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.
Professor Michael Weishaupt works at the University of Zurich in the School of Veterinary Medicine and is in charge of the Equine Performance Centre. His PhD in 2004 researched the compensatory mechanisms of weight bearing lameness in horses, and in 2010 this received the Venia legendi of the University of Zurich for Equine Sports Medicine and Exercise Physiology. His special interests include assessment of poor performing horses, orthopaedics, shoeing, and the biomechanics of lameness. He has published two e-learning tools, one on equine upper airway diseases (Equad) and one on shoeing and diseases of the hoof (e-hoof.com).
Professor Weishaupt discussed forelimb and hindlimb lameness in terms of the forces through the individual limbs. There is a difference between the clinical view (watching the horse) and the scientific measurement of ground reaction forces. In a lame horse, the ground reaction force between left and right fore, or between left and right hind, will be different. The degree of difference depends on the degree of lameness. The treadmill that Professor Weishaupt and his team have developed can measure the ground reaction force through all four limbs. Measurements include peak vertical force, stance time, vertical impulse, swing duration, and step duration. The technology can then calculate the symmetry / asymmetry of the movement.
There is an ongoing discussion about what the definition of asymmetry is, and what level of asymmetry is within the range of ‘normal’. So for example, the peak vertical force measurement of the left fore will not be exactly the same on every stride, it will vary by perhaps 2-3% above and below the average measurement.
When we see lameness in the horse, we can think about whether the contralateral (opposite) limb is being overloaded, or whether the affected (lame) limb is being loaded for a longer or shorter period of time. We can consider how forelimb lameness transmits to the hindlimbs, how hindlimb lameness transmits to the forelimbs, and how we hear lameness.
A study undertaken by Professor Weishaupt measured the compensatory mechanisms of weight-bearing lamenesses, using sound horses, and horses with slight, mild, and moderate lameness in trot. The stride frequency increased and the stride duration decreased as the lameness increased. This means that there were more steps in a given time, and each step lasted less time.
The study found that a horse who is left fore lame will weight bear for less time through the left fore and through the left hind. In a left fore lameness, the decreased vertical limb impulse through the left fore corresponds with a slightly increased vertical impulse through the right hind, a significantly increased vertical impulse through the right fore, and a slightly decreased vertical impulse through the left hind. This tells us that the horse has shifted his centre of mass slightly to the right, away from the lame limb.
In forelimb lameness, the peak vertical force decreases in the lame limb as the level of lameness increases, but does not change in the contralateral forelimb. So a horse who is left fore lame will have a lower peak vertical force through the left fore and the left hind, no change in the right fore, and slightly increased peak vertical force through the right hind.
With increasing forelimb lameness, the stance duration increases through both forelimbs as the lameness increases. In the hindlimb, stance duration is unchanged as the level of forelimb lameness increases.
In conclusion, there are four mechanisms which contribute to reduce peak vertical forces in the affected (lame) limb. There is a reduction in stride impulse as the stride frequency is increased. The increased stance duration changes the rate of loading. The load is redistributed between the two diagonals, and there is a redistribution of the load within the diagonal. This means that there is no equivalent compensatory overload. However, a forelimb lameness can give a projected false lameness in the ipsilateral (same) hindlimb.
The data shows that in forelimb lameness, increasing asymmetry between the peak vertical force of the forelimbs leads to increasing asymmetry between the peak vertical forces of the hindlimbs. However, in hindlimb lameness, increasing asymmetry between the peak vertical force of the hindlimbs does not lead to increasing asymmetry between the peak vertical force of the forelimbs.
In relation to the walk, horses who were moderately lame in trot increased their stride frequency in walk, and decreased the stride impulse in walk. The vertical impulse decreased in all limbs, with the vertical impulse in the lame limb decreasing by 12%. The stance duration was decreased in all limbs in walk.
In conclusion, the load shifting mechanism is not only effective in reducing peak forces in the affected limb, but even suppresses an equivalent compensatory overload in the other limbs. However, there is a slight increase in peak vertical force in the diagonal hindlimb in forelimb lameness, and in the contralateral hindlimb in hindlimb lameness. The level of change will depend on the individual horse. For example, a horse who is sore through his back may adapt differently to a horse who is less sore through his back. As the horse undergoes rehabilitation, his movement patterns will change. Also, the alignment of the hoof will have an impact on the ground reaction forces, and so corrective farriery will influence the movement of the horse.
To return to a question posed at the beginning of the presentation, is it possible to define parameters within which the asymmetries of movement can be defined as ‘normal’? Professor Weishaupt’s conclusion is that in clinically sound horses, the variation of peak vertical force around the average rarely exceeds 3-4% of bodyweight, which corresponds to a weight shift of 15-20kg in a 500kg horse.
For more information, great webinars and a whole heap of relevant research, visit www.centaurbiomechanics.co.uk.
© Sue Palmer, The Horse Physio, 2021
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