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.