In 350BC the famous philosopher Aristotle wrote and published his work ‘On the Motion of Animals’, outlining what he believed to be the general laws of locomotion. One of his principle ideas was that all of earth’s creatures (us included) were right-handed, for “…the nature of the right is to initiate movement, that of the left to be moved.”. Therefore, he argued, every creature on earth must be right-handed, since “All animals… start movement from the right…” Clearly there can’t have been many lefties in Ancient Greece, and since many have historically been viewed as ‘untrustworthy’ (the word ‘left’ comes from the Latin ‘sinistre’), the ideal was for anyone and by extension any thing to be dominant on the right side. Nowadays we are a little more open minded on the matter, and although the subject is not given an enormous amount of thought when riding, it can have a significant influence on our horse’s way of going. That being said, it is important to remember that ‘handedness’ in horses, as in humans, is perfectly natural. For the majority of horses, it will be almost unidentifiable and have little impact – however, spotting and understanding the signs of lateral bias can be beneficial for both horse and rider.
Best foot underneath...
The horse’s brain can be divided into two lateralities: the left and right hemispheres. Each hemisphere connects to the opposite side of the horse’s body (this is the same for the majority of animals and us, too), and from day one the brain dictates which side the horse will be stronger on. A horse who shows repeated bias towards their left foreleg will have right-sided cerebral hemisphere dominance (RHD), whilst a right-foreleg preference shows left-sided cerebral hemisphere dominance (LHD). So Aristotle's theory of right-sided limb dominance is actually founded on a dominance of the left side of the brain!
Take a look at your four-legged friend out enjoying their paddock – completely unrestrained, out in the open, able to move and act as naturally as possible. Ever noticed that they habitually prefer to stretch out one leg more than the other? Horses naturally stand with one leg extended out in front of them to allow their necks to sink lower to the ground, in what we know as the grazing stance. Their centre of mass subsequently shifts forwards, adding more pressure onto the forequarters than if they were to stand square with their heads in a neutral position. The leg placed beneath their body is the weight bearing limb, and this is the favoured side. The weight bearing limb supports their body weight in balance and is positioned in readiness for any flight response – never forget that our domesticated equine friends still harbour all the characteristics of a highly-evolved prey animal… even if their greatest nemesis today is a lone leaf. For the majority of horses, we can see them alternate their loading legs as they eat their way across their paddock, however those with strong dominant sides will display greater bias towards their favoured side.
As you can see, H weight bears on her left foreleg; she will also scratch her face with only her right leg when she is trying to avoid doing any work...
The grazing stance can also be observed in the stable where horses are fed from the ground. Here Kai shows that he prefers to bear weight on his right foreleg.
The weight bearing limb throughout the grazing pattern will normally be exposed to greater levels of pressure and this will in turn affect the feet: one will eventually develop to be bigger than the other. However, at times the difference is so small that the only way you can tell is to feel the width of the coronet band: that on the dominant side will be broader and thicker.
Locomotor Dominance - bias in motion!
With the grazing stance, we concentrated on the foreleg positions, however quadrupedal (i.e. four-legged) movement is initiated from the hindquarters: so a right-leg lead (indicating LHD) is actually the culmination of movement initiated by the left-hind, therefore suggesting RHD.
Somewhat easier to assess is whether or not there is any bias towards a forelimb when ridden, most effectively seen at canter or gallop. This can present as a mild bias whereby the horse works more efficiently on one rein than the other, or a strong dominance whereby the horse will habitually strike off on one lead, especially when ridden on a straight line.
Lateral bias can also be seen when lunging: should a horse be reluctant to stay on a larger circle, and appear to be consistently ‘falling in’ on one rein, it is highly likely that they are dominant on the opposite rein. Of course as riders we strive to keep our horses balanced beneath us, particularly when schooling: the whole reason we sit on the outside diagonal when trotting in a school is because the horse’s weight shifts to the outside when turning in order to balance themselves.
Therefore, we cannot ignore the level of rider influence involved (both positive and negative), particularly if the rider shows evidence of strong lateral dominance themselves. However, a study focussing on leading-leg preferences in over 2000 thoroughbred racehorses found that there were significant levels of laterality – 51.26% consistently led with their left fore from a walk to gallop start, and 48.74% led with their right fore in the same conditions. Of this massive sample, 44 were studied further by repeating the study parameters over and over: 22 exhibited right laterality, 21 left laterality and one was considered to be ambilateral! (Cully et al. (2018).
Although most laterality can go unnoticed, one forelimb can become repeatedly overloaded if handedness is not addressed. This will ultimately lead to pain and pathology over time, such as osteoarthritis, sidebone or ringbone, especially if the horse’s movement is sufficiently asymmetrical over a long period of time. This is why it’s important to work your horse equally on both reins and ‘neutralise’ any laterality issues. Even correct lunging and long-reining can help to balance them through both reins. More advanced work such as shoulder-in and leg yielding can further improve balance and strengthen the carrying power of each hindlimb, but consistency is key.
Make sure you address any handedness or asymmetries you may have to eliminate any over-influencing of your horse. Have your trainer assess you and if necessary book an appointment to see your musculoskeletal professional!
Is that water bucket half-full, or half empty?
Incredibly, your horses’ lateral bias could be an indication of whether or not they are an optimist.
As I mentioned above, the right cerebral hemisphere of a horse’s brain is connected with the left side of the horse, but it is also associated with the fight/flight response. The left cerebral hemisphere (connected with the right side of the horse) is associated with more relaxed, investigative behaviour. Those who choose to investigate something new using their left eye are RHD and are therefore more likely to show reactive, fight or flight behaviour, than those with LHD. However, the horse is first and foremost a fight or flight animal, and that lone leaf shooting up from the ground in a surprise attack will create acute stress which will initiate their primal response.
In 2018 German researchers studied the way seventeen horses approached a new stimulus: they first presented the horses with a box which contained carrots and could easily be opened (a positive stimulus), and a box which was locked and offered no reward (negative). These were placed in the same location so that the horses could learn which box was which. The researchers then presented a third, ambiguous box in a neutral location to observe how the horses would behave towards a completely new and unknown stimulus. They consistently saw that those horses who readily approached the new box struck off on the right lead - suggesting that these seemingly 'optimistic' animals were LHD. Those who were hesitant to approach the box were deemed 'pessimistic' and any movement was initiated from the right side, suggesting that these were RHD.
Essentially:
Right-sided cerebral Hemisphere Dominance
Reactive, more emotional responses. Control of emergency responses (fight/flight). New stimuli are more likely to be investigated with the horse’s left eye, and the left side/rein is preferred for balance and weight bearing. Pessimistic, more likely to withdraw.
Left-sided cerebral Hemisphere Dominance
Proactive, more relaxed responses. Control of routine behaviour. New stimuli are more likely to be investigated with the right eye, and the right side/rein is preferred for balance and weight bearing. Optimistic, more likely to approach.
Standing on the left is SO eleventh century...
Although this is by no means a black and white scenario, the recognition and understanding of laterality could be a valuable tool when training, especially with young horses. Given what we know about RHD and reactive responses being connected to the left side of the horse, should we continue the centuries-old practice of doing everything on that side? It could be argued that to do so could actually have an influence on a horse’s laterality and therefore their whole attitude to being handled, mounted, exercised, etc. So even though natural handedness can never be truly removed, it’s worth thinking about. I mean, it’s not like we carry longswords anymore…
References and Further Reading
'Evidence for right-sided horses being more optimistic than left-sided horses' I. Marr, K. Farmer and K. Kruger, 2018. Accessed Jan 2021 from www.mdpi.com/2076-2615/8/12/219
'Side effects: Does your horse favour a certain side of his body?' Article by Gemma Pearson MRCVS, Horse and Hound magazine, 07/03/2019.
'Laterality: Implications for equine management and performance' Jane Williams, 2011. Accessed Nov 2020 from www.theveterinarynurse.com/research/article/laterality-implications-for-equine-management-and-performance/
'Handedness in horses: How to determine your horses' laterality' Ann Romsey BS CERA APF-1, 2019. Accessed nov 2020 from www.equine-rehab.com/handedness-in-horses/
'Farriery: The whole horse concept' David W. Gill, pgas 89-96, first published 2007 by Nottingham University Press.
'The laterality of the gallop gait in Thoroughbred racehorses' P. Cully, B. Nielsen and P. McGreevy, 2018. Accessed Nov 2020 from www.ncbi.nlm.nih.gov/pmc/articles/PMC5993273
'On the Motion of Animals' ('De Motu Animalium') Aristotle, circa. 350BC. Translated by A. S. L. Farquharson. Accessed Nov 2020 from www.classics.mit.edu/Aristotle/gait_anim.html
Image of the Bayeux tapestry obtained Jan 2021 from www.medievalists.net/2014/08/ten-things-may-noticed-bayeux-tapestry/ No copyright infringement is intended.
Jess Eyres
J. Eyres Equine Physiotherapy
January 2021
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