When you do blood flow restriction training, your muscle cells might swell up, and that swelling could be what makes your muscles grow — even if there’s not much buildup of fatigue-related chemicals.

When you wrap your arm tightly and lift light weights, your muscles still grow — and your body turns on the same growth signals as when you lift heavy weights, which hints that the burning feeling might be key.

When you do blood flow restriction training—like wearing tight bands on your arms or legs while lifting light weights—you build muscle mainly because your muscles fire harder and earlier, not because of the burn or buildup of waste products.

You can build muscle just as well by lifting light weights with your blood partially squeezed off, as you can by lifting heavy weights—so maybe it’s not the weight itself that matters most, but how much your muscles burn and swell up.

When your muscles swell up after a workout because of increased blood flow, that puffiness itself doesn’t actually make your muscles grow bigger over time.

Doing light weight training with restricted blood flow can still make your arm muscles grow bigger — and it doesn’t cause swelling or damage, according to a 3-week study on people’s biceps.