Scratching the surface of tendon injury and rehabilitation
In early 2017 I made an ambitious 12-month competition plan with Pokemon for that year. The goal was Under 25 Grand Prix for my last year in the category, with (what would obviously be…🤔) a seamless move into the Senior classes as our even more ambitious goal… 🤣
The #SilverPrince in full glory at Keysoe Premier League, 2017
In late 2017 he tore his Gastrocnemius tendon (equivalent to the equine Achilles' tendon, crucial for weight bearing, not ideal to damage!). He was treated with a PRP injection to the affected area and then prescribed a 12-month rehab plan, and a disclaimer of sorts from the Vet that the odds of him competing higher than Medium level again were heavily stacked against him.
I tore up my plan and indulged a big cry. It wasn’t the first time I had faced disappointment, but it was the closest I had ever been to achieving the magical Inter 2/Grand Prix transition, and Pokemon is an incredibly special soul to me. No matter what level you’re working towards as a rider, the setback of injury, no matter how severe, is a heavy blow to deal with.
Muscles of the horse, demonstrating location of the Gastrocnemius muscle. (http://infovets.com/books/equine/A/A028.htm )
Firstly, Structure of the tendon:
A tendon is predominantly made up of one of the universe's most incredible materials: Collagen. There are several types of collagen known; type I is associated with tendons and ligaments. Collagen molecules arrange in to bands/tubes called microfibrils, which arrange in to fibrils, then fibres then fascicles, as shown in the image below. These fibres are arranged in a parallel pattern, which is most energy efficient and the strongest formation applicable. The collagen bonds in these fibrils create a molecular-level crimp pattern which is suggested to play a role in the stretch available in tendons (7).
Structure of a tendon (https://beva.onlinelibrary.wiley.com/doi/10.1111/evj.13331)
Unlike what many simplified diagrams lead people to believe: tendon material is continuous with muscles and bones. This makes it a much stronger connection between bones and muscles, which is obviously a benefit considering the force that is transferred through these tissues.
There are very few cells and no blood supply found in tendons (1); which is one of the reasons their injury and healing process is a long one (6). Blood supply offers drainage and delivery of helpful inflammatory products (2). Interestingly, the particularly avascular zones of tendons are prone to greater risk of damage and/or injury (2).
Functions of the tendon:
1. Tendons consist of connective tissue between muscles and bones (4). This provides support and stability to joints and a connection for the movement produced by the muscle contraction.
2. The tendon behaves like an elastic band and stores energy, which when released provides movement at a highly energy-efficient rate. The repetitive stretch and recoil of the tendons is created by muscular contraction resulting in tension (4).
3. Tendons are able to be stretched extensively and will return to original form when released; they are capable of withstanding a large tensile force. This property does diminish over time, and under certain conditions (such as heat and excess tension).
Injury to the tendon:
Injury to a tendon can be referred to by different titles, depending on the type of damage. These titles are often used interchangeably in wrong ways, clarity on them may help understand the injuries.
1. Tendinitis = inflammation of the tendon
2. Tendinosis = tiny tears of the tendon including degeneration of collagen in the tendon
a. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3312643/ this article explains the differences.
3. Tendinopathy = umbrella for non-rupture damage to a tendon (damage to the collagen in the tendon) https://www.jospt.org/doi/full/10.2519/jospt.2015.5884
Tendon injury is one of the most common reasons for decreases in performance and even retirement from work/competition careers (6). Not only does the initial injury cut into competition and training plans, but the risk of re-injury is incredibly high (between 48-58% and up to 80% depending on the source (6)). Re-injury is high due to a weakened tendon which in itself is predisposed to further damage as well as the compensation that results from protecting it.
Tendon injury generally results from overload of some kind. This can either be from a repetitive loading cycle where not enough recovery time is accounted for, or from an acute/sudden overload injury which stretches the tissues too far in a motion. This overload is even possible from muscles that are stronger and contract harder than what the integral strength of the tendon can manage. (check out https://www.germanjournalsportsmedicine.com/archiv/archiv-2019/issue-4/functional-adaptation-of-connective-tissue-by-training/ for further explanation of this).
Tendons have a low rate of cellular turnover and regeneration which means that healing occurs by scar tissue formation, NOT tendon regeneration (4), which is how bones repair. This is a problem because scar tissue has a far more random structure of collagen compared to tendons, which greatly reduces its ability to withstand tension. This often means that the limb with damage will always be weaker and that the horse is likely to compensate and offload from it. In contrast to tendons, scar tissue is initially a highly vascularised tissue (necessary for the healing process) and these blood vessels have been questioned to play a role in the cause/effect of the poor collagen fibril alignment that occurs in tendon injury repair (4, 5).
Rehabilitation of a tendon injury:
Depending on the level of damage, tendon injury can take 3 months or up to/over 18 months to “heal” (6). Because of the nature of scar tissue, a tendon will never return to its former strength, because the biological processes are unable to regenerate the perfection that is the intact tendon, therefore it is hard to say this actually “heals”. This means that it is almost always going to be something that needs to be monitored and managed with workload and treatments, and will always be a weakness in the horse.
Box-rest, controlled re-introduction of exercise and regular check-ups to assess healing of the site are crucial for optimal outcome. Muscle strength and bone density are lost throughout the lengthy rehabilitation period and are part of the reason for controlled exercise. This should reflect back on the balance that is necessary between muscle - tendon - bone strength. Box rest is a vital stage of rehabilitation in a variety of cases. This gives the injury site time to mobilise inflammation factors and reduce movement there. A fresh tendon injury means a portion of the tendon is unable to bear weight/tension properly, which means that the other collagen fibres of that tendon will be compensating. Movement of the tendon increases strain on the remaining fibres and is also extremely painful, so limiting this in the early acute stages of damage is important.
The fact that a very long rehabilitation process is required to get an injured tendon to a less-than-perfect state makes it a difficult diagnosis to cope with. The remodelling capacity of bones is far greater, and therefore usually (excluding extreme circumstances) an easier diagnosis to receive.
Tendon rehabilitation takes a LOOONG time… It is a rollercoaster involving moments of great hope when they look sound; great anxiety not knowing what point the new maximum will be that you get to ride/compete at; the disappointment during the 3 steps forward, 2 steps back process that seems intertwined with rehabilitation… Like when you start trotting after months of walking and they feel more broken than when you first received diagnosis? We know that feeling all too well. I tried to start a running program at the same time as Pokemon’s rehabilitation program. I had full-time University as well as two other horses to ride, so it was an inconsistent program at best, but I definitely developed an appreciation and sympathy for Pokemon on the days I felt sore and less able to go as far or as comfortably. Surely they experience the same!
Rehabilitation is a prime example of a marathon, not a sprint.
Pokemon enjoying a light "wellness" massage during his clinic visit.
The good news is that there is science and technology that is ever improving for your horse’s benefit... Physiotherapy being one of them! Shock-wave therapy, ultrasound therapy, LASER therapy, H-wave, Stretching, massage, controlled strengthening exercises… a variety of options are available to maintain comfort, muscle mass, joint health as well as tendon and ligament function. The rehabilitation road is a long and exhausting one; but it can be fascinating and important. Every time I have had to rehabilitate one of my horses, I have come out the other side with a better execution of their basics of Dressage, and a greater appreciation of what the good riding days become.
Good luck with your rehab, I haven’t met you but I have my fingers crossed for you! Let me know if I can help! 😉
Did you know: The half-life (length of time it would take for half of the material to disintegrate) of collagen in mature equine tendons is estimated to be around 200 years?! (3)
Some really interesting links for those interested.
"Inside Nature's Giants: The Racehorse" is an impressively informative video, however DISCLAIMER it is not for the faint-hearted/weak-stomached! A limb cadaver is used to demonstrate the pressure a tendon can withstand. The tendon info starts around the 15:50 mark, but I can thoroughly recommend the whole lot for anyone who wants to know more about how incredible horses are from a physiology perspective.
Description published by the British Equine Veterinary Association on Microdamage in tendons:
Information on the tendon vs. muscle strength training:
and the links I used and quoted throughout this:
2: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4650849/ Tempfer
3: Aspartic acid recematization and collagen degradation markers reveal an accumulation of damage in tendon collagen that is enhanced with ageing. Journal of Biological Chemistry. Thorpe et al. 2010.
4: Comparative study of the characteristics and properties of tendinocytes derived from three tendons in the equine forelimb. Journal of Tissue and Cell. 2009. Hosaka et al.
5: . Presence of lymphatics in a rat tendon lesion model. Histochemical Cell biology. 2015. Tempfer et al.
6: Evaluation of Return Rates to Races in Racehorses After Tendon Injuries: Lesion-Related Parameters Journal of Equine Veterinary Science. 2020. Kan Gulsum et al.
7: Mechanical and functional properties of the equine superficial digital flexor tendon. The Veterinary Journal. 2005. B.A. Dowling, A.J. Dart *