Iliopsoas: the main hip flexor
Iliopsoas is the main hip flexor: a pair of muscles sharing one tendon that lift the thigh forwards and upwards.
Remember
Iliopsoas flexes the hip, so weak hip flexion with normal knee extension points to it, whereas gluteus maximus and the hamstrings mainly extend the hip and the quadriceps extend the knee.
Iliopsoas is really two muscles that share a tendon. The psoas major begins on the lumbar vertebrae (the bones of the lower back), and the iliacus begins on the iliac fossa (the shallow inner surface of the pelvic bone).
Both muscles pass under the inguinal ligament (a fibrous band in the groin) into the front of the thigh. They attach together to the lesser trochanter, a small bony knob on the upper inner femur. Because the attachment is on the femur above the knee, the work is done at the hip.
The main action of iliopsoas is flexion of the hip: it pulls the thigh forwards and upwards, much like lifting a knee to take a step. Weakness of hip flexion therefore shows up when climbing stairs, because each leg has to be lifted to the next step. As a simple memory aid, muscles in front of the hip joint bend it forwards, whereas muscles behind it straighten it; hip extension comes mainly from gluteus maximus and the hamstrings.
Unlike many of its neighbours at the front of the thigh, iliopsoas does not extend the knee. The quadriceps do that. Rectus femoris, sartorius and pectineus are other muscles that also flex the hip, but iliopsoas is the main one.
Each part has its own nerve supply. The anterior rami (the front branches of the spinal nerves) of L1 to L3 supply psoas major, and the femoral nerve supplies iliacus. An injury to the L1 to L3 roots weakens hip flexion and so makes lifting the thigh difficult.
Key facts
- Parts
- psoas major plus iliacus, joined in one tendon.
- Attachment
- both insert together on the lesser trochanter, a bony knob on the upper femur.
- Main action
- it flexes the thigh at the hip, lifting it forwards and upwards.
- Knee
- iliopsoas does not extend the knee; the quadriceps do that.
- Nerve supply
- the anterior rami of L1 to L3 supply psoas major, and the femoral nerve supplies iliacus.
- Hip extensors
- gluteus maximus and the hamstrings mainly extend the hip, the opposite action to flexion.
Exam traps
- Hip extension is not an iliopsoas movement, because it is the opposite of flexion and is produced mainly by gluteus maximus and the hamstrings.
- Iliopsoas lies in the same part of the thigh as the quadriceps, but it does not extend the knee: extending the knee is the job of the quadriceps.
- Being at the front of the thigh does not mean that the quadriceps explain weak hip flexion: although rectus femoris, one of the quadriceps, also crosses the hip, the quadriceps extend the knee, and weak hip flexion with normal knee extension points to iliopsoas.
Read the full notes
Tested in 6 SimpleMed+ questions
Practice it in Musculoskeletal System, with the answer and a full explanation after every question.
Practice these questions