“In horses, and most other mammalian quadrupeds, 57% of the vertical impulse is applied through the thoracic limbs, and only 43% through the hind limbs.” (H. W. Merkens, H. C. Schamhardt,G. J. van Osch, A. J. van den Bogert, 1993).

 

Equestrian education states that as soon as ground contact, the alighting hind leg is propelling the horse’s body upward and forward. Measurements suggest otherwise. In reality, during the first half of the stance, which is the sequence of the stride where the supporting hind leg is exerting pressure on the ground, the hind leg on support is decelerating the horse’s body. Gravity is pulling the horse’s body down, inertia is pushing the body forward and the supporting hind leg absorbs and resists both forces flexing the joints. This sequence of the stride is referred to as the braking phase and last the initial 45% of the stance. At the contrary of previous beliefs, the supporting hind leg does not enhance balance propelling the horse’s body upward but instead, resisting attraction of gravity and inertia forces during the decelerating (braking) phase.

The concept is easier to understand at the piaff where the supporting hind leg is exerting a considerable braking activity resisting forward displacement of the body over the forelegs. “The hind legs have a considerable braking activity to avoid forward movement of the body over the forelegs.(…) The  forelimbs have a larger propulsive activity.” (1)

Activating the hind legs with a dressage whip is therefore a misconception. The stimulus increases the propulsive activity of the hind leg which lifts the croup and shifts the weight over the forelegs. Talented horses usually adapt to the training error shifting their front legs backward underneath themselves and finding balance over the forelegs. As a result, the forelegs are not capable to propel the horse’s body upward and after the second or third step, the horse is unable to sustain the diagonal and rhythmic motion of the limbs and the piaff became a walking gait. On this short video, you can compare a horse trained in respect of actual knowledge of the equine physiology with on the back ground, Olympic horses trained with the whip activating the hind legs. While the reference horse is sustaining a regular diagonal and rhythmic motion, Olympic contenders are losing the rhythm as soon as the second or third stride and are shifting the front legs backward underneath themselves. 

Back to forward movement, the hind leg exerts first a braking activity. After the peak vertical, which is the instant where the hind limb is acting vertically onto the ground, the hind leg is propelling the horse’s body forward. The limb is then behind the horse’s body. The effect is therefore a force in the direction of the motion. In the conclusion of their study, Schriver and his colleagues wrote, “The hind legs provide both support and propulsion.  The net effect is a force in the direction of the motion.” (2)  The study analyzed in details the two consecutive hind limbs’ actions during ground contact, the braking phase followed by the pushing phase. The decelerating phase is providing support and the pushing phase is providing propulsion. However, reading only the conclusion could accredit the old belief that the hind legs are propelling the horse into lightness. The two phases, deceleration and push are extremely rapid and there is no stimulus that could discriminate one or the other. Duration and intensity of the decelerating phase is therefore decided within the horse’s brain in respect of the body situation and the demand. If the horse’s brain is concentrating on balance control through appropriated coordination of the back muscles, the horse central nervous system is likely to enhance balance increasing the hind leg’s decelerating effort.   

During the second half of the stance, the active hind leg produces a force in the direction of the motion. The thrust generated by the hind legs is traveling forward through the spine from one vertebra to the next. Two phenomena are going to redirect a part of the hind leg’s thrust on the forelegs. One is the attraction of gravity. The other is the elevation of the croup created by the extension of the hind limb’s joints. “Raising the pelvis increases weight exerted at the anterior end and the pushing force is added to gravitational forces at the anterior end” (3)

The front limbs are designed to counteract the loading effect of the hind legs producing an upward propulsive force. James Rooney’s diagram illustrates the different front limbs’ positions during the stance showing the upward and forward direction of the front legs’ propulsive activity. Using the same technique, the pathologist illustrated the more horizontal force generated by the hind legs.

The forelegs are often compared to a pogo stick storing and returning energy. The distal limb of the horse has been shown to function like a pogo stick, storing and returning energy in long, spring-like tendons throughout the gait cycle (4) A child pogo stick is designed to compress and recoils under a load of approximately 80 pounds. The weight of an adult would certainly compress the spring but would not allow much recoil. Likewise, the front legs will not produce efficient vertical impulse if the load on the forelegs exceeds the springing capacity of the front limbs’ structure.

Balance results from the capacity of the back muscles to convert the thrust of the hind legs into vertical forces. When the back is working properly, the forelegs are submitted to a loading force that they have the capacity to handle and are therefore propelling the horse’s body upward. The whole process occurs forward through the spine, from the hind legs to the forelegs. Half halts attempting to rebalance the horse shifting the weight backward are ineffective gestures attempting to create a backward shift which, in fact, does not exist. 

This grey horse is carrying too much weight on the forelegs. The front limbs’ adaptation to excessive load is to shift the whole stance of the supporting foreleg backward. Impact occurs under the vertical of the point of the shoulder and push off is completed under the rider’s heels. Instead, when the same horse is properly balanced, the front limb alights more forward and the push off is not executed further back than the rider’s stirrup.

When the forelegs’ upward propulsive activity effectively counteracts the loading effect of the hind legs, both the croup and the shoulders are lifting equally. When the grey horse was carrying too much weight on the forelegs, the croup was lifting higher than the shoulders. Instead when the same horse was properly balanced, the shoulders and the croup were lifting equally. Since both pictures were taken at the beginning and the end of the same training session, the horse’s evolution was solely due to the riding technique. The technique applied was an adaptation of la Gueriniere’s half halt to actual knowledge of the equine physiology.

The classic author advises holding slightly more the horse in hands. This was in 1731. Great progresses have been made since in the understanding of the horse’s biological mechanism. Instead of holding with the hands, much better results can be achieved holding with the rider’s abdominal and back muscles. The rider’s hands are not actively exercising any gesture. Instead, they are used as sensors feeling how the horse deals with the rider’s abdominal and back muscles adjustments. 

Several conditions are required, which are directed by modern understanding of the equine physiology. If one wants to believe that pulling on the reins or acting with the hands will rebalance the horse shifting the weight backward, one should stop reading this study right now. With such antiquated beliefs one can only, like did Don Quichotte de la Manche and his mare Rossinante, fight wind mills. At the contrary, if one considers using the insights of modern science to better train better horses, here is how the horse’s body works and how the rider’s equitation needs to adapt to new knowledge. 

The horse’s back muscles have the capacity to convert the thrust generated by the hind legs into horizontal forces creating forward movement and vertical forces resisting attraction of gravity, “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 vertebra to the next”.  (5) Numerous studies have since furthered Tucker’s idea suggesting a more subtle equitation. The fundamental principle is reducing the range of motion of the rider’s vertebral column. On the saddle, the rider is submitted to a large diversity of movements which results from the limbs action and overall body movements.

The main function of the horse’s back muscles is to resist the forces acting on the thoracolumbar spine in order to maintain the amplitude of the thoracolumbar column movements within the limits of its possible range of motion. In 1959, Milton Hildebrand wrote, “The thoracolumbar spine of the horse is kept virtually rigid during locomotion; the small movements that may be seen take place in the lumbosacral joint.” In 1979, Hans Carlson demonstrated that the main function of the back muscles during locomotion was to preserve the integrity of the thoracolumbar column, resisting limbs actions and other forces acting on it. “Electromyography 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 that to create movement.” (6) In 1980, Leo Jeffcott measured the possible range of motion of the horse’s thoracolumbar column. “Thus the total range of movement in the dorso-ventral directions of the equine back was only 53,1 mm under these experimental conditions.”(7) Jeffcott’s experimental conditions were later contested and following different protocols other studies found dissimilar distributions of movements. For instance, Jeffcott situated the transversal rotations of the horse’s thoracolumbar spine between T1 and T11, while other studies registered that such rotation occur between T9 and T14. In spite of these variables, five decades of studies have consistently demonstrated that the horse’s thoracolumbar column functions within the limits of a quite restricted range of motion.

In 1999, Kevin Haussler wrote, “The amount of joint range of motion at any vertebral motion segment is small, but the cumulative vertebral movements can be considerable.” (8)  The author does not dispute the thought that the horse’s thoracolumbar spine functions within the limits of a very restricted range of motion. He underlines the fact that within such limits, the thoracolumbar spine is submitted to a large diversity of movements. Anyone who wants to believe that the horse’s vertebral column swings can interpret Hausler’s words as supporting one’s thought. The psychologist Tom Gilovich clearly explains the phenomenon. “When we want to believe a proposition, we ask, ‘Can I believe it?’ - and we look only for evidence that the proposition might be true. If we find a single piece of evidence then we’re done. We stop. We have a reason we can trot out to support our belief. But if we don’t want to believe a proposition, we ask, ‘Must I believe it?’- and we look for an escape hatch, a single reason why maybe, just maybe, the proposition is false.” The problem is that the work of the horse’s vertebral column is not about moving more but rather about resisting excessive movement. There are no words or metaphors that can simply explain such complex mechanism. In the matter, simplicity often leads the horse to lameness. The horse’s back muscles can be compared as a large orchestra where each instrument has to be tuned individually and as well to every other instrument. The overall performance cannot be achieved with the cellos or the piano only. This can be compared to the rider pretending orchestrating the horse’s thoracolumbar column with the hands or through neck posture. Even the touch of the triangle, which produces a tiny sound on the back of the orchestra, is part of the symphony.  One who is interested by the beauty of the horse’s symphony may follows James Crook’s advises, “A man who wants to lead the orchestra must turn his back on the crowed.”   

The situation is that on the saddle, the rider perceives large amplitude of movements, which are produced by the back and front limbs actions as well as overall body movements. The scale of these movements is much greater than the possible range of motion of the horse’s thoracolumbar spine. One must therefore, as does the horse’s back, absorbs and resists these forces reducing one’s vertebral column’s range of motion. The adaptation of the rider’s seat to actual knowledge of the equine physiology prompted Valdemar Seunig to describe the rider’s spine as oscillating minutely, “a movement so tiny that it is hardly perceptible to the naked eye.” Seunig clearly distinguish his view of a soft seat from the concept of a relaxed back. “This ‘soft seat’ differs fundamentally from a “doughy” seat, in which we find a spine that is too flexible and allowed to undulate freely in response to the horse’s movement.”

When at the 21st century, the plasticity of muscles function becomes well understood, the sophistication and complexity of the back muscular system suggested even greater involvement of the horse’s central nervous system, the brain. Instead of muscles lengthening or shortening to move vertebrae, the work of the back muscles is more at the level of forces created within the muscles. Muscles cells and connective tissues, (fascias,) are creating a large amount of multidirectional forces, which are perceived by the rider as movements. We placed seven sensors on the back of a horse at the walk and trot. The sensors were connected to a computer program measuring exclusively vertical forces. The forces recorded were the sum of the hind and front legs’ vertical impulse as well as the work of the thoracolumbar column’s muscles. The diagram of forces swinging up and down on the computer screen prompted Antony van den Bogert, who directed the experiment, to precise that the moving diagram was not illustrating vertebral column movements, but instead, intensity and frequency of vertical forces.  Click image to enlarge

Watching the diagram, one might think about waves. This is the phenomenon that has fooled our ancestors. The misconception continues to fool riders, trainers, judges and authors who are not capable to evolve. The feeling of swing is the feeling of forces.  Frist of all, the rider is placed exactly where the concentration of forces is the greatest. Also, the rider is placed where the horse’s vertebral column that is moving the most. The rest of the horse’s vertebral column does not have the same capacity of movement. The second problem is that forces might feel like movement. However, there is the difference between kinematics and dynamics. “Kinematics is the geometry of movement, a graphical record of the initial, intermediate, and final positions of the various parts and pieces of the system – one in relation to the other. Dynamics, on the other hand, is concerned with the forces applied to and generated by the system and, therefore, includes acceleration, mass, potential and kinetic energy, momentum, etc.” (9) The coordination of the forces acting on the spine cannot be orchestrated by increasing the vertebral column’s range of motion.

Upgrading la Gueriniere half halt to actual knowledge of the equine vertebral column mechanism truly educates the horse. Instead of holding slightly more the horse in hands, much better results can be achieved holding slightly more the horse with the rider’s abdominal and back muscles. The hands are involved but more as sensors feeling how well or not so well, the horse’s back muscles are controlling translocations of gravity through the thoracolumbar spine. If the rider is in neutral balance over the seat bones, the horse will respond to slight straightening of the rider’s vertebral column. At the contrary, if the rider is shifting her or his weight back to front or front to back, the horse will be unable to respond.

This point is a clear evolution from traditional thinking. “Undoubtedly” wrote Estienne Saurel, “any shift of the rider’s weight is important for balancing the horse for controlled movements.” (10) The main back muscles of the horse’s thoracolumbar column are set and functions in opposite direction.  Therefore, back to front or front to back shifts of the rider’s weight hamper the horse’s ability to precisely synchronize the simultaneous work of the main back muscles.

Once in neutral balance, which means having the middle of the vertebral column exactly at the vertical of the seat bones, the rider straightens the lumbar vertebrae using greater but measured tone of the psoas muscles. We are not referring to a posture but rather to a sophisticated control of reduced movements. The move creates a slight elevation of the rider’s lumbar region. Simultaneously, the natural kyphosis of the rider’s thoracic vertebrae is straightened advancing slightly the thoracic vertebrae and therefore opening the chest.

The horse does not resist to such adjustment of the rider’s back since this is not about submission but rather harmonic motion and therefore comfort. As soon as the range of motion of the rider’s back will match the range of motion of the horse’s thoracolumbar column, the horse will slow down indicating to the rider than the connection is established. If at the contrary, the horse does not respond, the rider is unconsciously creating stimulus confusing the horse. The confusion might be created by a rider seating too far back on the fannies, or a vertebral column too lose or too tight. Disturbances can be created by heavy contact on the bit, or hands too close or forearms or shoulders too tight. The rider’s legs might be too close or unstable. Any simple formula is almost systematically wrong. One needs to adapt the overall concept to one’s physique, energy and mental processing. The horse tunes up the rider. Remember, the horse does not resist. At the contrary, the horse enjoys this sophisticated harmony of movement. Lack of results cannot be corrected by doing more but instead, by refining the rider’s equitation and doing less.     

Once the rider and the horse’s back are tuned to each other, the horse will react to any adjustments of the rider’s back processing in his brain and body how to orchestrate the back muscles in order to stay in harmony with the rider. Down to the proteins creating the fibers composing the fascias, the horse’s biology is fundamentally designed for efficiency, which is optimum orchestration permitting minimum muscular work. The horse’s nature is designed for sophisticated equitation, include half halt. Hands gestures trying to shift the weight backward are gross violations of the horse’s refinement.

Jean Luc  Cornille

Copyright 2011 ©Jean Luc Cornille

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References:

(1)                  (Eric Barrey, Sophie Biau, Locomotion of dressage horses Conference on Equine Sports Medicine and Science - 2002)

(2)                  (H. F. Schriver. DVM, PhD.  D. L. Bartel. PhD.  N Langrana. PhD.  J. F. Loice. DVM. - 1978).

(3)             (5) (Richard Tucker, Contribution to the Biomechanics of the vertebral Column, Acta Thoeriologica, VOL. IX, 13: 171-192, BIALOWIEZA, 30. XL. 1964)

(4)                  (Biewener, 1998; Wilson et al., 2001).

(6)                 (Hans Carlson, Halbertsma J. and Zomlefer, M. !979, Control of the trunk during walking in the cat. Acta physiol. Scand. 105, 251-253)

(7)                 (Leo B. Jeffcott, Natural Rigidity of the horse’s backbone, 1980. Equine vet J. 1980, 12 (3), 101-108)

(8)                 (Kevin K. Hausler, DVM, DC, PhD  Preface, Veterinry Clinics of North America, 1999)

(9)                (James R. Rooney, Biomehcanics of lameness in horses)

(10)        (Pratique de l’ équitation  d’ après les maitres français. Flammarion, Paris, 1964)    

Science Of Motion

Jean Luc Cornille

scienceofmotion.com