The hip: joint structure, movements and muscles

The Colombian singer Shakira uses her hips to tell whether a song is ready to go out in public. If they start moving and she starts dancing – it is good to go. She believes her ‘hips don’t lie’ when it comes to identifying a good tune. Even if we don’t rely on our hips as wise advisers it is still beneficial to know their structure, movements and muscles.

Joint structure

The hip joint is formed by the articulation of the head of the femur and the acetabulum of the innominate bone. It is a large and deep ball and socket articulation, allowing for a significant range of movement and stability at the same time.
 
It can move around three axes: transverse (flexion and extension), anteroposterior (abduction and adduction) and vertical (rotation in and out). A layer of cartilage covers these surfaces with the exception of the depression on the head of the femur and the centre of the acetabulum.

Ligaments
 
The Transverse Ligament bridges across the notch in the margin of the acetabulum, leaving a gap through which vessels and nerves can pass. It is composed of strong fibres.
 
The Cotyloid Ligament surrounds the margin of the acetabulum deepening the socket. It grasps the head of the femur with its somewhat constricted free edge.
 
The joint is completely surrounded by a capsule which is considerably thicker superiorly. It is different from that of the shoulder joint as it doesn’t allow the ball to be drawn from the socket much. Above, the ligament is attached to the margin of the acetabulum. Below, it attaches to the cotyloid and transverse ligaments.

It is also attached to the neck of the femur:

• superiorly – to the root of the greater trochanter,
• anteriorly – to the intertrochanteric line
• inferiorly – to the root of the lesser trochanter,
• posteriorly – a little way above the intertrochanteric ridge

 
The fibres of the capsule run both in a circle and lengthwise providing additional strength. There are several identifiable longitudinal fibres that are strengthened:
 

• The iliofemoral ligament is also called the Y-shaped ligament of Bigelow. This triangular ligament attaches to the root of the anterior inferior spine and margin of the acetabulum by its apex. By its base, it attaches to the intertrochanteric line. Its name derives from the fact that at the base the sides are thicker than the centre.

 

• The pubofemoral ligament attaches to the acetabular end of the ramus of the pubis and the inferior aspect of the neck of the femur.

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• The ischiocapsular (ischiofemoral) ligament is also called the ligament of Bertin. It attaches superiorly to the ischium just in front of the small sciatic notch. Its lower end merges in the capsule.

 

• The ligamentum teres is a flattened band of tissue that attaches to the fossa on the summit of the head of the femur and to the sides of the notch in the margin of the acetabulum.

 
At the bottom of the acetabulum, there is a large mass of adipose tissue. These pads of fat are called Haversian glands and can be found to some degree in many joints.
 
The joint and the ligaments are completely surrounded by a synovial membrane. It hangs in loose folds between the articular margin and the attachment of the capsule on the neck of the femur.
 
Muscles and movements
 
As previously stated, the hip is a ball and socket joint capable of a wide range of movements.
 
Flexion is performed by iliacus, psoas major, sartorius, rectus femoris, pectineus, gracilis, adductor longus and obturator externus. We flex our hip when we initiate a step forward.
 
The muscles responsible for extending the hip are the glutes (maximus, medius, and minimus), the hamstrings (biceps femoris, semimembranosus, semitendinosus) and adductor magnus. We extend our hip when we initiate a step backward.

​Adduction is carried out by pectineus, the adductor (longus, magnus, brevis), gracilis, pectineus and the lower fibres of gluteus maximus. When we try to catch something by rapidly closing our legs to prevent it from falling on the ground we are adducting our hips.
 
Tensor fascia latae, gluteus medius and minimus and obturator externus are the main abductors. Piriformis, obturator internus, gemelli, sartorius and the upper fibres of gluteus maximus perform the same action during flexion. When we do a side split we are adducting our hip to its maximum range of motion. This takes a significant amount of flexibility in the opposing muscles.
 
Medial rotation is enabled by tensor fascia latae, gluteus medius and minimus. An example would be turning our leg and foot inward.
 
Lateral rotation is performed by obturator externus, inferior fibres of gluteus maximus, posterior fibres gluteus medius and minimus, sartorius, ilio-psoas, pectineus, and the adductors. During extension piriformis, gemelli and obturator internus also play a role in lateral rotation. We do this movement when turning our leg and foot outward.
 
Circumduction is moving our leg in a circle and is enabled by a combination of these muscles.

To bring some of these movements to live, let’s consider walking for example. During gait, a fancy word for walking, the body’s centre of gravity is distributed medially to the supporting limb. The ligaments of the hip are not enough to stabilize the body during the walking phase when we are on one foot only. The glutei muscles (maximus, medius and minimus) together with the iliotibial band (ITB) and tensor fascia latae (TFL) provide medial-lateral stability. These soft tissues are situated lateral to the hip joint.
 
The other side of the fulcrum is balanced thanks to the contraction of the adductors – magnus, longus, brevis, pectineus, gracilis, and obturator externus. The relative ratio of the lengths these two opposing forces is 2:1, which means that the glutei exert contractile effort equal to 2 time our body weight to stabilise the pelvis during unilateral stance. The necessity for such precision hopefully gives you a taste of the importance of the hip joint and its surrounding tissues. 
 
Take good care of your hips and they will never lie to you. 

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