WEEK 2: HIP + PELVIS UPDATED

Rectus femoris

Primary hip action: Hip flexion. Secondary action: Knee extension.

Iliacus

Primary hip action: Hip flexion. Secondary actions include assisting lateral rotation and helping to keep the femoral head positioned in the acetabulum during hip flexion.

Psoas major

Primary hip action: Hip flexion. Secondary actions include assisting lateral rotation and helping to stabilize the femoral head in the acetabulum during hip flexion.

Why is iliopsoas a good hip flexor?

It is an efficient flexor and also helps prevent the femoral head moving too far anteriorly during flexion.

Sartorius

Primary hip action: Hip flexion. Secondary actions: Hip abduction and lateral rotation.

Tensor fascia latae (TFL)

Primary hip action: Hip abduction. Secondary actions: Hip flexion and medial rotation.

Why is TFL not ideal for isolated hip flexion?

It produces more abduction torque than flexion torque, so it creates unwanted abduction.

Pectineus

Primary hip action: Hip adduction. Secondary actions: Hip flexion and medial rotation.

Adductor longus

Primary hip action: Hip adduction. Secondary action in anatomical position: Hip flexion.

How does adductor longus’s sagittal-plane action change in hip flexion?

At substantial hip flexion, it can become a hip extensor because its line of action moves posterior to the hip axis.

Adductor brevis

Primary hip action: Hip adduction. Secondary action in anatomical position: Hip flexion.

How does adductor brevis’s sagittal-plane action change in hip flexion?

It can become a hip extensor when its line of action moves posterior to the hip axis.

Adductor magnus

Primary hip action: Hip adduction. Key secondary action: Hip extension.

Why is adductor magnus an extensor regardless of hip flexion/extension position?

Its line of action stays posterior to the hip joint axis.

Which part of adductor magnus is especially associated with hip extension?

The hamstring part.

Gracilis

Primary hip action: Hip adduction. Secondary actions: Assists hip flexion; also flexes and medially rotates the knee.

Gluteus maximus

Primary hip action: Hip extension. Secondary hip action: Hip lateral rotation.

Upper gluteus maximus fibres — coronal-plane action

Hip abduction.

Lower gluteus maximus fibres — coronal-plane action

Hip adduction.

Gluteus medius

Primary hip action: Hip abduction + internal rotation of the thigh

What happens to gluteus medius’s rotational action in hip flexion?

All fibres become medial rotators.

Gluteus minimus

Primary hip action: Hip abduction.

What happens to gluteus minimus’s rotational action in hip flexion?

All fibres become medial rotators.

Piriformis

Primary hip action in anatomical position: Hip lateral rotation.

Piriformis — actions when the hip is flexed

Assists hip abduction and medial rotation.

Obturator internus

Primary hip action: Hip lateral rotation. Secondary action: Assists hip abduction, especially when the hip is flexed.

Superior gemellus

Primary hip action: Hip lateral rotation. Secondary action: Assists hip abduction, especially in hip flexion.

Inferior gemellus

Primary hip action: Hip lateral rotation. Secondary action: Assists hip abduction, especially in hip flexion.

Quadratus femoris

Primary hip action: Hip lateral rotation. Secondary hip action: Hip adduction.

Obturator externus

Primary hip action: Hip lateral rotation. Secondary hip action: Assists hip adduction.

Biceps femoris long head

Primary hip action: Hip extension. Secondary hip action: Assists hip adduction.

Semitendinosus

Primary hip action: Hip extension. Secondary hip action: Assists hip adduction.

Semimembranosus

Primary hip action: Hip extension. Secondary hip action: Assists hip adduction.

Which hamstring does not act at the hip? Why?

Biceps femoris short head, because it does not cross the hip joint.

Strongest hip muscles — Four strongest hip flexors from anatomical position

Rectus femoris, iliacus, psoas major, and adductor longus.

Strongest individual hip flexor from anatomical position

Rectus femoris.

Of the strongest hip flexors, which one does not primarily flex the hip?

Adductor longus — its primary action is adduction.

Four strongest hip extensors from anatomical position

Gluteus maximus, semimembranosus, biceps femoris long head, and gluteus medius.

Strongest individual hip extensor

Gluteus maximus.

Strongest hamstring hip extensor

Semimembranosus.

Why is semimembranosus the strongest hip-extensor hamstring?

It has the greatest physiological cross-sectional area, so it can produce the most force.

Which bony feature increases the hamstrings’ moment arm for hip extension?

The ischial tuberosity.

Two strongest hip adductors

Adductor magnus and adductor longus.

Strongest hip abductor

Gluteus medius.

Hip abductor with the longest moment arm

Gluteus medius.

Strongest hip lateral rotator

Gluteus maximus.

Second-strongest hip lateral rotator

Posterior fibres of gluteus medius.

Strongest hip medial rotator

Anterior fibres of gluteus medius.

Are there any muscles whose primary action is hip medial rotation?

No — medial rotation is always a secondary action.

Muscle-action principles — What must a muscle’s line of action do to flex the hip?

Have a vertical component and cross anterior to the hip joint.

What must a muscle’s line of action do to extend the hip?

Have a vertical component and cross posterior to the hip joint.

What must a muscle’s line of action do to abduct the hip?

Have a vertical component and cross lateral to the hip joint.

What must a muscle’s line of action do to adduct the hip?

Have a vertical component and cross medial to the hip joint.

What line-of-action pattern produces hip lateral rotation?

A horizontal component crossing posterior to the hip joint.

Why can one hip muscle have multiple actions?

Its line of action has components in more than one plane.

Why can a muscle’s action change in different hip positions?

Its line of action can move to the other side of a joint axis as the hip moves.

What is an extraneous torque?

An unwanted torque produced in another direction while a muscle performs the desired action.

What determines rotational strength/torque?

Torque = muscle force × moment arm.

Strongest hip muscle group overall

Hip extensors.

Why is correct muscle synergy important?

It makes movement efficient, produces the correct pattern, and distributes joint forces normally.

Gait and pelvic control — During gait, which side of the pelvis lowers slightly?

The swing-limb side.

Why does the swing-side pelvis lower during gait?

To minimise the rise in the body’s centre of mass.

Which muscles control pelvic drop in single-leg stance?

Gluteus medius, gluteus minimus, and TFL on the stance limb.

How do the stance-limb abductors control pelvic drop?

They contract eccentrically to control and limit lowering of the swing-side pelvis.

What may occur with weak stance-limb hip abductors?

Excessive contralateral pelvic drop — a Trendelenburg-type sign.

Hip joint and ligaments — Hip joint classification

Synovial, ball-and-socket, multiaxial diarthrosis.

What articulates to form the hip joint?

The femoral head and acetabulum.

Why is the hip more stable but less mobile than the shoulder?

The acetabulum is much deeper and covers more of the femoral head than the glenoid covers the humeral head.

Which part of the acetabulum is weight-bearing?

The lunate surface.

Which part of the acetabulum is non-weight-bearing?

The acetabular fossa.

What spans the acetabular notch?

The transverse acetabular ligament.

What is the main role of the acetabular labrum?

It deepens the acetabulum and improves femoral-head stability.

Which hip movements move the femoral head deeper into the acetabulum?

Hip flexion and abduction.

Why is hip flexion greater than extension?

The acetabulum’s orientation favours flexion, while the anterior femur can impinge at end-range flexion and extension is more restricted.

What are the three hip capsular ligaments?

Iliofemoral, pubofemoral, and ischiofemoral ligaments.

What position makes all hip capsular ligaments taut?

Hip extension.

Why is standing at the hip mechanically economical?

The spiral capsular ligaments tighten in extension and compress the femoral head into the acetabulum, reducing the muscle effort needed.

Strongest hip capsular ligament

Iliofemoral ligament.

Primary passive restraint to hip extension/hyperextension

Iliofemoral ligament.

Which ligament most restrains anterior translation of the femoral head in hip extension?

Iliofemoral ligament.

Which movement does the pubofemoral ligament limit?

Hip abduction.

When does the ischiofemoral ligament tighten?

With medial rotation, especially in approximately 10–20° hip abduction.

Hip loose-packed position

About 30° flexion, 30° abduction, and slight lateral rotation.

Why adopt the loose-packed position with an intra-articular hip effusion?

It maximises joint volume and reduces capsular tension, allowing room for extra fluid.

Ligamentum teres — proximal attachment

Transverse acetabular ligament, acetabular fossa, and margins of the acetabular notch.

Ligamentum teres — distal attachment

Fovea of the femoral head.

Ligamentum teres — function in an infant

A conduit for blood supply to the femoral head.

Ligamentum teres — stabilising role

It tightens like a sling around the femoral head and helps draw the femur and acetabulum together.

Ligamentum teres resists what in hip flexion plus abduction?

Inferior subluxation of the femoral head.

Ligamentum teres resists what with hip medial rotation?

Posterior subluxation.

Ligamentum teres resists what with hip lateral rotation?

Anterior subluxation.

Ligamentum teres is a secondary restraint to what movement?

Hip lateral rotation in flexion.

SIJ and pubic symphysis — What bones make up the pelvis?

Two hip bones, sacrum, and coccyx.

What three bones fuse to form each hip bone?

Ilium, ischium, and pubis.

What is the triradiate cartilage?

The Y-shaped cartilage where the ilium, ischium, and pubis meet at the acetabulum.

Why may fractures pass through triradiate cartilage in teenagers?

The three hip bones may not yet be fully fused.

What articulates at the SIJ?

The auricular surfaces of the sacrum and ilium.

Why does the SIJ have very limited movement?

Its articular surfaces interlock and it has extensive ligamentous support.

Why does the SIJ have a high coefficient of friction?

Its ridges and grooves interdigitate, strongly resisting shear.

Strongest SIJ-supporting ligament

Interosseous sacroiliac ligament.