The spine as a mechanical muscular gear

Human spinal mechanics is inextricably linked with the functioning of the muscular system. Its functional use is possible only thanks to the movement of a person on a solid surface under the influence of gravity. In the previous entry from 2023-06-30 titled Is Stuart McGill Method For Human Body Mechanics Correct? I started presenting my own interpretation of the functioning of the spine as a Mechanical Torso Muscle Transmission based on sets of levers regulating the muscle tension around the torso circumference. Today’s article is about the full mechanism of its operation. In the following entries, the way in which the gear regulates muscle tension in classic bending and sitting positions will be considered.

Mechanical Muscle Gear-Spinal Mechanics

The spine as a gear consists of 23 default segments functionally connected in a continuous manner, each of which is assigned to a separate spinal disc. It covers the muscular system consisting of over 300 trunk muscles, regulating their tension depending on the adopted body position. Human spinal mechanics regulates the work of muscles in such a way that under the influence of changes in the shape of the body, their tensions are ordered, and the vector sums of their forces balance the work performed in each assumed body position.

In the standing position (Figures 1), the Gear Segments in the frontal plane are adjustable horizontally, but they differ in adapting to the changing body circumference at different heights of the spine and its bends in the neutral position. Each segment is bounded by two default planes, Lower (1) and Upper (2) perpendicular to the Variable 3D Spine Axis, one of which passes through the lower half of the vertebra adjacent to the disc, and the other through the upper half. These planes, projected onto the sagittal plane, have a slope adapted to the course of the Variable 3D Spine Axis. In this way, they “cut out” the corresponding fragment of the trunk in the shape of an irregular cylinder. In figure 1, the shape of one of the gear segments is marked with a horizontal yellow stripe with a width of “h”. In the lateral position in Figure 3, it has the shape of a yellow wedge.. In the middle of these belts run the axes of the Gear Segment. Their intersection in the middle of the intervertebral disc defines the Main Plane of the Segment (Figure 2 – Cross-Section A-A). For better clarity, only the axes identifying the course of their main planes were marked at the height of the trunk. When assuming different flexion positions, the Mechanical Gear of the Spine changes its layout. Its Segments also adjust their shape to this change. This is done mainly through the mutual movement of the Lower and Upper Planes of the Gear Segments. Their mutual proximity means squeezing muscle fibers, and their distance – stretching. In the illustration, 3 individual Gear Segments have taken the form of wedges divided by color. They are blue in the compression zone and green in the tension zone.

F 1.pdf 5 7. The spine as a mechanical muscular gear-Spinal Mechanics
F 2 7. The spine as a mechanical muscular gear-Spinal Mechanics

Figure 1-4 (description)

  1. Variable Spine 3D Axis
  2. Intervertebral disc
  3. Spinal vertebra
  4. Gear segment axis
  5. Soft tissues of the abdomen (viscera)
  6. The group of muscles around the circumference of the trunk
  7. Muscle lever arm
  8. Single muscle fibers in balance (white)

a – axis of the Gear Segment

b – Lower Plane of the Segment

c – Top Plane of the Segment

h – segment height

N – Outer contour of the neutral zone of the muscles

X – linea mediana anterior

Y – linea median posterior

C – Compression zone

Fan mechanism of muscular transmission – sideways torso tilt

In the side bending position, the symmetrical system of muscle transmission undergoes a fan-shaped transformation (Figure 3), causing the muscles on the convex side of the torso to stretch (green) and the muscles on the concave side to be compressed (blue). In fact, the muscles around the entire torso form a kind of peripheral muscle spring. They wrap around the entire torso from the pelvis to the neck. The shape of this spring changes over this section. The coils of the spring (correspond to the gear segments) in the lumbar and thoracic parts are a flattened oval with a variable circumference, and at the neck level it significantly reduces its circumference and takes on a rounded shape. The variable 3D axis of a classic, properly functioning spring is always located in its neutral layer. The same applies to the 3D axis of the spine of a person with a normal body structure and bending according to correct movement patterns. This situation occurs regardless of the type of torso bending performed.

A flexible spine is neither a column nor a beam that carries loads. The spine is a mechanical transmission of the trunk muscles, which allows these muscles to transfer loads to the pelvis and via the lower limbs to the ground. These forces are always absorbed by the compressive layers of these muscles and properly balanced by the muscles on the stretched side.

Mechanical Muscle Gear Segment – Human spinal mechanics

Each segment is a functional transverse layer of the trunk corresponding to the height of a single intervertebral disc and two halves of the adjacent vertebrae, as well as fibers of the circumferential musculature ring of the trunk or neck, separated by the lower plane and the upper plane of the gear segment. Figures 1 show an example segment with a height of “h”, and in Figure 3, it is formed by green and blue wedges also marked with the letter “h”.

The top view of a single segment is shown in Figure 2: Cross Section A-A for the standing position and in Figure 4: Cross Section B-B for lateral flexion.

Each segment consists of a set of apparent levers created with the participation of the nervous system in such a way that they all run diagonally through the center of the intervertebral disc—they cross on the 3D axis of the spine—connecting the points corresponding to the muscle fibers on the right and left sides of the model’s body. For the purposes of the presented analysis, it can be assumed that the number of muscle fibers forming the levers of the segment corresponds to the distribution of points in red, by means of which the trunk muscles were graphically separated on its perimeter.

Due to the scale of the drawing, only a few randomly selected levers were marked, in which their white arms correspond to the neutral zone shown in Figures 1 —the standing position—and the green-blue arms of the lever corresponding to the stretching and compression zones seen in Figure 3—the side bend position. Human spinal mechanics is the active operation of the 23 Segments of the mechanical muscle gear, causing the muscles to stretch and compress when bending the body. Each of the gears functions by activating the full set of muscle levers belonging to it.

Shape and arrangement of segments of the trunk muscle transmission in the side bend position - Human Spinal Mechanics
Figure 3. Shape and arrangement of segments of the trunk muscle transmission in the side bend position – Human Spinal Mechanics
An example segment of the trunk muscle transmission for the side bend position (cross-section A-A at the lumbar level) - Human Spinal Mechanics
Figure 4. An example segment of the trunk muscle transmission for the side bend position (cross-section A-A at the lumbar level) – Human Spinal Mechanics

Figure 5-6 (description)

  1. Variable Spine 3D Axis
  2. Intervertebral disc
  3. Spinal vertebra
  4. Axis of the gear segment in the tension zone
  5. Gear segment axis in the compressed zone
  6. The group of muscles around the circumference of the trunk
  7. Part of the muscle lever arm in the compression zone
  8. Part of the muscle lever arm in the stretching zone
  9. Single muscle fibers in the compressed zone
  10. Single muscle fibers in the stretch zone

X – Y – inert layer

X – linea mediana anterior

Y – linea median posterior

C – Outer outline of the Muscle Compressed Zone

S – Outer outline of the Muscle Stretch Zone

Mechanical muscle Lever of The Segment

For an introductory description of how a single muscle lever works, see Figures 18-20 in the 2023-06-30 entry Is Stuart McGill Method For Human Body Mechanics Correct?

Figures 2 and 4 show a cross section of a gear segment. They are marked with randomly selected levers on the circumference of the trunk muscles. They are arranged in a characteristic diagonal way and connect oppositely arranged muscle fibers corresponding to each other, which, depending on the activity performed, are alternately stretched (expanded) and shortened (compressed). The levers perform alternating movements within the height range of the Gear Segment marked in the illustrations with the letter “h”.

Activation of the sets of levers occurs under the influence of gravitational lowering of one part of the torso with the participation of impulses coming from the nervous system. As a result, on the concave side of the bend, the muscles contract, and on the convex side, they stretch.

Conclusion

The spine acts as a Mechanical Muscle Gear and is located during the bending of the torso in its neutral zone separating the zone of compression of the trunk muscles from the stretching one. With the correct musculoskeletal structure and the use of appropriate movement patterns, it is not subject to loads, and even more so to destructive forces.

The correct human spinal mechanics can only work when the above conditions are met. The search for a miracle drug to relieve pain or guarantee happy lives without using the body properly is a utopia. In addition, muscle contraction and relaxation is the basic mechanism that dynamises metabolism and respiration at the cellular level. This mechanism significantly improves the regeneration of tissues and the entire body. 

Human Spinal Mechanics according to SM McGill and JP Callaghan

When writing about human spinal mechanics, I consider it necessary to comment on incorrect movement patterns leading to the degradation of the spine. One of the most damaging of these is the trunk flexion and hip-hinge lifting technique (Figure 5-7). It is disseminated by a wide range of therapists, trainers and doctors around the world. The great authority in this field is Stuart McGill, PhD, is a professor at the University of Waterloo at Waterloo, Ontario and his research team.

The hip hinge is the movement when the pelvis and upper body bend downwards driven from the hip joint. (1) Refer to Figures 5-7 for Hip Hinge Technique.

Deadlift using the "hip hinge" technique. An unnatural and unsafe technique that transforms the torso and spine into a cantilever beam - Human Spinal Mechanics.
Figure 5. Deadlift using the “hip hinge” technique. An unnatural and unsafe technique that transforms the torso and spine into a cantilever beam – Human Spinal Mechanics.
Unnatural forward bend using the hip hinge technique. A torso bending technique that disturbs the balance of the front and rear muscles of the torso. Variable 3D axis of the spine partially moved beyond the neutral layer of the torso - Human Spinal Mechanics.
Figure 6. Unnatural forward bend using the hip hinge technique. A torso bending technique that disturbs the balance of the front and rear muscles of the torso. Variable 3D axis of the spine partially moved beyond the neutral layer of the torso – Human Spinal Mechanics.
Independent learning of unnatural bending that limits body mobility using the "hip hinge" technique. This exercise should be performed by people with significant disabilities. Mostly after spinal fusion (surgical fusion using metal rods) -- Human Spinal Mechanics.
Figure 7. Independent learning of unnatural bending that limits body mobility using the “hip hinge” technique. This exercise should be performed by people with significant disabilities. Mostly after spinal fusion (surgical fusion using metal rods) — Human Spinal Mechanics.

The justification for the use of this technique was a study by SM McGill and JP Callaghan in 2001 entitled: Intervertebral disc herniation: studies on a porcine model exposed to highly repetitive flexion/extension motion with compressive force Its authors used for the study two adjacent vertebrae of the cervical spine of a pig (C3-C4) joined by a fibrous ring, with the intervertebral disc contained inside this structure. The purpose of the study was defined as follows:

[…] Purpose: To determine whether repetitive motion with low joint forces and flexion/extension moments consistently causes hernia in a non-degenerated, controlled motion segment of the porcine spine. […] (2)

The method consisted of clamping these discs in a mechanical press (Figures 8 and 9) and subjecting them to breaking forces as described below:

[…] Methods: Mobile cervical segments of pigs (C3-C4) were mounted in a custom fixture that applied axial compression loads with pure flexion/extension moments. Dynamic tests were carried out up to 86,400 bending cycles with a frequency of 1 Hz, while torques, angular rotations and axial strains were recorded during the test. […] (2)

Diagram of a compression press used to examine two porcine neck vertebrae connected by an intervertebral disc by JP Callaghan and SM McGill. (F-8)
Figure 8. Diagram of a compression press used to examine two porcine neck vertebrae connected by an intervertebral disc by JP Callaghan and SM McGill. (F-8)
A diagram illustrating in more detail the positioning and mounting of the test material shown in Figure 8 (area circled with a blue dashed line). (F-9)
Figure 9. A diagram illustrating in more detail the positioning and mounting of the test material shown in Figure 8 (area circled with a blue dashed line). (F-9)

In order to bring the technical side of the research closer, I have attached two illustrations showing the method of attaching the sample that has been tested. Figure 8 shows a diagram of the central part of the press, and to the right of it is a detailed fixation of two vertebrae of the spine with an intervertebral disc in the middle. This was the study of the mechanics of the spine, the result of which revolutionized movement patterns in the world.

As a result of a series of tests according to the described method, the following conclusions were formulated:

[…] Conclusions: The results support the notion that disc herniation may be more related to repetitive flexion extension movements than to applied joint compression, at least in younger, non-degenerated samples. Meaning. Although disc herniation is observed clinically, consistent reproduction of this injury in the laboratory has been elusive. The aim of this study was to investigate the biomechanical response and damage mechanics of spinal motion segments to highly repetitive, low-magnitude complex loads. […] (2)

These conclusions have been used by the authors of the research to formulate many controversial recommendations regarding human spinal mechanics. The most important is the recommendation to replace the intuitive ways of bending the body, which have been functioning for centuries, with the mechanism of the “hip hinge”.

Evaluation of a scientific study

The most serious mistake, in my opinion, is the treatment of the spine by scientists as a constructive element, giving rigidity to the torso and carrying loads. According to this concept, the spinal vertebrae under the influence of loads, mainly vertical forces, crush and destroy the intervertebral discs in many ways. Contrary to this perception of the role of the spine is one of SM McGill’s statements, in which he himself stated that the spine is flexible and subject to bending, so it is illogical to treat it as a rigid column or beam that can be loaded with vertical or bending forces.(3 )

Therefore, following the logic, in my opinion, such research and, above all, the formulation of controversial conclusions should have been abandoned.

I believe that no spine is suitable for such an examination. It is a set of small bones loosely connected by articular processes and united by elastic connective tissue. As a flexible, unstable structure separated from the muscular system, it is subjected to unnatural “adjustment” treatments. In the case of this study, the adjustment consisted in separating only this small section from the spine structure, so that it could be relatively easily fixed in a mechanical press and subjected to destructive forces in accordance with the research concept of scientists (Figure 8)

In fact, this is not a study of human spinal mechanics, but a study of the material strength of three small elements separated from a pig carcass. In no way does the applied research environment resemble the energy and mechanical system of a living human being. The examined tissues are devoid of active muscles around the entire torso and the spine itself, which have a protective effect on its structure. These muscles in a living organism operate under the influence of gravity and mitochondrial energy, which are intelligently coordinated with the skeletal system through the nervous system so that the tissues function optimally and are not destroyed.

In order to be in line with their beliefs that the spine is indeed a structural element in the human body, these scientists should use a similar research method for the spine shaped by the hinge bending technique. Such an examination should consist in the preparation of the spine dissected from the corpse and its rigid cantilever fixation (Figure 10). Like it is fixed between the bones of the pelvis. Then subjecting it to force “F”. The same weight that puts a load on the athlete’s body below the barbell lifter. Of course, it is known that such an experiment is impossible. The spine separated from the corpse will not only not bear any weight, but will collapse or bend under its own weight. However, it would be a real test of the mechanical capabilities of the spine.

Studying the parameters of the mechanics of a living organism on fragments of tissues isolated from corpses is not a good idea. Even worse is the formulation of conclusions based on such a study that lead to radical changes in the use of movement patterns by the human population.

Impact of bending with the hip hinge on the spine (human spinal mechanics) – analysis

Graphical separation of the spine as a cantilever beam wedged between the hip plates during a deadlift performed using the "hip hinge" technique. Its lumbar part is most exposed to destructive bending and shear forces - Human Spinal Mechanics.
Figure 10. Graphical separation of the spine as a cantilever beam wedged between the hip plates during a deadlift performed using the “hip hinge” technique. Its lumbar part is most exposed to destructive bending and shear forces – Human Spinal Mechanics.

SM McGill and JP Callaghan, introducing new body positions, did not carry out any geometric analysis or analysis of the distribution of forces acting on such a system. The human spinal mechanics should be considered considering the knowledge in the field of general mechanics supplemented with research on the living human body. In the next post, I will expand on this topic based on my own thoughts.

References

  1. Florian Michaud , * Manuel Pérez Soto , Urbano Lugrís i Javier Cuadrado,  Lower Back Injury Prevention and Sensitization of Hip Hinge with Neutral Spine Using Wearable Sensors during Lifting Exercises, ,[PubMed Central]
  2. J P Callaghan, S M McGill, Intervertebral disc herniation: studies on a porcine model exposed to highly repetitive flexion/extension motion with compressive force, 2001 Jan;16, [PubMed]
  3.  By : Dr. Stuart McGill, Why Everyone needs Core Training November 30, 2014, [Blog]

Figures

  • Figure 8 shows the method of attaching two pig vertebrae presented in the publication:J P Callaghan, S M McGill, Intervertebral disc herniation: studies on a porcine model exposed to highly repetitive flexion/extension motion with compressive force, 2001 Jan;16, [PubMed], Figures were used as polemical quotations (Fair Use).
  • Figure 10 is used by analogy to Figure 10 for a more detailed explanation of the testing method used by J P Callaghana, S M McGilla (References 2). I believe that interested people have the right to be made aware of an experiment that may be decisive for millions of people trying to take care of the health of their spine and body posture. For this purpose, I used an illustration from a study performed by: Olof Thoreson, Lars Ekström, Hans-Arne Hansson, Carl Todd, Wisam Witwit, Anna Swärd Aminoff, Pall Jonasson, Adad Baranto, The effect of repetitive flexion and extension fatigue loading on the young porcine lumbar spine, a feasibility study of MRI and histological analyses, Journal of Experimental Orthopaedics 4(1), May 2017, License CC BY 4.0

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