Sustained skeletal muscle hypertrophy in resistance-trained individuals is strongly associated with the accumulation of ribosomal RNA and expansion of nucleolar volume, rather than transient activation of mTORC1 signaling, because the physical capacity to synthesize proteins depends on the total number of ribosomes available, not just their activity rate.

When muscles are stretched and pulled during strength training, the force travels directly into the nucleus, turning on genes that build more ribosomes. These ribosomes are the protein-making machines that allow the muscle to grow bigger over time. The more ribosomes a muscle cell has, the more protein it can make, and this increase in ribosome number lasts for weeks or months. Training also adds new nuclei to muscle cells, each one adding more ribosome-making capacity. Without enough ribosomes, even strong signals to grow do nothing. Once built, these ribosomes stay in place unless the muscle is unused or starved, at which point they are broken down.