For male bodybuilders doing leg extensions, bending the hips to 40 degrees might work the top part of the front thigh muscle better than bending to 80 degrees, based on brain scan-like results...

When male bodybuilders do leg extensions with their hips bent at 40 degrees, the front thigh muscle works harder in certain areas than when their hips are bent more at 80 degrees.

When male bodybuilders do leg extensions with their hips bent at 80 degrees, the lower part of their front thigh muscle works harder than the middle part, according to MRI scans.

When male bodybuilders do leg extensions with their hips more bent, the top and middle parts of their front thigh muscle don’t work as hard — they show less activity and fatigue compared to when the hips are straighter.

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°).

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.