When new people start lifting weights — whether with light weights and restricted blood flow or heavy weights — their muscle fibers grow evenly inside, keeping the same balance between the parts that contract and the rest. This means their muscles grow the usual way, not by just filling up with fluid.

When you lift weights and your muscles grow, all the tiny parts inside the muscle cells grow together in balance — so the overall mix inside the cell stays the same.

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.

When you lift heavy weights regularly, your muscles get bigger because the individual muscle fibers inside them grow thicker and more numerous.

When young guys who haven't lifted weights before do strength training for 10 weeks, their muscle fibers grow in a way that makes them stronger, not just bigger with extra fluid. It's like building real muscle instead of just puffing it up.

Even though the muscles grew bigger, the body didn’t make more muscle or connective tissue proteins at a faster rate — meaning the growth happened without the usual increase in protein building.