Postexercise hypertrophic adaptations: a reexamination of the hormone hypothesis and its applicability to resistance training program design.

It has been well documented in the literature that resistance training can promote marked increases in skeletal muscle mass. Postexercise hypertrophic adaptations are mediated by a complex enzymatic cascade whereby mechanical tension is molecularly transduced into anabolic and catabolic signals that ultimately lead to a compensatory response, shifting muscle protein balance to favor synthesis over degradation. Myocellular signaling is influenced, in part, by the endocrine system. Various hormones have been shown to alter the dynamic balance between anabolic and catabolic stimuli in muscle, helping to mediate an increase or decrease in muscle protein accretion. Resistance training can have an acute impact on the postexercise secretion of several of these hormones including insulin-like growth factor, testosterone, and growth hormone (GH). Studies show that hormonal spikes are magnified after hypertrophy-type exercise that involves training at moderate intensities with shortened rest intervals as compared with high-intensity strength-oriented training. The observed positive relationship between anabolic hormones and hypertrophy-type training has led to the hormone hypothesis, which postulates that acute postexercise hormonal secretions mediate increases in muscle size. Several researchers have suggested that these transient hormonal elevations may be more critical to hypertrophic adaptations than chronic changes in resting hormonal concentrations. Theoretically, high levels of circulating hormones increase the likelihood of interaction with receptors, which may have particular hypertrophic importance in the postworkout period when muscles are primed for anabolism. Moreover, hormonal spikes may enhance intracellular signaling so that postexercise protein breakdown is rapidly attenuated and anabolic processes are heightened, thereby leading to a greater supercompensatory response. Although the hormone hypothesis has received considerable support in the literature, several researchers have questioned its veracity, with some speculating that the purpose of postexercise hormonal elevations is to mobilize fuel stores rather than promote tissue anabolism. Therefore, the purpose of this article will be to critically and objectively review the current literature, and then draw relevant conclusions as to the potential role of acute systemic factors on muscle protein accretion.