The dominant limb - can we train a completely symmetrical horse?
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By Georgie White
The dominant limb - can we train a completely symmetrical horse?
Limb dominance, handedness and laterality are all terms closely associated with the functions of the left and right hemispheres of the brain and how these may influence preferred use of one side of the body; a direct example in humans would be the preferred hand he/she learns to write with or foot to kick a ball with. The racehorse is often commemorated on their speed or ability to clear a fence, but you would be surprised how relevant limb dominance is to the performance and longevity of a racehorse’s career. This preference is often evident in an individual horse’s preferred canter lead, preferred leading limb in gallop, leading and landing limb over a fence. It can be argued that the former activities can be influenced by training, but to what degree and at what point should we utilize this preference? And when should we pay attention to the risk of injury? This article is going to discuss this relevance and explain the potential implications of limb dominance.
As with many corners of equine research, the observed phenomenon first began from studying the human and has since been studied extensively in performance and leisure horses, respectively. The goal of many a ridden horse is symmetry of movement, the discipline in which they train will dictate the degree of importance of this; the dressage horse can lose palpable marks when they are not symmetrical in their movement, whilst a racehorse may not be as well versed nor will it be a direct measure of performance quality.
Anatomical structure of a horse’s brain
The primary motor cortex is the central hub of dynamic movement, generating nerve impulses that control the execution of all movements and crossing the body’s midline to activate skeletal muscles. Every part of the body is represented and arranged somatotopically, meaning the area controlling the hoof is close to the area controlling the limb. The amount of brain matter or cortical space devoted to a body part represents the degree of control the horse has over that body part. For example, the human will have far greater cortical space devoted to intricate movements of the hands and fingers compared to the horse. In contrast the horse will have far greater cortical space reserved for movement of the ears, lips and nose to represent the vast number of social cues, foraging behaviors and body language exhibited. Although not yet proven in horses, human athletes have been shown to have increased cortical thickness in areas correlating with athletic ability or skill; this is likely to be the case in equines; for example, racehorses would likely see increases in areas devoted to limb control.
From a lateralization perspective, the brain is divided into two hemispheres or lobes which are referred to the left and right hemispheres and divided by the corpus callosum. The corpus callosum is an essential structure composed of a thick band of nerve fibers providing communication between the two spheres. Lateralization refers to these hemispheres having distinct functional differences, each offering specialized neural functions or cognitive processes. Lateralization occurs contralaterally, meaning that a horse who is left hemisphere dominant will experience a right-sided motor preference and vice versa.
What research is there on limb dominance?
In horses, grazing patterns are often studied, specifically noting which limb is placed forward most often; however, research is varied with some conclusions stating that innate skeletal asymmetry is highly prevalent in the foal and can be exacerbated by environmental factors. It is recommended that a farrier and vet work closely to correct visual asymmetry in goals as early as possible. A valuable longitudinal study completed by Van Heel et al., (2010) found a relationship with uneven forefeet in foals and subsequent unevenness as a yearling and then as a three-year-old; this was then found to correlate with sidedness in trot and canter transitions. This study highlights several points of interest, firstly the distinct need to correct limb asymmetries and abnormalities from an early age and secondly the potential future inclusion of quantitative laterality data at the time of studbook admission to assist breeders and owners in selection criteria.
Later research by Austin and Rogers (2012) stated there was no significant differences in limb dominance in a group of feral and wild Przewalski’s horses, which suggested that differences can be attributed to domestication at least to some degree. This varying body of evidence opens the discussion on the effect of domestication and relevance of cultural husbandry practices; in the western world it is common practice to complete many routine tasks from the left side of the horse, e.g., leading, tacking up and mounting. Future studies that attempt to measure this cultural influence may choose to select a large group of equines of generally the same breed and discipline to act as a control and prevent skewed results.
Many studies have attempted to decipher the causes of variation in motor bias. This is yet to have solid evidence behind it, with several studies showing links in gender and breed initially with training likely to play a part as they embark on their ridden career. As with all equine research, it is important to take note of cause and effect; for example, it may not be the fact that the horse is of Thoroughbred breeding, but the likely type of work or environment that a Thoroughbred is placed in that has supplied the correlation.
The gallop
A balanced change of leading limb can be the difference between winning a race and coming a close second.
As many a rider and trainer will describe, a horse will naturally have a leading limb preference and over a fence, they are generally left to continue with their preference; on the flat, generally left to the jockey’s discretion but the consensus seems to be less interference the better. Then comes the bends. In the transverse gallop, the horse will have a four-beat gait—a pattern of right hind, left hind, right fore, left fore—with the left fore essentially leading the movement and acting as the leading limb round a left hand bend. In the words of Dr. Ray Baran, the correct lead is “the easiest method to get the shortest distance between two points the quickest way”; as a result the horse is in balance.
Clockwise or counterclockwise?
Laterality research in the racing Thoroughbred has paid special attention to racetrack direction. …
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