Author: Sian Townson

From the main blog: https://equinemechanics.com/

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.

From the main blog: Equinemechanics.com:

When I first studied anatomy I was told that tendons had a very specific job – they attach muscle to bone, and so transmit the force of the muscle pulling to the bone, often creating movement at a joint. Ligaments, I was told, attach bone to bone and create stability at joints, holding it all together. However in horses there are some special cases where the tendons start at a bone, travel a long way and then have accessory ligaments anchoring them back to another bone; only part of the tendon is actually attached to any muscle at all. Anatomists can get very excited about which structures are tendons and which are ligaments, but we can agree that sometimes tendons have other jobs, and the muscle is just there to help adjust their tension.

Long tendons are part of what make a horse so interesting to study, and one of the ways in which the horse is specialised for locomotion. Horses don’t walk on flat feet like humans, they walk on their toenails. The muscles that control their legs are placed right at the top of the limbs, leaving the lower or distal parts to be lightweight, fast, and full of shock-absorbing joints and long tendons to store and release elastic energy. The digital flexor tendons of a horse are familiar to most horse owners because they include the most common sites of injury, as they take the brunt of impact and are predominantly stretched by the movement of the joints rather than tension in the muscles. These tendons are not there to transmit muscle pull and cause movement, they’re there to absorb motion, stretch like a rubber band as the hoof hits the ground and then ping back to length as the heel comes off, pulling the leg along with them.

In humans most tendons are short, strong and transmit force by acting like a rope. In horses’ legs these tendons are longer and more elastic. The digital flexor tendons and suspensory ligament run down the back of the horses’ legs. A sesamoid bone at the fetlock acts as a pivot and allows the tendon to transmit tension smoothly around the joint. As the hoof hits the ground, usually with enough force to break the cannon bone, the joints of the leg flex, and only the hoof feels the full brunt of the impact. The hoof travels fast and when it hits the ground it stops suddenly. Ideally the surface allows some slipping, but if the horse is wearing studs then the hoof stops very suddenly indeed. This deceleration causes an impact force. Flexing of the joint above the hoof (distal interphalangeal joint) allows the pastern bones (medial & proximal phalanx) to decelerate more slowly and flexing at the fetlock allows the cannon bone (metacarpal bone) to decelerate more slowly still, so these bones experience less impact. As the joints flex they stretch the digital flexor tendons, storing elastic energy. As soon as the heel leaves the ground the superficial digital flexor tendon can start to spring back to its shorter length, releasing this elastic energy, and helping the leg bounce along in very efficient way. Similarly the deep digital flexor tendon will recoil at toe off and return to its former length.

Except sometimes they don’t. Tendons need to be springy and elastic, but not stretch so far that they fail. One of the ways they do this is by not being straight: their collagen fibrils form a tight wave pattern – they are literally crimped. Tendon crimp means that whilst the tendon is never slack its initial bit of stretching just involves straightening out – the toe region of a length force graph. The more pronounced the crimping, the more force and stretch a tendon can take before it fails. Similarly as the horse warms up its tendons “become more able to” straighten out ready for work and are capable of sustaining larger forces and longer stretches without failure. Generally tendons are expected to work within this toe or linear region. Even within the elastic region that the tendon is capable of working in, not all the elastic energy is returned and some of it is lost as heat (hysteresis, shaded in pic). Heat build up in tendons is another major cause of damage and the reason that many old fashioned brushing boots have become unpopular.

Lesions and microtrauma in the tendon show up as imperfections in the crimp pattern, making it irregular, disturbed or with less crimp angle. These range from micro-lesions to full on blown tendons leaving big lumps of scar tissue, and as you’d imagine they all affect the tendons ability to stretch. Micro-lesions are built up by pushing the tendon to the point where individual fibres start to fail, and if not allowed to recover eventually these may cause complete failure of the tissue. The majority of catastrophic failures though require some external influence to push the length beyond the limits – such as an over-reaching hoof kicking into the tendon whilst it is already stretched.

With use tendons become stronger, more elastic, with their fibres better aligned. Basically they can withstand more before they fail. However with overuse or just plain ageing damage makes tendons weaker. The trick then, is to give a tendon plenty of training within its current elastic capabilities. Training actually affects different tendons in different ways, and responses to age-related degeneration and exercise are actually very different between for example the common digital extensor tendon and the superficial digital flexor tendon in the same leg of the same horse. Similarly there is a certain limit in how strong and elastic any given tendon will develop, based on getting the magic optimum amount of exercise in the foal. These posts sit on my desk unfinished for a long time if I try to include too much in them, so let’s stop there and cover those topics next.

Images courtesy of Jean-Marie Denoix, Ecole National Veterinaire d’Alfort, and from Robi et al., in Hamlin et al., 2013, with thanks.

When I started as a scientist publication was all, it still is. I now understand though that if I write a paper, it will be published. Maybe not in my first choice of journal, but however good or bad it is, there is somewhere that will publish it. Now journals accept papers on the same day or the day after submission: (bit.ly/1KWaV2c). Do we really think they’re getting peer reviewed? When I want to know more about a condition, I don’t just Google something I look at the academic literature, try and find out what the evidence suggests, but is that also just someone’s opinion now?

When I was a young, naive PhD student I was slightly shocked that journals asked me to suggest my own reviewers. Not that naive, I just named the people I was currently flirting with, but still it did seem to lack a little subjectivity. Fast-forward a decade or two and the profit-based world of publication has exploded.  Editors need to get papers in and out again through the pipeline. Authors need to get those manuscripts off their desks and on their CVs and grant reports. Quantity over quality every time. I have now written papers which are essentially just my opinion, waffling away, and they’re published not in journals on discourse but scientific journals that lend my thoughts and air of peer-reviewed authority. These are no longer my thoughts, these are now evidence-based facts.

So, if you’re reading up on a clinical condition, what should you look for?

1. Randomized Controlled Trials (RCT)

Just having collected some data and analysed some statistics doesn’t make it good evidence. If I flip a coin and it lands on heads four times in a row, is that a rare and noteworthy event? No, it’s a sign of randomness. If I flipped a coin and it neatly alternated between heads and tails, now that would be noteworthy. I once co-wrote a paper with an amazing statistician, demonstrating how the statistics in one of my previous papers were a misleading mistake. No one would publish it. Randomized controlled trials are the best we can do for looking for actual significance.

2. Cochrane reviews.

These are a special type of review of the existing papers. They collect together eligible studies into a meta-study, telling you what the balance of the evidence says. These try to include only peer-reviewed papers that have a rigorous method and so collect together a much larger, combined subject group. They use some fairly robust statistics, and are a hell of a lot more accurate than just trying to read and counter-balance all the papers yourself, by eye.

3. A bit of time for peer review

Most published manuscripts have date of submission and date of acceptance stamped on them. I used to think of this as a badge of shame: if it takes me a year to get my paper out everyone knows that. Oh, how the world changes. Look for something that’s obviously been through a  discussion process, preferably a long one, not just same day/next day acceptance. Publication bias means it’s actually much harder and slower to publish a paper with a negative result – “we looked but actually it wasn’t there”, “this doesn’t work” – or a paper that goes against the mainstream. Doesn’t make it a bad paper, on the contrary, unexpectedly negative findings can often be the important ones.

4. Big name journals.

There’s a lot to be said about impact factors and how to judge a good journal which doesn’t really belong here. Fact remains, how often a  journal is cited and how often a paper is cited says a lot about how seriously academics are taking any paper. Of course it doesn’t tell you whether people are agreeing or disagreeing with it, but any academic mention is still more reassuring than a paper that is politely ignored by the scientific community. Those lesser-known ones, in the journals with the ridiculous names, those are the ones that you need to politely ignore too. Scientists don’t actually ignore papers just because they disagree – it doesn’t take a second to write “Contrary to the findings of Blogg and Blogg (2015), we showed…” Having someone to disprove makes your paper more publishable – you’re now showing something that actually needed to be shown.

5. Conference papers

Any collection of papers that has a nice holiday attached a.k.a. conference papers, is going to get our slightly less completed thoughts. Conference papers are often published in mainstream journals and look like full papers on your CV, but depending on your field it’s fairly hard to get a conference paper rejected. They only need preliminary findings as these should be the cutting edge of your ongoing research, your chance to tell the field what you’re currently working on and get the feedback and awkward questions that’ll make the next round of tests and the final full paper a far more reliable affair. I for one am on the committee of a lot of the conferences I want to go to, and at the Keynote career stage. This means that I often get asked to submit a paper for a talk that I’ve already been asked to give. You can peer-review that all you like, it’s already been accepted.

These days scientific publication falls under the more general category of things we read on the internet  – where even if it’s an intelligent person’s best guess at the time it’s not necessarily true. That’s OK, it never was – scientists don’t believe in facts, they believe in best current theory and convergence towards a solution. Just remember that scientific papers are not the protected species that they used to be, and should be read with the scepticism and critical thinking that allows the good ones to shine.

Before and whilst I had Rossvean (above) I must have had 20 or 30 other horses. I was only a kid but I rode for competition yards, I rode for private owners, I rode for myself. Rossvean was my first real rescue horse, others were more scopey and more straightforward but he had a brave heart and I trusted him completely. Ross’ was the mane I cried into when my parents left. Eventually I sold him and my heart broke. I found the next horse, and the next. I asked my trainer (Karen Gibson) for help as something about these horses just did not work any more. She looked at me very sternly and explained that “there’s never going to be another Rossvean”.

I have an excellent memory, I don’t forget words, sometimes I wish I could. The feeling of being metaphorically punched never left me either. I sold up and I quit. For over a decade I quit. Eventually I started to ride again, very occasionally and only for other people. Don’t you miss it? they’d ask. Every day, I’d say. Then one day I fell in love with a horse. Hobo was exactly what I look for, a horse worth coming out of retirement for. Big floaty uphill paces and a jump that defined perfect. I used his photo in lectures on conformation. I studied his jump for research papers. Like Ross he was a little psychotic but when he trusted you he would overcome his own nature. Horses that can take on a 1.50m track are hard enough to find, here was a proper 1.60m horse and I loved his every quirk.

Hobo got his name by being chucked off yard after yard for being dangerous, and he was, he had a tendency to panic with no self-preservation, and an exceptional talent for putting people in hospital, but this was never a horse who was nasty, just very afraid. 1.60m horses are not just bred, they are made. Everything must go well in training and then you still have to be lucky. Hobo was big, nervous and slow to mature. To go with his talent he needed big track rideability: my mantra became “for him I have time”. Eventually the offers started coming in and under pressure to sell I bought Sox the stallion to ease the transition. Still I could not bear to part with him and now I had two (albeit one who was intended for stud). Sox is a great horse, turns his hoof to anything, never lame, affiliated dressage and jumping, safe to hack and easy to handle. Next week I’ll go and collect a championship trophy from his only ever shot at showing. But Sox is not Hobo, Sox is a riding club horse and I never wanted to be a riding club rider. I’ve ridden him with a broken wrist, with one side limp, post-caesarian, and, I now know, with a wolf tooth fragment stuck in his jaw. He’s a grumpy old git, but I trust him with my kids and he’ll never hurt them. He doesn’t owe me anything but the world will manage without his genes and he is now gelded.

More pressure to sell came and went. I turned down big money offers for both horses, met a “them or me” ultimatum from the husband and he became an ex-husband. For years the divorce went on and the horses stood stabled, in divorce limbo, going round a horse-walker and losing health. By the time it was over Hobo never made it fully back into work. This year I retired him, which is an odd expression for a horse that hasn’t really started his career, but I can’t keep bringing him into work and watching him go lame, I have to let him go.

Hobo is not dead. Every morning I lead him out of his stable he comes out in a big uphill march of a walk. I spent years on ground work with this horse, he leads beautifully, but most of all he moves beautifully. He has presence: he comes out of the stable roughed off and covered in mud and fur and I still think here is a horse. Here is what all horses should look like. I lead him up to the field and every time I watch his hooves – do they land heel first? Are they even? I fantasize that months of leading him up and down our stony path will harden his legs and he will come back, and I will feel that amazing jump, and the fences will be enormous and I will take photos to replace those lost to divorce, photos and memories which I can keep forever. Then we reach the field and I send him off and he hobbles, and I remember that I am an idiot, and I have to let him go.

As a child at this time of year I read an issue of Horse and Hound where the equine stars of the day listed what they’d want for their dream Christmas, and adverts ran alongside. I don’t remember what most people wanted. I think Mary King wanted an equine swimming pool and a new set of jumps. When they came to Eddie Macken he only had one sentence: “I just want Boomerang back”. Suddenly I saw capitalism and advertorials through the eyes of this man who didn’t know what we’re talking about. His heart is broken and seemingly his career with it, and we understood, because we all wanted Boomerang back.

I am very lucky and I know that. Pining over a (still-living) horse smacks of white, middle-class privilege, and hell yeah, I’m rich and thin. My Christmas list is ridiculously hopeful and contains saddles and bridles because I have four wonderful horses: Sox, Hobo, Denim (Hobo’s companion pony) and now the lovely Remus, a four year old. People are always asking how Remus is going, excited that for the first time in nearly a decade I’ve bought another horse. He’s going great I say, and he is. Better than I ever imagined, I say, because it’s true. Remus is exceptional, I hope one day he’ll be international. Maybe he’s even a 1.50m track horse and they’re hard to find. Certainly enough scope for me, don’t get the wrong idea – I’m no olympic rider. What I can’t politely say is he’s not Hobo, he’s not even in Hobo’s league. He’s lovely, I’m fond of him, I’m impressed with him, but like people horses don’t replace each other. So Dear Santa, come on, this year, even if only for one day, I just want my Hobo back.

I’ve put a post on the main blog which talks you through checking your own horse at home, using your mobile as a gait analysis tool. I may repost it here but for now have a link: https://equinemechanics.com/post/98984225743/checking-for-lameness-with-your-iphone