If you're already used to lifting weights, it doesn't really matter if you change up how heavy you lift, how many reps you do, or how long you rest—your muscles will grow about the same either way.

If you're already used to lifting weights, mixing up your routine—like changing how heavy you lift, how many reps you do, or how long you rest—won't make your muscles grow any bigger than sticking to the same routine. Both ways work about the same.

When people lift weights, how much their muscles grow depends way more on their own body than on the type of workout they do—because even when people switch up their routines, their muscle growth stays pretty much the same.

Even when you train one leg differently from the other, both legs tend to grow muscle in similar ways — which means your body’s natural biology, not the workout itself, is what really controls muscle growth.

Individual hypertrophic responsiveness to resistance training exhibits moderate inter-muscle correlation, but a substantial proportion of variability is attributable to non-exercise-specific biological factors.

Individual hypertrophic responses to resistance training are highly variable across muscle groups and are not consistently correlated between upper and lower body musculature.