The hypertrophic response to resistance training is conserved within individuals across different muscle groups, such that individuals who exhibit a high hypertrophic response in one muscle group (e.g., upper body) also exhibit a similarly high response in another muscle group (e.g., lower body), regardless of training load.

When muscles are stretched and contracted under load, the force on muscle fibers triggers a molecular signal that turns on the machinery to build more contractile proteins. This signal is stronger in some people than others, and once activated, it increases protein building in all muscles the person trains, no matter which muscles or how heavy the weights are. People who naturally build more muscle in one area also build more in others because the same internal signal controls protein growth everywhere.