Equine Mechanics

Category: Biomechanics Theory

  • Five Steps to Straightness

    Five Steps to Straightness

    Caught collapsing
    Different horse, same left hip too far in advance.

    Do you collapse on one side? Perhaps your shoulders look unlevel and try as you can you can’t seem to correct it. Maybe one heel doesn’t stay down, or your horse is just stiff and resistant on one side and you haven’t gotten around to blaming yourself yet. As you can see in these pics I sit asymmetrically like the barber who never gets his own hair cut so I thought it’s about time I corrected my own biomechanics. You’re welcome to watch & comment and hopefully we’ll all learn something.

    Here are two pictures from a couple of weeks ago showing the side of shame at its worst. My left hip is too far advanced and as a result my leg is crunched up, little man has too much bend in the neck and his left shoulder has drifted out. Note also the crinkled numnah as I’m twisting it with my crooked riding of shame!

    Tension, resistance and fall out of left shoulder.

    So: Step 1) Hips. I look like I have slightly asymmetrical shoulders. Technically I do but what’s more important and normally entirely overlooked is that I have very asymmetric hips. What you’re seeing in my shoulders is a result of that pelvis rotation, not a cause in itself. Similarly it looks like I’m sitting to the right, but this is just because I’m rotated to the right. If I pull my left hip back so that my pelvis is actually straight my horse stops resisting on the left. I can tell when I forget, because luckily my main horse is an unforgiving git – or if you prefer an excellent teaching horse who is sensitive to my aids. If I sort out my pelvis, my shoulders should look after themselves. Same goes for you, rubbish left leg.

    Step 2) Sleeping position. Have you ever noticed that people that crunch forward when they ride also sleep in the foetal position? I like to sleep on my right side, with my right leg straight, left leg bent up. It’s lovely, we snuggle. It’s allowing me to practice my folding up on the left bad habit to the extent that it feels natural & safe to sit that way. So I’m now going to stop reinforcing this asymmetry every minute of the day. I’ll try to sleep in a better position – stretching my left side; try not to sit on the sofa with my legs tucked up to the left; try not to carry my bag on my right shoulder… Basically if something feels natural I’m going to reverse it!

    Step 3) Halt. Not the horse, little horse recently has stopping as a forte – I need to practise my own halt. This morning I looked at my shoulders in the mirror, and noticed that when I’m on the ground they are indeed unlevel. Then I looked down and saw that my hands are uneven – I can see more of my left hand – so I tried to roll my left shoulder back. Then it twigged and I looked even further down. I’ve only got two legs and still I had not pulled off the square halt. My left foot was way in front. If I shut my eyes and stop left foot first it looks fine. Right foot first and I don’t naturally halt square. I’m not mentioning any names, but one of us is going to practise until I do.

    Step 4) Horse is going to go straight. Little boy and I had a good deal going – he likes to go crooked – I like to be crooked in a matching way! Great! He likes to put my weight on his right, where we both reinforce each other’s bad habits and just hope we never need to go left. So now that there’s a new regime in town we are going to go straight. Even when on a circle or going sideways we are going to be straight. My hip will be back, my weight will be down his left side as well, and I’ll know when it is as our lateral right will be as easy and round as our lateral work left, and when we land from a fence, we’ll go straight between two ground poles. For our piece de resistance when we turn left towards a jump or in counter canter in a test, he is not going to get gobby and complain. And I will pat him; with my right hand.

    Step 5) Biomechanics. I often video and check what’s going on. I video my riding & I video trot ups. Sometimes I’ll calculate joint angles, but this isn’t biomechanics. Joint angles and videos are kinematics and they’re useful – particularly if you have a robust way of calculating accurate and repeatable angles and aren’t just “joining the dots”. Biomechanics is the combination of these kinematics with kinetics – the study of the forces. By including the forces we can work out where the rotation originates – what is a primary problem and what is a secondary compensation, which muscle is too short, which one is too weak. Clinically and in training it’s important to consider the biomechanics, not just the kinematics. In my case that’s correct the pelvis, not the shoulder or leg position. If I just force my leg into the right place without correcting the pelvis I’ll make things worse. I need to correct the muscle imbalance & tightness that causes the pelvic rotation in the first place. That’s why I’m also working at it on the ground. So: I’m not going to force my left heel down, I’m going to drop my weight. I’m not going to correct my shoulders I’m going to straighten my spine. I’ll get my thumbs on top by correcting my elbows. I’ll go to the gym and stretch and correct until I’m neutral. I’m relying on you guys to keep me on the straight and narrow!

    Comments?

  • Dressage and the Equine Back

    Dressage and the Equine Back

    The Bow and String Model, Slijper 1946
    The nuchal ligament, and its action on the withers. Adapted from Denoix 2001

    The equine back is gaining popularity as research topic, but it has a long way to go. It’s difficult to research, diagnose and treat because it’s so inaccessible and so complex. The most popular current model of the back is the bow and string introduced in ’46 by Slijper. In this theory the vertebral column (spine) is a bow held in tension by the ventral abdominal wall – the part we call the stomach. Contraction of the abdominal (stomach) muscles, particularly the rectus abdominus, will tense the bow flexing/rounding the back, as will the engagement of the legs underneath the horse (retraction of the forelimbs and particularly protraction of the hind limbs). The string will be tensed (i.e. the back extended/hollowed) by protraction of the forelimbs and retraction of the hind limbs – when the forelimbs flash out in front or the hind legs trail behind. The string is also tensed by the weight of the abdominal organs, hence the many old brood mares with their often sunken backs.

    The bow and string theory puts a lot of emphasis on head and neck movement, a subject that people already get very excited about. Theoretically, if the head is lowered, the nuchodorsal ligament will pull on the withers and flex/round the spinal column. Vice versa, lifting of the head will extend/hollow the back.

    In dressage the desired position of the head and neck is described by the Fédération Equestre Internationale (FEI) as: “The neck should be raised, the poll high and the head slightly in front of the vertical”, implying a much more upright position than in the natural situation. Both in academia and on forums across the internet the welfare implications of training techniques using neck positions strongly diverging from the natural position are always a popular topic. Studies carried out by an international group have established that there is a significant influence of head/neck position on spine movement. Positions with an elevated neck tend to induce extension (hollowing) in the thoracic region and flexion (rounding) in the lumbar region. Lower neck positions produce the opposite. High neck positions generally lead to reduced movement of vertebrae, especially in the lumbar area, but low-neck positions increase motion. A very high neck position seems to greatly disturb normal motion, much more than a strongly flexed position (Gómez Álvarez et al., 2006; Weishaupt, 2010, Rhodin et al. 2005). The 2006 workshop organized by the FEI in Lausanne discussed the acceptability of the Rollkür or hyper-flexion training technique, in which the horse is ridden with a strongly flexed mid-cervical region that brings the head almost down between the front limbs. The preliminary outcome was that “there was clearly no evidence at the present time that any structural damage is caused by this training exercise, when used appropriately by expert riders” (Jeffcott 2006). The effects on vision, respiration and psychology, I’ll leave to another post…

    Carl Hester and Uthopia in competition 2011

    The great majority of the muscles that attach to bony elements of neck or back run from one part of this skeleton to the other, not attaching to the limbs, prioritising active, internal stabilisation to compliment the passive, internal stabilisation provided by the ligaments. The back will try to compensate abnormal or one-sided loading of it (e.g. by lameness or rider) by increased muscle tension. As a result, painful muscle spasms are common and early clinical signs of back problems even though they are generally secondary in nature.

    The different gaits have characteristically different movements of the spine and muscle activity. The walk is largely under the influence of passive mechanisms, where movement of the head, neck and limbs affect the spine kinematics. At the walk the back does not experience the twisting of the thoracolumbar junction seen in the trot and canter. The trot shows a very stable back with a reduced range of movement, and with major restraining influence of the muscles. At the canter, the back is influenced by passive mechanisms as in the walk and is restrained by the muscles in the total range of movement as in the trot. Muscle activity has a restraining function instead of an initiating function. Diagonal support of the trot and canter sees extension and twisting of the spine in the areas where pathologies are often found. Abdominal muscle strength, as well as hip extensors, are important in stabilising the back and preventing these injuries. There are clear relationships between back conformation and movement that are likely to be important to the orthopaedic health of the horse. Another topic for a future post…

  • Pushing off the ground – Newton and take off

     When a horse pushes off the ground to take off or just to travel, they have to obey the laws of physics:

    In order to take off:

    1, There must be a force (push)

    (Newton’s First Law – change requires a force)

    2,  The force must be enough to accelerate the horse’s mass. Think about trying to throw the horse up there yourself: it’s the change in speed and the mass of the horse that matter. 

    (Newton’s Second Law, Force=mass x acceleration)

    3, The floor must resist the push, or the horse will just sink. 

    (Newton’s Third Law, equal and opposite actions)

     In our picture our rider is ahead of the horse, and lifting with his hands.

    As he’s ahead of the horse his centre of mass is in front of the horse’s centre of mass. This prevents the horse from raising its front end effectively (and puts him at risk of a fall). Likely outcome: pole down in front.

    However he’s trying to compensate by lifting with his hands. This shortens the horse’s neck, bringing the head in. As the head and neck are a major part of the horse’s weight, this shifts the centre of mass backwards (caudally). This could help avoid the pole in take-off BUT also has implications for the rest of the jump. The head and neck act as a counterbalance over the fence, rotating the back end up. The longer the lever of the neck the more turning effect will apply to the horse (Moment =force x distance) so the higher the back end will go. A shortened neck prevents this adn lowers the back end. Likely outcome: pole down behind.

    So which was it in the case of this rider? I’ll leave you to decide.

  • Gait analysis for shoeing

    Computational model of the equine limb By Lawson, Chateau, Pourcelot, Crevier-Denoix and Denoix.

    Biomechanics is “The science that examines the forces acting on or within a biological structure and the effects produced by such forces. ” ~Hay, 1973

    However equine biomechanics is generally used to mean the study of equine movement and the equine musculo-skeletal system, ignoring most of the field. It is becoming widespread, and “experts” are springing up all over.

    Motion capture systems are now available which will allow the performance horse to have “optimised” shoeing,  examining the before and after effects of any changes. So far these have been used intelligently, but they are not sophisticated enough to distinguish between primary and secondary deviations – cause and compensation – and so it would be easy to overlook the effects of trying to change joint angles.

    The equine distal limb joints are shock absorbers, particularly the fetlock (metacarpo-phalangeal joint). It flexes in proportion to the vertical ground reaction force of the limb loading (McGuigan and Wilson, 2003). Changes to the distal joint angles affect strain in the deep digital flexor tendon, the superficial digital flexor tendon and the suspensory ligaments (Lawson et al., 2007a), common sites of injury in the horse. Changes in hoof medio-lateral balance and width affects strain in the collateral ligaments (Lawson et al., 2008). Changes in shoeing affect both tendon and ligament strains (Lawson et al., 2007b) and inter-articular pressure in the distal interphalangeal (coffin) joint (Viitanen et al., 2003). For example an elevated heel increases distal interphalangeal joint pressure, increases superficial digital flexor tendon strain, unloads (and therefore shortens) the deep digital flexor tendon, and yet is often employed in showjumpers “to help the hock”.

    Some gait analysis systems now are being marketed as easy enough for any vet or farrier to use. Let’s hope that they come with a lot of training and promote rather than discredit the field.

  • Why Study Horses?

    Why Study Horses?

     As cursorial (adapted for running) locomotion goes, bottom of the food chain is really the lizards. They’re ahead of the snake, they’ve got limbs and even elbows to lift themselves off the ground, further increasing efficiency. However their shoulder design is a little primitive and they resort to side-to-side (medio-lateral) wiggling to move forward, wasting a lot of energy in the process.

    At the other end of the scale is the cheetah. The wiggle has gone and the spine dorsi-flexes (up-down) like a caterpillar. This increases stride length and hence speed by so much that the cheetah could do six mph even if it didn’t use its legs (Hildebrand, 1959). The cheetah is a sprinter, fast over short distances.

    The horse however, is not just adapted for speed but efficiency.

    – It has a fairly rigid spine, handy if you want to carry a rider.

    The stride length is increased by running on the toenails (unguligrade), with the heels (hock) and wrist (knee halfway up the leg), lengthening the legs. 

    – Extra bones have been lost, leaving just the middle toe, and all the muscles are at the proximal (top) end of the leg, with long tendons running down the limb. This makes the distal (bottom) ends light, increasing stride frequency.

    – Elastic energy is stored and released by the tendons, via the “extra” shock-absorbing joints that the horse has gained by running on its toes, particularly the fetlock, with the small proximal muscles acting as dampers (Wilson et al., 2001, Lawson et al., 2007). This makes the horse efficient.

    – Joint constraints keep the limb motion parasagittal. Stable and efficient.

     – The horse has no clavicle (collar bone). The scapula (shoulder blade) is held on by a musclar sling and hence can slide along the thorax increasing stride length and efficiency (Lawson and Marlin, in press).

    In fact the horse is an engineering marvel. If you want to understand the musculo-skeletal system, study the horse.

Equine Mechanics

Insights and resources on equine biomechanics for horse owners, trainers, and enthusiasts.