Coupled Motion

• When standing in the neutral anatomic position and during gait:
• Anterior pelvic movement is countered by posterior sacral and upper lumbar movement, flattening the upper lumbar spine.
• The posterior pelvic movement is countered by anterior sacral movement, increasing the lumbar lordosis.
• Primary (pelvic) axial rotation is countered by contralateral sacral and upper lumbar rotation.
  • The upper lumbars rotate with the sacrum.
  • The lower lumbars rotate with the ilia.
• Lumbar lateral flexion is countered by contralateral primary axial rotation.
  • The upper lumbars rotate into the concavity.
  • The lower lumbars rotate into the convexity.

Coupling Principles
As far back as 1905, Lovett [1] studied coupled motion of the spine in cadavers and in a living model, as well as in two hundred children who exhibited lateral curves and rounded shoulders. He found that the vertebral bodies would rotate to the concavity of the curve in both the cervical and lumbar spine, but toward the convexity in the thoracic spine. He also found that each area had a dominant movement with regards to flexion, rotation, extension, or lateral flexion. From there, he determined that the starting position of movement (posture) determined the accompanying coupling motion in the lumbar spine. For example, he found that the lumbar spine rotates toward the side of lateral flexion (bodies toward the concavity) when the person is initially standing, or in extension, but towards the opposite side (bodies toward the convexity) when the person is initially in flexion. In other words, when starting in the neutral anatomic position, lateral bending causes rotation of the lumbar vertebral bodies to the same side, decreasing the lumbar lordosis on that side, demonstrating counternutation. He does not mention differences in rotation between the upper and lower lumbar vertebrae but he does mention that the upper and lower thoracic areas rotate and flex in opposing directions in a normal compound curve during movement (as in gait).

Bogduk [2]p90, reflecting on the complications in the understanding coupling, stated that “No reliable rules can be formulated to determine whether an individual exhibits abnormal ranges or directions of coupling in the lumbar spine…The presence of coupling indicates that certain processes must operate during axial rotation to produce inadvertent lateral flexion and vice versa. However, the details of these processes have not been determined. From first principles, they probably involve a combination of the way zygapophyseal joints move and are impacted during axial rotation or lateral flexion, the way in which discs are subjected to torsional strain and lateral shear, the action of gravity, the line of action of the muscles that produce either axial rotation or lateral flexion, the shape of the lumbar lordosis and the location of the moving segment within the lordotic curve.” However, he agreed with Lovett, Pearcy & Tibrewal, and Panjabi et al. that general patterns do exist.

So, to understand coupling, we have to understand a number of initiating factors, including posture, local muscle contraction, gravity, vertebral level, and direction of movement. As such, gait, which follows innate proprioceptive patterns, is the most normal physiological movement in which we can study coupling patterns for related muscular activity. Since we understand many of the initiating factors during gait, we can apply this knowledge to understanding the function of muscles such as the psoas and the quadratus lumborum, which are both pure lateral flexors that indirectly cause vertebral rotation through coupling actions. It should be noted that all of the processes of gait are not fully understood and are sometimes controversial. Perhaps, the application of the principles of nutation/counternutation will help clarify those controversies.

Gracovetsky & Farfan [3], using electromyography (EMG), found a pattern of trunk movement controlled by muscular actions during gait. Although their discussion is about gait, we can look at it as an example of an alternating nutation/counternutation pattern. In their example, left-sided muscles develop a counternutation pattern while, simultaneously, right-sided muscles develop a nutation pattern. They state: “As the left leg advances and the right leg is in extension, contraction of the lateral flexors forces the spine to flex to the left, as viewed from the back. The left facets engage and the spine flexes as it bends to the left thereby reducing lordosis. The coupled motion of the spine induces a clockwise torque, as viewed from above, as well as the reduction in lordosis. Hence, when viewed from above, L5 rotates clockwise with respect to L1. Therefore, the pelvis rotates clockwise, and the left hip moves forward, while the trunk, shoulders, and upper extremities move in the opposite direction. The counter-rotation of the shoulders is enhanced by the simultaneous action of the right pectoralis major, anterior deltoid, and anterior serratus, and the action of the left trapezius, posterior deltoid, and latissimus dorsi. The left shoulder moves backward, as the spine winds up and flexes to the left.” Reduction of the left lordosis is synchronous with the bodies of the upper lumbars moving into the concavity of the curve [1] as the upper lumbars rotate counterclockwise with respect to L5.

Upper vs. Lower Lumbar Spine
More specifically, Pearcy & Tibrewal [4] [2]p89-90 studied coupling patterns in the lumbar spine by having individuals either axially rotate or laterally flex their trunks (the primary movements). Primary axial rotation is described as the direction of pelvic movement. They found that “At the upper three levels, the axial rotation was accompanied by lateral bending in the opposite direction. That is, if the voluntary axial rotation was to the right, the accompanying lateral bend was to the left…and vice versa. At L4-5, some individuals exhibited lateral bending in the same direction as the axial rotations, and at L5-S1, if lateral bending occurred, it was always in the same direction as the axial rotation.” They continued “Although some degree of mechanical coupling may be present, it is more likely that the lordotic shapes of the lumbar spine, together with muscular control, are the two principal factors affecting the accompanying rotations.” Pearcy & Tibrewal’s study is consistent with Lovett’s [1] finding that the upper lumbar bodies rotate into the concavity of the lumbar curve, while L4-L5 may rotate either way or be neutral, and L5-S1 rotates toward the convexity of the lumbar curve. Because L4-L5 area is a transition area between the opposing upper and lower lumbar areas, it may rotate either way, which may be influenced by the shape and starting position of the lumbar curve. This transitional aspect of L4-L5, along with greater movement at that level, creates higher stresses than at other lumbar levels and maybe the reason that L4-L5 has the highest incidence of disc degeneration. However, due to its ties to the ilia through the iliolumbar ligaments, L4 may, more often, move with L5.

Panjabi et al. [5] confirmed some aspects of Lovett’s experiment with much greater precision as to the vertebral level and movement. He agreed with Pearcy & Tibrewal by stating that, in the upper lumbar vertebrae, lateral bending to one side produced spinal rotation to the same side and, at L5 to S1, lateral bending produced rotation to the contralateral side. Panjabi et al. added “We have shown that the spinal column exhibits coupled motions in a consistent manner. It seems that coupling is an inherent property of the lumbar spine, advocated by Lovett.” Further, he stated that “it is extremely important to define the posture, together with the intervertebral level, while presenting motion information at any intervertebral level.

In the textbook, Clinical Biomechanics of the Spine [6]p108-9, in seeming contrast to his own statements, above, and those of Pearcy & Tibrewal, and Lovett, who noted that the vertebral bodies rotate toward the concavity, Panjabi stated that “The direction of lateral bending coupled with axial rotation is such that the spinous processes point in the same direction as the lateral bending.” In support, he shows an illustration that demonstrates this movement at L5-S1. While this is true at the lumbosacral area, he also states that the upper three lumbar vertebrae rotate in the opposite direction. Thus, when considering the movement of individual lumbar vertebrae, Panjabi agrees with all of the above authors that the lower lumbar spine undergoes movement patterns opposite to the upper lumbar spine.

A Different Perspective: Spine and Pelvis Moving as a Unit
Yet, elsewhere in their textbook, [6]p104, White and Panjabi illustrate L3, representing the entire lumbar spine, rotating as a unit with the spinouses rotating into the convexity of lateral flexion. It appears that the authors are making conflicting statements, but, in actuality, they are expressing viewpoints of the same movement from different perspectives. These differing outlooks exemplify the confusion that exists in describing vertebral movements when not considering the principles of nutation and counternutation.

While other authors mentioned above [1, 3, 4, 7] measured the orientation of each vertebra relative to other lumbar vertebrae, in this instance, Panjabi measured the orientation of the lumbar vertebrae in lateral flexion relative to the entire pelvis and spine rotating as a unit.

As the spine laterally flexes to the right, the primary rotation of the pelvis is to the left. As the pelvis rotates to the left, it carries the lumbar spine with it and gives the impression that the entire lumbar spine is rotating to the left (spinouses right) with the pelvis. An x-ray view, taken at a moment in time, would indicate this gross movement of the spine and pelvis and not indicate the relative movements of the individual vertebrae; this forms the basis of the confusion about coupled motion.

Muscular Involvement
The lower quadratus lumborum fibers, attaching L4, and L5 to the ilium, are ligamentous in adults [8]. As such, L4 and L5 rotate and laterally flex with the ilium due to the non-elastic bond [3, 4]. Thus, while the spinouses of L1 to L3 rotate towards the convexity, the spinouses of L4 and L5 rotate towards the concavity. Other muscles also cause these counter-movements of the upper and lower lumbar vertebrae, including the psoas [2, 9], multifidus, and lumbar parts of the longissimus and iliocostalis [10].

Flexion and Extension of the Spine
In spinal flexion, as the body’s weight moves anteriorly with the innominates, they carry the sacrum and spine with them and may seem to be increasing the lumbar lordosis (lumbar extension). However, according to the principles of nutation/counternutation, the sacrum moves posteriorly relative to the ilia. As the pelvis moves anteriorly, the lumbar curve initially increases but, the more the pelvis moves anteriorly, the lumbar lordosis decreases [11]p148, resulting in straightening of the lumbar spine. In this manner, the center of gravity moves posteriorly, relative to the innominates, and assists in countering the forward weight shift.

In spinal extension, the lumbar lordosis increases as the sacrum nutates. As the bend is increased, the ilia rotate posteriorly to maintain structural balance.

This movement pattern demonstrates the inherent balance of nutation/counternutation in weight distribution by keeping the body’s mass centered. This is a key point in understanding spinal motion in the context of nutation and counternutation. Perhaps, the concept of nutation/counternutation patterning, in explaining posture and movement, will gain recognition in the future.

References

1. Lovett, R., The Mechanism of the Normal Spine and its Relation to Scoliosis. Boston Medical and Surgical Journal, 1905. CLIII(13): p. 349-358.
2. Bogduk, N., Clinical Anatomy of the Lumbar Spine and Sacrum. 2005: Elsevier Churchill Livingstone.
3.  Gracovetsky, S. and H. Farfan, The optimum spine. Spine, 1984. 11(6): p. 543-73.
4. Pearcy, M.J. and S.B. Tibrewal, Axial rotation and lateral bending in the normal lumbar spine measured by three-dimensional radiography. Spine, 1984. 9(6): p. 582-7.
5. Panjabi, M., et al., How does posture affect coupling in the lumbar spine? Spine, 1989. 14(9): p. 1002-11.
6. White, A. and M. Panjabi, Clinical Biomechanics of the Spine. 2nd ed. 1990, Philadelphia, PA: J.B. Lippincott Company.
7. Gracovetsky, S., An hypothesis for the role of the spine in human locomotion: a challenge to current thinking. Journal of Biomedical Engineering, 1985. 7(3): p. 205-16.
8.  Luk, K.D., H.C. Ho, and J.C. Leong, The iliolumbar ligament. A study of its anatomy, development and clinical significance. The Journal of Bone and Joint Surgery. British volume, 1986. 68(2): p. 197-200.
9. Santaguida, P.L. and S.M. McGill, The psoas major muscle: a three-dimensional geometric study. Journal of Biomechanics, 1995. 28(3): p. 339-45.
10. Adams, M.A., et al., The Biomechanics of Back Pain. 2002: Churchill Livingstone.
11. Junghanns, H., ed. Cinical Implications of Normal Biomechanical Stresses on Spinal Function. English ed., ed. H.J. Hager. 1990, Aspen Publications: Rockville, MD.