Hormonal responses to exercise and training

Current knowledge and understanding of the hormonal response to exercise are limited, whether in relation to horses, humans, or other species. The changes in plasma concentration of some hormones occur early in exercise, apparently owing to a neuronal stimulation, whereas others, being pituitary dependent, require hormonal stimulation. Also, although it is possible to observe changes in plasma concentrations of hormones, the mechanism by which this is achieved is not always understood, and unless the nonprotein-bound, or active, form of the hormone is also determined, changes in plasma concentration are less informative. Both the intensity and duration of exercise may be of importance in initiating or maintaining the hormonal response. Impulses from either the working muscles or motor centers, via the central nervous system, modify the response of the glands of the endocrine system directly via pituitary hormones or indirectly via the sympathoadrenal system. The initial response to the onset of exercise is enhancement of sympathoadrenal activity and secretion of pituitary hormones, which result in a reduction in the plasma concentration of insulin and a rise in that of virtually all other hormones. Because of this shift in hormone balance, a modification of the metabolism of intra- and extra-muscular triglycerides and glycogen as fuels for muscular exercise occurs. The variation in mobilization of one fuel source may well influence its combustion, together with both the mobilization and combustion of the other. When exercise is prolonged, the hormonal response is influenced by additional factors such as temperature, glucose availability, oxygen tension, and changes in plasma volume. The effect of training on hormonal responses is generally an ameliorating one that reflects an increased efficiency of muscular energy metabolism as a result of the training process.