When lying down, your leg muscles don't get stronger even if one key muscle gets longer — and that's mostly because your brain isn't fully turning on those muscles, not because opposing muscles are getting in the way.

When healthy guys push with their legs, they're just as strong sitting or lying down—even though one position stretches the muscle more. That means muscle strength isn’t just about how stretched the muscle is.

Leaning back during leg extensions might make your front thigh muscle work harder and grow more because it stretches the muscle more during the move.

If you want to make the front part of your thigh (rectus femoris) bigger, leg extensions work better than leg presses — after 12 weeks, leg extensions made it grow 12% more than leg presses did.

If you're a guy who lifts weights, your front thigh muscle works way harder during leg extensions than during leg presses — probably because the movement challenges how that muscle crosses two joints at once.

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.