Mechanoresponsiveness 28

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The relation between equitation and science, is not about adapting science to actual equitation; it is about updating equitation to actual science. When the conversation with the horse is refined to subtle nuances in muscle tone, guiding the horse mental processing toward the body coordination optimally adapted to the athletic demand of the performance, the meaning of lightness, harmony, sobriety emphasized in classical literature reaches a new dimension. This dimension cannot be reached through literature alone and the application of the correct aids. Equine athletic performances are about physics; subtle orchestration of forces. Aids can no longer be regarded as magic gestures but instead, descriptions of specific muscular coordination. It is no longer the placement or movement of the rider legs that is important; it is instead, the nuances in the tone of muscles inside the legs. A recent study measuring the horse’s very high tactile perception in the area under the rider legs demonstrates that a horse is capable to perceive and understand subtle contact of the rider legs. Laziness, is in most instances, a defense mechanism against the rudeness of legs actions such as lifting the heels or kicking.   

In 1946, L. J. Slijper proposed the “bow and string” concept. “The horse has a very flat shaped bow which is made up of the vertebral column, its epaxial muscles and ligaments. The whole structure is kept rigid and under tension from the string formed by the sternum, abdominal muscles, linea alba and the muscles of the limbs.” (E. J. Slijper, 1946) The bow and string concept authorized the belief that flexion of the equine thoracolumbar spine can be mechanically created engaging the hind legs at one end and lowering the neck and holding the bit at the other end. It is the equitation of driving seat and deep neck. Even if refined; this equitation is primitive and does not permit the subtle dialogue that we are talking about. The lower line flexing the upper line, is unrelated to actual knowledge of equine biomechanics and limits the rider capacity to properly coordinate the horse physique for the demand. With the evolution of knowledge, the concept of the core flexing the back was pertinently questioned, as abdominal muscles (the string) don’t have the power to overcome the mass and work of the back muscles (the bow). The second objection was that abdominal muscles don’t have the capacity to orchestrate the large diversity of minute movements taking place in the thoracolumbar spine. “The amount of joint range of motion at any vertebral motion segment is small, but the cumulative vertebral movements can be considerable.” (Kevin K. Haussler, DVM, DC, PhD, 1999)

In 1964, Richard Tucker completed the first dynamics analysis of the equine thoracolumbar column. “An initial thrust on the column is translated into a series of predominantly vertical and horizontal forces which diminish progressively as they pass from one vertebrae to the next”. (Richard Tucker-1964). I was a progress from the simplistic bow and string concept. The study also suggested that instead of the rider’s legs and hands, a great part of the conversation with the horse occurred between the horse and the rider back. Tucker explained the creation of vertical forces through rotations of the vertebral bodies. In his study, Tucker refers to internal and external rotary systems where muscles pulling on the dorsal spines rotate the vertebral bodies inducing upward forces. Observations in the necropsy room questioned the thought as there is greater wear and tear on the attachment of the muscles on the articular facets of the vertebrae, indicating that the muscles pull on the articular facets more than on the dorsal spines. Also, the idea of vertebral rotations would create an overall motion of the thoracolumbar spine far greater than its possible range of motion. Milton Hildebrand had already suggested that, in motion, the equine thoracolumbar spine was relatively rigid, and only a small amplitude of movements was taking place.

The thought that the rider’s back could have a more sophisticated influence on the equine thoracolumbar spine was attractive. In regard of the large complexity but also limited amplitude of vertebral movements, the concept of a swinging and relaxed back was overly simplistic. Newcastle at the XVII century and numerous classical masters after him, promoted stable pelvis and sober movements.

 “A rider’s body should be divided in three parts, two of which are mobile and one which is not. The first of the two movable parts is the body down to the waist; the other is the leg from the knee downward. Therefore, the immovable part of the body is from the waist to the knees.” (Duke of Newcastle, 1592-1676)  Hans Carlson demonstrated in 1979 that the main function of the back muscles was resisting forces, protecting the thoracolumbar column from an amplitude of movement that would exceed the throracolumbar spine possible range of motion. “Electromyographic studies and movements data presented above strongly suggest that the primary function of the back muscles during walking is to control the stiffening of the back rather than to create movement.”  (Hans Carlson, Halbertsma J. and Zomlefer, M. !979, Control of the trunk during walking in the cat. Acta physiol. Scand. 105, 251-253)

Already, in 1979, the equitation of the loose relaxed and swinging back needed to be updated to actual scientific knowledge. The next year, Leo Jeffcott measured the possible range of motion of the equine thoracolumbar spine finding that in the dorso-ventral direction, the range of motion was only 53,1 mm, which is a little less than two inches and a quarter. While traditional equitation, as well as therapies, emphasized stretching, release and relaxation, actual science painted a mechanism based on supple resistance, storage and reuse of elastic strain energy.  The elastic energy stored in and recovered from tendons during cyclical locomotion can reduce the metabolic cost of locomotion (Cavagna et al.,1977)

Limbs create forces that are converted into forward movement and performances by the back muscles. The large amount of forces that our ancestors attributed to the horse vertebral column movements, are in fact the sum of forces developed by the limbs and the thoracolumbar spine. Traditional equitation did not distinguish the forces created by the limbs and the forces created by the thoracolumbar spine, advising riders to absorb the whole amplitude of movement through the flexibility of the rider’s vertebral column. The misconception lead to an equitation where the amplitude of the rider movements exceeds the possible range of motion of the horse thoracolumbar column. Harmony demands instead, reducing the sum of the movements induced in the rider vertebral column to match the limited motion of the horse thoracolumbar spine. It is an equitation of supple resistance, where there is no shift of the rider weight a reduced motion of the rider back and, paraphrasing Newcastle, an ”immovable” pelvis. Newcastle choice of the term “immovable” s particularly clever as there is no possible precise and subtle adjustment of the rider’s back and abdominal and pectoral muscles without a stable pelvis and the stability of the pelvis does not mean total absence of movement. The pelvis is stimulated by the movements of the horse thoracolumbar column and the oscillations are reduced to almost immobile, by the muscles surrounding the pelvis.

While in the late seventies, the concept of elastic strain energy concentrated on the long tendons of the lower legs, further understanding of the plasticity of muscles function demonstrated that muscles were capable of storing and using elastic strain energy even in the absence of tendons. “Apart from the role of tendons and collagen in energy storage, the muscle itself stores and recovers elastic strain energy, as elastic strain energy can occur in the absence of tendons.” The findings suggested that the sobriety emphasized by classical authors was not just the aim of superior equitation, but the perquisite of any sound equitation. Muscles are composed of muscle fibers and tendinous materials vibrating in response to excessive motion of the rider’s vertebral column and never the less, to any shift of the rider’s weight.

Updating equitation to actual knowledge demanded then considering the concept of the horse natural cadence. “In a sense, because the muscle is composed of both muscle fibers and tendinous materials, all of these structures must be collectively ‘tuned’ to the spring properties for the muscle-tendon system to store and recover elastic strain energy during locomotion.” The problem is that not only there is a cadence proper to each horse, but the horse natural cadence is not just one frequency, like a metronome. It is instead a collection of many different oscillations and vibrations; “A given multicellular organism (like a person) is a collection of many different oscillations and vibrations (Hertz is cycles per second, and our body operates on many different cycles per [time period]--heart rhythm, breathing, brain waves, blood pressure, etc), many of which are out of our control (and some of which you don't WANT to change... like the Q-T segment of a heartbeat!). (Leslie Ordal Clinical research manager in a neuroscience-based rehabilitation lab).

Besides being sober and carefully measured, rider adjustments need to be progressive and calm. “Muscle force can only increase and decrease gradually, muscles cannot be either ‘on’ or ‘off’ momentarily.” (Liduin S. Meerrshoek and Anton J. van den Bogert. Mechanical Analysis of locomotion, 2003) Upgraded to actual science, the equitation is about subtle adjustments of muscle tone, tuned to the horse frequency and aiming at guiding the horse brain toward the body coordination most efficiently adapted to the effort. Equines like humans, are composed of systems within systems, within systems. “The existence of discrete network within discrete networks in bones, cartilages, tendons and ligaments optimizes their structural efficiency as well as energy absorption.” (Christopher S. Chen and Donald E. Ingber. Tensegrity and mechanoregulation: from skeleton to cytoskeleton, 1999)

Many systems are not controlled by the rider but they are part of the horse soundness and the elegance of the performance. When the quietness, progressiveness and soberly of the rider suggestions is within the comfort zone of the horse perception, the horse further the rider suggestions coordinating the deeper systems for greater ease and effortlessness. The refinement that our ancestors regarded as subtle obedience, is in fact a partnership where the horse intelligence participates to the education in search of greater ease through greater efficiency.   

It is a delightful equitation

Jean Luc Cornille