The Quadriceps Femoris composes of a majority of the anterior thigh, composed of 4 muscles.
These include Rectus Femoris, Vastus Lateralis, Vastus Medialis, & Vastus Intermedius. Of these only one crosses the hip joint, the Rectus Femoris. For this reason, changing the hip position will alter which quad you are stretching.
When these muscles are tight, knee and back pain may be a result.
The first stretch shows bringing the knee into flexion while the hip is in flexion. This will SLACKEN the rectus femoris and maximizing stretch at the 3 Vasti musculature.
The following stretching in sidelying is performed by bringing the knee into flexion while the hip is in EXTENSION this time. This is meant to maximize stretch of the Rectus Femoris.
Addiditonally, a contract relax technique of stretching was performed. This can be done with either stretch. To perform this: Contract the quads at end-range for 3-5 seconds, then relax to pull the knee into more flexion for 5 seconds, and repeat for 5 repetitions.
*Note- Avoid lumbar extension when stretching, and focus on getting the extension at the hip. Exercise Library