Proprioception - How do our hoses know how to put one foot in front of the other?
Proprioception refers to the innate ability of understanding where and how to position our bodies in relation to space and the objects around us. It is very closely linked to spatial awareness. An example of proprioception which I often give is walking up stairs - we generally only have to look at a staircase to be able to judge just how high we need to lift our feet. Proprioception gives our horses the ability to walk without thinking about where they are placing their feet, but also to provide stability to maintain balance and stay upright, perform fine movements and make precise and sudden changes if the situation changes. The whole body has to communicate to make sure that the animal stays upright. A prime example of proprioception and spatial awareness is when your horse bends round to itch a back foot. In the same action, the horse bends round to the side, lifts and brings forward the hindlimb and itches the exact spot with its muzzle.
So how do our horses know how to do this without falling over? Receptors which receive input about positioning are called proprioceptors. They can be found within ligaments, tendons, muscles and joints. These receptors respond to abnormal strain and tension. They consistently send input to the spinal cord and then the brain, which sends stimulation to the muscles in the body to synchronously move to try to ensure that the changes in posture and balance do not result in injury. Note that the inner ear also plays an essential role in balance, however, it is beyond the scope of this blog post. What influences normal muscular response? Input (response to strain) and output (muscular stabilization) are directed from the receptors to the brain and from the brain to the muscles respectively by nerves. Nerve conduction velocity relates to the speed at which an impulse travels along a nerve - imagine that the receptor is a post office, the brain is the central distributing hub and the muscles are the recipients of the letters. The speed of delivery is only as good as the means of getting the message to and from different parts of the body. As discussed earlier, with proprioception, this movement has to be instantaneous and subconscious. The speed of conduction is therefore influenced by nerve health, which is further influenced by the myelin. Myelin surrounds nerve cells to act like insulation, speeding up transmission of impulses. Inflammation, amongst other factors, can cause myelin to thin and therefore be less effective. Diet and supplementation can help to improve refurbishment of damaged myelin. The rate of impulse is also affected by the distance that it has to travel, and this is considerably longer in horses than it is in people. This is also developed by genetics, weight (heavier horses have increased joint strain), age (improves with maturity, decreases with old age) and type of upbringing - horses which are accustomed to different types of footing from a younger age subsequently have better proprioception (1). Why does proprioception start to fail? The main reason in horses is pain - after injury, the body tries to get into compensatory positions to limit strain which would predispose to further pain. This is done by decreasing joint mobility or by using, and developing, other muscles in other parts of the body, and restricting fascia, and therefore mobility, of other muscles. This can be made worse if a horse has to wear a cast or special bandaging which limits movement. Even once the injury has healed, and pain is no longer present, the memory of discomfort is still present, and the animal subconsciously protects that area from excessive strain. Treatment of a specific injury therefore not only involves improving the range of motion within the affected region, but also the muscles compensating for the discomfort. Retraining must also be gradual, giving the horse confidence in the integrity of the previously sore structure. Stiffness can also develop following a period of box rest, even if the reason for it was not orthopedic. Neck stretches and other exercises can help to maintain suppleness without exacerbating an issue. Although proprioceptive deficits are mainly a consequence of discomfort, it may also be due to a long list of neurological disorders, including equine motor neuron disease, cervical instability (such as Wobbler’s or osteoarthritis of the neck), and vitamin E deficiency. This is beyond the scope of this blog post, and severe deficits should be assessed by an experienced equine vet to rule out neurological dysfunction. What does failing proprioception look like? How many of us occasionally trip when going up the stairs, bump into a doorway which we have walked through countless times or have accidentally knocked over a glass of wine water? Although those are all signs of decreased proprioception, we should not be overly concerned unless this is happening consistently, and more regularly. Signs of decreased proprioception in horses can include increased toe dragging, forging or overreach, knocking of poles, and difficulty performing precise actions. This can be worse in the winter, as decreased blood flow due to the cold and increased nerve sensitivity due to an increase in atmospheric pressure make predisposed joints more sore, highlighting the importance of appropriate warm up.
Keeping a diary could help to create a better picture of what is going on with your horse - is your horse overreaching on one side more than the other? Are more poles being knocked with a particular limb? Is warming up taking longer than before? Is my horse struggling to stand for the farrier (potentially indicating discomfort when the lifted leg is flexed or due to increased weight bearing on the other leg)? Proprioceptive deficits could be an indicator that your horse is not quite right, and may warrant further assessment by your veterinarian, or suitably trained and qualified paraprofessional. Regular assessments, particularly under saddle, can help to track change and recognize early signs of degenerative diseases, like arthritis. How can we work on improving proprioception? Every horse is different, and cookie-cutter plans are dangerous, as they do not take into account the individual horse’s pathology, its temperament, the current degree of discomfort and the owner’s time and experience. As discussed earlier, a gradual increase in difficulty is essential to stop the anticipation of discomfort as the appropriate joint range of motion and muscling is acquired. Improvement is based on reversing the joint rigidity and decreasing the compensatory muscular tension. Exercises could also be used to maintain current proprioception, which is hopefully adequate. A combination of in hand and ridden work can be beneficial in ensuring that your horse continues to know ‘how to put one foot in front of the other’.
In hand exercises can include carrot stretches (use a lick if your horse nips!) to help activate dormant back muscles which assist with posture and obstacle courses over poles and around barrels on different footing. Obstacle courses can be changed daily to maintain mental stimulation and changes in terrain provide different stimulatory input. Pole work can also be performed, either when long reining, or under saddle. Pole work increases the range of motion in every joint in the limbs (2), whilst inducing flexion of the back and increasing stride length (3).
Using a bell boot on the limb with decreased proprioception during schooling for 3 weekly hour-long schooling sessions has been shown to increase muscle symmetry and improve proprioception (4). Conclusion It is nearly impossible to prevent our horses from experiencing pain or discomfort - be it from congenital problems as youngsters, strains during work, degenerative changes as they get older, trauma from a kick or self-inflicted in their stable or field. Eliminating pain and compensatory tension, as well as tailor-made rehabilitation plans to incrementally improve decreased proprioception are essential in maintaining musculoskeletal health. References
Denoix, J. and Pailloux, J., 2011. Concepts of neuromuscular physiology. In: J. Denoix and J. Pailloux, ed., Physical Therapy and Massage for the Horse, 2nd ed. Boca Raton, FL: CRC Press, pp.11-14.
Murrary, R., Walker, V., Guire, R., Spear, J., Newton, R., & Tranquille, C. (2020). Effect of walking over ground poles and raised poles compared to no poles on limb and back kinematics in horses with different postures. Equine Veterinary Journal, 52(S54), 9-10. doi: 10.1111/evj.10_13365
de Oliveira, K., Soutello, R. V., da Fonseca, R., Costa, C., Paulo, R. D. L., Fachiolli, D. F., & Clayton, H. M. (2015). Gymnastic training and dynamic mobilization exercises improve stride quality and increase epaxial muscle size in therapy horses. Journal of Equine Veterinary Science, 35(11-12), 888-893.
Jensen, A., Ahmed, W., Elbrønd, V. and Harrison, A., 2018. The Efficacy of Intermittent Long-term Bell Boot Application for the Correction of Muscle Asymmetry in Equine Subjects. Journal of Equine Veterinary Science, 68, pp.73-80.