Mechanoresponsiveness 23 Class
Jean Luc Cornille
“It is the extension of the hind legs, and not their engagement, that produces the elevation of the school gaits and of the airs that are derived from them.” (General Decarpentry, Academic Equitation, 1949)
In 1949, General Decarpentry hinted that the propulsive power developed by the hind legs was more important than the advanced position of the supporting hind leg under the body. “The more the hind-limb as a whole approaches the vertical at the moment it extends, the more it thrust the body upward.” (Academic Equitation) We often state that one does not benefit from the classical inspiration by repeating our ancestors words but instead by questioning and understanding their thoughts in the light of actual knowledge. The thought that greater thrust is created, as the hind limb approaches the vertical is corroborated by advanced research studies. During the stance, the supporting hind leg produces first a decelerating activity resisting gravity that is pulling the horse body down, and inertia, that is pushing the horse’s body forward. The combination of both forces can be referred to as acceleration of gravity. The decelerating phase, that is also called the “braking phase”, commences as soon as the hoof contacts the ground and last until the alighting hind limb approaches the vertical. Instead, the second half of the statement, “the more it thrust the body upward.” is contradicted by actual knowledge. As the limb approaches the vertical and then moves behind the body, the net effect of the hind legs propulsive activity is a force in the direction of the motion.
Our ancestors believed that the propulsive activity of the hind legs started as soon as ground contact. based on this belief, the consensus was that the alighting hind leg propels the body upward. Decarpentry realized that the theory was not accurate, but he did not have the availability of actual knowledge. Our ancestors theorized a direct relation between the advanced position of the alighting hind leg under the belly and its ability to propel the horse’s body upward. Actual knowledge portrays a more complex and totally different picture. “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). The thrust generated by the hind legs travels forward through the thoracolumbar spine where it is submitted to the attraction of gravity and a percentage of the thrust loads the forelegs. The front limbs compensate for the loading effect of the hind legs’ propulsive force propelling the body upward and forward. Between the hinds and the front limbs, the back muscles have the capacity to convert a percentage of the thrust generated by the hind legs into upward forces and consequently, reduce the load on the forelegs. “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).
Classic authors understood that greater upward forces were necessary for collected and more elevated gaits such as passage. “In the elevated gaits, the thrust of each hind leg displaces the horse’s body simultaneously forward and upward. An effort of horizontal propulsion and one of vertical projection can be distinguished in this thrust. “ (General Decarpentry, Academic Equitation, 1949) The problem was that beside greater engagement of the hind legs, no comprehensive explanation was made on how the horse converts the thrust generated by the hind legs into upward force. Greater engagement of the hind legs does not necessary produces upward forces and while the intuition of some riders as well as horses completed the lack of information, others were unable to figure the missing link. As an International Dressage Judge Decarpentry observed the phenomenon and wrote, “The officer who decides to prepare a horse for international dressage tests is going beyond the limits of his equestrian education.”
Albert Einstein praised intuitive minds over, what he referred to as “faithful servants”. Even today, based on old theories, only intuitive riders are capable to complete the holes of the equestrian education. Instead, we observe that providing a sound explanation of the way the horse physique effectively works, many riders re-awake their intuition and are capable to move away and above the limits created by an equitation of faithful servants.
Decarpentry was very close from actual understanding of the equine biomechanics. “The degree of elevation of the body during the moment of suspension will depend likewise on the vigor of the effort of the hind legs, and consequently on the amount of previous compressions of their joints.” Instead of compression of the joints, the forces propelling the horse’s body forward are, for a great part, the outcome of an elastic strain energy stored in the tendons, aponeurosis, ligaments and muscles during the decelerating phase and reused for the propulsive phase and the swing.
Instead of upgrading traditional beliefs to new knowledge, riders and trainers integrated new discoveries to antiquated views and often, researchers try compromises upgrading familiar equestrian language to actual knowledge. “The term ’stepping under,” as used by riders and trainers, may be correct if it is redefined to describe the position of the hind limb at the middle of the stance phase.” (Mikeal Holmstrom – 1994) The fact is that if the hind leg is a little more forward that under the vertical of the croup when the propulsive activity commences, it will be at the early stage of the propulsive phase and for a brief instant, a little more upward force. However, this type of thinking is more about preserving traditional beliefs than upgrading riding and training techniques to actual knowledge. The hind legs generate ultimately a force in the direction of the motion that is converted through the muscular system of the back, into hprizontal and upward forces. “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).
The engagement of the hind legs remains a necessity but for a different reason. At impact and during almost the first half of the stride, the supporting hind limb resist accelerations of gravities. This is part of balance control but depends more on the way the horse’s brain orchestrates the work of the decelerating hind leg than the position of the limb under the body. During piaff for instance, the supporting hind leg is less forward under the belly than during collected trot, but the decelerating action is considerable. “The hind legs have a considerable braking activity to avoid forward movement of the body over the forelegs.” (Eric Barrey, Sophie Biau, Locomotion of dressage horses Conference on Equine Sports Medicine and Science - 2002)
The decelerating phase of the hind leg is also the sequence where elastic strain energy is stored in the ligaments, fascia, tendons, aponeurosis and muscles, and reused for the propulsive phase and the forward swing of the limb. “The elastic energy stored in and recovered from tendons during cyclical locomotion can reduce the metabolic cost of locomotion” (Cavagna et al., 1977; Alexander, 1988; Roberts et al., 1997). Storage and reuse of elastic strain energy is a major component of efficient locomotion as well as performances and it is not limited to the advanced position of the hind leg under the body. During the first half of the stride, and therefore during the sequence of the stride where elastic strain energy is produced and store, the fetlock translates downward before inverting the motion and then translating upward. The downward translation of the fetlock is referred to as dorsiflexion, whyle the upward translation is referred to as palmar flexion. During the dorsiflexion, the canon bone, (Mt3), achieves a screw like motion. The twist induces at the other end of the bone, which is the hock joint, an inward rotation. When flexion and extension of the joints are properly coordinated with inward rotation, the move is sound, effortless and classy. Instead, when the synchronization between flexion and extension of the joints and inward rotations are not properly synchronized, shearing forces occur rendering the performance mechanical, restricted, difficult, painful and damaging. The fetlock and the hock are just examples. The knee, the stifle, the elbow and even the thoracolumbar spine functions combining lateral bending, flexion, extension and rotation.
Class is soundness and soundness is knowledge.
Jean Luc Cornille