What is a hip flexor strain? A hip flexor strain is an injury characterized by tearing of one or more of the hip flexor muscles and typically causes pain in the front of the hip or groin. The group of muscles at the front of the hip are called the hip flexors. The most commonly involved muscle in a hip flexor strain is the iliopsoas (figure 1). The iliopsoas muscle originates from the lower back and pelvis and inserts into the thigh bone (femur).