Hindlimb asymmetries / differences are common in both leisure and competition horses and may include differences between the left and the right limb in terms of any of the following;
Stride length or duration
Flight arc (the amount of clearance the limb makes over the ground when moving)
Flight path (whether the limb moves inwards, outwards or moves forwards and straight when in the air
Stance positioning (whether the limb places on the ground in a square alignment with the front limbs)
Pelvic alignment when standing as assessed manually and visually using bony landmarks such as the tuber ishii, coxae and sacrale
Muscle mass
There is a link between lameness and asymmetries such as those listed above but we are not sure yet which comes first, the chicken or the egg? Research is starting to improve our understanding, for example saddle slip has been linked to unilateral (one limb) hindlimb lameness (Greve and Dyson, 2013). It is likely that the initial cause varies animal to animal however it is broadly believed that asymmetries can lead to lameness. Research has indicated that pain leads to redistribution of weight which in turn results in repetitive strain injuries as additional loads over stress soft tissues and joint surfaces (Jensen, Ahmed, Elbrønd and Harrison, 2018). Therefore to reduce the risk of injury it is sensible to identify and manage muscular imbalances.
To reduce muscular imbalances we must influence the animals central pattern generator. This is the body's automated system that allows movement to occur habitually with little conscious effort, allowing several tasks to be performed simultaneously. The reason we must target this system is because asymmetric muscle activation can remain habitually after an injury has resolved, resulting in a negative cycle of asymmetric movement and compensatory weight redistribution and further injury. The last sentence is important - we must ensure that pain is not present before attempting to improve movement symmetry, this is because pain will render your rehabilitation efforts futile and because pain can indicate an injury that requires rest and veterinary management.
So how do we do this?
One way that we can tap in to the central pattern generator during rehabilitation is to improve proprioception (the awareness of limb location in time and space). Physical stimulation of the skin of the lower limbs is a useful way to do this.
There are general practices that are useful to include in your weekly routine that target your horses proprioception resulting in improved muscle symmetry over time;
Cross train. Don't repetitively perform the same type of exercise for example regularly schooling on the same menage surface. Do perform lots of different types of exercise and over different terrain.
Ground poles and raised poles.
Hacking and gentle hill work
Get off your horses back and do some ground work in hand, lunging or long-reining.
There are also methods that you can use under the guidance of a qualified professional such as your vet, physiotherapist or hydrotherapist.
Underwater treadmill
Weighted boots
Tactile / proprioceptive stimulation
Weighted Boots and Tactile / Proprioceptive Stimulation
Weighted boots and proprioceptive stimulation have the benefit of being able to target the weaker limb only, whilst leaving the stronger limb alone. In research, weighted boots have been either human weighted wrist wraps used around the pastern or brushing boots with strips of lead attached to create specific weights. Proprioceptive stimulation has been achieved using an overreach boot or a leather strap positioned around the pastern with short, lightweight chains dangling at regular intervals.
Clayton et al (2010) conducted the first investigation into the effects of a leather strap with proprioceptive chains hanging at regular intervals on the hindlimb's of 9 horses and found that this technique had a positive effect on the swing phase (when the limb is in the air) through increased flexion of the stifle, hock and fetlock.
In a later study, gluteal function (top bum muscle shown above) was measured before and after a 6 week training period using an overreach boot on the weaker hindlimb in 8 horses. They found 'significant and positive effects on hindlimb imbalances' (Jensen, Ahmed, Elbrønd and Harrison, 2018).
Clayton, Lavagnino, Kaiser and Stubbs (2010) investigated the effect of human wrist weights attached to the pasterns of 6 horses at the time of exercise and found that they significantly increased flexion of the hindlimb stifle, hock and fetlock and that they increased the activation of the associated muscles including those which influence the hip.
Wickler et al (2010) studied 6 horses and found that weighting the distal (lower) limb increased oxygen consumption (indicating greater muscle activation through energy expenditure) and significantly increased stride length and duration described as 'general expression'. They also found that weighted boots increased extension of the carpus (knee) and the range of motion of the stifle, hock and hind fetlock.
Although research relating to this subject has been performed on small groups, all studies have demonstrated positive effects on muscle activation and mobility and therefore this is a useful method to improve your horses symmetry and reduce the future risk of injury. It goes without saying that there are risks associated with the use of weighted boots and therefore this must be performed under guidance of your physical therapist and / or vet. These professionals are also trained to look at the whole picture, this is invaluable as you may be unknowingly performing some types of exercise that will counteract any effect of weighted boots or tactile stimulation therefore rendering your efforts a waste of time.
References
Clayton, H., Lavagnino, M., Kaiser, L. and Stubbs, N., 2010. Swing phase kinematic and kinetic response to weighting the hind pasterns. Equine Veterinary Journal, 43(2), pp.210-215.
Clayton, H., White, A., Kaiser, L., Nauwelaerts, S., Lavagnino, M. and Stubbs, N., 2010. Hindlimb response to tactile stimulation of the pastern and coronet. Equine Veterinary Journal, 42(3), pp.227-233.
Greve, L. and Dyson, S., 2013. An investigation of the relationship between hindlimb lameness and saddle slip. Equine Veterinary Journal, 45(5), pp.570-577.
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.
Wickler, S., Hoyt, D., Clayton, H., Mullineux, D., Cogger, E., Sandoval, E., Mcguire, R. and Lopez, C., 2010. Energetic and kinematic consequences of weighting the distal limb. Equine Veterinary Journal, 36(8), pp.772-777.
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