Interindividual variability in muscle hypertrophy responses to resistance training is substantially greater than intraindividual variability across limbs and loading conditions, indicating that inherent biological factors are the primary drivers of differential muscle growth responses.

When muscles are stretched and contracted under load, the muscle fibers sense the tension and turn on a molecular signal that tells the cell to build more contractile proteins. This process happens at a consistent rate for each person, no matter how heavy or light the weight is, or which muscle group is trained. People who naturally build more protein in response to this signal grow bigger muscles, while others build less, and this difference is set by their genes and not by how they train.