How Does a Healthy SI Joint Function?
When the SI joint is functioning properly, there is effective transfer of force and load across the SI joint from the lower extremities (legs) to the spine.28
A Properly Functioning SI Joint
There is a limited amount of motion in an SI joint which is functioning properly. The motion that does occur at the sacroiliac joint occurs in multiple planes. The sacrum translates (glides between the two ilea), the sacrum nutates (rocks forward and backward), and also pivots on a diagonal axis all at the same time. The motions are very small with the gliding motion being about 1.6 millimeters and with 1-2 degrees of sacral rocking motion in males, and 3-4 degrees of sacral rocking motion in females.27
The Three Components of a Healthy SI Joint
There are three components necessary for optimal SI joint function. Each of these components are described below:
- Form closure: passive forces
- Force closure: dynamic forces
- Motor control: the coordination between the nervous system and the muscles that stabilize the lower back and pelvic region29
Form Closure is the passive SI joint stabilization that is imparted by the shape of the bones comprising the joint and the capsule and ligaments supporting the joint.30,31,32 For example the irregular joint surface with its ridges and grooves and the wedged shape of the sacrum itself limit motion and provide passive stability to the joint. The other major contributors to form closure are the joint capsule and the multiple strong thick ligaments (dense fibrous tissue that joins two bones together). These ligaments support both the front and the back of the SI joint.
Force Closure is the dynamic stabilizing pressure that actively compresses the SI joints. Force closure is generated by contraction of the stabilizing muscles and their attachments to the bones and ligaments of the SI joint.30,31,32
The primary group of muscles that provide force closure to the SI joints are the core muscles or the local stabilizing muscles. Three of the four muscle groups making up the core muscles attach directly to the sacrum and/or ilium. These muscles are the transversus abdominus in the front, the multifidus or erector spinae in the back and the pelvic floor muscles at the bottom.
The diaphragm is the fourth of the core muscles and functions essentially as the lid of the force barrel that stabilizes the pelvis and both SI joints. The core muscles contract before motion of the spine or extremities.
There is a secondary group of muscles that selectively stabilize each of the SI joints. These muscles and their attachments are described as slings. These muscle groups cross the midline and also cross the SI joint from above to below.
An example of a muscular sling would be the left latissimus and the right gluteus maximus muscles stabilizing the right SI joint. Multiple muscular slings stabilize each SI joint. These muscle slings contract during motion of the spine or lower extremities. Proper tension, strength, mobility and balance in these muscles and integrity of their attachments are necessary for adequate force closure of the SI joints.
Motor Control is the process by which people use their nervous system to activate and coordinate the muscle firing and limb movement involved in the performance of a motor skill.28,33,34 It is the integration of sensory information, both about the world and the current state of the body, to determine the appropriate set of muscle forces and joint activations to generate some desired movement or action. This process requires cooperative interaction between the nervous system and the musculoskeletal system.35,36
With regard to the SI joint, the core muscles should prepare the system for the impending load or action by contracting before a load reaches the low back and pelvis. When someone catches a ball, if they have good motor control, the core muscles contract automatically to stabilize the low back and pelvis including the SI joint as the ball is caught. Before they throw the ball to another person, the core muscles will contract to stabilize the pelvis and SI joint automatically.