‘HANDSOME IS AS HANDSOME DOES’ is a saying we all know. However, we are constantly reminded that good conformation is the key to good movement. What good conformation is exactly is hotly debated and, to a large extent, athletic conformation depends on the sporting discipline at which the horse is aimed.
For example, the long back of the Cleveland Bay horse would be considered to be a conformation weakness in a riding horse, but for a carriage horse (which is what the Cleveland Bay was bred for) it is a sought after trait because it improves the ability to pull weights and makes the horse less likely to sustain overreach injuries.
A straight hind leg is also frowned upon, but we know that a straight hind limb enables a greater stride frequency, so it is a benefit in the speed horse. So perhaps ‘Handsome is as handsome does’ is a good principle to bear in mind, and we should not discard a horse because he does not conform to some erratically researched rules. There are myriad books available dealing with conformation in detail; detail we will not go into here as this book relates only to function.
Athletic ability and good function in the horse is governed by the principle of levers. In moving animals, levers govern the movements of joints and the more efficient the levers, the more efficient the movement; and the principles of ‘conformation’ were derived from an ideal vision of equine levers. A lever is a rigid structure, fixed at a single point, to which two forces are applied at two different points. One force is ‘resistance’ and the other is ‘effort’. The fixed point is known as the ‘fulcrum’. There are three classes of levers, and the one that dominates movement of equine joints is a thirdclass lever where the resistance and the fulcrum are on opposite sides of the effort, colloquially known as the ‘wheelbarrow’.
This lever system provides two important functions. Firstly it increases the effect of an applied force in much the same way as lifting the handles of a laden wheelbarrow makes moving the contents easier. The second function is to increase the speed or velocity of joint movement. Imagine you are picking up a mug of coffee. The weight of the mug is the resistance; the effort to pick it up is applied by your biceps muscle, and the fulcrum is the centre of rotation of your elbow joint. To follow this line would take us into the highly complex mathematical science of biomechanics, but this is also not the aim of this book. We hope to be able to deliver ‘applied’ biomechanics in such a way for you to appreciate the function of the horse, and use that knowledge to improve your horse’s movement, posture and athleticism.
In Chapters 2, 3 and 4 we listed the major muscles of movement and their functions, but in this chapter we will introduce the science of equine movement as quantified by gait analysis, and we thank our friend Russell Guire of Centaur Biomechanics for supplying the majority of the images you will see in this chapter. However, this is just a thumbnail sketch of the work that is being done in research into equine movement.
Figure 7.1 Muybridge’s famous sequence of photos: The Horse in Motion.
Interest into equine movement really began with the seminal work of Muybridge in 1887 which stimulated interest in the discipline with remarkable photographs of equine movement. Indeed it was Muybridge who first demonstrated using his photographs that in the gallop gait there was an actual moment of suspension when no feet were on the ground at all (Figure 7.1).
However, this brief flurry of research activity into equine movement ground swiftly to a halt when the general use of the horse was replaced by steam and motor power. The number and use of horses declined dramatically as man turned his attention to greater providers of horsepower.
A revival of interest began in the 1970s with the introduction of the concept of the sport horse and the advent of cheap computer power, and since that time the scientific discipline of equine locomotion has come of age.
For a long time expensive computerised gait analysis equipment could only be used in a laboratory setting with horses working on treadmills. Still much of the scientific work is conducted in locomotion laboratories with a three-dimensional system capturing up to 1000 frames of data per second which can be used for sophisticated analysis, and not generally available for commercial use with the average riding horse. Also, it is known that horses working on treadmills move differently from horses working over ground and comparisons to actual movement in athletic performance are difficult to extrapolate.
Other systems are available for general use outside the laboratory but these are only working in two dimensions so are unable to give the sophisticated data of the 3-D systems. The most easily available 2-D system used by the Centaur Biomechanics Team is the Quintic System (www.quintic.com). This bridges the gap between the laboratory-based limitations of a 3-D system as the data output is taken from a 2-D plane and mainly looks at the angular displacement of the joints. Two-dimensional analyses can also be carried out in the outdoor or competition environment therefore giving a better impression of an individual horse’s movement. Indeed the Centaur Biomechanics Team were very busy taking data from horses competing in the 2012 Olympics for research purposes, and were responsible for the research behind the revolutionary Fairfax girth (see Chapter 6) which was widely acclaimed for giving Team GB Equine a competitive advantage.
Photo 7.2 Standard anatomical marker placement for gait analysis.
For data collection, markers that can be tracked by computer are attached to the horse in standard, palpable anatomical landmarks (Photo 7.2). The camera captures up to 240 images per second, whereas the human brain can only process about 12 images per second. Computerised gaitanalysis systems can, therefore, pinpoint even small errors or asymmetries in movement that could not be perceived by eye alone, thus detecting the so-called ‘sub-clinical’ lameness. In terms of the athletic horse, where correct dynamic and static posture are paramount, the ability of being able to pick up subtle asymmetries in movement is key to maintaining soundness and performance.
Even better, if the equine athlete has baseline movement data, against which his development can be monitored, this can be the basis for his athletic conditioning and physiotherapeutic support.
Gait analysis and movement issues
So what type of movement issues can the everyday horse rider, competitor or physiotherapist use gait analysis for, and how does it work?
Photo 7.3 Computerised image of foreleg limb segments.
Photo 7.4 The computer software tracks the markers as the horse moves, to obtain a visual representation of how the limb moves throughout one stride. (Courtesy of www.quintic.com
Figure 7.5 Screenshot of marker tracking throughout one stride.
As the horse moves, a video camera captures the movement of the markers applied to the horse and the system software digitises that movement so it can be analysed within minutes and displayed on a computer screen. The software effectively ‘joins the dots’ between the markers and displays specific segments of the horse’s body. Photo 7.3 shows the segmented forelimb. One immediate visual effect is a demonstration of how the limb moves through time and space, as the markers are tracked through one stride (Photo 7.4). All markers can also be tracked by the system throughout one stride and visualised on screen (Figure 7.5).
Figure 7.6 On the left is a graph demonstrating the changes in the knee joint angle of the horse walking on the right. The red trace is the right knee and the blue trace is the left knee. (Courtesy of www.quintic.com)
There are obvious limitations to a 2-D system when compared with a 3-D system. Firstly, you do not get an impression of any simultaneous adduction or abduction of the limb, which in terms of athletic ability can mean that you are only getting half of the picture; secondly, to make a left/right comparison, the sides of the horse need to be tracked on separate occasions, so that there is no actual simultaneous recording of both limbs. Therefore the Centaur team tends to focus mainly on measurable range of movement (ROM) of limb joints and making a comparison between left and right, but allowing for errors. For example, Figure 7.6 shows a comparison between left and right carpal (knee) movement.
There is a minimal difference in the joint angle movement between left and right in this horse, which may be down to the left and right sides being recorded at different times. However, leading the horse from different sides (to prevent the handler from coming between the camera and the horse) can also have an effect on the way the horse moves.
We are still taught that horses should only be mounted and led from the left for no better reason than that Cavalry Officers wore their swords on their left and if they mounted from the right, the sword would have got in the way of the left leg clearing the saddle.
However, this constant emphasis on always being on the left of the horse ultimately creates a subtle asymmetry. Indeed Gail commonly encounters horses that show a very slight weakness in the right hind, which can frequently be seen on the lunge on the right rein, as these horses tend to circumduct the right hind on to the midline or, in bad cases, on to the outside, and rotate the pelvis towards the centre of the circle. Gail refers to this as ‘weak right hind syndrome’. Therefore it is recommended that to prevent this anomaly, you should vary the sides that you mount (and dismount) and lead.
Figure 7.7 Analysis of hock ROM. The red trace is the right hock and the blue trace is the left hock. (Courtesy of www.quintic.com)
Figure 7.8 Fetlock ROM from a horse displaying asymmetrical joint movement which required further investigation.
The hock ROM can be compared in a similar manner (Figure 7.7). When significant asymmetries are detected between left and right joints, then this would require investigation. For example, in the ROM graphs of the fetlock in Figure 7.8, you can see an asymmetrical graph of fetlock ROM which required further veterinary investigation.
Although an appreciation of subtle asymmetries in joint movement is helpful, how else can we use 2-D gait analysis in our quest for an understanding of functional anatomy? We go back to two of our basic principles of the equine athlete.
Photo 7.9 Measuring shoulder movement and step length.
Freedom of shoulder movement is positively correlated with limb-stride length. We can use the gait-analysis system to measure and monitor this. In Photo 7.9 the system is being used for just this purpose.
We know from published research that a long sloping shoulder facilitates the forward and upward movement of the forelimb which in turn gives greater freedom of forelimb movement. Slope of the scapula has been correlated with higher gait scores in dressage horses.
Balance of dorsal and ventral muscular chains and lumbosacral joint movement
Figure 7.10 Measuring hind-limb protraction.
Again the system can make a quantitative analysis of length of hind-limb step, and how this can improve with correct schooling (Figure 7.10). The more hind-limb protraction in the cranial part of the stride, the greater the flexion of the lumbosacral joint in the canter, gallop and jump. However if retraction is lengthened in the caudal phase of the hind-limb stride, this would be indicative of restricted lumbosacral joint function and/or imbalance of dorsal and ventral muscular chains.
At this point we must mention one other issue that comes towards the top of our list for its relationship to injury, poor posture and loss of athletic function – foot balance and farriery. Again, however, we shall only be able to give a brief account of how important equine foot care is in maintenance of soundness, and we recommend that you read Gail’s last book, No Foot – No Horse: Foot Balance to Soundness and Performance, to gain a full understanding of this important part of equine welfare.
This is not a new phenomenon, because in the third century BC the famous Greek General and horse master, Xenophon, wrote:
Just as a house would be good for nothing if it were very handsome above but lacked the proper foundations, so too a horse, even if all his other points were fine, would yet be good for nothing if he had bad feet for he could not use a single one of his fine points.
That is just as true now as it was 2400 years ago. Indeed studies carried out on the relationship of foot imbalance to lameness conclude that up to 95 per cent of all horses have some form of foot imbalance which predisposes them to injury.
Attaining good foot balance will result in a foot that is of a shape and strength to support the weight of the horse whilst providing a base for optimum movement. The foot/shoe/surface interface is the complete dynamic base from which your horse moves; after all it is (hopefully) the only part of the horse and rider that touches the ground!
As we have discussed, as a prey animal the horse’s major means of defence is to run away. To propel the horse during locomotion the major locomotor muscles are sited close to the centre of mass, just as the engine in a racing car is situated close to its centre of mass. The further away from the centre of mass you get, the lighter the structures must become so that the large locomotor muscles can move the limbs through the air with as much speed as possible. There are no muscles below the knee and hock in the horse because muscles are heavy structures and would become energetically disadvantageous at the extremities of limbs. So the lower limbs are worked by a series of levers and pulleys – tendons and ligaments.
The bones in the lower limb also become lighter towards the extremities both in terms of their diameter and density, again because weight at the extremities is energetically inefficient. We can conclude therefore that the further away from the centre of mass a body part is, the lighter, less dense and more susceptible to injury it is, because there are no margins for error in the system.
If Lewis Hamilton’s racing car was sent out to race with imbalanced wheels, he would probably not make it beyond the first bend. More than likely his wheels will fall off because of abnormal forces being transmitted through structures which have no margins for such error. So it is with horses. Horse movement involves a series of collisions of the feet with the ground at high velocities and unless there is near-perfect foot balance, abnormal forces are transmitted up the limbs and cause abnormal forces in other structures throughout the body. As it is the structures in the lower limb that have the least margin for error, they are primarily affected, but breakdown can occur anywhere in the body after a series of postural compensations caused by poor foot balance.
Another important consideration is elastic energy. Abnormal loading from the foot/shoe/surface interface leads to inappropriate loading of the lower leg tendons which return that energy inappropriately and cause deviations in distal limb movement. For example a horse that ‘dishes’, i.e. the forefoot and distal limb swing outwards during limb flight, is more likely to sustain injury. Dishing is thought to be associated with a pigeontoed conformation and generally that is true. However, most toe-in conformation issues result not from deviations within the actual skeleton but from deviations in the hoof capsule caused by poor farriery or foot imbalance.
Photo 7.11 Toe-in conformation can be caused by a skeletal malformation or a foot balance deviation and it is important to know which it is.
In Photo 7.11 we see a horse displaying such a toe-in conformation problem. This was a racehorse that the trainer was unable to keep sound and veterinary investigation revealed a chronic foot imbalance which was putting strain on the lower-limb tendons and creating a dynamic postural problem, resulting in lameness in the lower limbs and back pain.
Photo 7.12 Hanging the left foot shows that actually the limb is straight.
Photo 7.13 Hanging the right foot shows that this limb is also straight.
So how can the average horse owner know if their pigeon-toed horse actually has a hoof capsule deformation or a genuine skeletal irregularity? To do this, simply pick up your horse’s front limbs as shown in Photos 7.12 and 7.13. By dangling the legs from the knee, you can look down the unweighted limb and see if the hoof capsule genuinely hangs straight, which both do in these photos. Therefore this horse had a chronic lameness problem caused by a hoof capsule deviation arising from many years of incorrect foot trimming and farriery.
Figures 7.14a and b This horse is just about to land on the outside of the hoof, and not flat-footed as he should: a) front foot; b) hind foot.
Sometimes it is possible to visualise poor foot balance by eye alone, but we can also use gait analysis to ensure that a horse’s foot flight is the best it can be. For example, Figures 7.14a and b show frames taken from an analysis of a moving horse by the Centaur Team; front foot on the left and hind foot on the right. The feet are just about to land on the concrete and, as is evidenced from the orientation of the front and hind feet, they are going to land on the outside of the hoof first.
This orientation of hoof landing creates abnormal forces of secondary loading in the foot created when the inside of the foot contacts the ground after the outside, which then leads to abnormal loading of the limbs. In fact if you look carefully at the front foot you can see that the inside wall of the hoof is beginning to become concave because of this abnormal foot fall.
Figure 7.15 Idealised image of good mediolateral foot balance.
Figure 7.15 is an illustration of what to look for in mediolateral foot balance. A perpendicular line dropped through the midpoint of the limb dissects the hoof into equal parts, with equal weight bearing on either side. Further, the slope of the quarters is equal on both sides. However, we must be careful about using this type of image as our ideal for one main reason: the coronary band is not a fixed structure and can deviate with chronic foot imbalance.
Figure 7.16 This horse’s heels are collapsed and underrun. Ideally all the lines should be parallel to each other. This leaves the back of the foot lacking full support so the shoe needs to be lengthened.
Figure 7.17 Lack of heel support/collapsed heels put abnormal pressure on the back of the foot, ultimately leading to pain and lameness, as shown by the red areas.
We can also look at dorsopalmar foot balance (Figure 7.16). For good balance in this plane the angles of the toe, quarters and heel should be equal. In this photograph, we can see that the heels are collapsed and underrun. In this case the back of the foot is left without proper support, and in this particular case the farrier has applied the shoe with extra length. If we imagine a line between the back of the coronary band and the back of the shoe, it will now be more in line. We can visualise the effect of lack of heel support with thermal imaging (Figure 7.17).
Gait analysis and static posture
Figure 7.18 Comparison of left (blue line) and right (red line) hock angles during standing.
We have taken a look at how we can assess movement, but gait analysis can also help us with assessing static posture and the effects of postural sway by recording any changes in joint movement whilst the horse is stationary. In Figure 7.18 we see changes in hock angles whilst the horse is standing. The problem here is that we have very little in the way of peerreviewed scientific evidence about the posture of stationary horses and what issues affect postural sway, other than we know that hind limb neurological deficit shows evidence of increased postural sway when measured using a force plate.
We do, therefore, have much to learn about how we can scientifically quantify static and dynamic posture in the horse, and how it affects athletic ability, susceptibility to injury or the evaluation of a rehabilitation programme. We know that 75 per cent of sport horses will sustain at least one lameness event in any one season, but their long-term assessment is largely ignored – many vets feeling that if a horse is not clinically lame there is no impedance to continued athleticism. However, postural compensations may already have been made, even though not clinically evident, and this is where the input of your ASSVAP equine sports physiotherapist is vital.
It is not sufficient to declare horses as either ‘sound’ or ‘lame’. In the absence of trauma, these are the two ends of a very wide spectrum with a range of dysfunctional movement in between. The equine sports physiotherapist will look at your horse in terms of ‘function’ or ‘dysfunction’.