The pelvic structure is the center of our body. Our center of gravity is found there. The movement (or lack of movement) at the pelvis directly influences the movement available to the legs below and the spine above. Its primary function is to support the spine above and facilitate movement of the legs below.

Bones

The overall shape of the pelvic structure varies in the population as a whole. It also varies between men and women because women’s pelvises are shaped to accommodate childbirth. There are two sides, or halves, to the pelvis. These two sides connect at the sacrum on the posterior side and at the pubic symphysis on the anterior side. 

Each half is composed of three bones:

Ilium —— The ilium is the elephant ear-shaped, or fan-shaped, bone on the lateral side of each side of the pelvis. The crest or upper ridge of the ilium runs from the anterior superior iliac spine (ASIS - an important bony landmark for muscle attachment) on the front, to the posterior superior iliac spine (PSIS) on the back.

Ischium —— The ischium is the posterior and inferior bony aspect.

Pubic bone —— The pubic bone is the anterior, inferior bony part of the pelvis. The pubic bone is also an important bony landmark for muscle attachment.

These three bones are truly separate bones at birth, when they are separated by cartilage, but they become fused by the time we reach adulthood.

Additionally, there are two more bones which connect the two sides:

Sacrum  —— The sacrum is a triangular-shaped bone that is actually considered to be composed of five fused vertebrae. It fits like a wedge between the two ilia.

Coccyx —— The coccyx is more commonly referred to as our “tail bone”. It’s made up of three, four, or sometimes five fused vertebrae.

The sacrum and coccyx bones fit between each half of the pelvis on the posterior side.

Joints

There are several important joints at the pelvis. They function to connect the two halves together and also to connect it to the rest of the body.

Sacroiliac joints (SI joints) —— This is where the sacrum articulates with the ilium bone on each side of the pelvis. The SI joints connect the two halves on the posterior side of the body. The SI joints are also the points where the pelvis technically connects to the upper body because the sacrum is connected to the spine on its superior end.

Pubic symphysis —— This is where the two halves articulate with one another on the anterior side of the body.

Acetabulofemoral joints —— These two joints are better known as the hip joints. This is technically where the pelvis connects to the lower body.

Muscles and ligaments

Strong ligaments are an important part of what connects the two halves of the pelvic structure. They include:

• Iliolumbar ligaments
• Sacroiliac ligaments
• Sacrotuberous ligaments
• Sacrococcygeal ligaments
• Sacrospinous ligaments

As the pelvis is central to movement in both the legs below and the spine above, a long list of muscles connect to it both above and below. 

They include:

• Gluteus maximus, gluteus medius, gluteus minimus, and tensor fascia latae
• All five adductors
• All three hamstrings
• Rectus femoris
• Sartorius
• All six lateral rotators of the hip
• A group of pelvic floor muscles
• Iliopsoas
• Quadratus lumborum
• Rectus abdominis
• External and internal obliques
• Transverse abdominis
• Latissimus dorsi
• Erector spinae group

Movements at the pelvis

Since there are two halves of the pelvis, each of them can move independently of one another. This is important, as this independent movement is necessary for walking, running, and many other movements.

• The pelvis can tilt anteriorly or posteriorly. That movement happens at the acetabulofemoral joints.
• Each side of the pelvis can also do elevation and depression. Elevation is more commonly referred to as “hip hiking”. 
• Each side of the pelvis can rotate.
• At the SI joints, the sacrum can move in relationship to the rest of the pelvis. This forward or back tipping movement is called nutation and counternutation.

The pelvis is an important part of our body structure and necessary for our most basic movement. If you’d like to learn more about the muscles that attach to it, check out our posts on the piriformis muscle and the iliopsoas. If you want to dive deeper into this complex structure, these articles from the National Library of Medicine are a great resource: https://www.ncbi.nlm.nih.gov/books/NBK482258/ and https://www.ncbi.nlm.nih.gov/books/NBK551580/