Individual hypertrophic responsiveness to resistance training shows moderate correlation across different muscles, but a substantial proportion of the variability in this responsiveness is due to biological factors not specific to exercise.

Study: Similar muscle hypertrophy following eight weeks of resistance training to momentary muscular failure or with repetitions-in-reserve in resistance-trained individuals

When people lift weights, some muscles grow more than others — even when they do the same workout. This study shows that even under identical training, different leg muscles grow differently, which means your body’s natural biology, not just the workout, plays a big role in how you build muscle.

Study: Muscle Transcriptional Networks Linked to Resistance Exercise Training Hypertrophic Response Heterogeneity.

People’s muscles grow differently after weight training not just because of how they train, but mostly because of their natural, pre-existing biology—like their genes and how their muscles are already set up.

Study: Resistance training load does not determine resistance training-induced hypertrophy across upper and lower limbs in healthy young males.

Even when people lift different weights, their muscles grow at similar rates compared to their own other muscles—meaning your body’s natural biology, not how you train, mostly determines how much you grow.

Study: Molecular signatures underlying heterogenous hypertrophy responsiveness to resistance training in older men and women: a within-subject design.

Even when people do the same workout, some gain muscle and others don’t—and this study shows it’s because of differences in their bodies’ internal biology, not just how hard they trained.