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In a previous column, I described how we have used the force plate to measure the horse's weight distribution between the fore and hind limbs, and how this changes when the horse lowers or raises his head and neck. Another way in which we use the force plate is to assess a standing horse's balance and to detect changes in balance over time. Before describing how we measure standing balance in horses, let's look at how this is measured and used in people. If a person stands with both their feet on the force plate it registers the center of pressure, which is the central point (centroid) of the entire force distribution on its surface. Assuming the person's weight is fairly evenly distributed between the left and right feet, the center of pressure is approximately midway between the two feet. If more weight is placed on one foot than the other, the center of pressure moves toward that foot. Even when a person is standing quietly with no perceptible movements of their body, tension in the muscles is constantly changing. This causes very slight adjustments in the weight distribution and in location of the center of pressure. This is called postural sway. The movements of the center of pressure over a period of time can be measured and displayed as a graph, which is called a stabilogram. People who have neurological diseases, such as Parkinson's disease, often have balance problems and these can be detected by analyzing the stabilogram. One of the projects that is underway in the McPhail Center is an investigation of postural sway in horses. Eventually, we hope it may offer a means of detecting subtle disturbances in balance due to neurological diseases, such as EPM. But before we apply it as a diagnostic tool, we have to develop a suitable technique and gather baseline data from normal horses for comparison with those suspected of having neurological diseases. Due to the size limitations of the force plate, large horses are not able to get all four hooves on the force plate simultaneously, so we evaluate postural sway separately for the fore and hind limbs. The horses stand with the fore hooves on the force plate and we record the location of the center of pressure for 10 seconds with a recording frequency of 100 times per second. The procedure is repeated four times and average values are calculated. The horse then stands with the hind hooves on the force plate and four 10 second recordings are made. The stabilograms for the fore and hind limbs of a normal horse look fairly similar as shown in the picture. In order to make comparisons between different horses, we measure certain characteristics of the stabilogram. These include the range of motion from front-to-back, the range of motion from side-to-side, the area of the entire stabilogram and the average velocity between successive positions of the center of pressure. In horses, the range of motion from side-to-side is normally larger than the range from from front-to-back. This is a consequence of the fact that the base of support (the area encompassed by the hooves that are in contact with the ground) is narrower from side-to-side than from front to back, which equates with less stability in this direction.
Collecting postural sway data from the forelimbs, with and without blindfolds.
Balance is controlled by the horse's awareness and perception of his environment. This awareness is provided by visual feedback from the eyes, by the sense of touch from pressure sensors in various parts of the body and from information about the body's orientation in space that comes from specialized organs of balance. Removal of visual feedback by the application of a blindfold exacerbates the balance deficits in people afflicted by certain neurological diseases. The resulting increase in postural sway can be detected by analysis of the stabilogram. By analyzing stabilograms from horses with and without a blindfold, we have found no increase in postural sway when normal horses are blindfolded. The situation may be different, however, in horses with neurological diseases. Certain neurological diseases in horses are associated with obvious balance deficits that can be detected by clinical examination without the need for a stabilogram. The stabilogram of a horse with vestibular disease is shown in the picture together with a normal stabilogram for comparison. In this particular case, the horse was obviously ataxic (unbalanced) and we didn't need a force plate to detect the loss of balance! The next step will be to assess the sensitivity of postural sway analysis for detecting more subtle balance deficits in horses with mild neurological diseases, which present a diagnostic challenge to the veterinarian.
 A stabilogram showing a normal pattern (in pink) and a pattern from a horse with a vestibular
problem (in blue). The graph's scale is in cm.
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