Injury Series: Eccentric exercise and tendon remodeling, part I: Achilles tendonitis

Attention readers: I have published a significantly revised and updated article on midpoint Achilles tendonitis.  I strongly recommend you read that instead! The information below is incomplete and out of date! Click here to go to the updated Achilles tendonitis article.

Note: if you are looking for information on insertional Achilles tendonitis, see this article

We're shifting gears a bit today.  As high school and college cross country seasons approach, lots of runners are hitting their peak mileage right about now.  At the same time, there's a lot of runners who wish they could be out there hitting the road every day, but are sidelined by injury.  This will the the first post in a series on injuries: their cause, prevention, and treatment.  In the past 10-20 years, there have been some very important changes in the way the medical community approaches and treats many common running injuries.  In a few cases, highly effective treatments have been discovered that were not known even a decade or two ago.  Unfortunately, many physicians and physical therapists don't stay on top of the injury research that's being published in several of the major medical journals, so the clinical implementation of these scientifically proven treatments is lagging.  At the same time, many treatments that enjoy wide acceptance have not withstood a scientifically rigorous examination.  While few are harmful, wasting time on ineffective treatments is something neither the patient nor the doctor wants.  At the same time, I realize that treatment based solely on scientifically proven methods is often limited.  I've also amassed a fairly large bag of "tricks" either through experimentation or advice from fellow runners.  In truth, it's usually a combination of "tricks" and "treatment" that get you healthy and running again.  I'll do my best to keep it clear what is scientifically rigorous and what is hocus-pocus-magic.  This is quite a large undertaking, so (much to your delight, I'm sure) I'm going to break with my usual long-winded posts and break this series up in to smaller and more numerous posts, each on a specific injury and its causes, prevention, and treatment.  Today's topic: Achilles tendonitis.

Introduction and Background

Injuries to the Achilles tendon are often cited as one of the "big five" most common running injuries (the others being plantar fasciitis, patellofemoral pain syndrome (runner's knee), medial tibial stress syndrome (shin splints), and iliotibial band syndrome).  Whether to label Achilles injuries as "tendonitis" is controversial.  The suffix -itis implies the main problem is inflammation, as is the case in conditions like appendicitis, gingavitis, etc.  But Achilles tendon issues often present without any signs of cellular inflammation, especially in chronic cases.  Some podiatrists prefer the label "tendonosis," which implies a more general dysfunction in the Achilles.   Some even differentiate between tendonitis and tendonosis when diagnosing Achilles tendon injuries.  Regardless, "tendonitis" is the most common term, and it's the term I'll be using in this post.  However, it is important to remember that the root problem behind Achilles injuries is not inflammation--it is real, physical damage to the fibers of the tendon.

Terminology

Before we delve into Achilles tendonitis, I need to give a quick primer on concentric and eccentric muscle contractions.  Concentric contractions are simple.  It's when the joint movement is in the same direction as the muscle's contraction.  Using your biceps to curl a dumbbell up towards your shoulders is a concentric contraction.  In contrast, an eccentric contraction is when a muscle is working to oppose the motion of a joint.  Slowly lowering the dumbbell you've curled up to your shoulder is an eccentric motion.  If you were to completely relax your biceps, the weight would quickly slam down.  Your biceps work eccentrically to slow down the motion.  Most "down" motions are eccentric contractions working to oppose gravity: the down phase of a pushup, lowering a barbell down while doing a bench press, and the down phase of a squat all involve eccentric muscle contractions.  These contractions are stressful on muscles and are responsible for most injuries and muscle soreness--it's why running down a long hill several times will often make your quads more sore than if you'd ran up it. 

Anatomy

The Achilles tendon is the biggest and strongest of all the tendons in the body.  It connects the gastrocnemius and soleus muscles to the calcaneus, or heel bone, and allows them to perform their main task: plantar flexing the foot. Its role in running is essential--it allows the calf muscles (the gastrocnemius and soleus) to elastically store energy via the stretch-shortening cycle, which is released upon toe-off.  The tendon itself also stores energy by functioning as a very stiff spring.  And I do mean very stiff--upon loading with 120 pounds of force, it only lengthens by a few millimeters.  In fact, its stiffness tops that of suspension springs in high-end sports cars--it would take over 900 pounds of force to stretch your Achilles an inch!

The Achilles tendon connects the calf muscles--both the gastrocnemius and the soleus--to the heel.  Some doctors and researchers refer to both muscles as one unit: the triceps surae

Caffeine and running: effectiveness, ethics, and the NCAA

In my previous post, I mentioned how caffeine can boost athletic performance by stimulating the central nervous system, which in turn makes a given effort seem easier–the rated perceived exertion (RPE) drops.  I was leading you on a bit: the drop in RPE isn't the whole story with caffeine.  In fact, it only accounts for about 30% of the performance boost that comes from using caffeine.  I've received recieved a few questions specifically about caffeine since then, so today I'm going to go in-depth on the ins and outs of caffeine as an ergogenic (perfomance-boosting) aid.  This article is fairly thorough, since you can encounter a lot of myths about caffeine and sport on the web.  One site will claim "to fail the NCAA's drug test, you'd have to drink twelve cups of coffee in two hours" (page 10, I'm not making this up) while another will claim "just a cup or two of coffee or one energy drink can cause you to fail your drug test!"  I'd like to clear up some of that confusion.  To do so, there's a bit of math ahead–if you find yourself over your head, skip ahead to the next bolded sentence–that's the important stuff. 

Introduction

Caffeine might just be the "world's greatest drug."  It's certainly the world's most popular.  In the United States alone, 90% of the population consumes caffeine in some form every day.  Worldwide, billions of people consume caffeine on a regular basis.  By all definitions, caffeine is a psychoactive substance, and like all drugs, it has several effects (good and bad) that occur with different doses.

 The negative effects of caffeine at high doses include headaches, difficulty sleeping, tremors, and irritability.  It increases urine production, although regular caffeine users rapidly become immune to this effect.  The "caffeine causes dehydration" myth has persisted for a long time, but it is simply not true.  In massive doses, caffeine can cause seizures and death. Additionally, withdrawal in heavy users can cause headaches, drowsiness, and insomnia, and may persist for up to five days. 

However, it has a very large therapeutic index–the ratio of the toxic dose to the effective dose.  And its benefits are manifold at doses far below those that cause negative effects.  Caffeine boosts mood, alertness, vigilance, and cognitive function.  In an excellent 2008 review of double-blind, placebo-controlled trials, C.H.S. Ruxton concluded:

"From a review of double-blind, placebo-controlled studies published over the past 15 years, it would appear that the range of caffeine intake that could maximise benefit and minimise risk in relation to mood, cognitive function, performance and hydration is 38 to 400 mg per day, equating to 1 to 8 cups of tea, or 0.3 to 4 cups of brewed coffee per day. Current levels of caffeine intake in the UK fall well within this range, suggesting that risk, for example from dehydration, is likely to be minimal."

In a different review, A. Smith concluded that "Regular caffeine usage appears to be beneficial, with higher users having better mental functioning. [...] The evidence clearly shows that levels of caffeine consumed by most people have largely positive effects on behavior."

So even fairly heavy users of caffeine are safe from its negative effects and will reap the benefits of its boost to cognitive performance.  But caffeine also has an effect on athletic performance–a very pronounced one in the case of endurance events.  In a 2005 meta-analysis, M. Doherty and P. M. Smith wrote,

"In comparison to placebo, caffeine also improved performance by approximately 11%, a finding that concurs with another recent meta-analysis of the effects of caffeine on exercise test outcome (Doherty & Smith, 2004). As RPE was lower during exercise but unchanged at the end of exhausting exercise, it may be that part of the explanation for the improvement in performance has to do with a dampening of the perceptual response during exercise. Indeed, our regression analysis revealed that RPE during exercise could explain nearly one-third of the variation in the subsequent improvement in performance." [emphasis added]

Eleven percent is a whole heck of a lot.  And Doherty and Smith's review is only one in a litany of papers that report the benefits of caffeine on endurance exercise.  Away from the lab, for a recreational athlete, the boost will be more like 5%, according to Dr. Mark Tarnopolsky of McMaster University in Canada.  But even then–five percent is the difference between 16:00 and 15:12 in the 5k.  Often, ergogenic aids boost the performance of recreational athletes much more than elites, so perhaps a top-flight athlete wouldn't benefit as much.  But according to Matt Fitzgerald (who didn't cite it, tsk tsk) a treadmill study of elite runners still found a 1.9% increase in time to exhaustion.  Putting that in perspective, 1.9% is the difference between a 4:30 and 4:25 mile.  How does this happen? Is it legal? Is it right?

The different types of fatigue

In my last post, I discussed how a preponderance of studies show that your running mechanics change when you are fatigued.  The specifics of what changes occur are difficult to discern, as they likely vary from person to person.  Before moving on, I noted a problem with conflating all of these studies, a problem that I'd like to elaborate on today:  "fatigue" is not a monolith! Feeling tired after a 1500m race is not the same kind of fatigue you feel after a 12-mile run on a hilly trail, and that is not the same fatigue you feel 22 miles into a marathon.  This post is more of a grab-bag than usual, and there's a good bit of tangential material, so buckle up!

Overall fatigue can be thought of as the sum of four distinct specific fatigues--muscular fatigue, metabolic fatigue, energy depletion, and central nervous system fatigue.

Muscular fatigue

Muscular fatigue results from microtrauma to muscles--the soreness and weakness in your legs 30 minutes after a hilly run are an example of this.  It is thought that the microtrauma that causes muscular fatigue is also what causes delayed-onset muscle soreness (DOMS).  This damage to the muscles directly impacts performance by reducing strength and power output.  Interestingly, it seems that DOMS is brought on exclusively by eccentric movements, which is why a middle-distance runner gets sore calves and hamstrings after a race, but usually not sore shin muscles or quadriceps.  The quadriceps do work eccentrically when running downhill or over very soft terrain, which is why hilly or mushy runs seem to beat up your quads.

 Doing lots of push-ups will give you muscular fatigue, although the tiredness you feel immediately after each individual set of push-ups is mostly the result of metabolic fatigue in your upper-body muscles.  Doing push-ups will also make you more like Chris Solinsky, which is never a bad thing.

There is little you can do during a workout or race to reduce the eccentric activities that damage muscles.  Running on a softer surface may reduce microtrauma to muscles by reducing the peak impact loading rate and spreading out the impact that must be absorbed over a longer period, but there's nothing more than anecdotal evidence for this.  Most strategies for decreasing muscular fatigue involve limiting the damage after a workout ends.  Icebaths, compression socks, putting your legs up against a wall, and Alberto Salazar's much-mocked cryosauna all attempt to decrease inflammation in the legs and increase the efflux of fluids from the muscles back into the blood and the rest of the body.  Inflammation occurs for two reasons: 1) the damaged muscles leak their contents out into their surroundings and 2) the body rushes blood and lymph fluid to the damaged areas to repair them.  Repair may seem like a good thing, but the combination of the cellular fluids leaking out from the damaged muscles and the fluids rushing in from elsewhere in the body can increase pressure to the point where further damage and internal fluid leakage occurs.  In cases of major internal damage, such as surgery or bodily trauma, the swelling in response to the injury can cause life-threatening complications, and compression, elevation, and ice effectively combat this swelling. There's no reason to worry, however--if internal bleeding and swelling from surgery is a fire hose, post-workout microtrauma is a faucet drip.  But anyone who's had his or her feet swell up after a long hike knows that long workouts do cause inflammation.

There is some evidence that moderate-temperature "ice" baths will reduce soreness and increase performance in the days following a hard workout.  Interestingly, most studies use 12-15°C (54-59°F) water baths--rather puny compared with training-room tough guys who like to crank it down to 50°F or colder.  But there's no evidence colder is better! In fact, some people speculate that very cold temperatures can actually increase damage.  I like to think that the hydrostatic pressure from the weight of the water itself is a significant contributor to the recovery boost.  Standing in an ice bath with 24" of water provides a pressure differential of 45 mmHg between your feet and your knees--that's more than compression socks would provide!  


Likewise, compression wear also has a body of evidence supporting its use.  Compression wear was actually invented to control swelling and reduce the possibility of deep-vein thrombosis in hospital patients after surgery.  Veins have a series of one-way valves that are supposed to allow blood to flow only one direction (back towards the heart).  But the veins in your legs are susceptible to pooling, where blood stagnates because gravity is preventing the blood from achieving a high enough pressure differential for the valves to operate properly.  Compression counters the hydrostatic pressure inside your body due to gravity, allowing better bloodflow.  Compression wear has been getting a lot of attention recently because it might also increase performance during exercise via better bloodflow, increased proprioception, or decreased muscle vibration.

Fortunately for the impoverished runner, the evidence for these approaches is equivocal--some studies find no difference in recovery betwen groups who use compression or ice-bathing and those who do not.  If there are benefits, they are relatively small.  Further, leg elevation is a low-tech approach that likely has many of the same effects as compression and ice-bathing (namely, increasing fluid efflux from and reducing fluid influx to the lower body).  My entire high school team used to put our legs up against a wall for 5 minutes or so following our cool-downs.

The cumulative damage theory of injuries

I've had injuries on the brain lately.  Why do they happen? My high school's training room had a sign outside that said "Running injury? TOO MUCH, TOO FAR, TOO SOON."  Needless to say, the trainer wasn't very helpful.  But the medical/scientific consensus isn't much more helpful than that.  Overuse injuries are "tissue damage that results from repetitive demand over the course of time" according to emedicine, which is a slightly less harsh wording of the same idea my high school trainer had.  But things are more complicated than that (this is going to become a theme on this blog).  It seems like the rules are always changing--mileage or workouts that were okay last year are problematic this year, or the other way around.  The topic of "why injuries happen" is way too broad to over all at once, but today I'd like to look at one small piece: the process by which damage accumulates.

Each of the various tissues in your body has its own injury threshold.  This is the amount of stress it can take before becoming injured, and it varies from person to person and from time to time.  Healthy training will increase the injury threshold of most tissue.  So a runner who has been running 50 miles a week will be able to handle a given stress (say, a 10 mile easy run) better than a runner who has only been doing 25 miles a week.  However, the higher-mileage runner is also incurring a greater stress on a day-to-day basis.  Statistically, higher-mileage runners get injured more often.  The problem is that, in most studies, "high mileage" is typically defined as 20+ or 25+ miles a week.  Most competitive runners don't consider anything under 30 miles a week to even be "training."  I wish I had the resources to do a large-scale study on factors that can predict running injuries.  There have been some very interesting studies, though they tend to either use a large number of serious runners and find very few conclusive results (these studies are often done via surveys) or they use a small number of recreational runners and get good results, but their applicability to serious athletes is questionable.

Here's an example: Let's say a study finds that recreational runners who weigh over 200 pounds have a greater risk of injury.  We might conclude "lighter is better, since it puts less stress on the body."  But in competitive runners (particularly females), a low body mass index is associated with an increased risk of injury.

Injuries occur when internal factors coincide with external factors.  I might have a biomechanically defective elbow (an internal factor), but since I don't play tennis (an external factor), I don't get lateral epicondylitis--tennis elbow.  I've started to put some things together for a post on the various possible internal causes of injury (e.g. weak hip stabilizers, low bone density), but today I'm going to look at external factors. 

For a given runner, how does stress on the body scale? That's what I want to answer.  What is the relative increase in injury risk for a 4, 6, 8, 10, or 12 mile run? There are three possibilities: the injury risk has a linear increase, a compounding increase, or a compounding recovery.