Equine Mechanics

Tag: biomechanics theory

  • Five Steps to Straightness

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    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 above 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!

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    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?

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  • Gait analysis, that uses markers, right?

    King Arthur, 2004

    Looking for the updated Free Gait Analysis Software post? Scroll down or click here

    Attaching markers is a clever idea that started when we (the biomechanists) wanted to assess human gait but videos were too big for available computer power. We needed to track how people moved, so we put retro-reflective markers on people and tracked the markers using infra-red cameras. Now we only had to deal with a set of point co-ordinates, not big bulky videos, and the computers could cope. Tracking more than seven co-ordinates was a major technical challenge so we placed the markers directly over the joints of interest and kept the number of markers low. 

    Move forward twenty years and computers can cope, cameras can cope, and we have fire-wire cables. Not only can we use video but we can use high-speed and multi-camera video set ups. I used to use spherical (not flat) markers and opto-electronic cameras for a lot of film industry applications because we need data that’s fast and easy, backwards compatible and not hugely accurate. The picture above is from the set of King Arthur in 2004, and clearly I was still using markers then, but I use this pic a lot to remind me what a nightmare the markers were on that shoot! These days I’d rarely attach markers for film, research or investigation.

    How can you track without markers?

    Tracking without markers is a lot more accurate, as your video contains an image of the horse, not just a point. If you only track a marker that is attached to the skin of the horse, then you’re reliant on how accurately it’s placed, and you have to cope with the fact that it moves will respect to the horse’s skeleton due to skin and muscle movement. However accurately you think you can place a marker, you can’t put it over the joint’s centre of rotation, as this is a functional, changing point not a fixed physical part of the joint. The very basic models place markers over the “joint centres” and just join the dots so their outputs should be taken with a pinch of salt. Moving one marker by a single centimetre results in a massive change in joint angles, even before skin movement comes in. I don’t think anyone has used these for clinical human assessments for over a decade!

    To track without markers the computational model simply fits a skeleton to the horse in the video, and works out functionally where the bones, joint centres, muscles and tendons must be. This is easy to do as we know that the bones don’t change length, that joints don’t dislocate and that skeleton moves in a predictable way. Combine these things and you get a system which can work out where a horse’s skeleton is accurately and reliably. If you are interested in the even more technical side you can have a browse through the publication list, and see joint angle accuracy of less than one degree, and even single millimetre accuracy on the joint centres in the really flash version! 

  • Dressage and the Equine Back


    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 Model, Slijper 1946

    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.

    The nuchal ligament, and its action on the withers. Adapted from Denoix 2001

    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…

    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…

    Carl Hester and Uthopia in competition 2011

  • Biomechanics Clinic before and after shots Monday afternoon/ evening sessions.

  • Biomechanics clinic before & after shots Monday lunchtime sessions.

  • Hand in glove: can wearing gloves make you a better rider?

    I like gloves because I like my hands, and I particularly like my hands with the skin on – not ripped off by some leadrope or lunge rein. Also I hate the cold and I live in the U.K. so I have expensive gloves and I use them. However this evening I started to think about their effect on my riding, when little stallion had a few rather unkind things to say about my hands and then I realised I had forgotten to put them on.

    Why do gloves improve your riding?

    (Good) gloves increase the friction between your hand and the reins. That means that you can keep a more relaxed hand and still have the reins secure. In turn this relaxed hand means a more relaxed forearm, as like horses we keep the muscles further up the limb, away from the digits. If your forearm is relaxed you have softer more responsive hands and your stallion stops whinging (or whining and complaining in American). Everyone’s a winner. My gelding also tells tales on me, but apparently I wore gloves to ride him.

    I have ambitions, Sox merely likes grass and doesn’t see why he has to put up with fools.

  • Engaging the hindquarters and lengthening the stride


    Photo from BritishDressage.co.uk

    BD Convention supported by The National Saddle Centre

    The recent BD convention featured the training talents of the Academy Bartels. It was covered by an informative write-up in the most recent British Dressage magazine. There was some very refreshing talk of responsiveness and relaxation – a horse that waits for, not predicts, the commands and once asked to go forward continues in these longer strides until given an aid to change without continual nagging from the legs. Similarly a half pass ends in a new set of commands not in the removal of the aid. The removal of any aid has already occurred as soon as the horse complied. This strict philosophy gives a strong foundation, and shows how the Bartels achieve such relaxed, engaged horses.

    The classic changes of stride length and tempo exercises are used to reinforce this. However I found these two types of exercises a little confused in the report. The report appears to refer to longer strides being faster strides, and this being part of the tempo exercise. The accepted wisdom in dressage, entirely supported by the scientific literature, is that in order to maintain the same tempo, longer strides should in fact feel a little slower. Tempo exercises changing the stride timing are very useful in creating engagement and responsiveness, as are exercises changing the stride length, but these are not normally seen as the same thing. As early as ’94 we knew academically that in high-level horses stride duration is maintained (canter) or extended (trot) with increases in stride length (Clayton, 1994a; Clayton, 1994b). Biomechanically-speaking speed is the product of stride length and stride frequency, so it is entirely possible for speed to increase, as a result of longer strides, whilst the horse’s stride frequency (tempo) slows. However the article refers to faster strides rather than faster speeds. Was the journalist making an error here, or do the Bartels really change the stride frequency as well as the stride length when in training? Anyone?

    Interesting the research also shows that in order to achieve increased stride length, the footfall rhythm in canter is altered by increasing the time spent in suspension and hence the distance covered between leading front leg and the next hind leg placement. The timing of the hoof-falls within the hind leg-diagonal pair-front leg part of the pattern is proportionally quicker, with longer spent in suspension. Perhaps this is why lengthened strides, which have the same duration and in fact feel slower to ride, are often described as faster.

    In trot, stride length is increased by increasing over-reach distance, and stride duration increases (tempo slows) with increasing stride length. In other words medium and extended trots do have a higher speed, but a slower stride frequency. So in fact in both gaits higher engagement of the hindquarters is needed for lengthened strides, and this would be undermined if the strides were faster rather than longer.

    I wish I had attended the convention and will try to make the next one. The report was a great read, and I particularly liked the following points, which fit well with my last post on the equine back:

    On straightness: “The hind legs must step into the same tracks as the forelegs, whether on the straight line or a curve. Tineke used the helpful image of the horse having to be like a train that stays on the railway lines…..The aim frequently spoken was to ride into the two reins.”

    On roundness: “..they put great emphasis on getting the muscles from the tail, under the hindquarters, under the saddle, into the area in front of the wither and up the neck, soft and malleable. Being round and soft on a connection helped soften these muscles and remove blockages. Any blocks of tension in the upper line would mean there is no connection from the hindquarters to the bit….. Until the horse took the rein down there was no work on the movements.”

  • Be your own Biomechanist: What to look for in your photos

    If you can afford regular lessons with a good instructor you’re laughing. If not eyes on the ground or at least a video can be a real help. Even if all you have is the odd photo you can still plan and make progress.

    So, without further ado, here is why Carl Hester is better than me, from a biomechanics perspective.

    Here’s that photo from the previous post, of Carl and Uthopia in competition (right) and me slobbing about on the left:

    Carl’s horse is obvious happier, it’s more uphill, it’s more engaged and swinging through. All this is easy to see when you have a comparative photo opposite. But let’s assume you don’t. Here’s a few basic biomechanics things you can look at in any picture, with or without a gold standard.

    1) Would it make a statue?
    Even though Uthopia (right) is on one leg, you could imagine it as an ambitious statue. If my photo was made into a statue, little horse would fall on his nose. In biomechanics terms, what you’re looking at is whether the horse and rider’s centre of gravity is over the base of support. Imagining the statue helps you do this, without the maths, and in my case makes it obvious that he’s not that well-balanced. I find thinking about this helps when I’m riding too.

    2) Check your verticals (pictures below)
    Carl’s horse’s nose is slightly in front of the vertical, as is correct for a dressage test. He is sitting slightly behind the vertical (the line of his back is more or less parallel with his horse’s nose). My horse is behind the vertical and I need to straighten up and get my shoulders back too.

    3) Check your horizontals – is the horse uphill? 
    Badly timed or angled photos can easily hide how uphill a horse actually is, so if you compare your horse’s back to the fence line look at the whole horse -. If we look at the top of my horse’s back it doesn’t looks so bad, but if we look at the whole horse and check for lowered (engaged) quarters and raised shoulders the illusion is shattered. Compare the little guy to Carl’s horse.

    Moving on….

    4) Is the horse tracking up and symmetrical?
    For the horse to use its back effectively (see previous posts) the hind legs should be stepping well underneath it. Check whether the hind hoof is landing in the print of the fore hoof. In walk your horse should over-track – the hind hoof should land well in front of where the front hoof left the ground. My horse, on the left, is trotting so we can also check for symmetry – are the diagonal pairs synchronised? It’s pretty common to start to spot problems in extended or very collected work. My horse is looking nicely symmetrical and similarly Uthopia is clearly in the midst of a clear three beat canter, with the diagonal pair well matched.

    5) Is the horse happy and relaxed?
    Yes, this is a biomechanics thing! Muscle tension is the enemy of athletic performance and my little stallion is not just giving his grumpiest just-been-told-off face and tail twitch but you can see the muscle tension in his neck, shoulders and back. Carl’s horse, Uthopia, on the other hand.. oh, you get the idea…..

    6) Is the rider secure? Dancing with the horse or fighting against it?
    Ok, first let’s all concede that my photo is taken from a very unflattering angle! Ok. So if you look at Carl’s seat and leg, ignoring the lower leg, you can see his security comes from the seat and length of thigh. Even if his leg ended where his boot started, you can see he’s staying on. Look how ineffective my seat and thigh look in comparison. Admittedly my legs are shorter than Carl’s but I’m not using what I’ve got. My security is coming from the lower leg, a show jumping habit, whereas it should be coming from higher up.

    People are intuitively good at maths. You use it every time you catch a ball – having successfully predicted it’s path and timing, allowing for gravity, momentum, and air drag. Horse riders tend to be doubly so. Carry on using it.

  • 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.

  • 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.