Biomechanics and Dynamics of the Pelvic Girdle
Original Editor - Deborah Riczo
- 1 Sacral Biomechanics
- 2 Musculoskeletal Dynamics
- 2.1 Slings
- 2.2 Integrated Model of Function
- 2.3 Other considerations to note: Primary movers in anterior and posterior pelvic tilt
- 3 Neurodynamics
- 4 Lymph Nodes in the Abdominal/Pelvis Area
- 5 Resources
- 6 References
The sacroiliac joint (SIJ) is the largest axial joint in the body. It connects the spine to the pelvis and transfers load between the lumbar spine and the lower extremities. Research has shown the sacrum has very little movement. Numerous ligaments across the joint support and limit movement of the SIJ.
The sacrum tightly wedges between the ilia and the ligaments provide resistance to shear loads. Additional resistance is also provided by grooves and ridges to protect the joint against shearing.Although several of the body’s largest and powerful muscles surround the SIJ, no muscle directly affects sacral movements.
- Muscles that play a role in providing SIJ stability through increasing compression load across the joint are the transversus abdominis and the pelvic floor muscles (levator ani and coccygeus muscles).
- Ligaments that play an important role in SIJ stability are the iliolumbar ligament (ventral side), the posterior sacroiliac ligaments, interosseus ligaments, sacrotuberous and sacrospinous ligaments. The pubic ligaments, the anterior sacroiliac ligaments and the dorsal side of the iliolumbar ligaments have the least effect on pelvic stability.
Range of Motion
The sacrum can move with respect to the ilium in all directions, but the magnitude of motion is minimal at about 6° of freedom. The SIJ range has three planes of motion in flexion-extension (nutation/counternutation) is approximately 3°, axial rotation is around 1.5° and lateral bending 0.8°.
The two main movements occur when the sacrum moves relative to the iliac bones in the sagittal plane.
- Nutation describes when the sacrum is rotated forwards relative to the iliac bones 
- Counternutation describes when the sacrum is rotated backwards relative to the iliac bones which occurs in supine and non-weight-bearing.
According to Willard et al nutation can be regarded as anticipation for joint loading, as it is more stable than counternutation. During nutation, the posterior parts of the iliac bones are compressed into the “keystone-like” shape, and the joint is in the locked and close-packed position. This normally occurs during increased load-bearing situations e.g. standing and sitting, to increase stability.
- Anterior and posterior rotation as the innominate movements occurs in the sagittal plane. Anterior rotation of the innominate should not occur in weight-bearing. To keep the leg stable in single-leg standing, the innominate will posteriorly rotate, to put the sacrum in a nutated position. This will be the close pack position
- Inflare and outflare are normal innominate movements around the transverse plane and that occurs during rotation and lateral bending tasks.
Ligaments involved in nutation and counternutation:
Ligaments that tighten with sacral nutation:
The female sacrum is wider, more uneven, less curved and more backwards rotated. The male pelvis tends to be relatively long and narrow, with a longer and more conical pelvic cavity compared to the female pelvis. The female pelvis has a wider sciatic notch and the acetabula are wider apart compared to the male pelvis.
These sex differences are also seen in the biomechanics of the SIJ with the female SIJ having higher mobility and greater stresses, greater loads and more pelvis ligament strains compared to the male SIJ. Furthermore, the influence of hormones such as relaxin, which increases the mobility of the SIJ by providing ligament laxity for birth, needs to be considered. All these features may contribute to women being more prone to incidences of SIJ and pelvic pain due to high mobility.
Ligament and muscle-forces are needed to provide compression of the SI joint. This mechanism of compression of the SI joints due to extra forces, to keep an equilibrium, is called ‘force closure.
Superficial and Deep Anatomical Slings
The four muscle slings around the pelvic girdle that contribute to force closure of the pelvis are:
Anterior Oblique Sling (AOS)
- External and internal obliques connecting with contralateral adductor muscles via the adductor-abdominal fascia
- Contraction of these group of muscles provides stability, compressing the pelvic girdle, resulting in force closure of the pubic symphysis
- Multidirectional sports such as tennis, soccer, football, basketball rugby and hockey places a great demand on the AOS as it must not only contribute to accelerating the body but also to rotating and decelerating it during the change of direction.
Posterior Oblique Sling (POS)
- Latissimus Dorsi, gluteus maximus, biceps femoris and interconnecting thoracolumbar fascia
- Muscles work as synergists to directly stabilise the pelvic girdle
- Force closure increases indirectly due to the anatomical attachments of the gluteus maximus and the thoracolumbar fascia with the sacrotuberous ligament
- POS works with AOS to create balance
Deep Longitudinal Sling
- Erector Spinae, multifidus, thoracolumbar fascia, sacrotuberous ligament and biceps femoris
- The sling allows for movement in the sagittal plane while also influencing local stability
- Contraction of the sacral part of multifidus causes sacral nutation thereby increasing tension in the interosseus and short dorsal ligaments and thus creating increased force closure of the SIJ
- The iliac attachments of multifidus together with erector spinae pull the posterior parts of the iliac bones toward each other and therefore limiting further nutation
- Contraction of erector spinae and long head of biceps may increase force closure due to their anatomical attachments with the sacrotuberous ligament.
- Contraction of erector spinae and multifidus has broadening effect (inflation of the fascial cylinder), this will increase tension and assist with force closure.
- Gluteus medius, gluteus minimus, tensor fascia latae (TFL) and iliotibial band (ITB)
- Sling provides stability in the coronal plane and involved in pelvo-femoral stability in dynamic movements (gait, lunges, stair climbing)
- To understand the relevance of the lateral sling, the actions of the muscles need to be understood
- Hip abduction and medial rotation – gluteus medius and minimus
- Tensor Fascia Latae works in synergy with these muscles to hold pelvis level in single leg movements
- Tensor Fascia Latae also works with gluteus maximus on ITB to stabilise the hip joint by holding the head of the femur in the acetabulum
- Movements involving single-leg stance, such as walking, lateral sling comes into tension to maintain pelvis stable over stance leg and preventing pelvic drop on the opposite side of the pelvis
- Failed control of lateral sling often presents a hip drop/ Trendelenburg sign during the stance phase of gait and single-leg stance
A good understanding of the concept of these anatomical slings and the ways in which they function to influence lumbopelvic stability and function will be helpful in deciding on which treatment strategy to apply in patients with pelvic girdle dysfunction.
For a further in-depth look at anatomy slings see this PP page: Anatomy Slings and their Relationship to Lower Back Pain
Integrated Model of Function
The integrated model of function has been derived from anatomical and biomechanical studies of the pelvis as well as clinical experience. This model aims to address why the pelvis is painful and not able to sustain and transfer loads as opposed to a model which only seeks to identify specific pain generating structures. The four components of this model are:
- Form closure (structure) - the stability of the joint from the design of the pelvic anatomy.
- Force closure - forces produced by myofascial action across the joint to create stability
- Motor control (specific timing of muscle action/inaction during loading)
This model suggests that multiple factors affect joint mechanics. Some of these factors may be intrinsic to the joint itself, whereas other factors are produced by muscle action. These in turn can be influenced by the emotional state. To effectively manage pelvic girdle pain or dysfunction it is necessary to address all four components to guide patients towards a healthier way to live and move.
For more detail on the integrated model of function read this chapter: Principles of the Integrated Model of Function and its Application to the Lumbopelvic-hip Region Chapter 5 in: The Pelvic Girdle
Other considerations to note: Primary movers in anterior and posterior pelvic tilt
Muscles for anterior pelvis tilt:
- erector spinae
- rectus femoris;
Muscles for posterior pelvis tilt:
- rectus abdominis
- external oblique
- glute max
Innervation of the pelvis
- Sacral plexus:
- derives from L4-S4 nerve roots
- sits on the internal surface of the piriformis muscle
- most of the sacral nerves stemming from the sacral plexus exit through the greater trochanteric notch
- the sciatic nerve forms out of sacral plexus and may be compressed by muscle, causing radicular pain down the leg
- Coccygeal plexus
- derives from the S4 - S5 nerve roots
- lies along the coccygeus muscle on the pelvic surface
Nerves susceptible to neurocompression in the pelvic girdle
- Responsible for low back, buttock, posterior thigh/leg/foot pain and weakness
- Widest and longest nerve
- Branches from L4 to S3
- Lateral hip rotators (except for piriformis)
- Posterior compartment of the thigh
- Branches into common peroneal and tibial nerve
- Difficulty ambulating
- Possible lower extremity instability
- Deep ache in adductors by the pubic bone
- Exercise-related pain or groin pain
- Pain and weakness worsens with exercise
- Impaired jumping ability in athletes
Superior Gluteal Nerve
- gluteus medius
- gluteus minimus
- Tensor fascia latae
- Piriformis entrapment
- Aching claudication-type buttock pain
- Tenderness to palpation
- A weakness of abduction and a waddling gait
- Pelvic pain with sitting except on toilet
- Discomfort with tight clothing
- Bladder/bowel symptoms
- Hesitancy, frequency, urgency, retention, constipation, pain
- Dyspareunia - recurring pain in the genital area or within the pelvis during sexual intercourse
- Genital pain
- Anal pain
- Condition is often misdiagnosed and mistreated
- Causes may be excessive sitting or cycling
Lymph Nodes in the Abdominal/Pelvis Arealymph nodes are in the abdominal area. When treating a patient with pelvic girdle dysfunction it is important to consider the vascular system and the lymphatics' role in inflammation. 
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