Hormonal
Activation of the PI3K-mTOR signaling pathway by insulin, amino acids, and resistance exercise is the central mechanism regulating skeletal muscle protein synthesis and hypertrophy.
To maximize muscle growth, you must combine resistance exercise with adequate protein intake. The body uses a common signaling pathway (mTOR) to integrate these signals. Simply taking protein supplements or exercising without nutrition is less effective than doing both together, as they synergistically activate the pathway responsible for building muscle.
Although insulin, amino acids and exercise individually activate multiple signal transduction pathways in skeletal muscle, one pathway, the phosphatidylinositol 3-kinase (PI3K)–mammalian target of rapamycin (mTOR) signalling pathway, is a target of all three.
Why this rating
The paper is a review of multiple studies (in vitro, animal models, human subjects) establishing a strong mechanistic consensus, though it lacks a single large-scale clinical trial.
Source
Regulation of protein synthesis associated with skeletal muscle hypertrophy by insulin-, amino acid- and exercise-induced signalling
Douglas R. Bolster et al. · Proceedings of The Nutrition Society · 2004
DOI 10.1079/pns2004355
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