When you squat, your front thigh muscle can't work as hard because it's stretched across two joints at once.

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.

Even though squats work your legs hard, muscles like the hamstrings and front thigh muscle don't grow much because they get pulled in two directions at once during the movement.

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.

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.