This is a very complicated question. If you are deliberately trying to stress one sacroiliac joint via a Yin Yoga pose, I would probably recommend postures like Reclining Twist or Swan. This may help to increase mobility in that SIJ. However, how much is too much? Only the student can determine that. Maybe start with shorter holds and see how it feels over the next day or so.
To understand the complexity of your question, I am posting below an extract from Your Spine, Your Yoga. Unfortunately, I can't include the figures or footnotes, so for those, you will need to refer to the book (starting on page 85). You will find the answer to your question on how to move the pelvis and SIJ at the same time. I hope this helps.
STRESSING AND SUPPORTING THE SACROILIAC JOINTS IN YOGA POSTURES
We can stress a joint at two times: when it’s under a load, and when it’s relaxed. When under a load, we want to stiffen a joint to protect it, and this applies to our SI joints as well. When a joint is not under load, that’s when we normally work to mobilize it and perhaps increase its range of motion. But does this apply to the SI joints? In general, no—there is no reason to try to increase the range of mobility for already healthy SI joints. But some people with certain pathologies may need to increase mobility. In either case, and even if we don’t want to mobilize the SI joints further, we still need to stress them from time to time, to maintain their health (if we never stress a joint it will atrophy—be aware that babying a joint out of fear of overstressing it may lead to problems in that joint).
There are two main ways to create stress in the SI joints: (1) gravity can create a stress in the SI joints, depending upon our orientation to it; and (2) certain movements may also create a stress across the SI joints. Whenever we are sitting or standing, the SI joints are being stressed, simply through the weight of the upper body pressing onto the sacrum, which in turn is being supported by the ilia. Only when we are lying down does no stress occur in the SI joints. When we move the spine, we tend to move the sacrum, which may create stress across the SI joints; this may or may not result in movement of the pelvis. The more the pelvis is fixed and the spine moves, the greater the stress in the SI joints. Alternately, when we move the pelvis against a fixed spine, we also stress the SI joints.
A study done in 1955 illustrates the way gravity can create movement of the sacrum. Th e researcher measured the position of the pelvis and sacrum of healthy individuals in a variety of postures (standing, lying down on the belly and on the back, with the spine and hips in neutral and in exion and extension). Table 3.104 summarizes the findings using yoga names for the positions examined, and gives the percentage of the subjects who exhibited particular sacral movements. The positions, with some poetic yogic license and adaptations, are shown in figure 3.124. One generalized finding was that when we are standing, the sacrum has usually moved to its nutation limit, and further nutation from there is not possible; conversely, when we are lying down, the sacrum has usually moved to its counternutation limit and further counternutation from there is not possible. This is important to understand because, for example, when we are standing, the sacrum cannot move more into nutation, it can only counternutate from there, and when we’re lying down, it can only move toward nutation because it is already counternutated. However, this does not mean that it will move in those directions; it may stay in its initial state.
In general, the study found that the sacrum tends to move into counternutation when the spine is extending and into nutation when the spine flexes. (Again, we find that the sacrum follows the spine.) There is an even greater likelihood of nutation if both the spine flexes and the hips flex. There were no differences in sacral movement between lying on the back, with neutral, extended or flexed hips, or when doing these movements while lying on the stomach; a Supported Bridge Pose (Setubandhasana) moved the sacrum in the same way as Cobra Pose (Bhujangasana) does. There were no gender differences in the direction of movement or the amount of movement, with the exception that women who had very recently given birth had a greater amount of movement than the other participants. One final important point to note: not everyone showed the same direction of movement. There is human variation even here. For example, while most people tend to nutate their sacrum when standing, not everyone does, and a small percentage of people actually counternutate. So, again and again, we see the “Flaw of averages,” and we should not expect everyone to be average. The participants in this study were healthy; so, although rare, counternutation while standing may not be a pathology or something everyone who experiences it needs to x.
From this study we can easily see why so much confusion and so many discrepancies arise between views on whether the sacrum is nutating or counternutating in various yoga posture: it all depends! At best we can say that there is a tendency in one direction for most people, but this doesn’t apply to every body.
Symmetric and asymmetric stress
The stresses we have been discussing may be applied equally across both joints (a symmetric stress) or unequally (an asymmetric stress). We produce asymmetric stresses to the SI joints quite frequently in our yoga practice. For example, consider the Pigeon Pose (Rajakapotasana) as shown in figure 3.125. In (a), we see a side view of the upright Pigeon, while in (b), we have an aerial view. In (a), the hands are on the floor, which takes much of the weight off the hips, reducing the stress on the SI joints. There are versions of the Pigeon in which the hands are off the floor (for example, when the hands are clasped behind the back), which put more stress on the sacral and hip joints, and versions where the upper body is folded completely to the floor (the Sleeping Pigeon) and there is very little stress in the joints. Notice in (a) that the back leg has created extension in that hip joint, while the front leg has created flexion in that hip joint. Flexion will tend to create nutation of the SI joints, while extension will tend to create counternutation. The stresses are not symmetric: counternutation loosens the joints, and if they are bearing a large load, the joints could become unstable. So the risk in the Pigeon Pose for someone with unstable SI joints is to the back leg’s joint. There may be too much stress there, and the joint may not be stiff enough to handle the stress. Keeping the hands on the floor will reduce the weight (stress) in that hip. If the hands are off the floor, co-contraction of the muscles around the SI joint is advised to stabilize it.
We can find symmetric or asymmetric stresses occurring in any orientation: standing, sitting, lying down, etc. In standing, when the legs are symmetrically positioned, such as in Mountain Pose (Tadasana), there is usually a symmetric stress on both SI joints, simply due to the weight of the upper body resting on the sacrum. If we bend forward from here into Forward Fold (Uttanasana—see figure 3.126a), the sacrum will tend to nutate symmetrically at both SI joints. Similarly, if we do a standing backbend, keeping the legs symmetrically positioned in the frontal plane, we will symmetrically stress the SI joints, leading toward counternutation of the sacrum. However, if we move one leg forward and the other backward and we fold forward, as we do in Pyramid Pose (Parsvottanasana—see figure 3.126b), there is an asymmetric stress across the SI joints. The front hip is more deeply flexed, stressing the hamstrings more in most people, which will tend to posteriorly rotate that ilia backward relative to the sacrum; this is nutation. The back hip is not as deeply flexed, so there is less stress from that hamstring, and thus less posterior rotation of that ilia; this still creates nutation, as the spine is deeply flexed, but it may be less nutation than in the front leg. Depending upon the student and her unique biology, some teachers believe that the back SI joint may actually experience counternutation. That may be the case for some students, but as the back hip is still in flexion, as is the spine, I believe the most common tendency will still be toward nutation.
Asymmetric stresses also occur at the SI joints when we add asymmetric movements to the spine (twists and side bends) or when we move one hip differently than the other. Consider Triangle Pose (Trikonasana) as shown in figure 3.127: the front foot’s hip is abducted and externally rotated, while the back foot’s hip is adducted and neutral or slightly internally rotated. If there is no flexion or extension of the spine in this posture, there is no reason for the sacrum to be either nutated or counternutated, even though it may experience a side tilt (this is the version shown in figure 3.127a). However, that is not how most students do this pose; for them, the hips are slightly flexed, and the spine is slightly side flexed, forward flexed and rotated as well. These positions at the hips and spine will create different stress pat- terns at the SI joints. While all researchers would agree that the back and front SI joints are stressed differently in Triangle Pose, which movement is happening at each joint is debatable! It depends upon which student is doing the pose and how. If the spine is slightly extended (a mild backbend), such as occurs in the Iyengar-style Triangle Pose, then the front leg’s SI joint might be slightly counternutated. But if the student has flexed the spine a lot (which most beginners do), the SI joint may be nutated. In addition, if the spine is laterally flexed as well, the sacrum at the front SI joint may also be rotated toward the back leg, but since the spine is also rotated toward the front foot, the SI joint may be rotated in that direction as well. Do these two rotations cancel each other out? Who knows? It’s complicated and variable.
In the aerial view of the Pigeon Pose in figure 3.125b, we can see another asymmetry: the line of the shoulders is parallel to the front of the yoga mat, but the line of the hips is angled away from the front of the mat. Very few yogis can “square” their hips perfectly in this posture (“square” means that the line of the pelvis is parallel to the line of the shoulders); most people will have to twist the spine to keep the shoulders square to front of the mat. is creates an additional asymmetric stress in the SI joints; the hips are fixed, so as the spine twists, the sacrum will try to rotate against the fixed ilia. The forward SI joint experiences a backward stress (the sacrum is trying to turn backward against the fixed ilium), while the back joint experiences a forward stress (the sacrum is trying to turn forward against the fixed ilium)—again, asymmetric stresses.
These stresses can potentially arise in all forms of twists, but there are different ways to create twists: we can move the shoulders and keep the pelvis still; we can move the pelvis and keep the shoulders still; or we can do a little of both. In the Pigeon Pose, the hips are pinned by the weight of the body, and the shoulders are free to move. This is also the case in Seated Twists such as Ardhamatsyendrasana (see figure 3.128). However, we can choose to pin the shoulders and move the hips, such as in some versions of the Reclining Twist (Jatharaparivartanasana). In general, because straight legs are longer and heavier levers than the arms, pinning the shoulders/arms and moving the hips/legs can generate more twisting torque along the spine and into the SI joints than pinning the hips/legs and moving the shoulders/arms. Figure 3.129 shows two variations of the Reclining Twist: (a) has the hips pinned and movement occurring from the shoulders (less leverage), while (b) shows the shoulders pinned and movement occurring from the hips (more leverage). In these cases, the leverage of the legs is reduced due to the knee being bent (which is why this is an easier or beginner’s option); if the leg were straight, more torque would be generated. It is possible, of course, to do the posture with neither hips nor shoulders pinned (c) and move them both, in which case the stress along the spine may be more equal throughout.
Twists in yoga and their the effect on the sacroiliac joints
Most often, the twists we perform in yoga asana practice fix the legs and pelvis and torque the spine by moving the arms, shoulders or upper torso. If you consider the standing twists, such as Revolved Triangle (Parivrttatrikonasana), twist while in wide leg forward fold (Prasaritapadottanasana) or even in Warrior 1 (Virabhadrasana A), or consider most seated twists, such as Ardhamatsyendrasana, or Marichyasana C and D, the hips and legs are pinned and movement is generated from the upper body. Since the sacrum is part of the spine, it will try to turn with the spine against the pinned pelvis, creating the tendency toward nutation in one SI joint and counternutation in the other. The opposite occurs in a few postures, such as one version of reclining twist (Jatharaparivartanasana), where the shoulders are pinned and the pelvis turns, but there are fewer of these postures. Generally, twists start with the shoulders or torso, which means that the sacrum tries to turn against a xed pelvis, rather than the pelvis turning against a fixed spine.
The distinction can be important! Depending upon which strategy you choose to come into the twist (hips pinned or shoulders pinned), you may increase or decrease the amount of stress experienced in the SI joints. To help us understand how this works, it is useful to visualize a spring with a bar attached to the top and bottom, as shown in figure 3.130. (As with all analogies, please take this one with a few grains of salt; it is meant to illustrate a principle rather than make a definitive statement of how twists work. For example, it ignores the role of the muscles and fascia below the shoulders, which affect the torque along the spine, and we do not initiate every twist by turning the shoulders; we may involve the lower torso as well.)
As we rotate the top bar, twisting stress (known as torque) works its way down the spring. The more we twist the top bar, the lower down the spine the torque goes. In (c), we see the same effect but from the bottom working upward.
Just because we twist one end of the spring does not mean that this torque will be felt everywhere along the spine equally. How much the torque travels down the spring depends upon the stiffness of the spring; if it were extremely stiff , like a metal rod, the torque would be transmitted completely and instantly from the top to the bottom (imagine twisting a broomstick—the top and bottom move together). However, if the stiffness were very low (as shown in b and d), almost no torque would be transmitted from the top to the bottom. (Visualize twisting a string connected to two bars; the bottom bar would not feel anything until you had twisted the top bar in circles many times.)
If your spine were a stiff, straight rod, the torque of the twist would be felt equally all along the length of the spine, but your spine is not a stiff , one-piece rod. To use an overly simplified analogy, it is like a combination of cogs (vertebrae) and springs (muscles, ligaments and fascia) aligned in series. A torque applied at one end of the spine will move the “cog” at that end first, until it either reaches its elastic limit to twisting (tension) or compresses into the next cog (which tends to happen at the facets); at that point, the next cog will begin to rotate until it reaches its limit and begins to apply a torque to the next cog. In this way, the twist starts at the end where the initial torque is applied and will work its way along the spine. Understanding this, we can address an important question:
Which twisting technique places the least amount of stress in the sacroiliac joints?
The answer is: “Postures that pin the hips and move the shoulders, like a Seated Twist or Pigeon, generate less stress in the SI joints than a Reclining Twist, where the shoulders are pinned and the pelvis moves.” More generally stated,
When a twist is initiated by movement of the upper body, there is generally less stress in the SI joints than with a twist initiated by movement of the pelvis or lower body.
This may be an important consideration for a yoga student with instability at the SI joints. If we begin to move the pelvis first, keeping the shoulders pinned, as in the Reclining Twist shown in figure 3.129b, the ilia will move first and whatever slack is present in the SI joint will be quickly taken up; the ilia will compress into the sacrum. This will cause the sacrum to begin to move before any other segment of the spine. As the sacrum continues to move, it will quickly apply a torque to the facets of the lowest vertebrae (L5 for most people). If stress continues to be applied, the L5 superior facets will compress against the L4 inferior facets, and so on up the spine. Depending upon how much torque is applied by the turning of the pelvis, the upper spine may not experience any torsion at all; the torque may peter out before it reaches the shoulders. You have probably experienced this. When you are in a Reclining Twist with shoulders pinned, then start to slowly rotate the hips, where do you feel the stress first and most? The majority of people will feel the stress initially and more strongly in the lower spine. Sometimes a nice “pop” or “crack” will happen too, felt in the sacral area, indicating a release of fixation in the SI joint. (See the sidebar “It’s Complicated: Snaps, cracks and pops—noisy sacrum” to understand why we hear cracks and other sounds.)
Contrast this with what happens in figure 3.129a, or in a Seated Twist, as show in figure 3.128, or in the Pigeon Pose. Here the hips are pinned and the shoulders and upper body move; when we rotate the shoulders, the torque is initiated in the upper thoracic spine and works its way down toward the sacrum, through the SI joints and into the ilia. Depending upon how much torque is applied by the upper body, there may not be any torque applied to the SI joints. You have probably experienced this too; when you are in a Seated Twist and begin to slowly turn the upper body, you probably feel the stresses mostly in the upper back and ribcage before feeling anything in the lower spine.
Of course, we can come into a twist by initiating the torque from both the shoulders and the hips at the same time! This happens in a reclining twist (see gure 3.129c) when we let both the knees and the shoulders float off the floor. Th effect of this twist, which is illustrated in figure 3.130e, is to have the most torque at both ends of the spine, with less felt in the middle.
Since human variations will and do occur, some students have relatively stiff spines, and the stress from moving the shoulders will travel quite quickly down into the SI joints. Other students, those with very flexible spines, may never feel anything in the SI joints. All we can o er are general guidelines; you will need to find out how stresses from twists are distributed in your body.
Remember, stressing the joints is not bad per se. All joints need stress to stay healthy and prevent fragility, and that includes the SI joints. But if a student does suffer instability in the SI joints, she may find that a Seated Twist is better for her than a Reclining Twist with the shoulders pinned. On the other hand, she may also choose to do a Reclining Twist where the hips are pinned and the shoulders are floating, because in this pose, the torque is again generated by the upper body. Some yoga teachers advise students to take care while in Seated Twists, fearing too much stress in the SI joints. Th is may or may not be a valid concern (see the sidebar “It’s Complicated: Changing the alignment of your hips before twisting”); it depends upon the student and her unique situation, but if minimizing stress at the SI joints is a concern, a Seated Twist may be better than twists where movement is initiated by the pelvis.