Physiological implications of domestication on the horse
Very often, certainly in the argument of barefoot versus shod, comparisons re made between the wild and domestic horse. But in reality, are they in fact different species, or at least physiologically different?
When considering the implications of domestication, two main areas come to mind. Hoof morphology and posture. Shoes become a necessary evil of domestication with inevitable physiological implications, but this necessity for shoes itself is a product of the creation of a different physiology.
Starting from birth there are influences on the development of the young horse from their domestic setting. From their feet to their poll, domestication is affecting their development. Gard et al. (2015) studied the differences in the hooves of calves that were allowed to roam with their parents and calves that were barn raised. Results indicated that exercise on alternative terrain increased the volume and surface area of the digital cushion of the feet of dairy calves, which should make them less susceptible to lameness. We can extrapolate that domestic horses will not, for the most part, have as strong and robust hoof capsule anatomy as their wild counter parts. meaning a lower elastic modulus and hardness.
Fig.1 Gard et al. showed that foals allowed to roam with their parents in a natural environment will have larger, stronger more robust haemodynamic systems.
Just as important as the lack of movement, is the lack of varied substrate and therefore stimulation, and the increased levels of hydration the domestic hoof is often subjected to. A certain amount of moisture is documented as being important for the flexibility of the hoof, keratin materials are brittle without water molecules to plasticize the material. However, excessive soaking has been said to cause the hoof to become too flexible affecting its load bearing capabilities and possibly leading to increased plastic (permanent) deformation over the hoofs natural elastic (temporary) deformation under load.
Studies have shown us that both the tensile strength and hardness in the tubular and inter-tubular areas decreased significantly after full hydration. Even more interestingly and perhaps relevant is that the stiffness and hardness of tubules become less than in the inter-tubular areas, which is due to the higher water absorption in the tubular areas, this means these structures lose their reinforcing characteristic.
A study showed that in a fully saturated hoof sample, the tensile strength and hardness decreased by as much as 98%. In reality a hoof would probably not reach that level of saturation, but it illustrates possible implications of domestic boggy fields. To worsen the situation, while wild horses have the benefit of a more stable moisture content, the domestic horse often has huge fluctuations in moisture content affecting the hoof wall matrix. This leads to “crumbly” walls, wall separation and makes room for bacterial invasion.
Fig.2 The disruption of the hoof wall matrix as a result of the un-natural wet dry cycles the domestic hoof experiences. This also opens up the areas to anaerobic bacterial invasion.
The diet of the domestic horse of course also plays a huge role in the robustness of its hoof. In the wild, evolution has allowed the horses biological systems to fit into its natural ecosystem. As the vegetation and seasons the horse traverses ever changes, so to do the vitamins, minerals, and nutrition it exhumes naturally change.
Poor hoof quality, according to many studies, could be an expression of a lack of any one of the following: Crude protein, Sulphur containing amino acids (methionine primarily, cysteine), Essential fatty acids, Zinc, Copper, Selenium, Vitamin E, Biotin, pyridoxine etc. This shows the complexity of nutrition that enables ideal hoof wall growth and composition. Domestication immediately inhibits the natural relationship the horse has with its food and therefore its ability to grow the ideal hoof unless substantial effort is put into understanding and addressing the complexities. This domestic diet also has huge implications on the teeth, which we will outline later in this discussion.
Then of course we have to factor in when humans domesticated the horse their workload and intensity of work increased exponentially and equestrian sports that are simply unsafe to perform without the increased traction of shoes were created. Add to this the disregard for the concept of survival of the fittest and you can suddenly see the necessity of protecting the feet of beasts of burden. Horses who couldn’t cope with escaping predators because of disease caused by poor conformation or other pathological issues were easy prey, but humans do not have survival of the fittest in the forefront of their mind. Economic reason, or sentiment, far outweighs Darwinian principle.
So, with the domestic horse having inherently weaker feet due to the lack of development, living in conditions that create a softer hoof, with poor conformations that predispose to injury and playing sports that require extra grip, we see the man-made physiological need for shoes.
But shoes present with a whole new set of physiological implications. The hoof is a miracle of evolutionary bioengineering. Its structures and mechanisms work as a singularity to absorb the huge concussive forces it can be subjected too. Applying shoes restricts this function. If we look all the way back to snow and Birdsall 1990 they discussed how the shoe was restricting the natural deformation of the hoof. This was measured in 2001 by Roepstorff et al. who showed that the expansion and contraction of the hoof was restricted by the application of a steel open heel shoe, but the application of frog support padding helped to return this closer to the barefoot in terms of expansion but not contraction. How relevant contraction of the foot is to its health needs to be researched. What this does suggest is the negation of the haemodynamic mechanism is directly related to the amount of natural deformation occurring.
Fig.3 Frog support padding helps to re-establish expansion of the hoof and reduce shock. Here shod in Mustad comfort mix.
A more recent study Gunkelman and hammer (2017) discussed how the ability of the hoof to dissipate shock directly affected its morphology. This reduction in deformation that has been measured plays a part in the increased shock measured by other studies, but also the hardness of steel itself will obviously play a role in this. The higher stiffness and hardness of steel inevitably creates increased shock through the hoof, higher landing velocities, higher peak forces and higher and longer impact vibrations in traditionally shod feet (Willemen et al 1999, Gustas et al. 1999, Parkes and Witte 2015).
The other mechanism shoes restrict is the natural weight counteraction mechanism of the palmar hoof structures. All of this causes the prolapse and crushing of the heels, frog and bulbs of the heel. This perhaps helps us to understand the ubiquity of long toe low heel conformations as outlined by Dyson et al. (2011) and Clements et al. (2019) in the shod domestic population.
Fig.4 Shoes have been shown to increase shock. They also negate the natural weight counteraction mechanism of the caudal hoof structures. Both lead to crushed heels and prolapsed structures.
We also need to understand that the hoof is a neuro-sensory organ, it is the primary avenue for the horse to obtain information about the physical features of the ground surface upon which it stands and moves. These sensory structures detecting touch and vibratory stimuli are important to enable smooth transitions between movements of the joints and limbs during a performance, as well as maintain the appropriate postural stance.
Hoof balance therefore directly affects posture, and this brings us nicely onto the next consideration of domestication. Posture.
Fig.5 The proprioceptive feedback to the central nervous system from the hoof dictates the horses posture.
Posture is also influenced by many of the factors outlined for the necessity of shoes. The lack of movement, confinement, diet and what humans do with the horse.
Gellman and shoemaker talk extensively about dentition, the TMJ joint and the upper cervical area. These are huge proprioception input areas that are affected by domestication and influence the horse’s posture. They discuss how A horse’s standing posture, is a window into the overall integration of the complex neuromusculoskeletal system and the integrity of the stomatognathic system in domestic horses is commonly compromised by human intervention.
Dr Gellman outlines the grazing habits of wild versus domestic horses being responsible for why domestic horses have such a high prevalence of dental abnormalities. “Most horses living in domestication have several profound differences from wild horses: they commonly prehend and chew their food with raised heads, they eat concentrated, partially digested carbohydrates (grain pellets and sweet feeds), and they acquire only a small part of their overall nutrition from traditional grazing. In traditional grazing, horses shear more abrasive material with the premolars (i.e., plant roots with accompanying soil), keeping the front cheek teeth more evenly worn with the back molars. This domestic diet changes their occlusion which has a huge effect on their posture.”
Fig.6 Dental Occlusion and the TMJ are major proprioception centres with direct influence on posture.
Fureix et al (2011) used the application of geometric morphometrics to measure horse postures to evaluate the impact of environmental factors and to compare individuals and groups. They found that the postures of horses kept under natural conditions clearly differed from postures of riding-school horses kept in stables. They discussed how repeated exercise is known to impact the physical state of the horse, modifying its kinematics and muscular development, which is likely to influence the horses’ posture. Similarly, le simple et al. studies the same difference in domestic environment, finding that the riding school horses had a higher head carriage with a flatter neck.
Fig.7-8 Feureix et al. and Lesimple et al. measured the angles made by the neck and head carriage associated with different domestic situations. Higher head carriage and a flatter neck, associated with confinement and riding was linked to back problems.
Then we have further studies like a recent one Raspa et al. 2021 that looked at hay net position and how it affected back, neck and mandible position. They talked about how stabled horses are constantly subjected to postural modifications and these influences have a greater effect then aging.
More recent studies (Tabor et al. 2018, Taylor et al. 2019) have outlined further that these postural parameters then have a knock-on effect in the thoracolumbar region and possibly leading to kissing spine.
This “domestication” posture, to include the limb orientation has then been further linked to poor hind hoof balance and pathologies along the dorsal myofascial line (Mannsman et al. 2010, Pezzanite et al. 2018, Clements et al. 2019, Sharp 2021).
Fig.9 Showing the results of my recent research indicating the areas of pathology associated with negative plantar angles. The study agreed with the findings of the previous research.
These are all domestic implications on static posture, which becomes very important for the horse and its ability to rest. When the horse is standing with a vertical metacarpal/tarsal, suggested as correct posture by emerging studies, the bone is working in pure compression with minimal shear forces. It has been shown that larger animals shift from a crouched to a more upright limb posture, increasing the mechanical effectiveness of their limb extensors. By adopting an upright posture, large animals align their limbs more closely with the ground reaction force, substantially reducing the forces that their muscles must exert (proportional to body mass) and hence, the forces that their bones must resist, to counteract joint moments.
The benefits of upright posture extend beyond efficient counter action of gravity. This limb orientation provides stability with minimal postural sway.
Maintenance of posture is done through a relationship of body parts to one another in their functional response to gravity (Gellman et al. 2019). The wider the base of support the smaller the muscular effort needed to maintain centralised centre of mass. The horse, by evolutionary design, maintains the ability to quickly evade danger while utilising as little energy as possible. This means being able to rest while standing up. Cue the stay apparatus.
Fig. 10 Gomez-Costa et al showed more instability associated with a base narrow posture. We can extrapolate the findings to the camped under posture in the top left showing a hugely reduced base of support. These findings were also backed up by reading increased muscular activity.
Studies such as Gomez-Costa et al. (2015), measured the instability of base narrow horses, showing a more volatile centre of mass, increased postural sway and a resultant increase in muscular activity by using pressure mats and electromyography. While the same studies into camped under horses hasn’t been done, we can assume similar results from a reduced base of support.
The so‐called passive stay apparatus ensures limb stabilization with very minimal muscle activation. The key structure of the stay apparatus in the hind limb is the stifle joint. It can be prevented from flexing by fixation of the patella behind a ‘hook’, formed by the medial portion of the femoral. Due to the reciprocal apparatus, locking of the stifle means automatic locking of the hock. The more distal joints are stabilized passively by tendons and ligaments. A recent study (Mushonga and Bishi 2019) agreed with a previous study (Schuurman et al. 2003) that found during weight‐bearing the vastus medialis (but no other muscle) was active, providing minimal muscle tension to stabilize the stifle. The required tension was estimated to be less than 2% of the force that would be needed in absence of a lock mechanism.
Interestingly the 2003 study orientated the limb with a vertical metatarsal in what it called “normal posture” in order to test the stay apparatus. Although further research is needed to test the efficacy of the stay apparatus in a camped under posture, experiential opinion, logic and physics suggest it cannot be efficiently utilised with a non-vertical metatarsal.
Limb orientation also plays a role in joint loading. Moger et al. (2009) showed re-orientation of collagen fibres in response to surface loading in equine articular cartilage and Bourne et al. (2015), found Static, high intensity, short muscular pre-load protected cells from impact injury, whereas repetitive, low intensity, prolonged muscular pre-loading to the point of muscular fatigue left chondrocytes vulnerable to injury. Considering the increased muscular effort associated with abnormal compensatory posture and the inevitability of different joint surface loading, one has to question the predispositions of this posture on the joints. Could this be why we see it linked with both joint pathology (Pezzanite et al. 2018, Clements et al. 2019) and muscle soreness (Mannsman et al. 2010).
Inevitably as we mount our noble steeds, we also have direct effect on their dynamic posture. The static effect on the horse outlined at rest will create a pre-fatigued state and pre-dispose them to injury. But then what we ride the horse in and how, compounds this even more so.
According to the bow and string theory (Denoix, 1999), every action at the level of the head and neck will have consequences on the back kinematics. Yet, when ridden, horses are often stuck into nonnatural postures, with consequences on the back kinematics (Gomez-Alvarez et al., 2006; Rhodin et al 2005). In a large-scale study conducted on photographs of young, advertised and feral horses, McGreevy et al. (2010) showed that the head-neck angle requested by riders is significantly smaller than when the horses move freely, whatever the gait and the horse’s “experience” in riding.
Fig.11 Riding creates interference with the upper cervical area, also affecting proprioception, posture, and spinal kinematics.
Seneque et al. (2018) stated inappropriate riding techniques induce a stiffness of the spine and abnormal postures, due to constant opposition of the back muscles to the actions of the rider’s hands and legs. Alvarez et al. (2008) expressed how an elevated head and neck (From riding) induced extension in the thoracic region and flexion in the lumbar regions well as reducing sagittal range of motion.
These findings are going to inevitably effect major proprioceptive input area of the poll and upper cervical.
We can also add rider experience and skill levels to the mix with further implications on physiology. Williams and Tabor outlined the rider impact on equitation, stating an unbalanced rider will not be able to give clear and consistent aids to the horse therefore affecting their behaviour when ridden. The horse will have to adapt their locomotion to account for the moving weight of the unbalanced rider which increases the physiological demands of exercise.
These physiological implications have been quantified recently by Mackechnie-Guire et al. (2020) which suggested asymmetric rider position appeared to have an effect on the kinematics of the thoracolumbar spine. Rider size and riding style have also been shown to affect the horse’s physiology, Dyson et al. (2020) stated that large riders can induce temporary lameness and behaviours consistent with musculoskeletal pain. Christensen et al. (2021) which stated that riders significantly affected the horses’ physiological responses expressed that further studies are needed to identify which elements of riding style are particularly important for sports horse welfare, as well as to improve the objectivity, consistency and reliability of rideability assessment in sports horses.
Mcgreevy et al. (2011) explored the implications of elite sport horse training and its potential risks. It expressed how the industrial focus is predominantly on keeping horses in work, while possibly overlooking the training factors involved in creating physiological dysfunctions. Just the fact that we sit on their backs has implications for physiology, De Cocq et al. (2010) found that Weight and a saddle induce an overall extension of the back, possibly contributing to soft tissue injuries and kissing spine.
We can then add all of the studies into the effects of tack on the horses’ biomechanics, for instance Mackechnie-Guire et al. (2019) quantified the effects of saddle fit on the on the kinematics of the thoracolumbar spine, thoracolumbar epaxial musculature profiles, equine locomotion, and saddle pressure distribution.
The research into the effects of the rider and their tack on equine physiology is extensive with inevitable implications on the horse’s natural system state and biomechanics. Creating compensations and postural changes, creating predispositions to injury with further effect on hoof morphology.
This article only scratches the surface on the implications of domestication on our horses and it doesn’t even touch on the psychological implications and its effect on physiology. The reality is that farriers, physios, chiros and vets alike are for the most part dealing with and managing the realities of horses not living in their natural habitat. While in no way am I suggesting we no longer keep horses, keeping them does come with a certain obligation. To acknowledge the unintended consequences and to continue to research and do our best to employ changes to how we manage them.
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