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 stretch your hips back while doing leg extensions, your quad muscle gets longer and grows more—especially in the middle and upper parts—than if you do the exercise with your hips bent.

The rectus femoris muscle exhibits greater hypertrophic adaptation to isolated knee extension exercises than to compound movements involving hip and knee flexion due to biomechanical limitations in force production during multi-joint movements.

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.

For people who don't usually work out, using a knee extension machine grows the front thigh muscle much more than using a leg press machine.

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.