Not recommending any stretch while possibly injured until a Dr clears you for it.
In some people a tight/short rectus femorus might cause pain under the knee cap.
WARNING - hip flexor stretches might cause stress and injury to the lower back.
A tight rectus femorus might also contribute to anterior pelvic tilt. I don't know the incidence of some degree of anterior pelvic tilt in the population but I believe that it is a common characteristic of the posture of middle age and older people.
Anterior Pelvic Tilt