Biomechanics and the horse’s back - Veterinary Practice
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Biomechanics and the horse’s back

MARION McCULLAGH concludes her reports from the 2009 BEVA congress

EQUITATION science is what happens when you look at the horse as an athlete who partners a rider.

Riding is an inherently experimental process where there is a continuous interplay between cause and effect. Outcomes are recorded and analysed and new methods developed but equitation science uses subjects such as anatomy, physiology, behaviour or biomechanics to explain what goes on when a horse is being ridden.

Professor Pieter Brama of University College Dublin chaired Saturday’s session at the BEVA congress on “The equine back and performance”. Professor Rene van Weeren of the University of Utrecht outlined “The biomechanical concept of the equine back”.

The first person to record his theories on this subject was the Roman physician Galen, who lived in the first century AD. He regarded the back as an arch supported by the limbs acting as pillars. His model lasted until the middle of the 19th century when the back was seen as a bridge, the limbs being the land at either end.

The upper ledger was the supraspinous ligament, the vertebral bodies were seen as the lower ledger and the girders which connected the upper rigid part with the lower one, pointing either craniodorsal or caudorsal, were the spinous processes and the interspinous ligaments.

In engineering terms this was a poor model and it has been replaced by the concept of the back as a “bow-and-string.” This model is the first to introduce the concept of flexibility and it is the only dynamic model. It takes into account the whole trunk rather than looking at the vertebral column in isolation.

The bow is the vertebral column, and the string is the underline of the trunk, made up of the linea alba, the rectus abdominis and associated structures. In fact, this idea was first put forward by Barthez in 1798 but his work was ignored until Slijper rediscovered it in 1946.

It allows for the fact that the back is in dynamic balance under constantly changing tension. A force on one part alters the biomechanics of the other parts. It is still a crude model that needs refinement but it is more useful than its predecessors.

Gravity is the one constant as it always acts downwards. It tends to straighten the bow, hollowing the back. Pregnancy, tack and rider increase the mass that gravity acts on so it is not surprising that most broodmares have hollow backs.

Ridden horses are conditioned to withstand the effect of the considerable extra weight of tack and rider.

Nearly all the muscles of the back run from one part of the axial skeleton to another. The whole system is under intrinsic tension which means that any change in one part changes the rest of the system. This is a far cry from the rigid bridge theory.

Terms to describe the deformations are taken from aircraft design. “Pitch” is dorso-ventral movement, lateral movement is “yaw” and axial movement is “roll”. Contraction of the ventral musculature increases the upward arching of the back, while contraction of the massive epaxial musculature hollows the back.

Limb movements also influence the curvatures. The bow and string model does not help with analysing the pelvis or the interaction between sacro-lumbar spine, pelvis and hind limbs, but at present it is the best means of thinking about the highly complex, dynamic situation that is involved in the use of the horse’s back. It lies behind Professor van Weeren’s advice: “Treat your horse’s back well.”

The brain

Going from the back to the brain, Professor Paul McGreevy of the University of Sydney described “Behavioural influences on training and competitive success: the horse”. This was in the session on “Managing the top performance horse”, also chaired by Professor Brama.

A horse that wins at competition is not just a superb athlete, his physical ability needs to be backed up with his coping skills. He must thrive on training and settle enough at the show to perform well and he has to travel well.

Competitions are exciting and the horse needs to be sufficiently aroused to makeabig effort without being upset by his surroundings. Familiarisation of the animal with the circumstances of competition is essential, the horse is learning all the time and the trainer needs a knowledge both of learning theory and of animal behaviour to get things right.

To start with, we have physiology and conformation, which combine to give athleticism, then we need trainability and finally there are two unanswered questions: “Does the horse have a will to win?” and, “Does it have the will to please humans?”

While there may be no objective answers to these questions, it is certain that any exposure to poor handling at any time can be as harmful as poor riding technique. Poor weaning sets up many problems and any frightening incident can go deep into the subconscious and never be forgotten even if conditioning can damp down the response to some extent.

The importance of seasonality is often overlooked: the responses to other horses are influenced by oestrus and social challenges that are manageable outside the breeding season but may be a problem at this time.

Looking at the performance from the breeder’s point of view opens up another set of dilemmas. When we select horses to win races, we are going for speed and stamina which depend on a strong flight response. This ignores the needs of the pleasure rider who has limited abilities and needs a much steadier animal.

In the world of dressage, big stride wins competitions. The Warmblood is getting taller with a prettier head and a bigger shoulder. Short head goes with short sight and crowded dentition. Taken to theoretical extreme we may find ourselves breeding a dressage horse whose action has so much swing that it takes a skilled, supple rider to be able to ride it at all.

Our aim should be to reduce the conflict in the interaction between horse and rider and so reduce the large amount of misery that leads to behavioural wastage.

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