I recently read a great article on Jenni Rawlings’ awesome and informative blog titled “5 Things We Didn’t Learn About the SI Joint in Yoga Teacher Training”. In her post, Jenni details some misconceptions surrounding this joint. She reveals how the yoga community unnecessarily promotes the idea that the SIJ is unstable and requires active protection on the practitioner’s behalf. I would like to expand on her contribution by illuminating other misconceptions we have about the SIJ. In this article, I will uncover what the scientific literature has to say about SIJ movement and function. In discussing the function of this joint, I hope to reveal why characterizing SIJ movement is probably a lot less important than we may want to believe.
The Nature of the SIJ — An age old dilemma
The sacrum is the bony structure at the base of the spine wedged between the two sides of the ilium (i.e., upper part of the pelvis). The SIJ is aptly named as the joint borders the sacrum and the ilium hence “sacro-iliac”.
For centuries, a debate has been brewing about whether or not this joint actually articulates (i.e. moves) at all. Even Hippocrates (460-377 BCE) weighed in on the movement, or lack thereof, of the SIJ1. If you read my blog post on the psoas muscle, you might be like me wondering how a field of study as ancient and unchanging as anatomy can be so misunderstood! Today, studies continue to present difficulty finding common ground regarding the palpability of its movement, the nature of its range of motion, and our ability to detect these subtle movements using clinical tests2. For example, in my research for this blog post I found the SIJ describe as both a diarthrodial joint (i.e., freely moving joint), an amphiarthrodial joint (i.e., a joint with some free movement), and even a “diarthro-amphiarthrodial” joint (i.e., a joint with both characteristics of a freely moving and semi-immobile joint)1,3. Thus, even agreement regarding its basic joint type classification is non-existent.
SIJ Movement — Beyond Nutation and Counternutation
Any lay audience material would have you believe that SIJ mobility can be boiled down to two simple movements: nutation and counternutation. Nutation is the forward nodding of the sacrum relative to the ilia where the tailbone un-tucks while counternutation is the opposite. Sounds straightforward…right?
A quick glance of the scientific literature, however, reveals that nutation and counternutation are an oversimplified way of understanding sacral movement. The sacrum can move at the SIJ in at least three planes of direction1,4. SIJ movement in any direction is very small. While nutation and counternutation are the largest movements the sacrum experiences, the average individual can only nutate or counternutate their sacrum by 2° 5! As Jenni detailed in her blog post, the SIJ is an incredibly strong and stable joint. Its stability, and thus limited mobility, stems from an extensive ligamentous system surrounding the joint3 (see photo below). If movement of this joint beyond nutation and counternutation is so miniscule, it seems I am only labouring this point to be technically accurate. However, there is a method to my madness in characterizing SIJ motion. That is, I believe a more nuanced understanding of SIJ movement is helpful for understanding the function of this joint.
The SIJ helps transfer and dampen forces moving from the upper and lower body as they are transferred to the pelvis1. The SIJ therefore contributes to walking, postural control, and many functional whole-body movements. Unlike other joints, movement at the SIJ is the by-product of movement involving some other joint. The number of muscles thought to indirectly act on the SI joint is astronomical and is revealing of the joint’s complex nature. Muscles include (but are not limited to): multifidus, gluteus maximus and medius, quadratus lumborum, erector spinae, the hamstrings, lattisimus dorsi, the hips flexors (i.e., rectus femoris, iliacus, and sartorius), the internal obliques, and even the diaphragm1,6,7! Evidently, the sacrum responds to movement initiated by its neighbouring structures with minute position changes to maintain the stability of the entire lumbo-pelvic (i.e., low spine and hip) region.
SIJ Movement — The Myth of Accurate Palpation
Scanning yoga blogs and lay-audience material would have you believe that nutation and counternutation are predictable movements of the sacrum within the ilium. Under the microscope, however, studies find that SIJ motion is “strikingly inconsistent” in that a given movement may lead to nutation in some individuals and counternutation in others. For example, in a study looking at SIJ motion during forward flexion, nutation and counternutation occurred at the SIJ in different individuals5. This is a remarkable finding when you consider how other joints in the body are understood. Imagine if the knee joint was extended by the quadriceps in some individuals and flexed in others. Of course this is an impossible comparison given that sacral movement is not the direct result of a flexor/extensor (or nutator/counternutator) muscle. As we have seen, SIJ motion is not actively initiated but results as a response to muscular action acting outside the SIJ.
Because of these inconsistencies, it is unsurprising that studies have repeatedly shown how sacral palpation methods are unreliable and invalid2. Intertester reliability is low — in plain language, two people palpating the sacrum move will often reach different conclusions about the nature of SIJ movement. Thus, it is very unlikely that a clinician (or yoga teacher) has the ability to feel the sacrum move at the SIJ and determine the type of movement occurring. As one study states, “the results clearly show that movements in the SIJ […] are so minute that external detection by manual methods is virtually impossible”8. Moreover, as we previously saw, SIJ movements are “strikingly inconsistent” between individuals. Thus, not only are we unable to palpate the sacrum and determine is positioning, we cannot even predict how the sacrum will move in a given pose.
Of course, there is no question that it is possible to feel the sacrum move during asana. Herein lies the difficulty: the sacrum is easy to locate (at the base of the spine) and palpating its movement can result in a noticeable change in its position. However, feeling the sacrum move at the base of the spine is not equivalent to feeling the sacrum move at the SIJ. The sacrum is wedged between the two sides of the pelvis and thus movement of the pelvis necessarily moves the sacrum without necessarily affecting the SIJ. Take an easier joint to imagine: the wrist. I can move the position of my hand without articulating my wrist — all I have to do is move my arm! Perhaps this is where confusion about SIJ motion stems — sacral movement is easily mistaken for pelvic movement.
SIJ Movement — Just Let it Happen
While decades of research have been unable to characterize a model of SIJ movement in a comprehensive and agreeable way, I believe there is a take-home message for us yoga practitioners. I think that those who claims to understand their own (or their students) sacral movements are likely overestimating their understanding of this perplexing joint or possibly confusing their perceived sacral movement with movement of nearby structures (i.e., the lumbar spine or the pelvis). If you ever hear the cue to either “nutate” or “counternutate” your sacrum in a yoga class — take it with a grain of salt.
I believe the take home message lies in the function of the SIJ and the nature of how we move. We do not control specific muscles — we decide over movements. In other words, you don’t choose to flex your quadriceps to extend your knee. You decide to extend your knee and your brain creates a motor plan, which involves flexion of the quadriceps muscles. In the case of the SIJ, if your intention is to manipulate the position of the sacrum relative to the pelvis to achieve a desired sacral position, change your intention to match the natural function of this joint!
- Vleeming A, Schuenke MD, Masi AT, Carreiro JE, Danneels L, Willard FH. The sacroiliac joint: an overview of its anatomy, function and potential clinical implications. J Anat. 2012;221(6):537-567. doi:10.1111/j.1469-7580.2012.01564.x.
- Goode A, Hegedus EJ, Sizer P, et al. Three-dimensional movements of the sacroiliac joint: a systematic review of the literature and assessment of clinical utility. J Man Manip Ther. 2008;16(1):25-38. doi:10.1179/106698108790818639.
- Alderink GJ. The Sacroiliac Joint: Review of Anatomy, Mechanics, and Function. J Orthop Sport Phys Ther. 1991;13(2):71-84. doi:10.2519/jospt.19188.8.131.52.
- Sturesson B, Selvik G, Udén A. Movements of the sacroiliac joints. A roentgen stereophotogrammetric analysis. Spine (Phila Pa 1976). 1989;14(2):162-165. http://www.ncbi.nlm.nih.gov/pubmed/2922636. Accessed October 25, 2017.
- Jacob H, Kissling RO. The mobility of the sacroiliac joints in healthy volunteers between 20 and 50 years of age. Clin Biomech. 1995;10(7):352-361. doi:10.1016/0268-0033(95)00003-4.
- Vleeming A, Mooney V, Stoeckart R. Movement, Stability & Lumbopelvic Pain : Integration of Research and Therapy. Churchill Livingstone Elsevier; 2007. http://www.sciencedirect.com/science/book/9780443101786. Accessed October 22, 2017.
- Wilson P, Indahl A, Holm S. CHAPTER 7 – The sacroiliac joint: sensory–motor control and pain. In: Movement, Stability & Lumbopelvic Pain.; 2007:103-111. doi:10.1016/B978-044310178-6.50009-8.
- Sturesson B, Uden A, Vleeming A. A radiostereometric analysis of movements of the sacroiliac joints during the standing hip flexion test. Spine (Phila Pa 1976). 2000;25(3):364-368. doi:10.1097/00007632-200002010-00018.