The hip joint
The hip joint is a ball and socket joint, where the head of the femur fits into a deep cavity, called the acetabulum on the pelvic bone. The cavity on the pelvis is much deeper than the cavity on the scapula, so the hip joint is much more stable, but less mobile, than the shoulder joint. The hip joint is also reinforced by extremely strong ligaments, making it much more difficult to dislocate than the shoulder.
Movements possible at the hip include flexion, extension, abduction, adduction, rotation and circumduction.
The most anterior muscles of the hip and thigh tend to flex the hip and extend the knee. The posterior muscles of the hip and thigh mainly extend the hip and flex the knee. The third group of muscles, located in the medial region (or inside thigh) adduct the hip. These three groups of muscles – anterior, posterior and medial – are separated by walls of fascia into separate compartments.
Movements at the hip are largely caused by muscles which are anchored to the pelvic girdle. Hip flexors generally pass in front of the hip joint, the most important being iliopsoas, tensor fasciae latae and rectus femoris. These muscles are assisted by the adductor group of muscles, and the sartorius. The prime mover of hip flexion is the iliopsoas.
Hip extension is caused mainly by the hamstring group in the posterior thigh, and the gluteus maximus of the buttock. Buttock muscles located on the lateral region (the side) of the hip, abduct the hip and rotate the thigh medially. The muscles located here are the gluteus minimus and gluteus medius.
Medial rotation is opposed by six small deep muscles located in the gluteal region, which are collectively known as the lateral rotators.