Well, as you descend into the bottom of the squat or lunge (in hip flexion and knee flexion), the rectus femoris is trying to lengthen at the knee but shorten at the hip, and ends up staying roughly the same length. Then as you ascend (performing hip extension and knee extension), the muscle is trying to shorten at the knee but lengthen at the hip, and again ends up staying about the same length. (3)