Tag: biomechanicstheory

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 horse that is over at the knee has the
appearance of a permanent knee bend, and this is not really that serious. A
horse that is back 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.

Perfect horse. Dog needs work.