I have a teenager. He’s fairly sweet and not normally given to tough love. Here’s what he had to say about my most recent riding injury:
Me: It’s just unfair. I tucked, I rolled, I got out the way of the horse. I did what I was meant to do.
Teenager: And you broke your shoulder not your back. That’s exactly what’s meant to happen. Now you get over it.
Voila, the effect of being parented by a biomechanist who consults in forensics. My son instinctively understands a fail-safe. From an engineering perspective a fail-safe is a way of preventing a more serious failure. It is a safety net – it doesn’t stop you from falling, but it does prevent the full impact of hitting the floor. It’s inconvenient when your house plunges into darkness because of a trip switch or a fuse that’s blown, but on the plus side
your house did not burn down and you are not on fire. In the horse’s legs the accessory (check) ligaments of the flexor tendons are partly a fail safe. When they break it is a large problem and they take a while to heal but nothing like the carnage that would have occurred if the check ligament had not “taken the strain” literally and figuratively and the main tendon had been allowed to tear.
In the British horse industry in particular, we love a fail safe. We tie our horses up with a breakable link, designed to snap under tension, a practise often frowned upon in the U.S.A. I see the arguments on both sides but I will say this: I once cross-tied a horse with both a leadrope tied solid and a travel bungee. When the horse slipped the travel bungee did exactly what it was meant to do and snapped, the leadrope did not and it flipped the horse over. The physical and psychological damage was pretty dramatic, and even the pure financial cost was a lot more than a replacement bungee. Anecdotal evidence counts for little other than to illustrate that whilst there are few times when you want an unexpectedly loose horse, there are some.
I’ve seen a lot of people complain when their kit broke, and I myself have lamented my damaged hat, ripped rug, snapped headcollar, etc. Working in equine biomechanics as an expert witness though, I’ve seen a lot of what happens when there is no fail safe. Personally, I’ve broken my ankle against a stirrup, so last year when I merely broke my stirrup I was happy enough to pay out to replace it. Similarly when a rug rips or the catch breaks, it’s always frustrating, but I’ve seen the alternative and that’s really not pretty. We use leather headcollars not just because they look the business but because – in the case of no acceptable alternative – they snap. Your stirrups may now be safety stirrups and your stirrup bars have long been designed to release the leathers. It’s worth extending that logic to every part of your horse’s world – your tack, your haynets, your ring-feeders, your fencing – if this fails, does it fail safely? If not, sometimes it can be quick and simple to change that. A fail safe is often cheaper than what it can save you from having to replace.
One of the most successful developments in this area is the safety cup (showjumping) or frangible pin (eventers). Gone are the days when even in showjumping if you came downwards onto a pole, such as a horse hitting the back bar of a spread, the only way that pole was going to shift was if you broke it in two (as often we did). Now cups are designed to release whichever direction you hit them in and I would encourage anyone to ensure that this is the type of cup they use in their competitions and at home. This release means that the cup also “fails” by hitting the ground and needs to be reset along with the pole, but as riders we already endure greater hardships than this. Horses do not need to hit their legs hard to learn to be careful, and jumping should be about the confidence to make mistakes, not a high-risk sport. The rotational fall is our greatest cause of serious injury and could yet be eradicated – the horse should never strike a solid enough object to be thrown into a cartwheel.
If release is not an option, our second defence for injury prevention is to dissipate the incoming force. Crumple zones in cars make modern vehicles rather easy to dent, but by folding up in a predetermined way the car protects the central passenger section from the worst of the blow. Someone else explains that here.
Your helmet is designed along the same lines to protect your head by self-destructing to absorb the blow. It’s got a pretty case around the outside so sometimes you can’t see the damage but still if you bash it you need to replace it. Body protectors on the other hand work by being hard, and saving your body from minor injuries and fractured ribs. They are not a fail safe. They won’t actually protect your spine or prevent internal soft tissue or crush injuries, that’s not what they’re designed to do (e.g. Mills and Gilchrist, 1990; Kelly et al., 2004). The utterly misleading misnomer “back protectors” and their compulsory use in some equestrian disciplines has rather dented any development or acceptance of the spinal protectors seen in other sports. There are plenty of people who tell stories where “without my back protector I would have had a spinal injury” but at the moment the evidence doesn’t support that. Various companies are trying to improve safety and create body protectors that can protect your spine. There is evidence for the need for better safety gear – particularly supports for the cervical spine – and only greater public awareness will help get these products developed.
I jump in a helmet/hat which is lightweight, flexible, vented and peaked. Those elements make it comfortable and beautiful, but give the manufacturers a nightmare job in making it sufficiently safe. My hat adheres to current safety standards, none of which address protection from rotational (brain-tearing) injuries, and those are the cases that would break your heart. These companies are the people we trust with our lives, yet bombard with our mostly fashion-based demands. We undermine them not just with our need for practical wearability, but with social media posts “exposing” hats which break into pieces – as many of the most safe are designed to do, in order to deflect the impact and save our heads. I wish every manufacturer every luck with their task.
Sox: externally rotated hinds, otherwise conformation just right.
An endo skeleton is a skeleton
that is worn on the inside. Some creatures, like beetles, have exo-skeletons
armour-plating their outside, horses have endo-skeletons providing support and structure from within, framed around a backbone, which makes them vertebrates.
Vertebrate bones are incredible piece of engineering.
Designed to withstand forces from all directions whilst still being as light as
possible. To do this, large parts of the bones have a honeycombed trabecular structure which has
been much copied in man-made materials. Trabecular or cancellous bone is basically composed of a series of small beams, so there’s material
where it’s needed, in the form of little supportive struts, and none where it
isn’t needed, cutting down on any extra weight. The property that’s really,
crazy, blow-your-mind clever, that we struggle to replicate in man-made
materials is its ability to adapt. The much-quoted Wolff’s Law tells us that
bone will adapt to the loads placed on it. That means that as long as you and
your horse are alive your bones are constantly adding struts, thickening parts,
and removing (reabsorbing) other parts. The whole system is constantly under
reassessment.
Why do we need to care about this particular marvel of
anatomical science? It means a number of
things for your horse. It means that:
a) Bones remodel to the strength we tell them that they
need. This means that they need an advance heads-up. If you’re going to do
something high impact, or have suddenly increased impact following box rest
then BUILD UP. By using repeated loading
within the horse’s current capabilities you can increase bone mass and strength
and hence stretch what is safe for him.
b) Bones are strongest in compression, since that’s the
direction they’re designed to load in. If they’re suddenly, unexpectedly loaded
in a different direction, for example by a bending force, they can often just
snap.
c) Bones will do their best to remodel if not correctly
aligned due to conformational defects, but usually this will mean bypassing the
bone and putting the extra strain on the joint.
Joints allow the skeleton to move. They’re essential, they’re magical, they
create the part of biomechanics that most people are the most excited about,
and yet they are a terrible weak point in the system. The majority of
orthopaedic problems originate at the joints. When we talk conformational
defects we’re normally talking about joints. The bones are just the linkages
that make the joints easier to see. The bones may be too long or too short, or
headed off in the wrong direction, but that deviation originates and inserts at
a joint, where the price is paid.
The joints are held together by collateral ligaments and
joint capsules and usually move due to articulating surfaces. This means that
it’s the collateral ligaments, articular surfaces and joint capsules that often
fail, along with the tendons responsible for taking the strain when movement
occurs.
Whether the horse is still or moving, it has to cope with
forces. If a horse stands on the ground, it is pushing down into the ground
with its body weight. Have a horse stand on your foot, it hurts. Horses rarely
stand on your hand. You can get kicked on the hand, sure, but then your hand
moves out of the way unless the floor supports it. I’ve yet to see someone
exert enough force with their hand to hold the horse up. At the same time as the horse is squishing
the ground, the ground is pushing back on it with an equal and opposite force.
Sometimes it doesn’t and the horse just sinks into the ground, but usually,
eventually, the ground pushes back hard enough that the horse can stand on the
ground.
We can measure this reaction force, the most confusing of
Newton’s forces, using a force-plate mounted into the ground. This can tell you
how much weight a horse is putting through an individual leg, by telling you
how hard the plate is having to push back, and what direction it’s pushing in.
In an ideal world we’d have one in every yard and vet clinic, telling us about
the subtle changes in the way the horse feels and functions. Biomechanics is
all about reactions to forces, and these are some of the very
forces we’re interested in.
Align these forces correctly with the bones, so they pass
straight through the joints, and the skeleton functions at its most efficient.
If a joint is not well-aligned, it will experience extra strain, and
potentially disease and failure.
Conformation
Whether
you’re choosing a new horse or trying to make the most of the one you’ve got,
being able to judge conformation is a handy skill. No horse is perfect but if
you’re aware of your horse’s weak points there’s a lot you can do to mitigate
defects, maximise soundness and make sure he’s up to the job. Many aspects of
conformation vary with breed and so some breeds may be more suited to one
activity than another, as different equine sports have different requirements.
However there are also a few basic conformation flaws worth watching out for in
all ridden horses.
As we’ve previously covered the
horse’s skeleton is actually very similar to our own. In the horse instead of
wrist we say knee, and instead of heel we say hock, but most of the bones and
tissues are the same. The horse is adapted to be as light and fast-moving as possible,
so he runs on his third fingernail/toenail, not the flat of his foot, and has
lost all “unnecessary” bones, including all of the other fingers and
toes. These adaptations leave a lot of bouncy joints for shock absorption, and
a lot of scope for variation.
Distal (lower parts of) legs
To assess limb conformation
you need a horse to stand well, and view him from the side, front and
back. Basically you’re looking for a
straight, well-balanced leg, with no major twisting in any direction.
Examples from the MUST HAVE book “Equine Locomotion” (Holmstrom Chpt, Back & Clayton Eds).
Pastern length is one of the
first aspects to check in the fore and hindlimb. If the pastern is too long the
fetlock will flex more, leading to excessive strain in the tendons or their
insertion points (such as the navicular or coffin bone). If the pastern bones
are too short or “upright” there won’t be enough flexion at the
fetlock for effective shock absorption. This means that if your horse has
pasterns that are unusually short or long, then you should minimise high impact
activities such as trotting on roads or a lot of jumping.
Straightness in the forelimb
In the front leg, a horsethat is over at the knee has the
appearance of a permanent knee bend, and this is not really that serious. A
horse that isback at the knee looks
like the knee has bent the wrong way. This causes additional strain on the
tendons and ligaments that struggle to maintain posture and support the weight
of the horse, particularly in jumpers or
racehorses. For these horses it’s a good idea to focus on tendon strengthening
exercises such as hill work (see previous posts).
Hobo: straightness from the front, check. Ability to wear a rug, lacking.
Pigeon toed (turned-in toes)
and toed-out horses are common. Toed-out hindlimbs are present in 80% of
warmbloods, so can be considered normal, and can even help with half-pass and
shoulder in. Toes that don’t point straight ahead are still not ideal due to
the increased stress to the lower parts of the limb, but not serious. Horses
with toe-in or toe-out are often seen competing at higher levels and it’s not
strongly associated with break down, although more extreme examples may cause
problems. Base narrow, toe-out forelimb conformation can increase interference
(brushing) injuries including splints so is often avoided in dressage horses.
For all other activities the addition of brushing boots can go a long way to
minimising this problem!
Toe-in conformation is often
seen with bench (offset) knees, which although common may predispose the horse
to splints and fetlock problems. These horses need to avoid deep surfaces where
possible.
Toed-out hindlimbs are not
the same as a cow hocks (narrower at the hocks). Horses who are only toed-out
and not narrow at the hocks will present a vertically straight hindleg if you
stand behind the point of the hock (and not behind the horse). Look at where
the hoof is pointed, forgive the deviation and stand behind the heel and hock,
then decide if the legs bend in at the hocks or merely point the wrong way. Sickle
hocks are over-bent when the standing horse is viewed from the side. They do
allow a horse to step under himself, but prevent him from being able to carry
that weight effectively and so are rarely seen in elite dressage horses. Poor
hocks, especially sickle hocks and cow hocks, have been associated with
osteoarthritis, bone spavin and back problems, so in these cases it is worth
avoiding occasions that cause a lot of strain – such as a lot of jumping, or
very deep or hard surfaces. Whilst horses with poor hocks might not have the
longest hunting careers, they rarely cause a problem in racehorses.
Hobo has straight but externally rotated hindlimb. Not cow-hocked, but looks similar from this angle.
On the other hand, in the
forelimbs knock-kneed conformation may even be protective and has reduced the
incidence of carpal fractures in racehorses.
Remus as a slightly knock-kneed
youngster (with poor hoof trim).
Straightness in the hindimb
As we reach the hoof, the
research shows that as the heels become more ‘underrun’ (low heels and long toes), the odds increase of
joint problems further up the leg. It is interesting that there is little
evidence that hoof angles affect the likelihood of disease or injury, only
evidence for the effect of hoof balance (differences between front and back).
Head, neck, body, upper legs.
Many aspects of conformation
that relate to the head, neck and body are difficult to measure objectively,
and so can lack scientific evidence, but breed differences in this area show
the effect of selective breeding for different activities. Plough horses and
racehorses look very different for a reason!
There is currently no solid
evidence linking shoulder conformation to injury, only performance. Elite
showjumpers and dressage horses have been shown to have more sloping shoulders
than average, and sloping shoulders correlate well with gait scores in young
horse performance testing.
It is worth remembering when
assessing the slope of the shoulder or croup that in some horses the outward,
muscular appearance does a good job of mirroring and representing the
underlying skeleton, but in many horses it doesn’t. It can be helpful to place
a piece of tape on the upper and lower parts of the shoulder bone to allow you
to stand back and observe the actual line. However a seemingly long and sloping
shoulder with good withers will place the rider in a good position in better
balance with the horse, and so the appearance of the shoulder can be as
important as its real slope.
Judges often use terms such
as “freedom of the shoulders” but high-speed analysis shows that
differences in forelimb movements are mainly influenced by the elbow joint and
not by the shoulder. Consequently a long humerus (upper arm bone) is strongly
correlated with performance in dressage horses, but rarely remarked on.
Elite dressage horses and showjumpers have flatter pelvises
than average riding horses, however again many horses have a flat croup
(muscles) and a steep pelvis (bone) so appearances can be deceptive. A flatter
pelvis assists pelvic rotation, and this is the most important determinant of
gait elasticity and jumping ability. On the other hand pelvic conformation does
not appear to affect longevity in hacking horses, and weakness here is often
compensated for somewhere else.
In the hindleg, a long, forward-sloping femur (thigh bone)
has been reliably and frequently shown to give both soundness and performance.
When we say that a dressage horse should be well “camped under” this
is the leg position that results from a forward-sloping thigh, which places the
hind well under the horse, aiding collection, balance and power transfer. It is
also possible to judge the femur’s position by marking both the point of the
hip and the horse’s knee, to allow you to judge whether the thigh bone slopes
forward or straight down. This is particularly important in hacking and riding
club horses where vertical femurs have been linked to leg and back problems.
Increases in height up to
around 17hh are linked to performance in showjumpers and trotters but not
dressage horses, and in all sports there is a massive variation in successful
horses. Sadly increased height also
comes with decreased soundness. Research confirms the adage that a short back
is a strong, healthy back, and good for performance, but it also predisposes
the horse to overreach injuries so overreach and solid brushing boots should be
considered.
Once we get to the neck, It’s
hard to objectively judge the actual “set” (attachment point), given
variations such as topline muscle, posture and wither height. A low set neck
can make it difficult for the horse to lift the forehand and so higher neck
posture is preferred for dressage. A longer neck can improve jumping
performance, acting as a counter-balance to the hindlegs, but a shorter neck is
common for dressage. Long necks can also increase fetlock problems, but only in
horses that race.
A wide throat latch (jaw) is
thought to facilitate breathing, although there is little evidence on this.
There is evidence that a wider-than-normal poll to throat latch distance is
often seen in elite dressage horses and showjumpers, and thought to help with
collection.
Many frowned-on conformational variables, including being
croup-high and lengths of cannon bones, do not affect the likelihood of injury
or disease, in the research at least. It’s an odds game, all we can do is give
ourselves the best chance we can, and then work with what we have. Other
factors such as temperament are just as influential, and for every solid
conformational rule, they’ll always be a
horse that beats the odds.
My last post covered a little about tendon structure, hysteresis and crimp and what all this means for the equine digital flexor tendons (link here). This time I’m going a little deeper into how tendons work, tendon creep, and how to make tendons (and ligaments) stronger. The main tissues I’m going to talk about are the superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT), and the suspensory ligament or third interosseous muscle
(SL), which despite its confused names is usually classed as a tendon. These form the suspensory apparatus, and I’ve talked a bit about what they do in the last post, but I’ve put a picture above so that we’re all on the same page (from one of my previous papers Lawson et al., 2007). The picture also has the main bones of the distal limb marked – the third metacarpal (MC3), proximal sesamoid bone (PSB), distal sesamoid bone (DSB), P1, P2 and P3.
Tendons are made up of a lot of different proteins, but the two main ones are collagen which resists loads and elastin which allows stretching. In tendons the collagen fibres are mostly parallel. This makes them effective in resisting forces from one predictable direction, and in turn being exposed to unidirectional forces encourages fibres to align and be more parallel, and we have a happy cycle. The collagen fibres in ligaments are less organised, as they can be pulled from more than one direction (see picture below of my lovely collateral ligament model). This makes ligaments weaker if we’re comparing stretching in one direction. In tendons and ligaments, collagen fibres are crimped when not under load, so they have an increased ability to lengthen and resist force without failing when they’re aligned in the right direction, more about that in the earlier post.
The amount a tissue can stretch compared to its original length is measured using strain (the ratio of new length to original length). The other thing we’re fond of measuring is stress, which is the load it’s exposed to as a proportion of its size (force per area). The amount a tissue will deform for any given load (stress divided by strain) is called the modulus of elasticity. This varies between tendons, and is one of the properties that will determine ultimate strength. The stress vs strain graph pictured shows the effect of modulus of elasticity on point of failure (ultimate tensile strength). The more a tendon can lengthen, the more load it can take before it fails.
So as tendons are not equally strong in all directions (anisotropic),
the chance of them failing partly depends on the direction of the force and partly on the tendon’s (modulus of) elasticity. The final important factor is how quickly a force is applied (viscoelastic properties).
Creep is the effect that allows tendons to progressively lengthen under the same force which has the effect of releasing tension. It’s the reason you can get further if you hold a stretch and keep stretching. Does it matter? Well, it means that tendons are great with constant pull, lousy with sudden impact. This is the reason that I talk a lot about the initial impact spike of hoof hits ground, and its effect on the digital flexor tendons, despite the fact that peak tendon strain occurs in mid-stance (for SDFT and SL) or end-stance (DDFT). Sudden changes in length such as a tendon struck (and stretched) by a hoof or an unexpected footfall from uneven ground – whether that’s rabbit holes or inconsistent arena surfaces, this is what causes a lot of injuries. I have a deep resentment of inconsistent/patchy all-weather surfaces.
To try and understand how to develop stronger tendons, a lot of research has compared foals raised in stables and given different formal exercise regimes with those kept in fields. Pasture-kept foals, that live out 24/7, consistently grow stronger tendons not just in the short term but actually develop tendons with more collagen fibres – i.e. with a higher potential strength that they can achieve through training. Whilst increasing exercise in stable-kept foals does lead to stronger tendons, in all studies 24/7 pasture exercise induces more strengthening changes than controlled exercise combined with stabling. Foals that are shifted from box-kept to being field-kept will to some extent catch up, but are unlikely to ever reach the same extent of collagen structure and tenocyte metabolism as foals that have always been in the field. It’s worth mentioning that in research tests horses that live in but have access to 2-4 hours of field time a day are considered to be stable-confined. You just can’t get the same amount of stimulation in 3 hours as you can in 24. Similarly pasture-kept horses are rarely in “natural” conditions, as pasture-size and herd-size tends to be limited, which reduces the amount of movement horses experience. You get the picture.
Research looking at the effect of training on tendons of horses of different ages supports this. Contrary to popular belief early training whilst immature can reduce the risk of injury in the older horse, as younger tendon has a strong ability to adapt to exercise and older (mature) tendon less so. The equine digital extensor tendons do grow (hypertrophy) with sprint exercise, but not so much the flexor tendons. The SDFT and SL, with their role in elastic energy storage, do not appear to gain in collagen content or diameter with exercise once fully mature, and these tendons are mature by the time the horse reaches about two years old.
After maturity tendons get weaker with age, and so become more likely to fail at a lower strain rate. When the horse is 5 years old the SDFT has already started to degenerate, with changes in crimp angle and collagen levels causing a reduced total strength. From here on in exercise accelerates this age-related degeneration.
Some studies have shown exercise-induced increases in SDFT diameter in 2yo thoroughbreds (but not warmbloods), but not an improvement in crimp angles or biomechanical properties – in other words tendons got bigger but not stronger. Tendons can grow due to pathological changes, so in research studies an attempt is usually made to rule these out by biopsy or ultrasound (as it was in this case). Tendon can also increase in volume just due to gains in water content, which is an exercise-induced response, but not one that helps. In immature warmbloods, all tendons and ligaments adapt, improve and strengthen in response to exercise, all apart from the SDFT. In all horses the SDFT just behaves a little differently.
In elastic-energy storing tendons such as the SDFT, increased size
increases stiffness so an increase in size actually reduces function. In these tendons being able to lengthen elastically rather than snap is more important, and so too much stiffness is dangerous. The improvement in the common digital extensor tendon with exercise is more helpful in improving the performance of the SDFT as these two tendons work together to help the SDFT hit the right stiffness for elastic-energy storage from hoof strike to push off. As I covered in the last post, the introduction of scar tissue in the tendon from overuse or previous injury is another potential source of increased stiffness.
Compared to hugely-responsive tissues like bone, tendon adapts slowly to exercise, and possibly not at all once mature. It may be that research in this area just isn’t precise enough at the moment to detect changes and predict optimal training regimes. In the great hierarchy of research funding, grants are keenly fought over and some excellent projects will always go unfunded. Understanding equine tendons is not the crocodile nearest the boat for many funding bodies, and not all human research can be directly translated as equine tendons are so unique. Research continues; slowly.
Images from my own research and graphs as before from Robi et al. 2013.
