Unit V – The Pelvis

  

Objectives:

 At the completion of Unit V the student will be able to:

 1.      Identify the bony structures of the pelvis and their articulations which form the pelvic ring.

2.      Identify the key bony landmarks for the hip bones and the sacrum.

3.      Discuss the differences between the male and female pelvic structure.

4.      Identify the location and action of the primary muscle groups in the pelvic region.  

5.      Describe the inter-relationship of the pelvis, the lumbar spine, and the lower extremities for postural alignment and movement.

Introduction:

 In Units III and IV we explored the spine and respiration. Another important component of the trunk is the pelvic region. The pelvic region is the part of the trunk that lies distal to the lumbar spine and abdominal cavity. It is the transition between the trunk and the lower limbs. There are many important structures housed in the pelvic region of the body. This course will focus on the musculoskeletal anatomy and function in this region, but it is important to remember that a large part of the organs for digestion and elimination, as well as the major sexual and reproductive organs are located in the pelvic region.
   
 

Bony Connections of the Pelvis  - An Overview

The word pelvis means “basin” and is also called the pelvic ring. It is a cylindrical structure composed of 3 articulating bones, associated muscles, and ligaments that make up the pelvic floor. The pelvis receives articulation with the spine from above at the 5th lumbar vertebra and by muscle attachments in the spine from above. It serves as the transition to the lower body by its articulation with the femur bone of the leg. This articulation between pelvis and femur forms the hip joint. Because the pelvis lies between the trunk and the legs it must receive weight from the upper body and trunk, as well as absorb the stresses of the lower body from below with activities like walking, jumping, or running.

 
   
 

The Pelvic Ring – Specific Bony Anatomy

The pelvic ring is composed of 3 bones: 2 coxal or hip bones (1 left and 1 right) and 1 sacrum.

  
   
  The coxal or hip bones articulate posteriorly on the lateral surfaces of the sacrum to form the left and right sacroiliac joints.   The coxal or hip bones articulate anteriorly to form the pubic symphysis joint. (This joint is easy to remember if you ever recall falling on your bike and hitting your groin area on the support bar of the bike – part of the pain you felt was the contact with the pubic symphysis)    
 

Differences in the Pelvic Structure

 The shape of the pelvis varies considerably among normal individuals. Women have a broader, more flared pelvis and a larger pelvic inlet and outlet to allow for childbirth. Men have a heavier and thicker pelvis that is narrower, and more upright. The pelvic inlet of the male is more heartshaped, while the pelvic inlet of the female is more oval or rounded. Even within genders there is a great deal of variability in the shape of the pelvis.

  
 

 

 Examples of variability among pelvic inlet shapes  

The Hip Bones (also called the Coxal Bones or the Innominate Bones)  
The left and right hip bones are mirror images of each other so bony anatomy of one will be discussed. The bones have 3 segments:  the ilium, the ischium, and the pubis.
The junction of these 3 segments forms the acetabulum. The acetabulum is a deep, bony socket that articulates with the head of the femur (long leg bone) to form the hip joint. Another opening formed by the junction of the ischium and the pubis is the obturator foramen.  Through this opening pass several important anatomical structures, which will be discussed later. (Remember a "foramen" refers to an opening in anatomical terminology)
Ischial Tuberosity – is the bony prominence most inferior and posterior on the ischium; this is the bone we call the "sitz bones" that bears weight when we sit.

Medial surface of the Ilium – this is the internal bowl or scoop-shaped area of the ilium that houses the internal iliac fossa. In this fossa is the location of the iliacus muscle. The iliacus is one component of the iliopsoas which has been discussed  in previous units. (Remember a "fossa" is a depression or concave surface)

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There are several important bony landmarks on the lateral surfaces of the hip bones. They are as follows:
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Iliac Crest – the top edge of the ilium; it is the most superior aspect of the bone. You can feel this when you place your hands on your hips. It is the bone your hands rest on.

 

Anterior Superior Iliac Spine (ASIS) – this is the most anterior portion of the ilium; it is a small, sharp bony prominence. If you lie on your back it is the bony point that sticks out in front at your hip level.

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Posterior Superior Iliac Spine (PSIS) – this is the posterior bony prominence of the ilium. If you see a dimpled area on the skin above the buttock it is in the area of the PSIS

Greater Sciatic Notch (GSN)– this is the opening in the posterior aspect of the ilium. It provides the pathway by which the sciatic nerve passes into the lower extremity.

 

The Sacrum

  
   
 

The sacrum is the posterior wedge-shaped or triangular component of the pelvic ring, and is located between the 2 ilia of the hip bones. It has 5 fused vertebrae (S1-S5) whose components are still visible.

The anterior surface of the sacrum is concave and smooth, while the posterior surface of the sacrum is convex and much rougher - with bumps for muscle and ligament attachments. 

 The sacrum has several important bony landmarks: 

  

The Sacral Foramina – these are the small openings on the sacrum through which branches of the sacral nerves pass. These are visible as the small round holes in the picture to the left.

The Sacral Promontory – this is the upper, anterior edge of S1. The superior surface of S1 is called the sacral base and articulates with the 5th lumbar vertebrae.

The Apex of the Sacrum – this is the inferior surface of S5 and makes up the lower portion of the sacrum. The sacrum articulates with the coccyx. 

The Sacral Crest – this is the bony prominences that run down the middle of the posterior aspect of the sacrum. Compare these to the spinous processes in the vertebrae above.  If a person is thin you may be able to feel these prominences  on the posterior  sacrum ©
The Coccyx – this is a small section of bone located distal to the main body of the sacrum. These small connected, and sometimes fused, bones make up what is called the “tailbone”. They serve no specific function now and are remnants of a tail. These can be damaged by a fall on the buttock, or with repeated trauma from sports such as gymnastics with a lot of rolling, especially if their anatomical alignment places them more posteriorly.
   

Ligaments of the Pelvis

 
  A general overview of the two views of the pelvis to the left indicate the many ligaments that attach to the pelvis. The anterior sacroiliac ligaments are located on the anterior surface and attach from the ilium to the sacrum. There are posterior sacroiliac ligaments located in this same region on the posterior surface of the sacrum.   
  There are also ligamentous attachments to the lumbar spine above via the iliolumbar ligaments. All of these attachments are important collectively because they make the sacroiliac (SI) joints very stable. It is uncommon for a joint dislocation in the SI, and it usually only results from trauma such as a fall or car accident. If an individual has ligament laxity or general “loose-jointedness”, as might occur in dancers, there can be some dysfunction in the SI joint area that causes pain. The dancer may need to work on strengthening and stabilization, as well as modification of habit postures and movement patterns that could alleviate this problem.

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Muscles Important to the Pelvis Region

 
  Many of the muscle groups that have already been discussed in relation to the spine, also impact the pelvic region. Also, since the bony articulation for the femur, which makes the hip joint, is located on the pelvis, the muscles of the lower extremity also impact the pelvic region. It is important to remember the the pelvis plays a key role in the transition between trunk and lower body and provides both stabilization and mobility roles for the body.  
     
 

Trunk Muscles – From Above

 
  The key muscles that impact pelvic function that begin in the trunk and insert distally are the back extensors or erector spinae muscles, the abdominals, the quadratus lumborum , and the iliopsoas. All of these have been mentioned in earlier units, but it is important to consider them in relation to the sacroiliac joints and pelvic region.  
 

Erector Spinae Muscles – (The Back Extensors)

 
  Long muscles extending from the head to the sacrum on far left are the erector spinae group.  
The superficial back extensors have their origin at the thick fascial tissue overlying the posterior aspect of the sacrum, as well as the spinous processes of the lumbar spine. These muscles then pass superiorly to attach from the ribs to the base of the skull. Remember that the erector spinae are divided into three sections: the spinalis closest to the spine, the longissimus in the middle, and the iliocostalis most lateral along the edges of the ribs.  Because this muscle group attaches to the sacrum it provides an important distal function. The spine can be held at the distal aspect in the area of  the sacrum and the individual can backward bend from above. The individual can also perform an anterior pelvic tilting action of the pelvis on the spine by moving the erector spinae at the distal end.

* Try this now:  Stand with your feet shoulder width apart and weight evenly distributed on both feet. Place your hands on your hips. Bend backward using the backward movement of your head and shoulders as your moving portion. The pelvis area remains still. You can only go backward for a specific distance before you must put your arms back and shift your hips forward to catch yourself. A continuation of this movement would result in doing a backbend to the floor. Return to the upright position. Now try to keep your head and shoulders still and move your pelvis anteriorly under your lumbar spine. This increases the arch in your lower back and pushes your buttocks posteriorly in relation to the rest of the spine.  This motion is called an anterior pelvic tilt.

 

The Abdominals – the Flexors

Recall from our previous discussion that the abdominals as a muscle group have 4 components:  the rectus abdominis, the internal and external obliques and the transverse abdominis. They collectively produce flexion or forward bending of the spine.

The Rectus Abdominis (pictured below on far right) originates from the pubic symphysis of the pelvis and attaches superiorly to the 5th –7th ribs and xiphoid of the sternum.

The Transverse Abdominis (pictured below second from right) originates on the cartilages of the lower 6 ribs and the diaphragm and attaches to the lip of the iliac crest in the pelvis.  

The Internal (below far left) and External (second from left) Obliques originate on the lower ribs and pass in diagonal fashion to attach on the ASIS and pubic tubercle and the crest of the pubis – all in the pelvic region. As is evident, the abdominals are intimately connected to several structures in the pelvic region that serve as distal anchors for movement.

The abdominal group below:

As with the erector spinae, the abdominals serve two collective functions. If the pelvis is stabilized and the abdominals are moved starting from above they bring the trunk toward the pelvis in a forward bending motion. If the internal and external obliques are fired more primarily, there is a resulting rotational movement with the trunk flexion. The Transverse Abdominis is now thought to be important for stabilizing the lower trunk and pelvis. If the trunk is held still and stable and the abdominals are moved from the distal attachment the result is an posterior motion of the pelvis under the trunk. This is called a posterior pelvis tilt.

*Try this now -  Lie on your back with your knees bent and your feet on the floor. Place your hands across your chest. Using just your abdominals lift your head and shoulders toward your knees. You will note that you can only do a partial sitting up action using the abdominals. Once you raise above 45 degrees you must engage your hip flexors below to complete this action. For this reason  partial sit-ups are now encouraged, rather than full sit-ups in this position to strengthen the abdominal muscles. Now return to your starting position. Keep your trunk flat. Using your lower abdominal muscles flatten your stomach to the floor and feel your spine flatten against the floor as your pelvis tips backward. This is the posterior pelvic tilt action.  You can also do this motion in the standing position. Try that now as well.

These pelvis tilting motion are key for dancers to understand as they form some of the component movements for the dancer. Let’s look at a few other trunk muscles that impact the pelvis.

 

 

Quadratus Lumborum

 
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The Quadratus Lumborum is a deeper, smaller muscle that originates along the superior aspect of the iliac crest of the pelvis. It attaches above to the inferior borders of the last ribs and the transverse processes of lumbar vertebrae 1-4. It functions to laterally flex the lumbar spine toward the pelvis if moved from above. Or, if moved from the pelvis can perform a hip-hiking motion on one side of the body.  Do not confuse this hip-hiking motion with raising the hip using the outside leg muscles from below. This is a common problem in dance technique. The hiking you feel using the quadratus lumborum is smaller motion and is located in the posterior aspect of the spine, not on the outside of the leg.

*Try This Now-  To feel how the pelvis moves using the quadratus lumborum stand with feel apart and weight equal on both feet. Actively raise your right hip, without lifting the foot from the floor, toward your ribs. As you do this you will feel the pelvic region tilt upward. The primary mover for this activity is the quadratus lumborum. As stated above, this is a small motion that is felt in the posterior aspect of the spine, not on the outside of the leg. If you feel it down in your leg or on the outside of your leg and hip you are using the tensor fascia lata muscle and not the quadratus lumborum. 

 

The Iliopsoas

This important muscle group has also been discussed in previous units. It is divided into two parts – the psoas major and the iliacus. The origin of the psoas major is the transverse processes of the lumbar vertebrae and attaches on the lesser trochanter of the femur below. The iliacus originates in the iliac fossa of the pelvis, along the iliac crest and attaches also into the lesser trochanter of the femur. These muscles act collectively to either flex the spine on the trunk or to flex the hip and bring the leg towards the trunk. The psoas major when acting alone can produce increased lumbar lordosis in the spine. It can also assist with lateral flexion or sidebending of the spine when one side is activated.
The iliopsoas is an important transition muscle from trunk to lower extremity. It serves a stabilization role to stabilize the trunk and allow the legs to move freely under the trunk. It also acts as a prime mover for the leg under the trunk. It is important for the dancer to understand the actions of this muscle and know how to correctly activate during movement.

 
  Images for the Dancer  
The alignment of the pelvis has a lot to do with the support of the strong muscular abdominal wall on the anterior aspect of the torso. Pictured at right is the deepest layer of the abdominal wall - the transverse abdominis. 

For dancers, this is a crucial muscle layer that protects the lumbar spine from undo strain.  It  also maintains proper support of the pelvis ensuring that it will not “tip” forward or backward, or “hike” upward when lifting or moving the legs as in a grand battement.

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  See Diagram A.  This shows that the pelvis can drop forward along the axis of the hip sockets in conjunction with lumbar spinal flexion, if the abdominal muscles are not in use.  This can spell disaster for the low back (lumbar spine) as all the weight of the body now falls onto the joints between the vertebrae rather than having it supported through the muscles of the torso. 

Diagram B shows that abdominal support actually “lifts” the front of the pelvis into proper vertical alignment, taking the strain off the spine.

Diagram C shows abdominal support in conjunction with the Psoas muscle.  The two work together which allows the psoas to properly flex the leg with efficiency and strength gained from a properly aligned pelvis.

  
  Picture at right shows the important psoas as it travels from the spine to the femur bone.  It is a major hip flexor, which depends upon the abdominal muscles to support the front of the pelvis  
  A good image for aligning the pelvis while standing, is to imagine two plumb lines, or beams of light, dropping down from the sitz bones of the pelvis to the heels.  The sitz bones are the bony landmarks for the base of the pelvis.  If they are directed downward over the heels, you can be sure that you are not tipping the pelvis forward or backward  
  Also, imagine the two bony landmarks on the front of the pelvis, the most anterior point of the iliac crests on both sides, lining up on a horizontal axis or “ruler”.  This insures that you are not “hiking” up the hip as you lift the leg on either side.  Hiking the hip displaces the pelvis, therefore sacrificing aligned and supported movement. .  

Lower Extremity Muscles and the Pelvis – From Below
As mentioned, the pelvis is the transition structure between the trunk and the lower extremity. There are several groups of muscles that attach in the area of the pelvis, but are lower extremity muscles. Since these muscle groups impact the pelvis function as a whole it is important to know these as well. These muscles will be mentioned briefly in relation to the pelvis, and will be discussed in detail in the next unit on the hip.

Motions of the Hip Under the Trunk
The hip and leg perform several motions and must have proper strength and flexibility to do these motions.
  • The motions of hip flexion and extension, hip abduction and adduction, and internal and external rotation of the hip require use of muscles that have attachments superiorly in the area of the pelvis. 

Although these muscles act to move the leg, they can also affect the position and stability of the pelvis if they are weak, tight, or asymmetrical left to right.

These muscles include: 

  • the iliopsoas, the gluteus maximus, the gluteus medius, tensor fascia latae, the internal and external rotators of the hip, and the adductors of the leg.

Relevance to the dancer:  It is important to remember that though these muscles are discussed in relation to the hip and leg, they play a role at the pelvis. These muscle groups have multiple attachments to the pelvis and are important for the integration of movement and stability of the spine, trunk and  lower body.  The body requires a delicate balance of strength and flexibility of the trunk, pelvis and lower body muscles that must work together for quality movement patterns. Changes in any of these muscles can result in changes in movement quality, and may cause injury or chronic posture problems. These lower body muscles will be discussed in detail in the next unit on the hip.

This concludes Unit V. Return to Blackboard to take Quiz #2 for Units III - V. Once you have successfully completed the quiz you can proceed to Unit VI.