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 Marie Rhodin DVM is an associate Professor in equine clinical biomechanics at the department of Anatomy, Physiology and Biochemistry, Swedish University of Agricultural Sciences. She has experience in objective motion analysis for lameness detection with techniques as high speed cameras and inertial measurement units. Her main research focus is orthopaedic diagnostics, orthopaedic pain, and objective motion analysis in horses, and horse-rider interaction.
Vertical movement asymmetry is a sensitive measure of lameness. Objective motion analysis tools are good at measuring symmetry and detecting lameness. They can help to identify the lame limb / limbs, to identify the type of lameness, to quantify the effect of diagnostic analgesia, and in assessing follow up treatment and rehabilitation.
We must take into account what’s known as compensatory lameness. A hindlimb lameness can show up as a forelimb lameness on the same side. A forelimb lameness can show up as a hindlimb lameness on the opposite side. It can be tricky to determine which is the primary lameness and which is the compensatory lameness. One way of determining which is which, is that the compensatory lameness will not respond to nerve blocks.
A study evaluating 60 vets assessing lameness on the lunge found that vets were fairly good at detecting which leg the horse was lame on, when the lameness was in a forelimb. In hindlimb lameness the vets were much less likely to be correct, and often they suspected a forelimb lameness rather than the hindlimb.
Looking at the kinematics and kinetics on a treadmill, a compensatory head nod was seen in many horses when a hindlimb lameness was induced. Withers movement is asymmetrical in both the forelimb and the hindlimb lame horse. In the forelimb lame horse, the withers come up as the head comes up, and go down as the head goes down. In a hindlimb lame horse, the withers come up as the head goes down, and go down as the head comes up. This is another way of detecting whether the primary lameness is in the forelimb or the hindlimb.
In a multi-centre study of 314 lame horses, 30% of the compensatory forelimb lamenesses were bigger than the primary hindlimb lameness. In 80% of the forelimb lame horses, the withers went down when the head went down, and vice versa. In 90% of the hindlimb lame horses, the withers went down when the head came up, and vice versa.
In one study, 222 riding horses in training were recruited. There were perceived as non-lame by the riders, and had not been treated for lameness within the past 6 months. The horses included show jumpers, dressage horses, eventing and pleasure riding. Objective motion analysis was performed on the straight line and on the circle. Vertical head and pelvic movement was measured.
This study found that 73% of the horses presented with motion asymmetries on a straight line. The degree of asymmetry was similar in magnitude to that of clinically lame horses. Various other studies have found that a large percentage of apparently sound horses present with asymmetries. A study into movement asymmetry in working polo horses found that 60-67% had motion asymmetries, and a study of racing Thoroughbreds trotting in hand found 92% with motion asymmetries.
Preliminary data from a recent study into 125 high performing horses in dressage, show jumping and eventing shows that 77% present with movement asymmetries. Dressage horses showed slightly bigger hindlimb asymmetries, it’s not known why this is.
The big question is, does movement asymmetry equal lameness with pain and pathology? There are a variety of influences on the symmetry of a horse. In the study mentioned earlier, where 60 veterinarians were assessing lame horses, there is very low agreement between the vets on whether a horse was sound. This suggests that evaluating the horse on the lunge to determine whether it is sound or lame is extremely difficult.
Therefore a study looked at 200 horses, who were perceived as sound by their owner. Straight line measurements were taken, and 94 symmetrical horses were chosen. What the researchers found was that lungeing sound horses increased vertical head and pelvic movement asymmetry during lungeing. Common pelvic asymmetric patterns that were seen were less upward movement during push off of the outside hindlimb, and less downward movement of the pelvis during impact of the hindlimb to the inside. This suggests that on the circle, asymmetry can exist without pain.
How does the rider influence the asymmetry of the horse? A study looked at 26 horses ridden in a straight line and on a circle, by one intermediate level rider. The horses were assessed unridden, in sitting trot, in two point seat, and in rising trot on both diagonals. Objective motion analysis was used to measure the movement of the horse. The researchers found that in rising trot, the rider induces asymmetric movement in the horse. This is not the case in sitting trot or in the two point seat. As the rider goes from sitting to standing, the horse is less able to push off. If we are riding on the correct diagonal, this counteracts the asymmetries that are created by the circle.
In the horse with a mild hindlimb lameness in a straight line in hand, on a straight line the horse will become more asymmetrical when the rider sits on the ‘lame’ diagonal, and less asymmetrical when the rider sits on the ‘sound’ diagonal. On a circle with the ‘lame’ limb to the inside, the ‘lameness’ with decrease when the rider is rising on the correct diagonal, and increase slightly when the rider is rising on the incorrect diagonal. On a circle with the ‘lame’ limb to the outside, the lameness will increase when the rider rises on the correct diagonal, and decreases when the rider rises on the incorrect diagonal.
Many horses in training have movement asymmetries when measured with objective systems. Many of these asymmetries are in a similar range to those of clinically lame horses. We have sound horses who are asymmetrical, and we have lame horses who are asymmetrical. However, we also have lots of horses who are asymmetrical, but we don’t know whether or not they are in pain. Therefore, a study investigated whether pain relief affected movement asymmetry.
The study included 66 riding horses in training, all of whom had movement asymmetries. The horses were assessed with objective motion analysis on the straight and on the circle, on different ground surfaces, at different speeds, and on different circle sizes. The horses were treated with meloxicam (pain relief drug) and a placebo, in a cross over design. There was 14-16 days between the treatment and the placebo. Treatment and placebo were both given for 4 days. There were no significant differences in movement asymmetry between the placebo and the treatment, on a straight line on a hard or a soft surface. This could be because the asymmetries are not due to pain, or it could be because meloxicam is not an effective analgesic in this situation.
Therefore another study looked at whether meloxicam reduced lameness. This study found that in many clinically lame horses, meloxicam was not effective in resolving lameness. 24 lame horses included, 1/5 or 2/5 lame, who were treated with meloxicam for 7 days. 16 of these horses were still lame after treatment. 11 horses had a reduction in asymmetry with diagnostic analgesia, and so we know that their lameness was due to pain. In the other 5 horses, it wasn’t possible to localise the pain or the pathology. These results suggest that we need better medication to help determine whether or not an asymmetry is pain related.
To help understand the prevalence of movement asymmetries, and changes over time in movement symmetry, Professor Rhodin and her team studied 106 unbacked riding horses that were between 2 and 3 years old. 73% of these horses presented with movement asymmetries. 73 horses were monitored for two years, and examined every third month. The regular examination included taking a history, performing a clinical examination, doing motion analysis on the straight and on a circle, asking a laterality questionnaire (did the rider feel that there was any one-sidedness in the horse. Also a laterality test was done on a subset of these horses. In total this led to 516 examinations, with an average of 7 per horse. No horse had symmetrical movements for all measurements. Only for 8% of the measurements was motion symmetry symmetrical for all variables.
The level of asymmetry did not increase over time, as the horses began and progressed with their ridden work. The most asymmetric horses did not necessarily continue to be the most asymmetric, but were spread evenly over time between more and less symmetrical. The horses who showed significant findings on clinical examination were not necessarily more or less symmetrical than those who did not show significant findings on clinical examination.
Another study looked at 103 young trotters, 1.5yrs old. 77 of these were measured in hand and when driven, 24 were measured in hand, and 2 were measured when driven. 93% presented with motion asymmetries in hand, 94% presented with motion asymmetries when driven. The horses were followed for 2 years, and this data is being studied.
74 Icelandic horses in training were measured in hand at trot. 87% presented with movement asymmetries. Some showed very large movement asymmetries, these were probably lame and in pain.
These studies highlight the difficulties for riders and trainers in detection movement asymmetries.
Moving on to foals, a study did motion analysis and laterality tests on 74 foals. 91% of these foals presented with motion asymmetries. These foals consisted of riding horse and trotters. The trotters were more asymmetric, compared to the riding horses.
Professor Rhodin and her team have started a new project called ‘Does it hurt?’, which is about identification of orthopaedic pain in horses. There have been studies describing the facial expression of horses who are in pain. A current study is looking at whether the pain face of a horse in the stable correlates with the level of asymmetry from an induced lameness. The data is suggesting that it does. This study will go further to investigate whether this is relevant in the ridden horse.
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
