure. Update on the matter and preliminary analysis of data from sedentary individuals and runners

Abstract

This paper presents two separate sections, dealing with I. a synthesis of the recent advances in the knowledge and understanding of the biomechanical-structural relationships involved in the muscle-bone interactions, and II. a practical application of these concepts to the analysis and interpretation of some tomographic data of bone and muscle indicators obtained from individuals chronically exposed to different intensities of physical activity.The first section recalls that in all vertebrates the skeletal development is normally influenced by both genetic and epigenetic factors. Among the latter, virtually only mechanical and endocrine-metabolic determinants are acknowledged. Mechanical modulators exert chiefly directional effects and constitute the input of bone mechanostat, a feedback system which regulates the only bone property subjected to a cybernetic control, namely, the structural stiffness of every bone. This control is carried out with a high degree of regional specificity. Endocrine-metabolic modulators exert only systemic (non-directional) effects, with a wide spectrum of actions on virtually every bone property, but with no role at all in any kind of feedback regulation within the skeletal system. A technical reference to pQCT technology is also provided, concerning its relevance to the non-invasive analysis of the musculoskeletal status in humans.The application section presents some recent tomographic (pQCT) studies performed in healthy adults of both sexes who either had sedentary habits or have been chronically trained in long-distance running. The analysis of the collected data shows some original evidence of 1. the relevance of the vectorial influence of the mechanical environment of the skeleton to the biological regulation of the efficiency of bone structure; 2. the role of those mechanical factors as determinants of the input of the mechanostat system; 3. the high dependence of all the above relationships upon the activity of the regional muscles, and 4. the systemic influence of endocrine-metabolic factors as determinants of significant, sex-related differences in the observed results.