When male bodybuilders flex their knees at a 30-degree angle, the upper part of a specific thigh muscle gets stiffer if their hips are straighter, suggesting how stretched the hip is can change how the muscle works.

In male bodybuilders, the top part of a big thigh muscle reacts more to hip position than the lower parts when the knee is bent slightly, showing that the muscle doesn't work the same way from top to bottom.

When male bodybuilders flex their knees and push without moving their legs, the front thigh muscle feels stiffer than a neighboring muscle — and this difference depends on how much the hip is bent.

For male bodybuilders, changing the knee bend doesn't really affect a key thigh muscle's stiffness if the hip stays in the same position — but changing the hip position does seem to matter more.

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.

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.