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.

When performing leg extensions with the hip fully extended, the rectus femoris muscle grows larger than the vastus lateralis muscle because the rectus femoris spans two joints and is stretched more during this movement.

If you do leg extensions while leaning back, it might build more muscle in the top part of your front thigh compared to doing them upright.

Doing leg extensions with your hip bent at 40 degrees instead of 90 degrees probably makes the front thigh muscle (rectus femoris) grow more in guys who haven’t lifted weights before.

If you're a guy new to lifting and want your front thigh muscle to grow as much as possible, doing leg extensions with your hip more bent (40°) is probably better than keeping it straighter (90°).

In untrained young men, doing leg extensions with the hip bent at 40 degrees causes more growth in the upper and lower parts of the rectus femoris muscle than doing them with the hip bent at 90 degrees over 10 weeks, while the vastus lateralis muscle shows no difference in growth between the two positions.