The hip joint is made up of the pelvis and thigh. The pelvis is made of two coxal bones*, or hip bones. These are each divided further into the ischium, the ilium and the pubis. These three bones are fused. Each coxal bone articulates (joins with, meets) with a femur, or thigh bone. This articulation is known as the femoralcoxal joint**, or hip joint. So one coxal bone and one femur equals one hip joint.
There are two important bony landmarks in the hip joint, one on the coxal bone and one on the femur. On the coxal bone you have the acetabulum***, a bowl-shaped indentation where the ilium, ischium and pubis meet. On the femur, you have the head of the femur, shaped like a ball, which articulates with the acetabulum by nestling into its bowl. This type of joint is known as a ball-and-socket.
Ball-and-socket joints allow for several types of movement: flexion and extension, rotation (around an axis), circumduction (circular, all around) and abduction and adduction. Abduction is movement away from the midline of the body; think abd-, as in abdicating from the body. Adduction is movement toward the midline of the body; think add, as in "adding" something to the body.
The major muscles of the hip cross the hip joint and act as pulleys to the bones' levers to achieve all this motion. The pelvis is fixed while the femur moves. The head of the femur pivots around in the acetabulum.
Four quadriceps are located on the anterior (front) and lateral (outside) femur. The quads flex the hip, bringing the thigh to the torso.**** Three hamstrings muscles are on the posterior (back) thigh, and work to extend the hip, bringing the thigh out of flexion or hyperextending the thigh behind the body.
Three gluteal muscles shape the buttock. The gluteus maximus forcefully extends the hip and laterally rotates the thigh. Lateral rotation is when you turn the anterior thigh bone to the outside. Your foot will turn to the outside, too. Gluteus medius and gluteus minimus abduct the hip, moving the femur away from the body. They also medially rotate the hip, turning the anterior thigh bone to the inside (turning the foot to the inside, too).
Five adductors work to do just that: adduct the thigh, or bring it toward the body. These muscles sit in between the quads and hamstrings on the medial (inside) femur. Then there are six small, deep muscles beneath the gluteals called the lateral rotators. Can you guess what they do? The work with gluteus maximus to laterally rotate the thigh.
Flex, abduct, extend and adduct the femur and the result is circumduction, or a circular motion, like making circles with your arms in gym class.
So, the next time you are walking up the stairs, you'll know that you are alternately flexing and extending the hip joint, engaging your quads, hamstrings and gluteus maximus. Notice how the quads and hamstrings are directly opposite each other on the femur, and how they perform opposing actions. It's the same for gluteus medius and minimus, which abduct the thigh, and sit laterally on the hip. The adductors of the thigh sit medially. Opposite sides, opposite actions.
Enjoy your new understanding of the articulation of the coxal bone and femur!
These pictures are from Trail Guide to the Body, Second Edition, by Andrew Biel, illustrated by Robin Dorn (both massage therapists) and published by Books of Discovery.
*Two coxal bones make the pelvic girdle, and when you add the sacrum and the coccyx, you have the pelvic bowl. Pelvis is Latin for "basin".
**It is also referred to as the acetabularfemoral joint (acetabulum and femur), or the ilialfemoral joint (ilium and femur), or simply the coxal joint. Either way, the name tells you the location.
***Acetabulum (as-e-tab-u-lum) is Latin for "little vinegar saucer". The Romans would use vinegar daily and would keep it in large wooden saucers or bowls. These bowls would develop cracks, like spider veins. Scientists of the time refered to this bony landmark as a "little vinegar saucer" because of its bowl shape and how the articulation of the ilium, ischium and pubis looked like cracks.
****The quadratus femoris or "quads" muscle is so large that is actually made up of four different heads, each with its own name: rectus femoris ("upright" or "straight" and "femur"), vastus lateralis, vastus medialis, and vastus intermedius (vast = large, lateral = outside, medial = inside). And truly, only rectus femoris flexes the hip joint, as it's the 0nly head to cross it. The primary action of the quads is to extend the knee.
