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°).

For the outer thigh muscle (vastus lateralis), bending your hip to 40 or 90 degrees during leg extensions doesn’t seem to make a difference in how much the muscle grows in guys who haven’t lifted before.

Doing leg extensions makes the front thigh muscle bigger along its whole length more than doing squats, especially near the knee.

Doing leg extensions makes the front thigh muscle grow more evenly along its length than doing squats, which don’t stimulate that muscle as much.

Different leg exercises make different parts of your thigh muscles bigger — leg extensions grow the front part, squats grow the lower outer part.