A study of high-level dressage horses found to be clinically sound revealed evidence of inherent laterality – that is, the natural dominance of one side.
Swedish University of Agricultural Sciences researchers Anna Byström, Agneta Egenvall and their colleagues said the inherently symmetrical nature of the walk and trot made them potentially suitable for detecting left-right asymmetries.
The researchers used seven warmblood dressage horses, all of whom had been judged as clinically sound at the trot by an experienced veterinarian. They were fitted with a series of markers and walked and trotted unridden and unrestrained on a treadmill, with their movement tracked by 12 infrared cameras.
The study team found that, of the seven horses, five consistently dropped the withers more in early left forelimb stance. Another was found to be fairly symmetrical, and the seventh dropped its withers more in early right forelimb stance.
Their analysis showed that the asymmetry they found in vertical withers movement was related to a complex pattern of asymmetries in the stride cycle rather than to vertical load redistribution between the forelimbs.
This, they said, suggested that the asymmetry may be due to inherent laterality rather than weight-bearing lameness. What they had observed was consistent and repetitive vertical movement asymmetry of the forehand during walking.
“In classic horse training,” they noted, “it has been observed that most horses are more willing when lunged to the left than to the right, and many horses are reluctant, especially at first, to be lunged to the right at all.
“This observation was supported by a study of motor laterality in young riding horses through an analysis of derailment during trot on circles.
“In that study 70% of two-year-olds showed derailment when moving to the right, but none showed derailment to the left.
“Apart from behavioural observations, there are also biomechanical studies that have indicated that a majority of adult riding horses use the forelimbs differently.”
Discussing their findings, the authors said: “The extent to which this pattern of asymmetry in walk is characteristic of the equine population at large is not known.”
It was possible, they said, that this asymmetry might be associated with inherent or acquired laterality and asymmetries in trot, as well as in relation to equestrian training.
“Future studies need to investigate the effect of inherent asymmetry in ridden horses moving both on straight and curved lines, and in horses known to be clinically lame.
The full study team comprised Byström, Egenvall, Lars Roepstorff, Marie Rhodin and Elin Hernlund, all from the Swedish University of Agricultural Sciences; Filipe Bragança and René van Weeren, from Utrecht University in the Netherlands; Michael Weishaupt, from the University of Zurich in Switzerland; and Hilary Clayton, from Michigan State University.
Byström A, Egenvall A, Roepstorff L, Rhodin M, Bragança FS, Hernlund E, et al. (2018) Biomechanical findings in horses showing asymmetrical vertical excursions of the withers at walk. PLoS ONE 13(9): e0204548. https://doi.org/10.1371/journal.pone.0204548