Biomechanics of the Horse

This process, which is known as gait analysis, measures a horse's performance objectively and allows us to quantify some aspects of performance that are not visible to the human eye.

Biomechanics applies mechanical principles to the study of living systems. In this case the system we are interested in is the horse and, more specifically, we are using biomechanical techniques to study how the horse moves. This process, which is known as gait analysis, measures a horse's performance objectively and allows us to quantify some aspects of performance that are not visible to the human eye. Here are some examples of the applications of gait analysis in dressage horses:

• measurement of a horse's performance or level of training by comparing an individual with others of known ability
• detection of subtle lameness problems through finding asymmetries in the limb movements, or in the carrying or propulsive abilities of the left and right sides of the body
• monitoring recovery from lameness by repeated examinations of the horse during its recuperation
• measurement of strains in different parts of the horse?s body in response to exercise
• assessment of the effects of footing and shoeing on performance and soundness

BIOMECHANICAL TECHNIQUES

In my research, I use a variety of equipment and techniques, each of which provides a

different kind of information about the horse's movement. The methods include video analysis, force platforms, electromyography, strain gauges, pressure transducers, and accelerometers.

Video Analysis is a means of quantifying movement patterns through timing, distance and angular measurements. The timing variables describe the tempo, rhythm and regularity of the stride. The distance variables describe the stride length and the distances between individual limb placements, such as the amount of over-tracking. The angular variables describe the position of the bones and joints, for example, how much flexion occurs at different joints.

In dressage competition the judge's eye is an important component in determining the scores a horse receives. Analysis of videotapes is a way of quantifying what the judge sees. Video analysis involves digitizing specific points on the horse's body in each video frame. The points are then connected to form stick figures representing the horse's body and limbs (Figure 1).

A Force Platform is a rectangular, metal plate that is embedded in the ground, then covered with a suitable surface (usually rubber) over which the horse moves. When the horse steps on the force platform, it measures the carrying and propulsive forces as the hoof pushes against the ground.

Analysis of forces can be used to measure the contribution of each limb to the carrying and propulsive responsibilities. It detects differences between the front and hind limbs, and can monitor changes as a horse progresses through training. The force patterns also reveal differences between the left and right sides due to lameness or sidedness.

Electromyography (EMG) is a technique for determining which muscles are active at specific times in the stride. We need to know which muscles are used in the various movements and how muscle use in dressage differs from ?normal? locomotion. This knowledge will be useful for preventing and treating muscular injuries and soreness.

Strain gauges and accelerometers measure the strains and accelerations, respectively, of the surface to which they are attached. They can be attached directly to the hoof wall to detect concussion on the limb. An example of their use is in monitoring the effects on the horse?s limb of different types of footing.

Pressure transducers indicate the distribution of forces over a specific area. They can be used to measure the pressure of the saddle and the effect of the rider?s weight and aids on the horse's back.

As you can see, the techniques available for biomechanical research are capable of providing a great deal of pertinent information. In my laboratory we use several of these techniques simultaneously to give us a more complete understanding of the mechanics of the dressage horse.

COLLECTING DATA

Some of the equipment, such as the video cameras, is completely portable, and can be used to collect data at almost any location. This is a particularly useful feature for studying competition performances; the footage we recorded in the Barcelona and Atlanta Olympic Games has yielded valuable insight into the performance of dressage horses at the highest levels of competition. However, the cameras must be set up and operated with precision, if the resulting videotapes are to be used for scientific evaluation. It takes a lot of time to analyze videos recorded during competition because we aren?t able to use markers on the horses for automatic digitization by the computer. Another drawback in analyzing competitive performance is that it?s not possible to gather data from other sources (force platform, EMG) during competition, which limits the amount of information. To gather the full range of video, force platform and EMG data, experiments are conducted under controlled conditions in a gait laboratory. Reflective markers are glued to the horse?s hair over specific bony landmarks, and illuminated by a spotlight, which makes them glow brightly. When the videotapes are analyzed, the computer detects the markers automatically which speeds up the digitization process. The video data can be synchronized with recordings from a force platform and with EMG signals from selected muscles. Unfortunately, at the present time, there are no facilities in North America suitable for performing this type of analysis in horses under saddle. The only laboratory suitable for complete biomechanical analysis of ridden horses is at Utrecht University in The Netherlands. I spent a sabbatical leave there in 1995-96 with the objective of gaining a better understanding of the biomechanics of dressage horses.

CONCUSSION ON THE HORSE'S LIMBS

At the instant the hoof strikes the ground, it is rapidly decelerated, and this sends a shock wave up the horse's limb. The shock wave is characterized by having a high amplitude and rapid vibration frequency; these characteristics make it particularly damaging to the bones and joints. In people, the effects of impact shock are responsible for the development of problems such as arthritis. Activities that involve running or jumping, in which there is an airborne phase, are much more damaging than walking or stepping, in which there is always at least one foot on the ground. This is why people tend to get fewer injuries when doing low impact aerobics. In horses, impact of the hoof with the ground is the most important phase of the stride in relation to the development of degenerative joint diseases, such as arthritis, which is the most frequent cause of premature retirement from training and competition in dressage horses. The big movement and lofty suspensions that are favored in dressage, combined with the large size and weight of our horses, exaggerate the damaging effects of the impact shock and increase the likelihood of arthritic changes later in the horse?s career. Since footing and shoeing have a profound effect on impact shock, we need a lot more information about the surfaces and shoes that reduce impact shock.

It is possible to get some information about the hardness and other properties of the footing using mechanical testing devices (drop hammer tests, cone penetrometers), but the horse?s limb responds rather differently than a steel testing apparatus. A more realistic approach is to attach an accelerometer to the hoof wall to measure the hoof deceleration during impact. To interpret the likely effect on the horse, we need to know how the shock wave is transmitted from the hoof wall to the bones and joints of the limb. Our research shows that the soft tissues within the hoof, which include the laminae and the digital cushion, reduce some of the damaging effects of the impact vibrations before they reach the bones and joints. Precisely how this occurs, and how the characteristics of the hoof-ground interaction affect the shock wave reaching the bones has not yet been investigated.

MOVEMENT PATTERNS AT THE TROT

I have studied the timing, distance and angular characteristics of the walk, trot and canter, and have investigated how these characteristics differ among the collected, working, medium and extended gaits. The top competitors maintain almost the same tempo in the transitions between the collected, working, medium and extended gaits. Consequently, the increased speed of the extensions is a result of taking longer strides. Using the trot as an example, the stride length depends on the diagonal distance (the distance between the diagonal pair of limbs when they are on the ground) and the overtracking distance (the distance between the imprint of the front hoof and the subsequent imprint of the hind hoof on the same side). Only a small fraction of the increase in stride length is due to a longer diagonal distance; between working and medium trot the diagonal distance increases by about 2" associated with the lengthening of the horse's frame. However, the vast majority of the increase in stride length is a result of more over-tracking. In the collected trot, the horses are about 3" short of tracking up whereas in the extended trot there is about 16" of over-tracking. This is achieved as a result of a bigger suspension; the horse is propelled higher into the air, stays airborne longer, and covers a greater forward distance during the airborne period.

Incidentally, forging at the trot (catching the toe of the front shoe with the toe of the hind shoe) is a consequence of insufficient lifting into the suspension. It occurs in horses that are either not yet strong enough (or too lazy) to generate enough upward propulsion into the suspension.

Sometimes the problem disappears when a more experienced rider gets the horse moving more actively and with more impulsion. In other cases, strength training exercises, such as walking and trotting over raised rails or trotting up gradients, help to develop the appropriate muscles.

COLLECTION AND SELF CARRIAGE

Throughout dressage training, we seek to improve the collection and self carriage of the horse. As a result, the horse becomes better balanced and the movements are easier to perform. The trained horse moves with the hindquarters lowered and the hind limbs acting underneath the horse?s body, while the forehand is elevated, with the neck raised and arched. These characteristics can be measured by analysis of videotapes. However, self carriage is much more than a position of the horse?s body parts; to achieve the required lightness and balance, the horse must alter the way it pushes against the ground. I have always been curious as to how this was achieved in biomechanical terms, and I dedicated my sabbatical leave in Utrecht to unraveling the mysteries of self carriage.

This study required the participation of high-level dressage athletes, and I was fortunate that Tineke Bartels agreed to bring her own horses and those of her students to the laboratory. The horses Tineke provided included her Barcelona horse, Courage, and her Atlanta horse, Barbria.

Each horse performed working trot, collected trot, passage and piaffe on the force plate (Figure 2), while video recordings were made simultaneously.

The results show that as the horse becomes more collected, both the movement patterns and the force patterns change. The results from the force platform were particularly interesting. Traditionally, we assume collection involves the hind limbs carrying a greater share of the weight and providing progressively more of the forward propulsion. As a consequence of engaging the hind limbs, the front limbs carry less weight, which allows the forehand to become lighter and more mobile. Data from the force platform confirm some of our assumptions, but in other respects       provide a new insight into the way the horse achieves self carriage.

The carrying forces recorded by the force platform indicate how the weight is distributed between the front and hind limbs and whether there is a weight shift (movement of the center of gravity) as the horse becomes more collected. In a standing horse the front limbs carry about 55% of the horse?s weight, the hind limbs about 45%. In many top dressage horses, the weight does not shift significantly from the front to the hind limbs as the horse becomes more collected. However, a few horses do show a marked weight shift, and these seem to be the horses who are particularly well balanced. Therefore, balance may indeed be related to the horse?s ability to carry more weight on the hindquarters, but the absence of this ability does not preclude a horse from competing successfully at the highest levels of competition.

The force platform also measures the amount of braking and propulsion provided by the front and hind limbs. In a horse moving freely in hand, the body rolls over the limb while the hoof is in contact with the ground. Initially, each limb has a braking effect that tends to slow the forward movement. Around the time the cannon bone becomes vertical, the braking force changes to a propulsive force that drives the horse forward. These effects can be appreciated by visualizing the way the horse uses its limbs if it wants to stop suddenly by maximizing the braking effect ? all four limbs are fixed in a forward position like struts. The opposite occurs when the horse is trying to maximize propulsion, as in accelerating from a standing start, all four limbs tend to act behind the vertical position. Dressage training changes these basic patterns of braking and propulsion.

The hind limbs become almost entirely responsible for providing propulsion, which is in agreement with our traditional concepts. The role of the front limb, however, is not what we would have expected. The front limbs lose most of their propulsive thrust; instead they provide more braking, which is used in combination with the carrying force of the front limbs to push the shoulders and forehand upwards and backwards. Therefore, raising the forehand is much more than simply a result of lowering the hindquarters, it is an active process brought about by the action of the front limbs. A crucial component is the ability to use the braking activity of the front limbs to produce self carriage.

Passage shows a similar force pattern to the collected trot, but the piaffe is very different with the roles of the front and hind limbs being reversed in terms of the braking and propulsive forces (Figure 3). In piaffe, the front limbs provide propulsion and the hind limbs provide braking. The fact that the force patterns are very different in piaffe than in any other gait that we have studied (except the rein back), is a sign that it involves unique muscle activation patterns. This explains why the transitions between piaffe and passage are so difficult to perform.

A more complete understanding of the mechanics of self carriage and the mysteries of collection awaits EMG data, which I hope to gather in the not too distant future.

Hilary Clayton, BVMS, PhD, MRCVS, is a faculty member in the Department of Large Animal Clinical Sciences at Michigan State University's College of Veterinary Medicine. She was appointed as the first incumbent of the Mary Anne McPhail Dressage Chair in Equine Sports Medicine in July 1997. In that capacity, she performs scientific investigations that directly benefit the sport of dressage, with special emphasis on prevention and treatment of lameness problems. Dr. Clayton is a certified equestrian coach in the UK and Canada, and has competed in eventing, show jumping and dressage.