In Search of Excellence – Body Physics – The Spinal Engine
Clearly a part of training to be a high level competitive dancer is to become an outstanding athlete.
Training to develop as an athlete and dancer as well as coaching athletes and dancers requires a reasonable understanding of functional anatomy and biomechanics in order to prevent injuries, and optimize performance.
Often we see the precise usage and functionality of legs or the torso in its sub-components as in ribcage and pelvis or sides, and of course the usage of arms discussed. Rarely though is there deep consideration given to the complexity and actual functionality of the spine.
Yet a big part of the complex picture of human movement, if not the central part, involves the understanding of the functionality and purpose of the spine.
[section label=”Serge Gracovetsky” anchor=”theory”]
The Spinal Engine Theory
by Serge Gracovetsky
Serge Gracovetsky graduated from the Swiss Federal Institute of Technology in 1968 in nuclear physics and earned a PhD in Electrical Engineering from the University of British Columbia (Canada) in 1970.
He went on a tenured faculty at Concordia University in Montreal (Canada) for 27 years. His main interests varied from the control of paper machines, the analysis of the injury process experienced by military jet pilots during emergency ejection, the study of the human spine, the study of the reasoning process of physicians making a diagnosis for lower back pain and various other related (and unrelated) topics.
In addition, he founded and controlled four technological companies developing products in the field of measurement and the function of the spine. These companies exploited the concept of the spine as being the primary engine driving the pelvis during gait. The technique has been used on over 500,000 patients in many countries.
He holds 22 patents, has written a few dozen papers and some books and has presented at a few hundred conferences [clear] [section label=”The Standard Model” anchor=”locomotion”]
Human Locomotion – The Standard Model
Dr. Serge Gracovetsky offers with his “Spinal Engine” theory a compelling model that presents the holistic and integrated function of the whole body in human locomotion, versus older (but still omnipresent) theories, which attribute locomotion to be a function of the legs, and the spine being nothing but a weight bearing column.
In this “classic” representation of human gait the trunk is considered a passenger on moving legs. Hence it would not matter what the spine does.
If we applied that model to a champion level sprinter we must consider that it is vital for that runner to accelerate as quickly as possible from his starting position and attain his maximum speed in a minimum amount of time. Similarly competitive dancers are required to vary between extremely high and low speeds at almost an instance.
With that “classic” model in mind, where the legs do all the work in powering the body through space, Newton’s 2nd law would dictate that athletes like a sprinter or dancer with powerful legs and emaciated upper bodies should be the more superior.[clear]
But a quick glance at the starting field in either discipline shows that the best athletes have well proportioned muscular upper as well as lower bodies.The muscular power available in the upper body is somehow translated into movement thrust. How this power conversion takes place is one of the concerns of the “Spinal Engine” theory.
[blockquote]Big muscles run parallel to the spine[/blockquote] Broadly stated the problem of bipedal locomotion is centered on the need to rotate the pelvis in the horizontal plane. To do so would require a muscular system laid out in the horizontal plane. But the layout in the human anatomy is such that the majority of the muscles run parallel to the spine, like the erectores, or hip extensors and hamstrings.
Direct action of these muscles does not result in efficient pelvic rotation in the horizontal plane.
The mechanism by which such rotations are achieved is one of the core concepts of the spinal engine theory.[clear] [section label=”Spinal Sections” anchor=”sections”]
The spinal engine is comprised of three sections to achieve the dual purpose of locomotion and stabilization of the control platform (the head). Each section is akin to a specific curvature of the spine: lordosis for the lumbar spine, kyphosis for the thoracic spine, and lordosis of the cervical spine.[blockquote]The double S curvature is the key to the spinal engine[/blockquote]
Basically the spinal engine converts the “primitive” lateral bend of the fish into an axial torque which drives the pelvis (and the legs amplify), and at the same time stabilizes the head.[clear] [section label=”Coupled Motion” anchor=”coupled_motion”]
Coupled Motion – the gearbox of the Spinal Engine
The term coupled motion refers to the fact that a flexible rod, which is already bent in one plane, induces an axial torque if simultaneously flexed in a different plane.
The smallest functional unit of the spinal engine: The intervertebral joint
The intervertebral joint is the smallest functional unit of the spinal engine. Its purpose is the conversion of a lateral bend into an axial torque.
The spine is an assembly of these functional units grouped into three distinct sections. Each section achieves different objectives, yet shares many similarities.
A lateral bend of the lumbar spine to the left generates generally* a clockwise axial torque.
This arrangement would indeed permit rotation of the pelvis. And it would be tempting to imagine that the entire spinal engine would be made of a single block. Even though such an arrangement is feasible, it would suffer from several drawbacks:
1 – The head, neck and shoulders would then swing laterally with an increasing amplitude, as walking (weight shifting) speed is increased. This though is not what we observe.
2 – The head, which supports the primary visual and acceleration sensors could not be easily stabilized. However the need to stabilize the sensory platform to track a target is seen as essential to survival.
To overcome this problem we need a more sophisticated engine.
*This output is directly dictated by where inside the intervertebral joint the center of rotation is located relative to the facets of the joint, and can be controlled to reverse the output.
By adding a second similar building block, the design of the spinal engine can be improved.
There are two possible ways in which these two blocks can be combined:
1- By stacking them. This arrangement however results in one big lordosis and brings up the problems discussed prior.
2 – By inverting one, and then stacking them.
In this case the second section would have a reversed curvature to the first section (kyphosis).
To generate the same clockwise axial torque induced by the lumbar section bending to the left, the thoracic section must bend to the right.
With this arrangement the spine does not need to swing the shoulders laterally in order to induce axial torque. On the other hand the familiar S – curvature in the sagital (front/back) plane of the spine is required.
Although the lateral displacement of the base of the neck is reduced, the axial rotation of the shoulders is amplified. If the head were to be attached rigidly to the shoulder girdle it would also rotate.
In this case stabilizing the head means essentially de-rotating the motion of the shoulders, which can be achieved by adding a third building block of the previous two.
We can see in the illustration that implications of “incorrect” (left side) or “correct” (right side) stacking of the two building blocks:
As before there are two geometrical arrangements for stacking the third block on top of the two previous ones.
By stacking a block with lordosis, the lateral displacement of the head is reduced.
But to de-rotate the axial counter-clockwise rotation of the shoulders with a lateral bend to the left, requires the lordosis in the cervical spine to exhibit a coupled motion opposite to the one in the lumbar spine. And this is precisely what is the case.
The engine as described does not require any interaction with the earth’s gravitational field in order to function. As long as the trunk muscles are able to control both, the lumbar and cervical lordoses, together with the various lateral bends of the S-shaped spine, the pelvis will be rotated and the head held steady. However, this arrangement is not energy efficient and is limited to walking at low speeds.[section label=”Efficiency” anchor=”efficiency”]
Increasing the efficiency of the engine
A purely muscle driven engine without any energy storage and conversion system would use muscle power to both, accelerate and decelerate the body masses within their various directions which would present a highly inefficient energy usage.
However, if the system, once started, continues to oscillate until the energy losses of each cycle use up all the energy put into the system, and the energy losses of each cycle are small compared to the total energy stored in the system, the mechanical efficiency is high, and many cycles would occur till the systems ceases to oscillate.
As the spinal engine decelerates masses, their kinetic energy becomes available for storage and later use during the next re-acceleration. This elastic storage not only possible in the muscles themselves, but also in the spinal ligaments, such as the massive lumbodorsal fascia.[clear] [section label=”Power” anchor=”power”]
Increasing the power of the engine
This presented yet another interesting problem for evolution to solve. The muscles which can side bend the spine (and therefore axially rotate the pelvis) must compete for space with other vital organs in the abdominal content. Since the abdominal content prevented the back muscles from growing arbitrarily, the solution was to move the muscular power outside the abdominal content on to the legs. The hip extensors (gluteus maximus and upper hamstrings) form a large and powerful muscle mass, but they can not directly side-bend the spine.
As a result an indirect approach must be used. This requires the introduction of an additional step in the sequence of events relating muscular contraction to axial rotation of the spine. [clear] [section label=”Gravitational Field” anchor=”gravity”]
Exploiting the Earth’s gravitational field
When the hip extensors are activated, they lift the entire trunk upward. This effect is more obvious during running, when the runner “flies” through the air with each stride.
This raises the body’s center of gravity in the gravitational field, and stores the chemical energy liberated in potential form.
When the runner strikes the ground, he recovers that energy in kinetic form and transforms it into a heel strike pulse that travels up the leg, then pelvis and at last the spine.[clear] [section label=”Energy Converter” anchor=”converter”]
The spine is an energy converter
That pulse energy is distributed at all intervertebral joints. Each joint converts the energy it received into an axial torque that rotates and de-rotates the spine. Hence the intervertebral joint is not a shock absorber but an energy converter.
If the intervertebral joints were mere shock absorbers, then the energy would be dissipated in the form of heat. In addition it is well known that running on soft sand leads to rapid exhaustion since the hip extensor’s energy is dissipated and wasted in the sand, instead of being recuperated by the spinal machinery.[section label=”Dance” anchor=”dance”]
Sport and Dance exploit the coupled motion.
The coupled motion is fundamental to appreciating spinal motion. The various sports and movement disciplines, including dance use it. It can be argued that proper coaching is in part the teaching of proper use of the power delivered by the coupled motion of the spine, with the extremities amplifying the basic movement of the spine.
An important consideration in movement analysis from the spinal engine perspective is that the coupled motion is reversible. That means a combination of axial rotation and lateral bending yields the flexion or lordosis.
For example a combination of lordosis and lateral bending generate axial rotation.
Baseball exploits the ability of the coupled motion to generate axial rotation.
The spine cannot rotate without flexing. The spinal engine theory requires that the lumbar spine is flexed laterally if and axial torque is to be induced. The large torque reqired to propel the baseball demands such a large amount of lateral bending that the pelvis must be rocked as well. It is for that requirement that the pitcher lifts his left leg before the throw.
The power to throw the ball will be delivered as the pitcher changes his weight to the left foot, literally dropping all his body weight onto the lumbar spine as it beds sideways.
This lateral flexion combined with the presence of significant lordosis, induces an axial torque, driving the shoulder girdle into a counter clockwise rotation.
Therefore it can be seen that the possible options available to sports that need power are limited to three. Each one exploits one feature of the coupled motion. They are lordosis, lateral bending, axial rotation. Once any two of these are combined, the result is always the third:
Lateral bend + Axial rotation = Lordosis
Lateral bend + Lordosis = Axial rotation
Axial rotation + Lordosis = Lateral bending