People respond to weight training in wildly different ways—some grow muscles a lot, others hardly at all—and those differences between people are about 40 times bigger than the tiny differences you’d see if you trained one leg harder than the other in the same person.

Lifting weights at a challenging level makes your muscles build more protein, and this boost happens just as much in arms or legs that are used to exercising as in ones that aren't.

Lifting weights makes your muscles start building more protein right after the workout, and mixing up your routine a little bit gives your muscles a tiny extra boost right after lifting—but in the long run, both routines build about the same amount of muscle.

When you lift weights, your body’s protein-making response doesn’t change much from week to week — but it’s very different from person to person.

When young men who haven't trained before do one-arm exercises for eight weeks, their resting muscle building speed goes up by almost half in that arm, but the part that builds muscle fibers stays the same, showing different parts of muscle react differently to exercise.

When young men lift weights to build muscle, their muscle-building process speeds up at first but slows down after 10 weeks, even if they keep increasing the difficulty.