In studying the way the musculoskeletal system adapts, it is important to consider how the different tissues interact with each other. During growth, and in response to changes in posture, the functional length of muscle is adjusted by altering the number of sarcomeres in series to the optimum for force generation and power output. In some cases, insufficient adaptation gives rise to problems such as muscle contracture. Stretch has marked effects on muscle mass with eccentric contraction being the most effective form of exercise for building muscles. As far as the mechanism is concerned, the current authors have cloned the deoxyribonucleic acid (cDNA) of a splice variant of the insulinlike growth factor gene that is produced by active muscle that seems to be the factor that controls local tissue repair, maintenance, and remodeling. This new growth factor has been called mechanogrowth factor to distinguish it from the liver insulin growth factors, which have a systemic mode of action. The discovery of the locally produced insulinlike growth factor seems to provide the link between the mechanical stimulus and the activation of gene expression.

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