Variability in muscle gain observed across different training phases is predominantly influenced by modifiable factors—including training intensity, training volume, mental focus during exercise, and nutritional intake—rather than by fixed genetic differences among individuals.

Study: Can muscle typology explain the inter‐individual variability in resistance training adaptations?

The study found that people gain muscle differently not because of their genetics, but because of how they train and eat. Even people with 'slower' muscles can catch up by doing more reps or training more often.

Study: Relationship between protein intake and resistance training-induced muscle hypertrophy in middle-aged women: A pilot study.

The study shows that women who increased their protein intake at breakfast gained more muscle from strength training, which suggests that what you eat—not just your genes—can affect muscle growth.

Study: The Influence of Frequency, Intensity, Volume and Mode of Strength Training on Whole Muscle Cross-Sectional Area in Humans

The study shows that how hard, how often, and how much you train your muscles affects how much they grow—supporting the idea that effort and training choices matter more than genetics.