Hormones as Stimuli for Muscle Growth

Skeletal muscle is affected by many hormones and growth factors, both catabolic and anabolic in nature. Our understanding of the synthesis, release, transport and tissue sensitivities of these hormones has increased greatly in recent years. Despite this knowledge we still do not know the link between hormonal/growth factor release, training and muscle hypertrophy. The hormonal response to high resistance exercise involves an acute increase in cortisol, GH, testosterone, and catecholamines and a decrease or no change in insulin and thyroid status. In the longer term there may also be changes in muscle sensitivity. Much attention has focused on the anabolic effects of androgens. The evidence that exogenous derivatives of testosterone are anabolic in eugonadal men is still controversial. In supraphysiological doses, and combined with training, there is some evidence for their anabolic effects. This needs to be weighed against their potentially harmful side-effects. Studies on GH replacement in GH-deficiency strongly indicated an important role for GH in the maintenance of lean tissue mass. Despite its role in clinical situations, the controlled trials have failed to demonstrate a clear anabolic role for GH in either the healthy younger or older person. A likely candidate for causing muscle hypertrophy as a result of resistance training, is IGF1. This would appear to be independent of GH release and act in an autoocrine/paracrine fashion. A splice variant if IGF-1 has been identified and is expressed locally in muscle during repair and overload. This mechano growth factor (MGF) may be an important factor linking a mechanical stimulus and growth. More recently there has been renewed interest in searching for an “anabolic hormone” due to the increasing numbers of elderly people within populations and the associated frailty common in this group. Reduced physical activity levels and a shifting anabolic/catabolic ratio all contribute to this problem.