Tag: ddft

Tendon training, age and maturity. How do your horse’s tendons become stronger? How can you give the best start to your foal?

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.

May 24, 2015
Tendon biomechanics, equine digital flexor tendons, and crimp. How does all that work then?

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.

May 4, 2015

Tag: ddft

Tendon training, age and maturity. How do your horse’s tendons become stronger? How can you give the best start to your foal?

Tendon biomechanics, equine digital flexor tendons, and crimp. How does all that work then?

Equine Mechanics