If you're new to working out, doing leg presses or knee extensions twice a week for 12 weeks will grow your thigh muscles about the same amount—so pick the one you like!

If you're new to working out, doing leg presses twice a week might build more leg muscle in less time than just doing knee extensions, because leg presses work more muscles at once — like your quads, butt, and inner thighs.

If you're new to working out, doing leg presses twice a week for 12 weeks can noticeably grow your butt and inner thigh muscles, but doing just knee extensions won’t help those muscles much.

Doing leg presses and leg extensions for 12 weeks both make your main thigh muscles bigger about the same amount, but leg presses also make your butt and inner thigh muscles grow more, while leg extensions make the front part of your thigh (rectus femoris) grow much more.

Pushing a heavy leg press machine makes the outer part of your thigh muscle grow more than doing leg extensions, because it puts more overall stress on that part of the muscle.

Doing leg extensions might grow your front thigh muscle more than leg presses, even though both workouts work the other big thigh muscles just as hard.