In October 2021 I was lucky enough to attend the Centaur Biomechanics Equine Sports Seminar Virtual Summit. As always, the lectures were of a very high quality and there was an immense amount to take in. Here I share with you some of my understanding.
Dr Sarah Hobbs is a Reader in Equine and Human Biomechanics at the University of Central Lancashire, where she also gained a BEng (Hons) Mechanical Engineering degree in 2000 and a PhD in Equine Biomechanics in 2007. She is the leading author on the FEI Equine Surfaces White Paper and has led the development of a complete set of protocols for the certification of show jumping competition surfaces for the FEI. In addition, she has been commissioned to lead a project to evidence the FEI para-dressage classification system.
Dr Hobbs interest in the functional consequences of uneven feet began from conversations many years ago. The development of uneven feet had previously been linked to postural and loading preferences during standing, to pain avoidance, and to early retirement in the elite level sports horse (with show jumpers more likely to retire early). What we didn’t know was whether unevenness influenced loading patterns during locomotion, or whether individual foot conformation or the difference between feet was more important in relation to limb loading during locomotion.
To study this at Utrecht university, Dr Hobbs and her team looked at 34 sound horses, capturing detailed information on forelimb kinetics and kinematics as the horses trotted over a force plate. The first study was involved in defining unevenness. They measured 8 parameters, which were hoof area, hoof width, heel height in the unloaded condition, dorsal hoof wall angle, long pastern length, long pastern inclination, cannon bone length, and fetlock angle. These measurements went into a discriminant analysis to define unevenness, and to define where the threshold for unevenness should occur.
The researchers then looked at what the functional differences were, if the horse was defined as even or uneven. They were also interested in whether the difference in foot shape itself was an important part of alterations in function.
The variables studied included peak force, impulse (the area under the force / time curve), and stance duration. They also looked at stiffness parameters, including whether the fetlock dropped more on the flatter foot compared to the steeper foot, and the vertical force at maximum fetlock joint displacement.
In relation to foot shape in itself (flat, medium, or upright), the study found no significant changes in peak force, impulse, stance duration, or stiffness functional parameters.
In uneven horses, the flatter foot had a significantly larger peak vertical force than the steeper foot, and no difference in absolute time of the stance phase. The mean time for the stance phase for the flatter foot and the steeper foot was 0.32 seconds. There was a significantly larger peak braking force in the flatter foot, and a later relative braking to propulsion transition. The flatter foot had a significantly larger vertical fetlock displacement, with a more supple fetlock spring.
They learned that unevenness of the feet can be best determined by the absolute differences in dorsal hoof wall angle, with a difference of only 1.5 degrees to define an even horse compared to an uneven horse. Conformational differences between the forefeet were more important in relation to defining differences in function, than individual foot conformation itself.
Relative timing differences suggest altered functionality during the diagonal stance phase. Peak vertical force differences may suggest signs of subclinical lameness. More supple fetlock spring in the flat foot links to clinical observations at walk, that the fetlock on the flatter foot drops lower. It’s not known whether the functional differences in fetlock suppleness is due to altered loading patterns and / or to differences in tissue properties.
The second study looked at the Centre of Pressure (COP) path. The COP path was evaluated in 31 horses. A pressure mat measured the COP path, a 3D camera system located the position of the markers relative to the pressure mat, photographs of the solar hoof showing the position of the markers was taken prior to data collection, and Matlab was used to align the pressure data to the hoof segment, which included subtraction ‘slide’ of the hoof from the data.
Of 222 landings (centre of pressure paths) in the analysis, 175 landed laterally. 145 landed in front of the centre of the hoof, which is surprising since traditionally horses have been quoted as landing with a lateral heel landing. In only 35 of the trials was the centre of pressure close to the middle of the hoof in mid stance. The study showed that COP paths were highly repeatable within each limb. The COP paths were similar for most horses in the dorsopalmar direction (direction of motion). The COP position was more lateral throughout stance on the right compared to the left limb. Only 9 horses had mirrored mediolateral COP paths with dorsal hoof wall angles of between 1 and 8 degrees (some were even, some were uneven).
Traditionally, we would assume that we’d like a COP path to land slightly laterally and towards the heel to dissipate some of the impact through the digital cushion. In mid stance we’d like a more central alignment to reduce the moments at joints, and at breakover we’d like the centre of pressure to move further forwards to reduce the moments at the toe. This is not what the study found. This is perhaps because the COP is only one of part of the story of optimisation of the forces travelling through the limb.
In a third study, the groups were separated further by identification of the higher and lower dorsal hoof wall angles. The researchers explored the ground reaction force profiles of fore and hindlimbs using continuous data analysis techniques. They analysed the relationship between vector variables and dorsal hoof wall angle, and differences in ground reaction force patterns between contralateral limb pairs. They measured force vector analysis, vector magnitude, and vector angle. Statistical Parametric Mapping (SPM) analysis was used. The uneven group were separated into ‘high left fore’ and ‘high right fore’. Findings showed that functional differences were not mirrored in high left fore compared to high right fore groups. For high left fore and uneven groups, greater acceleration is evident during propulsion of the high diagonal. For the high right fore group, the mediolateral forces were mainly directed to the left side of the horse. That could perhaps make it more difficult for these high right fore horses to turn clockwise. Larger laterally directed force in the left hind in both high groups suggest that these horses have additional locomotor challenges.
In conclusion, the force profiles of uneven horses differ between forelimbs. The limb of the steeper foot is stiffer. The COP pattern in the forelimbs is not consistently influenced by unevenness. Uneven horses produce increased laterally directed force in the hindlimb. In general uneven horses accelerate more on the steeper footed diagonal particularly if this is left fore. Horses with a steeper right fore may find it more difficult to turn clockwise.
For more information, great webinars and a whole heap of relevant research, visit www.centaurbiomechanics.co.uk.
© Sue Palmer, The Horse Physio, 2021
Treating your horse with care, connection, curiosity and compassion