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In respect of actual knowledge of limbs kinematics and equine vertebral column mechanism, medium trot is a performance athletically more demanding than Piaff and Passage. Rationally, medium trot should not be asked before at the least Prix Saint Georges and probably even later. In fact is medium trot is performed as it should maintaining perfect control of the balance and therefore cadence all the way through the diagonal, the athletic demands of the performance is as difficult if not more difficult than the piaff. The reason why medium trot is asked at third level is that the designs of the dressage tests as well as the judging standards are based on the heretical belief that the alighting hind leg propels the horse’s body upward and forward as soon as ground contact. The same belief supports theories such as driving the horse onto the bit and rushing the horse forward. If, as these inaccurate theories believe, the alighting hind leg produced an upward propulsive force as soon as ground contact, increasing the hind legs propulsive activity would be effective.

The problem and the reason why these riding and training techniques are keeping talented horses below their real athletic abilities and cripple a large percentages of them is that the supporting hind leg does not propel the body upward as soon as ground contact. At the contrary, the supporting hind leg decelerates the horse’s body from impact and until about 45% of the support phase. The joints of the alighting hind leg fold resisting attraction of gravity and inertia forces. This cumulus of forces is often referred to as “impact forces.” This sequence of the stride is referred to as “braking phase.” The term braking is confusing. A good comparison is the work of your leading leg as you are walking down hill with a back pack of about 15 pounds. Your leading leg impacts and your knee extensors muscles decelerate your body resisting attraction of gravity and inertia forces. Without this decelerating action, gravity would make you run faster and faster until the bottom of the hill. Technically, your leading leg is ”braking.” However you leg is not rigid bracing against the ground; forward motion never stops. The joints of your leg fold resisting attraction of gravity and inertia forces. The work of your knee extensor muscles is essentially eccentric, which is a powerful type of muscle contraction. As you are walking and not jogging, the strain energy created by the eccentric contraction is not immediately reused and therefore is dissipate as heat. If you are not trained at hiking up and down hill, you will experience muscle soreness the next day.

At impact and until the peak vertical, which is the instant where the hind leg is acting vertically onto the ground, the supporting hind leg decelerates the horse’s body. The decelerating phase is the sequence of the stride where the supporting hind leg is involved in controlling balance. After the peak vertical, the supporting hind leg propels the horse’s body forward. The hind limb is no longer under the body but is rather behind the horse’s body. The net effect is therefore a force in the direction of the motion. The thrust generated by the hind legs travels forward through the horse’s thoracolumbar column and is submitted to the attraction of gravity.  A percentage of the thrust generated by the hind legs is therefore loading the forelegs. The forelegs are designed to compensate for this loading effect propelling the horse body essentially upward . This is why scientific measurements have demonstrated that during regular locomotion, the forelegs produce 57% of the vertical impulse while the hind legs only 43%.

The numbers vary with the demands. During piaff for instance, the supporting hind leg develops a considerable braking activity in order to resist forward displacement of the body over the forelegs. The forelegs in counterpart produce considerable vertical impulse. The problem is that pelvic muscles, and therefore the muscles of the hind legs producing propulsive forces are 4 times stronger than their equivalent muscles of the forelegs. If the main muscles of the thoracolumbar column are not converting a percentage of the propulsive force generated by the hind legs into vertical forces, resisting attraction of gravity and therefore enhancing balance control,  the load on the forelegs overwhelms the forelegs’ upward propulsive capacities. The forelegs then resist excessive forces increasing their braking action. The braking phase of the forelegs last from impact until the 40% of the stance. Stance describes the time where the front limb remains on the ground.

During piaff, the hind legs produce a considerable braking activity but little propulsive force. This is understandable since the propulsive activity of the hind leg lifts the croup and the elevation of the croup shifts the weight over the forelegs. The back muscles convert during piaff a percentage of the thrust generated by the hind legs into vertical forces. The work is eased by the fact that the hind legs produce little propulsive activity. Correct piaff results therefore from the large decelerating activity of the hind legs and the conversion through the main back muscles of the thrust generated by the hind legs into greater vertical forces. When the horse is properly educated, the decelerating action of the hind legs and the work of the main back muscles reduce the load on the forelegs allowing the forelegs’ extrinsic muscles to propel the front end of the horse’s body upward.

When the horse’s vertebral column is not properly educated and the training technique is heretical such as activating the propulsive activity of the hind legs with a dressage whip, excessive forces load the forelegs and the front legs are unable to propel the front part of the horse’s body upward. This is why improperly educated horses are unable to keep the rhythmic sequence of the limbs action and the piaff degenerates into a marching band type of gait. This is also why horses shift their forelegs backward underneath themselves. They balance themselves over the forelegs and are consequently unable to perform a correct piaff.

During piaff, or at the least when the training of piaff is correct, the back muscles only have a moderate amount of hind legs’ propulsive  to convert into vertical forces. During medium or extended trot, the work of the back muscles is more difficult since the thrust generated by the hind legs is greater. When medium trot is asked before educating the main back muscles in converting the thrust generated by the hind legs into greater vertical forces, the forelegs receive excessive load and are unable to propel the front end of the horse’s body upward. The front legs attempt then to increase their braking activity controlling balance by opposing the braking activity of the forelegs to the propulsive activity of the hind legs. According to the horse’s muscular power, stamina, style, different compensation are applied inducing excessive stress on the bony column or tendons and ligaments or both. The front legs are designed to recoil using the forces accumulated during the decelerating phases to increase the upward propulsive activity. The front limbs are not designed to brace against excessive load. The front limbs of a horse asked to perform medium trot without adequate education of the vertebral column mechanism endure each stride stresses exceeding the capacity of their bones, or tendons and ligaments structure and therefore, breakdown occurs.

Proper mechanism of the horse’s thoracolumbar spine is a perquisite for collection. It does not mean that a horse entering Prix Saint Georges has been properly educated but asking medium trot to a horse which is definitively not properly educated such as a third level dressage horse is very likely to induce excessive impact forces on the forelegs as the horse races through the diagonal throwing or not throwing his front limbs in front of him.  

Talented horses succeed to sustain some suspension in spite of bad training technique due to unusual strength of their forelegs’ extrinsic muscles. However they bounce their body upward but not forward. They increase then their knee action throwing their lower limbs forward. Lacking experience and knowledge, some judges reward these dysfunctional horses. This is regrettable as it orients dressage toward compulsories for which the horse is athletically unprepared instead of an art where talented athletes are soundly and adequately educated for the athletic demand of the performance.

Jean Luc Cornille