Mechanoresponsiveness 34

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“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, Anton. J. van den Bogert, 1993).

Oliver is a mini Schnauzer with a happily bouncing running style. I love watching him running as he demonstrates both, the inaccuracy of old theories of locomotion and how locomotion is effectively created. Old theories attributes gaits and performances to large angular variations of the limbs joints. For instance, flexions and extensions of hocks. Oliver does have very short legs only capable of little angular variations of the limbs joints. He weighs 17 pounds, and yet, bounce twice his own hight. The reason is explained by modern understanding of human and equine biomechanics. Gait and performances are, for a great part, the outcome of elastic energy stored in tendons, aponeurosis, muscles and fascia, during the first part of the stride and reuse during the second part.   

When measurements demonstrated that the forelegs created more upward force than the hind legs, 57% for the forelegs and only 43% for the hind legs, the concept was difficult to accept, as it contradicted the belief that motion is created through large angular variations of the limbs joints. From this perspective, the hind legs are better suited to produces vertical impulse than the forelegs. Instead of upgrading their understanding of equine locomotion, many opted for deny, remaining at the level of flexion and extension of the joints. They refuge in the comfort of old beliefs perpetuating the heretical thought that the hind legs produce the greater amount of upward force.

As often in the equestrian world, novelties are welcome, but only as long as they don’t question traditional beliefs. Traditionalists oppose literature to science omitting that most classical authors have been willing to review their thoughts in the light of new knowledge. Colonel Hans von Heydebreck, who, in 1935 edited the Fourth version of Steinbrecht’s “Gymnase” wrote in his preface, “There are a few theses in the book that did not held well in front of the discoveries of scientific researches.” A growing number of riders and trainers who truly respect horses, upgrade their views to actual knowledge. It is indeed, more a paradigm shift than an upgrade, as looking at equine locomotion and performances in terms of dynamics verses kinematics, deeply modifies riding and training principles.

The thought that elongating muscles would enhance the range of motion is a theory of the past. In their aware-winning publication, Alan Wilson and his colleagues explained that for a large part, gaits and performances are created through storage and reuse of elastic energy. “The muscular work of galloping in horses is halved by storing and returning elastic strain energy in spring-like muscle-tendon units.” (Alan M. Wilson, M. Polly McGulgan, Anne Su & Anton J. van den Bogert, Department of Biomedical Engineering, Cleveland Clinic Foundation - 2001) Many researches completed prior and since 2001, have explained that, when it comes to storage and reuse of elastic energy, the forelegs are better, suited than the hind legs. Alan Wilson explained the “catapult mechanism” of the biceps brachi and its internal tendon. Anton van den Bogert investigated the phenomenon of “power transport between joints.” Lower legs. Muscles spanning over more than one joint, can absorb power at one joint and simultaneously produce power at another joint.

This does not mean that the hind legs do not have upward propulsive power. The hind legs propel the horse body over the jump, but during locomotion, the net effect of the hind legs propulsive activity is a force in the direction of the motion. The point is, that locomotion and performances occur at a different level than traditional literature trained us to believe. For instance, power transport between joints is not exclusive to horses. In human vertical jumping, the gastrocnemius muscle generates, at the end of the push-off, positive power at ankle and negative power at the knee. “This function of biarticular muscles s is referred to as power transport between joints.” (Gregoire and all, 1984)  In spite of his very short legs, Oliver bounce twice as high as his own size using these phenomenon.

Muscles, absorb, manage and produce forces. Power absorption is usually associated with eccentric contraction and power production is usually produced by concentric contractions. Muscles are constructed of cells, which contract, and tendinous material. This is why a muscle can store and return elastic energy even in the absence of tendons. The concept of elastic strain energy is easy to understand looking at the long tendons of the lower legs. The long and stiff tendons of the lower leg are capable to store substantial amount of elastic energy. Furthermore, the deep palmar ligament of the carpus, which is the knee, also absorbs a substantial amount of energy during hyperextension of the carpus.  Power absorption is therefore, not always associated with eccentric muscle contraction. It can also be caused by elastic energy stored in tendons and ligaments. The subsequent power production can originate from the release of elastic energy instead of concentric muscles contraction.

We talk here about the lower legs but similar mechanisms occur through the whole limbs and thoracolumbar spine. There is, just for the front legs, the catapult mechanism of the biceps brachi. Higher on the forelegs, there are the aponeurosis of the serratus ventralis thoracic muscles, which support the trunk between the scapula. Aponeurosis act like tendons storing and returning elastic energy. A horse can take off over the second element of a bounce before the hind legs touch the ground with the power of the aponeurosis of the serratus muscles, aided by elastic energy stored in tendons, ligaments and catapult mechanism of the biceps. Oliver bounces during his run using the same storage and return of elastic energy.

Lengths of the stride and amplitude of the performances are not created elongating of relaxing muscles. These theories are simplistic and unaware of basic equine anatomy and function. For instance, the two main tendons of the lower legs are the superficial flexor and the deep digital flexor. Their respective muscles are the superficial flexor muscle and the deep digital flexor muscle. The muscles create proper tension of their respective tendon, If the muscles were stretched or relaxed, it would be little elastic energy stored in the tendons and very little propulsion and forward swing of the front limbs. Nature refined the process, adapting the muscles fibers to the elasticity of the correspondent tendon. As the superficial flexor tendon is more elastic, the fibers of the superficial flexor muscle are shorter and stronger. By contrast, as the deep digital flexor tendon is stiffer and the fibers of the deep digital flexor muscle are longer and more elastic.

In spite of very short legs, Oliver bounces twice his own hight, because no one ever tried to convince him that he would bounce higher using stretching and relaxation. As his brain is stimulated with having fun, Oliver further explores the capacities of his physique. Stimulated with the perspective of ease and effortlessness, the horse brain explores as well more efficient ways to reduce the metabolic cost of locomotion, storing and returning elastic energy. Of course, in order to succeed, the horse brain needs to be free of exploring further and consequently, free of making errors. Just these two points disqualify most riding and training techniques.  Jean Luc Cornille