This stretch focuses on your vastus medials, intermedius and lateralis as well as the rectum femoris. When doing this stretch correctly you should feel the stretch up the front of your upper leg all the way from the hip to your knee. You should also feel a small stretch you abdominal region but thats due to lifting the stretching leg whilst lying down.
Make sure that you find a flat surface to preform this stretch.
Don't ever under estimate the importance of stretching and the benefits it holds.
The on the side Quad Stretch can help with hip flexor strain, avulsion fracture, osteitis pubis, Iliopsoas tendonitis, trochanteric bursitis, a quad strain and tendonitis, patellofenoral pain and tendonitis and a subluxing kneecap.
Always remember that while stretching may be recommended to aid recovery out of injury, you should always be very careful when stretching through rehabilitation. Your soft tissues are likely to be more vulnerable to re-injury if you stretch too much or too hard during this time. Always check with your Health Care Professional who is guiding you through your recovery to make sure it is appropriate to start stretching, and please - ALWAYS FOLLOW THE RULES FOR SAFE STRETCHING (The first posting on this stretching blog).
Image taken from "Lower Body Stretches" wall chart by B. Walker.