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Balance is a key concept for riders. According to the FEI Rule Book, an one of the objectives of collection is to "further develop the balance and equilibrium of the horse, which has been more or less displaced by the additional weight of the rider". In the McPhail Lab, one of our current research projects is a study of the balance of horse and rider. This column will give you an introduction to the research we're doing in that area, together with general information about balance and the factors that affect it. First, some definitions:
When a horse is standing squarely, supported by four feet, the base of support is roughly rectangular in shape. If a plumb line were dropped from the center of gravity, it would contact the ground within the base of support. However, the center of gravity is not in the middle of the base of support. As you can see from the drawing, it lies closer to the fore feet than the hind feet, which indicates that more weight is carried by the fore limbs (approximately 55-60%) than the hind limbs (approximately 40-45%). The weight-bearing function of the fore limbs is reflected in their pillar-like construction, with relatively little angulation of the joints. The more angulated joints of the hind limbs are less well designed for weight-bearing, but they are well designed for providing propulsion. One of the objectives of dressage training is to move the center of gravity closer to the hind quarters so that the weight distribution between the fore and hind limbs becomes more equal. By relieving some of the weight from the fore limbs, the forehand becomes lighter and more mobile. One the current projects in the McPhail Lab is to calculate the position of the center of gravity of both the horse and the rider during different gaits and movements and to look at the effect of the rider on the horse's balance. Engineers Adam Arabian and Joel Lanovaz are exploring different methods of calculating the position of the center of gravity. One method is based on the position of the body segments, another method involves calculations based on measurement of the ground reaction forces. In the future we will apply these calculations to evaluate the location and movements of the center of gravity as the horse becomes more collected and performs different movements. The results will not only indicate the weight distribution between the fore and hind limbs, they will also allow us to determine the contributions the lowering of the haunches, the engagement of the hind limbs, and the elevation of the forehand to the overall shift in balance. Returning to the standing horse in which the line of gravity falls within the base of support, this horse is in a balanced position . we say he is in static equilibrium, which means he can maintain this position without tending to fall over (in more technical terms, the resultant of the forces acting on the body is zero). Factors that influence stability are:
During locomotion, a different situation pertains. The horse is in dynamic equilibrium, which means that balance is maintained through virtue of having forward motion. In spite of having only three, two, one or no feet on the ground, the horse maintains his balance because as his body falls toward the position of the center of gravity, another limb is placed on the ground which 'catches' the horse's weight, supports the body and projects it forward. This is repeated for each limb placement. The faster the horse is moving, the more he relies on dynamic equilibrium, and the less need he has for static equilibrium. Conversely, the slower the forward progression, the more the horse needs a large base of support to compensate for the loss of dynamic equilibrium, and this is achieved by having more feet in contact with the ground. If we take the walk as an example, the extended walk has sufficient forward motion that the alternating periods of support by two feet or three feet keep the horse perfectly well balanced. In the collected walk, the speed is slower and the periods of support by three feet become longer while the periods of support by two feet are shorter than in the extended walk. In the walk pirouette, it is even more difficult for the horse to maintain his balance. The time during which each hoof is in contact with the ground increases further, especially that of the inside hind which forms the key balancing foot around which the horse turns (for more information on the walk pirouette, see the November issue). A good way to understand balance is to think about riding a bicycle. As you cycle along at a reasonable speed, it's easy to keep your balance. But when you apply the brakes and go more slowly, it gets harder to keep your balance. When the bicycle comes to a stop, most of us topple to one side and put a foot on the ground, which greatly increases the area of support. Similarly, horses do very well with only one or two feet on the ground during a gallop, but as the speed is reduced, it's normal to see an increase in the number of supporting feet. Obviously, balance is an important component of the performance of dressage horses. Training plays a large role in developing balance. This involves learning to adjust the location of the center of gravity so that it is in an appropriate position relative to the base of support. Our research will lead to a better understanding of precisely how the horse accomplishes this.
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