The tendon post

It's the post you've been waiting for.......TENDONS.  How they condition (or not), how they heal (or not), and what you can do to prevent injury (or, unforunately.....not).

I've been putting this post off because I felt like I didn't have the time or energy to give the topic justice.  However, in the last week I've written 3 separate emails where I've touched on tendons and the conditioning process....and decided whether I feel like I'm "ready" or not, it's time for the topic to be discussed.

I DO have references and literature for this topic and if you are interested. I can't include links to the full documents because many are copyrighted for my personal use only through the school, however I can give you citations and abstracts.  This post is a bit repetitive in its main points - however I had a hard time wrapping my mind around some of these concepts in school since it contradicted what I had read for many years, and hearing the same concept in many different ways helped my understanding, and I hope it helps yours. :)

Tendons.  Let's start with the basics.  Tendons hold bone and muscle together.  Tendons are composed of organized fibers of collagen. 

There's a saying that I heard multiple times as a newbie endurance rider - muscles/cardio condition in 3 months, tendons in 6 months, and bone in a year.  The structures continue to strengthen and adapt to the amount of work and grow stronger over the years, unless you screw up, overload them, and end up with an injury, either through an acute event (ie - a wrong step in a hole) or through chronic fatigue (overriding a horse over the course of months or a year and ending up with too much micro damage too fast for the body to heal and adapt, leading to injury).

Turns out this is mostly true (and in the case of bone, doesn't give enough credit to how AMAZING it is as it adapts).....except for tendon. 

Here's the interesting thing.  I just finished the musculoskeletal block and we were taught (and I clarified with the professors) that no "conditioning" of the tendons actually occurs.  Conditioning/training actually CREATES these little lesions (fiber disruption - which is a sign of tendon injury) in the center of the tendon, and causes the collagen fibers in the tendon to change in apparance - they get smaller and more irregular.  This is NOT good and are considered degenerative changes.  As the horse ages, the tendon undergoes degenerative changes - exercise accelerates this process.  It doesn't matter whether the conditioning is "appropriate" - this degeneration occurs regardless.  In summary: There are differences in tendon anatomy/physiology as the horse ages.
Exercise accelerates this process and this process results in a weaker tendon


In addition to the fiber bundles changing shape and the fiber disruption in the center of the tendon, the cross sectional area of the tendon (how "thick" the tendon is) can increase.  While I found a paper that hypothesized that this was an adaptive change that made the tendon stronger, the majority of the more recent papers do NOT attribute this change to increased strength.

In summary, exercise/conditioning actually sets the tendon up for injury, even if you are doing it "right".  You are not less and less likely to get tendon injuries as the seasons pass - in fact, it's even more critical to examine bone and muscle conditioning and not override, or over-fatigue the well conditioned horse.  The golden rule remains - do not ask for in competition what you have not prepared your horse for in training, no matter how experienced or fit the horse is.

Tendons are injured when they are overstretched.  The fiber pattern becomes "disrupted" as fibers break.  I'm simplifying, but bear with me - trying to keep this succinct and "readable" to a broad audience, and also get to bed before 9:30 tonight!  Especially considering that a conditioned horse may already have a compromised tendon due to degenerative changes to the fiber pattern and collagen size... the trick is to protect that tendon as best as possible.

How do you protect a tendon?  Don't fatigue the support structures (muscles) to the point where the tendon is taking more load than it should - because the tendon will fail in that situation.

It is NOT about conditioning the tendon.  It's about conditioning bone and muscle to "take the load" off of the tendon and not subjecting the tendon to any more load/stretch than necessary.

Wanna hear a terrifying fact?  They have calculated the strain a tendon can take before it fails (specific to the tendon - for example, the SDF of the front leg of an equine).  At maximal exercise (like a gallop), that tendon is being strained almost to the failure point.  It would have been nice for Mother Nature to have given a little bit more room for failure.......

So it's a bone and muscle conditioning issue.  The trick is to not overload the bone to the point of stress fractures, and not overload the muscle to the point of tendon injuries (because the muscle is too fatigued) - and NOT actually about conditioning the tendon themselves.

Let's talk briefly about how a tendon "heals".  Assuming it's not a absolutely horrifying injury and it's a more typical chronic strain injury, assume you have fiber disruption.  As the tendon attempts to reconstruct the fiber pattern, it lays down a collagen type (there are many different collagen types in the body) that does not handle strain as well as the original collagen type.  The final remodeling of fiber pattern and the replacement of "temp" collagen with the correct collagen takes place late in the healing - a year or so - and can continue for a while, especially as the horse returns to work.   Ideally the fibers align on the correct plane and the tendon gradually progresses to a "normal" looking fiber pattern. Rarely does the ideal situation occur.  Normally, not all the "temporary" collagen is replaced with the "correct stuff", the fibers don't always align well, and other fibrous tissue components can remain in the tendon, which weakens it.  This "healing" process is why degenerative changes in the tendon due to age/exercise are so worrisome - even if there is no clinical lameness.  Tendons don't heal well.  Even repaired tendons that look normal on ultrasound have a disrupted fiber pattern under polorized light, which shows the "kink" or "wave" of collagen fibers.

It's a novel way of looking at tendons for me.  Instead of being less worried about tendons as the years progressed, I should stayed concerned and conservative.   Instead, I think I became a bit relaxed after not having a tendon injury in over a 1000 miles of competition - thinking that my tendons were "conditioned",  just like the other structures.  It didn't matter I had gone 4+ years without incident - because she was in training, conditioned, and being exercised, it was likely that degenerative changes were present, even discounting the previous healed injury.  Of course, in my favor and protecting the tendons were the bone and muscles that were better prepared to handle the work after years of competing, a horse that was less likely to misstep due to experience, and a rider that could read the trail better than 1000 miles ago.  But it's never OK to push past a certain level of fatigue because THAT is what bows tendons.  Here's what could have happened at 20MT in 2010 (there are many different scenerios, but this is a plausible one and illustrates the concept I'm trying to get across).   Sand tends to fatigue the apparatus in the horse's leg that's responsible for the return of energy and is why horses move more efficiently through the sand than you and I.  After cramping early on in the ride and traveling more on the forehand, combined with sand and soft footing, the muscles in the front legs probably became fatigued as they are now performing work they were not conditioned for (I don't typically train my horses to travel on the forehand for 100 miles).  At mile 92, the muscle was fatigued enough that it forced the tendon to carry additional load.  And......boom - I'm out for a year.  

In summary, muscle fatigue kills tendons and it's impossible to "condition" a tendon to perform under those conditions (that of muscle fatigue of being pushed further, faster, over different terrain etc than what the horse's muscles/bone were conditioned for).  Pushing through this type of fatigue at any level is when injury will occur - no matter what base I have put on my horse.

Does this make sense?  The same principles hold true that I've always believed in (condition for ride conditions, condition slowly and carefully)- BUT you are not protected from tendon injuries by virtue of time or distance - only by how well you have conditioned the rest of the animal (bone, muscle, heart etc.).



What does this new (to me) knowledge about tendons mean to me as I bring Farley back into the sport?  My response may surprise you.  Stay tuned for tomorrow's post - It's 9:45p and I'm going to bed. 

Disclaimer - first year vet student that talks to as many professors as possible about issues like this, but is certainly not infallible.  :)  Just a curious endurance rider who SWEARS she's not going into equine practice. 

Other tendon facts to consider:

•  Initial exam for a tendon injury should be performed 4-5 days post injury.  It's difficult to ultrasound to pick the extent of the injuries prior to this.
• Long heels, short heels etc dramatically impact the lever system of the horses' leg and HUGELY affect the strain the the SDF (for example) tendon is under.  One way to take pressure off of a SDF tendon that is too weak to support the fetlock (so the fetlock is drooping) is to put "sliders" on the hoof, that extend behind the hoof several inches.  It's crazy - but by just having this piece of plastic sticking out behind the heel, the SDF is under less strain and is able to raise the fetlock to a more normal position.   This brings up an important point about hoof protection.  A Hoof boot (or shoe) that extends the heel behind (or in front) of the "normal" heel of the horse will dramatically affect the tension and strain that the tendons are under - maybe for good, maybe for bad.  I think that ANY kind of hoof protection that changes the lever arm of the hoof (such as extending behind or in front of the hoof to any significant degree) is cause for pause and contemplation. 

 

If you are interested in a bit of light reading,  here's a nice review article:

The response of bone, articular cartilage and tendon to exercise in the horse.
E. Firth, 2006, J. of Anatomy, volume 208, issue 4, pages 513-26

This article states exactly what I was taught, and what was said by my professors when I talked to them individually.  It mostly addresses tendon response to exercise during growth, but mentions the response of the tendon to exercise as it relates to age generally as well.

I also recommend:

A review of tendon injury: why is the equine superficial digital flexor tendon most at risk?
C. Thorpe, P. Clegg, H. Birch.  Equine veterinary journal 2010, vol 42, Issue 2, pg 174-80

Contains some nice general information about tendon and injury and it's response to loading forces that could cause injury.