Effects of knee and hip flexion angle on region-specific and longitudinal compartmental muscle stiffness of the rectus femoris using shear wave elastography in male bodybuilders

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.

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.

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 bodybuilders push hard with their legs, the stiffness in a key thigh muscle changes depending on leg position—but the usual muscle activity sensors (EMG) don’t pick up these differences, suggesting they might miss some real muscle behavior.

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.

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.