hip joint | andreacollo

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The squat movement can be described as a compound exercise which involves multiple groups of muscles. It is usually performed by recreational and professional athletes to strengthen hip, knee and ankle muscles. The squat exercise consists of two main phases, lowering and standing.

The lowering phase

The body starts from a standing position and, replicating the motion performed while sitting on a chair, it is lowered until the squat configuration is achieved. All the lower limb joints are involved, with several groups of muscles that contract as they lengthen. This results in eccentric contractions. squatL

Hip: flexion movement. The hip extensors (gluteus maximus, semimembranosus, semitendinosis and biceps femoris) mainly control the speed of the body, whose lowering is naturally supported by gravity.
Knee: flexion movement. The knee extensors (rectus femoris, vastus medialis, vastus intermedius and vastus lateralis) mainly allow to tune the knee bending speed.
Ankle: dorsiflexion movement. The plantarflexor muscles (gastrocnemius and soleus) mainly counteract the pull of gravity and provide a stable support on the ground.

The standing phase

squatS The body leaves the squat configuration and returns to an upright position. The speed of this movement is continuously controlled, as well as the stable support provided by the feet. Once again, this is ensured by the combined action of all the lower limb joints. The same groups of muscles as for the lowering phase now shorten as they contract. This produces concentric contractions.

Hip: extension movement. The hip extensors mainly bring the trunk back to an upright position.
Knee: extension movement. The knee extensors help contracting and smoothly straightening the knee joints.
Ankle: plantarflexion movement. The plantarflexor muscles push down against the ground and are responsible for the overall stability of the body.

The femur head (Latin: caput femoris) is the highest part of the thigh bone (femur). It has a roughly semispherical shape, with a short “neck of the femur” angling the head anteriorly, medially and superiorly to fit into the acetabulum of the pelvis bone.

The acetabulum, also called socket, is the cavity in the pelvis which “hosts” the femur head. It is formed by three innominate bones: the ilium, the ischium and the pubis.

The femur head’s surface is smooth and normally coated with cartilage. It is supported by the neck of the femur and gives attachment to one single intracapsular ligament, the “ligament of head of femur” (ligamentum teres, on the top of the femur head in the figure on the left). It may be not that important as a ligament (it is only stretched when the hip is dislocated, and may then prevent further displacement) but can often be vitally important as a conduit of a small artery to the head of the femur. This small artery is not present in everyone but can become the only blood supply to the bone in the head of the femur when the neck of the femur is fractured or disrupted by injury in childhood.

The femur head together with the acetabulum form the Hip Joint. The hip joint has three degrees of freedom, since it can move in three different planes:

sagittal plane: flexion/extension of the leg.
With just this movement, approximately 3 to 3½ times the body weight acts on the hip joint. An example of this motion is shown by the figure on the right.
frontal plane: abduction/adduction of the leg. Regardless of the direction, the respective supporting leg is then subject to approximately 3 times the body weight. This kind of motion is represented by the figure on the left.
transverse plane: external/internal rotation of the femur with respect to the pelvis bone. This motion, typical when crossing legs, makes the femur head rotate in several directions. An example is shown by the figure on the right.

The head of the femur is attached to the femur shaft by a thin neck region that is often prone to fracture in the elderly, which is mainly due to the degenerative effects of osteoporosis. If there is a fracture of the neck of the femur, the blood supply through the ligament becomes crucial. In orthopedic surgery, the Total Hip Arthroplasty surgery consists in removing the femur head and the acetabulum and replacing them with a total prosthesis.

Normally, the two involved prosthetical components are:

the Acetabular Cup, a shell that fits the pelvis bone to replace the acetabulum. It is usually attached to the bone by using friction or cement. Additional fixation can be achieved by means of screws.
the Femoral Component, that is a stem with attached prosthetic femoral neck and head (a ball that fits the Acetabular Cup). Femoral bone is removed and the femur is shaped to accept the femoral stem.