The psoas and iliacus commonly known as the hip flexors are often not considered in cases of back pain or chronic quadriceps strain but is almost always involved. In fact it is difficult to have a low back pain condition where the psoas does not become involved. When the psoas is shortened, fibrotic and weakened it will cause the lumbar spine to become hyperlordotic (having an excess lower back curvature) and limit hip extension. This can cause a facet syndrome. A weak psoas will also cause the rectus femoris to work harder at flexing the hip.
In sprinters a weak and tight psoas will cause the rectus femoris to be prone to injury as it attempts to accomplish the majority of hip flexion, a task normally accomplished by the psoas. When people have psoas problems they will usually exhibit poor posture while standing. The hip will not extend completely so the pelvis tilts forward and the lumbar spine is hyperlordotic especially when rising to a standing position after being seated for any length of time.
Common situation that involve psoas injury are triathlons (where the psoas is short during the bike then must lengthen to run), any sport involving sprints, cyclists and figure skaters, jumpers and post lumbar spine surgery and people that have difficulty rising from the seated position.