Reclining the torso during knee extension exercises increases rectus femoris muscle activation by lengthening its muscle fibers, which enhances mechanical tension and promotes greater muscle hypertrophy.

Study: Hip flexion angle affects longitudinal muscle activity of the rectus femoris in leg extension exercise

The study tested whether leaning back more during leg extensions works the front thigh muscle more, but found that leaning back actually made that muscle less active.

Study: 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

The study found that leaning back during knee extensions actually reduced tension in the front thigh muscle, contrary to what the claim suggests.

Study: Enhancement of knee extension voluntary and electrically-evoked strength with the body tilted backward

The study found that leaning back helps you push harder during knee extensions, but it doesn’t make the thigh muscle activate more, which goes against the idea that it works better that way for muscle growth.