In a tensegrity structure, bones compose the compression units, while the soft tissues, such as the muscles and connective tissues, including ligaments, compose the tension units. However, the interplay between these tissues in maintaining joint tension has not been fully identified. Therefore, I propose that the sacrum is alternately suspended by ligaments and muscles as they share responsibility for maintaining joint tension.

For a joint to move smoothly, the muscles on one side must undergo concentric contraction while their antagonists on the opposite side undergo eccentric contraction; this is true whether it is flexion/extension of the elbow, right/left lateral flexion of the spine, or nutation/counternutation of the sacrum. I am simply extending the role of agonist/antagonist muscle relationships to include their controlling influence, the ligaments, as described in the arthrokinetic reflex [1] and ligamentomuscular reflex [2]. What I am adding is how this mechanism maintains tension in the joint due to the reciprocating roles of the muscles and ligaments, both on the same side, and on the opposite side, which is a small but significant step in conceiving how tensegrity applies to the musculoskeletal system.

The key to preserving integrity in our musculoskeletal system is maintaining balanced tension. In the sacroiliac joint, the sacrum is suspended by ligaments and muscles which share responsibility for maintaining tension in our structural core. In my model, during nutation, ligaments that restrict nutation become tight and those that restrict counternutation become lax. When the ligaments become lax, the supporting muscles tighten.

For example, as the sacrum goes into right nutation, the sacral apex moves away from the right ischial spine, the right sacrospinous ligament is tensed, and the right piriformis eases and lengthens via eccentric contraction.

On the left side, the sacrum goes into counternutation.  As the apex moves toward the left ischial spine, the left sacrospinous ligament becomes lax, and the left piriformis undergoes concentric contraction to take up the slack and maintain tension in the joint.

Keeping with the principles of tensegrity, continuous tension is maintained by the alternating roles of the ligaments and muscles during the reciprocating phases of nutation and counternutation.

Consequently, if the sacrospinous ligament sprains, the ipsilateral piriformis is available to assume greater responsibility for stability by maintaining tension throughout both the nutation and counternutation phases. I propose that the greater the degree of sprain, the greater the muscular response. These muscles will be in a continuous state of concentric contraction as long as the ligament is insufficient. At this point, the ligaments have exceeded their elastic limit and, due to poor blood supply, do not have the intrinsic ability to return to their normal length, so the joint may remain hypermobile and degenerate indefinitely [3].

This relationship between the sacrospinous ligament and the piriformis serves as just an example of the far-reaching effects of a nutation lesion. Since all the nutation ligaments sprain as a unit, all of the counternutation muscles will develop a constant state of tension as a unit.

In this model, as a response to a right sacroiliac nutation lesion, the sacrum is pulled into some degree of counternutation on the right, indefinitely. The effect can be seen distant to the sacroiliac joint and even on the opposite side. The pelvis will torque, the spine will twist, the extremities will rotate, and joints will move in asymmetrical patterns, etc. The structural alterations that develop eventually spread throughout the musculoskeletal system and forms the basis for many chronic dysfunctions and injuries, including those to the foot, knee, hip, spine, intervertebral discs, shoulders and, probably, the neck and cranium. Time and uneven wear produce chronic dysfunction. Please see The Sacroiliac Nutation Lesion, Spinal-Pelvic Syndromes and Associated Syndromes.

The Serola Sacroiliac Belt placed in the correct position, with the correct amount of tension, helps normalize motion and remove stress from the ligaments, nerves, and other structures by keeping the sacroiliac joint within normal range of motion. It takes the place of the injured ligaments and allows the muscles to return to normal function.

The Serola Sacroiliac Belt is the only belt designed with the principles of tensegrity in mind; to properly normalize sacroiliac joint function, reduce stress to the ligaments, and allow the muscles to return to a healthy balance. Please see The Serola Sacroiliac Belt for more information.

References:

1. Cohen, L.A. and M.L. Cohen, Arthrokinetic reflex of the knee. The American Journal of Physiology, 1956. 184(2): p. 433-7.
2. Palmer, I., Pathophysiology of the medial ligament of the knee joint. Acta Chirurgica Scandinavica, 1958. 115(4): p. 312-8.
3. Kennedy, J.C., I.J. Alexander, and K.C. Hayes, Nerve supply of the human knee and its functional importance. The American Journal of Sports Medicine, 1982. 10(6): p. 329-35.