What is movement symmetry?Movement symmetry implies that both forelimbs and both hind limbs are used equivalently, meaning that the footfall timing, stance duration, swing duration and sweep angle of the right forelimb are identical to that of the left forelimb. For the hind limbs the same applies. « The movement symmetry is the prerequisite of the horse training. It is also a main component of the performance assessment by the dressage judge who has to grade the gaits by evaluating their frankness and regularity ».
A sound horse on a straight line and on a flat surface should have symmetrical movements. To measure this, Seaver compares the up-and-down movement and the left-to-right movement of the torso of the horse during the placement of each limb on the ground (figure 1). In a sound horse, these movements should be symmetrical for each limb. « In case of asymmetry during the veterinary visit of an international competition, the horse will probably be barred from participation. If he competes, the horse will be penalized especially in the extensions where is weakness is likely to be all the more flagrant. Movements revealing an asymmetry won’t be given a satisfactory rating ».
However, it’s evident that this rule only applies to symmetrical gaits like walk, trot and pace. Since canter and gallop are asymmetrical gaits, the footfall of the left forelimb will be different from the one of the right forelimb (1). « An asymmetric horse may be limited in many aspects of his learning process and will necessarily be less competitive. Each exercise is likely to suffer from this anomaly. If walk and trot movements are the most affected, this can also occur when the horse is galloping. For example, the change of foot from left to right may be different from the change of foot from right to left in terms of amplitude and/or straightness if the horse is otherwise asymmetric ».
Why is movement symmetry important?If a horse moves symmetrical, it will bear the same weight on the left forelimb as the right forelimb and it will bear the same weight on the left hind limb as the right hind limb. Given that most lameness causes pain during weight bearing, horses will bear less weight on the affected limb to relieve the pain (2). « As a rider or coach, it is essential to be cautious about any asymmetry and quickly consult a specialist. Anticipation is essential to ensure that no health problem will endanger the well being of the horse. We must be able to detect any anomaly, quickly track down the cause and take the appropriate measures. In our jargon, if the horse falls to the left is that the pain is located in the right limb. Before questioning his riding and observing the problem from a purely technical point of view, the rider must check that his horse is clinically healthy ».
A threshold of approximately 25% movement asymmetry has been suggested for detecting a problem, meaning that a horse putting 25% less weight on one limb compared to the contralateral limb, might have a problem and may need to see a veterinarian. Seaver measures the weight distribution per leg, which will allow you to detect a problem in early stages. « The later a veterinary problem is considered, the more likely it is to cause complications. An asymmetry may not be observed, be neglected or misinterpreted. Being able to count on an accurate symmetry analysis reduces uncertainty and maximizes the chances of keeping your horse in good shape ».
Figure 2 shows an example of horse putting more weight on the right forelimb, indicating left forelimb lameness.
Factors influencing movement symmetryBeside lameness other factors can influence movement symmetry.
Symmetry measurements change in different ways when movements in a straight-line versus a circle are compared. When working in a circle, horses tend to lean to the inside of the circle with an angle of up to 20° compared to a straight position. Obviously, this means that the weight distribution of the horse will shift to the inside limbs. The lean angle of the horse’s body also depends on the diameter of the circle and the speed of the horse. Seaver is able to measure all of these parameters, which enables a thorough comparison of data during different exercises and directions. On the other hand, the amount of the lean body angle of the horse can also be used as an indicator of orthopedic problems when speed and diameter are constant. E.g. a horse with a problem on the inside limb will tend to lean less to the inside, to reduce the loading force on that limb.
Another factor that can influence the movement symmetry of the horse is the balance of the rider. If the rider doesn’t sit straight on the horse, the horse will get off-balance and the movement will be asymmetric.
Finally, the surface is very important. To be able to analyze the movement symmetry of the horse adequately, the surface should be flat and regular.
Doctor of Veterinary Medicine
* Also a big thank you to Camille, top dressage rider, for her precious help in writing this article :)
4th January 2017