Hoof Balance - Where and When?

Farriery, until recently, has been based on tradition, anecdotal evidence and personal experience, but with the introduction of objective locomotion assessment and importantly pressure plate systems, a new revolution of evidence - based farriery research is taking place (Oosterlinck et al 2019). Scientific study of the relationship between hoof balance and the wider musculoskeletal system is still in its infancy, however, it’s easy to see that this relationship is recognised as playing a huge role in achieving peak performance from our horses. keeping them sound and treating them after injury. The hoof is the horses point of contact with the ground, the biomechanics of this interaction dictate the physiological effects of movement on the given animal, however this doesn’t mean it’s the farrier’s fault if this relationship isn’t ideal. We must understand that the form of the hooves follows the forces that act upon it (Curtis 2002, Caldwell 2016). Conformation, hoof growth and the biomechanical feedback loop the hoof receives have direct effects on hoof morphology. From the second the farrier puts down a finished hoof, these components are having a negative effect on physiology.

A farrier can not change a horses conformation after the point at which its growth plates have closed, what one can do is facilitate a more balanced interaction with the ground by assessing the individual and shoeing it accordingly. Even with ideal conformation, normal hoof growth has negative effects on the internal structures of the hoof (Moleman et al 2006, Van Heel et al 2004,2005). Coupled with conformational defects that every horse will have, you have a complexed array of physiological effects. Curtis (2002) discussed how the forces acting upon the hoof are infinitely different in each horse, however you can predict the predispositions of certain conformations and how they will morph the hoof.

For the most part (assuming correct farriery) hoof distortion is caused by poor conformation and off-axis loading from above, not the other way round. However, it is the farrier’s job to recognise the imbalances and look to re-establish both static and dynamic efficiency.

Dr Kilmartin (2014) discussed the relationship between hoof balance and the wider musculoskeletal system, he stated that even a small amount of imbalance can cause a change in muscle development and tension in the upper body. A recent review (Larson 2019) echoed these findings concluding that hoof anatomy and biomechanics were closely interlinked and therefore farriery interventions can have a significant effect on equine locomotion.

Farriery can affect biomechanics at each stage of the stance by different interventions. The first, most important and most complexed key to evidence-based farriery and creating a positive feedback loop, is hoof balance. Conclusive findings on this subject of course remain elusive. Hoof balance is widely debated, remains subjective and new research adds even more complexities to understanding good practice. Balancing horses’ feet is a major factor in maintaining good performance and long-term soundness. The optimum balance is suggested as when the horse's weight is distributed equally over his foot (Farmer 2011).

But when?

Fig. 1 The phases of locomotion. Hoof balance has been shown to affect the loadings on the hoof throughout this stance phase.

The horse has different stages to a stance phase, which is the period of time the hoof is in contact with the ground as opposed to the swing phase, which is when the hoof is in the air. Hoof balance will affect loading at every phase of the stance! The farrier has minimal effect on the swing phase over and above the swing phase duration, as this is mainly dictated by asymmetries of the articular surfaces of joints, proximal muscle tension and central pattern generators (Hagen et al. 2017, Tabor 2020). However, the farrier plays a large role in the interaction between the hoof and the ground from first impact to how the hoof breaks over.

Up to this day, hoof care professionals use their eye to assess the impacts and loadings of the foot. This assessment largely dictates their trimming approach, yet we know that different farriers will trim differently, and trimming may result in significant changes in hoof conformation (Kummer et al 2006, van Heel et al. 2004). Changes in hoof conformation are then linked to lameness (Dyson 2011). Recent advances in gait and biomechanical analyses have provided several technologies to objectively evaluate these parameters (Faramazi et al 2018). Studies using these have bought into question the accuracy and relevance of what is seen, even by slow motion videos. A Study looking at dorso-palmar balance showed that trimming to restore hoof proportions resulted in a reduced Stance-phase duration, swing-phase duration, and gait-cycle duration (Faramazi et al 2018), this agreed with previous studies. van Heel et al (2004) showed a reduction in landing time and maximum lateral displacement of the centre of pressure after trimming, meaning the hoof was bearing load over its whole surface sooner and more centrally.

Fig.2 van Heel et al. (2004) COP trace.

van Heel et al (2004,2005) traced the centre of pressure (COP) in a trimmed, versus overgrown hoof and showed that the COP moved caudally in response, Moleman et al (2006) also outlined this caudal movement of COP. However, this is in contradiction to Weller (2020) who discussed the dorsal migration of the point of force (PoF)(Fig.2), which is the same calculated point as the COP. All the studies did however agree that an increase in toe length, decreased the hoof and palmar angle, increasing the moment force significantly around the distal interphalangeal joint (DIPJ).

Fig. 3 Poor dorso-palmar/plantar balance associated with hoof growth or a long toe low heel will increase the extending moment acting on the limb. This creates a need for increased flexor structure strain to counteract.

Van Heel et al showed that a slight lateral heel first landing (Fig.2) was most common and stated as normal, this finding has been echoed by more recent studies. Hagen et al (2017) stated normal as a flat or lateral landing, which would correlate with van Heel, however questioned the widely accepted heel landings as normal for non-lame horses. Mokry et al (2021) found a similar pattern, the most frequent hoof landing patterns at the walk were flat (39.6%) and lateral landing (35.4%) over all hoof conformations. Interestingly, Rogers and Back (2007) suggested a change from flat landing at lower velocities to heel landing at higher velocities and the other studies also outlined a change as gait speed increased. Clayton et al (1990) clearly showed an effect on each phase of stance from poor dorso-palmar/plantar balance. Clayton (1990) and Van Heel et al (2005) showed that hoof growth increased the landing time and breakover time and increased the incidence of toe first landings (Fig.4).

Fig. 4 Clayton et al. (1990) van Heel et al. (2004) the effects of poor dorso-palmar balance on the kinematics of the stride.

Its clear to see that improvement of dorso-palmar balance, where the proportions of the foot are made optimal around the centre of rotation, has positive effects on hoof biomechanics at every phase of the stance. It helps to reduce the incidence of the agreed pathological toe first landing and reduces the extending moment relieving strain from the flexor structures. The research into medio-lateral balance is less conclusive and more complexed. The studies above seem to agree on optimal initial impact being flat or lateral, however they don’t express a point at which a more pronounced lateral landing may become pathological. Considering current farriery practice focuses on flat landings to reduce the risk of pathologies associated with these landings, this, and the possible effects of forcing a lateral landing preference, to land flat, become important factors to determine in future research. Balance, put very simplistically, ensures the limb is evenly loaded. Each joint, bone, tendon, cartilage and ligament shares load as its designed to do and therefore has a much lower risk of injury (Oosternick 2019). Wilson et al (1998) highlighted the effects of medio-lateral imbalance, showing that the point of force moved toward high points of the hoof causing uneven loading of the internal structures of not just the hoof but the entire limb. Establishing hoof balance is the foundation of any shoeing job, remedial or otherwise. Establishing as close to ideal as possible, medio-lateral and dorso-palmer balance is critical both for the barefoot and before any shoe is applied (Oosternick 2019). However, new studies such as Johnson (2018) have asked questions of what true medio-lateral balance means. Adding a new measurement to the equation, impulse forces. These are a measurement of the cumulative loads on the hoof through the whole stance phase. With the ability to trace the COP and now calculate the additional impulse forces, new questions arise as to the importance of initial impact versus mid-stance and now versus the cumulative load through the entire stance phase. Asymmetry between medial and lateral wall heights has been linked with pathology of the distal interphalangeal joint due to abnormal loading of said joints (Oosterlink et al 2015). Farriery teaching therefore has focused its attention to level landings, yet, using quantitative techniques like pressure plate analysis, it has repeatedly been shown that lateral landing is the most common landing pattern (van Heel et al., 2004, Oosterlinck et al., 2013), although Mokry et al (2020) found a level landing slightly more frequent. With Hagen et al (2017) stating that trimming had minimal effect on landing beyond that of the human eye perhaps horses are showing an individual preference. Understanding the subsequent loading pattern throughout the rest of the stance phase may help uncover why. Hagen et al (2017) showed that the location of the COP was independent from the initial impact, indicating a flat landing is not automatically connected to a centrally located COP during mid-stance.

Fig.5 Hagen et al. showed that COP at midstance didn't correlate with initial impact.

The implications for this are that it becomes necessary to find an ideal trimming protocol for each case considering peak impact forces versus maximum load during the mid-stance phase. The study went on to say that the load and leading of the limb during motion are likely related to the conformation and development of higher structures such as the shoulder, the width of the chest or the angle of the carpal joint (varus, valgus). Hagen et al. (2017,2020) suggested that the swing phase and landing patterns could be more influenced by joint asymmetries of the limb, and central pattern generators (Tabor 2021). Importantly creating a flat landing in a horse with a preferential lateral landing was questioned, a change in the initial contact towards a flat landing could cause unequal loading of the hoof during the mid-stance phase, when the maximum vertical load affects the limb (Fig.5). Johnson (2018) asked similar questions, as stated earlier measuring impulse forces on the hoof. It also added yet another dimension, gait.

Fig.7 Impulse movies for the three different gaits measured from left to right, walk, trot and canter, divided into quadrants to calculate dorsopalmar and mediolateral distribution of impulse across the ground bearing surface of the foot during stance. Courtesy of Johnson (2018).

Johnson (2018) expanded on the different loading patterns at different gaits, citing an earlier study Reilly (2010) which found at walk, the hoof loaded 65% laterally, whilst at trot, the results showed a 50:50 split of force medial and lateral. Johnson (2018) found the impulses affecting the solar surface of the horse’s hoof at walk, trot and canter were shown to be different when comparing between the stance phases of the different gaits. This, like Hagen et al. (2017) has implications for our understanding of balance. Does a level landing mean a centralised load phase? And does balance for walk mean the same as balance for trot or canter?

As stated by Johnson (2018) “The implications of this to clinical practice are considerable, as farriers tend to assess dynamic hoof balance at walk and assess soundness at trot, a lateral loading horse at walk tends to be corrected by lowering the lateral quarter of the hoof to obtain even loading at walk. The results shown here could be extrapolated to show that this may then overload the medial aspect at trot, a considerably more concussive gate.” This sentiment was also expressed by a previous study. Aikins (2015) stated, “existing farriery protocols are based on a requirement to attain symmetry which may be incorrect and contraindicated by enforcing foot symmetry on horses that are predisposed to a natural inherent asymmetrical tendency.” This highlights the fact we must appreciate digit conformation when appraising initial impact and indeed mid stance load. For instance, Toed-out horses had significantly higher loading of the medial zone at the end of the stance phase compared to normal horses, but only at walk. It makes logical sense that we create optimum balance for the points at which the hoof, digit and limb will be under the greatest amount of load. While grossly imbalanced landings are pathological, and this article is in no way suggesting they are irrelevant, impact forces are significantly smaller then forces at midstance. For an athletic horse, a centralised COP at higher gaits may well be more important then a level landing at walk when considering the risk of injury. Medio-lateral balance is and always will be of paramount importance, but perhaps we need to reconsider our notions of when and where.

Having said all of that, we perhaps need to appreciate that these findings were the results of technologies measuring ever decreasing amounts of asymmetry, uneven landings too small for the human eye to see. Many farriers are now utilising these technologies in daily practice. In the light of these studies, we must be careful how we apply these studies to daily practice, both for those who use and do not use the technology. As we stated previously, uneven landings visible to the eye are probably indicative of dysfunctional physiology and are certainly known to lead to pathology both within the digit (Oosterlink et al. 2017) and higher structures (Kilmartin 2014). Asymmetrical landings could have many different aetiologies, conformational faults (Curtis 2002, Mokry et al. 2021) is a common causation, resulting in poor hoof morphology, namely lateral flare and medial contraction and bulb shunting. In the authors opinion, with presentation of these or similar obvious hoof morphologies intervention to influence landing and loading is suggested if only to manage the situation. However, there can be other factors at play that perhaps warrant further investigation to understand the genesis of certainly more profound asymmetrical landings, but also to see if they play a role in the high percentage of lateral landings shown by the previously mentioned studies. Common doesn’t always constitute correct. In the light of recent research into myofascial lines and their possible effects on locomotory patterns (Elbrond and Shultz 2015), we must also consider this influence as well as other higher musculoskeletal issues.

Having discussed the ever-mystical theory of balance and added new dimensions of minutia to the equation, the only conclusion we can come to is that there is more study to be done to establish the clinical relevance of these findings, and further to that, establish at which point asymmetrical landings become pathological.

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