This version of the quad stretch facilitates hip extension which will provide a better stretch to the longest quad muscle, the rectus femoris. Using gravity to help stretch these muscles makes it easier to relax and achieve a more effective stretch.
This stretch is aimed at stretching out the hip flexor muscles (predominately psoas and iliacus). These muscles can become tight with prolonged sitting and can be a factor in low back stiffness.
The gluteus medius muscle is an important muscle of the hip and is hugely important for knee health, hip health, and back health. This muscle can be weak (and is often weak) in individuals who have issues in any of these three joints. The exercise in the video is a good beginning movement for strengthening the gluteus medius muscle.
The single leg squat is effective for working the gluteus medius, gluteus maximus, and quadriceps muscle groups. Learn how weak glutes can facilitate low back, hip, and knee issues.