The group of muscles located at the front of the hip are called the hip flexor muscles. They include the Psoas Major, Psoas Minor, Iliacus, and Iliopsoas. These muscles are responsible for moving the thigh towards the abdomen. They are also active when the abdomen is being moved toward the thighs, e.g. sit-ups.
They are some of the biggest and strongest muscles in the body. What many people do not realize is that much lower back pain can be traced back to the muscles of the hip; that is, when the hip flexor muscles are overly tight, they can lead to pain in the lower back. There is, indeed, a relationship, between lower back pain and tight hip flexor muscles.