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 The information below is incomplete and out of date!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.
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.
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!
Like all tendons, the Achilles is made up of collagen, a long helix-shaped protein that makes up much of the tough, fiberous material in your body: skin, cartilage, ligaments, and tendons. In a tendon, the collagen fibers are arranged in a wavy, linear, and quite orderly fashion, as shown to the left. The wavy shape is likely what allows the tendon to have its spring-like behavior.
Naturally, any activity that involves explosive upward or forward motion relies heavily on the Achilles. A high jumper, a cross country runner, and a sprinter all require their Achilles to be ready to rapidly store and release loads many times the athlete's body weight. Because of its ubiquitous nature, injuries to the Achilles tendon are quite common across many sports. Though acute Achilles injuries, including ruptures, are often a concern in contact sports, runners usually deal with overuse injuries to the tendon. These almost invariably occur in the last three inches of the tendon. Due to the size and relatively poor blood supply to the tendon, Achilles injuries can seem quite intractable to the frustrated athlete.
In runners, the traditionally-blamed culprits for Achilles tendonitis are excessive speedwork, hillwork, or explosive strength training. Runners who have a forefoot-striking style also seem more likely to suffer Achilles tendon injuries. But even heel-striking runners doing all easy mileage on flat roads can suffer severe Achilles tendonitis. So what's the research say about the causes of Achilles tendinitis in runners?
As we've seen before, epidemiological studies are not particularly useful in determining the cause of running injuries. "Training errors" are cited in 60-70% of Achilles tendonitis cases, though this is not a helpful finding--presumably, something else caused the Achilles to be the weakest link. Runners only rarely come down with two or more completely unrelated issues at the same time; although a training error may have provoked the injury, there still must be some underlying reason the Achilles tendon gave out first. Among factors other than training errors that have been linked to Achilles tendonitis, the most relevant to our discussion include ankle range-of-motion limitations, weak calf muscles, and excessive pronation. More rare causes like reactions to local cortisone injections in the ankle and reactions to fluoroquinolone antibiotics are important to be aware of, but aren't a factor in most cases.
More than a few studies (1, 2, 3) have shown excessive pronation to be associated with Achilles tendonitis. All of these studies have a flaw, however--they are retrospective, meaning they look at athletes who already have Achilles tendonitis, instead of taking a healthy group of runners and seeing who gets injured. A prospective study like that would be very difficult--in a typical year, somewhere in the neighborhood of 30-50% of runners report an injury, but of these injuries, only 10% or so would be Achilles tendonitis. To get a workable study size, a very large group would need to be evaluated. As a result, we're left to extrapolate from retrospective work. The main problem with retrospective results is that they make it impossible to determine causal relationships--do runners with Achilles tendonitis pronate because their Achilles is injured, or did the pronation cause the Achilles injury? Unfortunately, this can't be answered yet. Even if we assume pronation does increase strain on the Achilles, one of my earlier posts showed that common interventions like supportive shoes and orthotics do not reliably alter pronation, so approaching treatment from that angle seems to be a non-starter.
Weakness in the calf muscles is at least a possible causal factor with no evidence against it. It makes intuitive sense--a weak gastrocnemius or soleus would require the Achilles to contribute a greater portion of energy return during toe-off, which would in turn increase the strain on the tendon. There is also evidence that strengthening the calf muscles improves recovery from Achilles tendonitis; we'll go over this later.
On the same token, poor ankle range of motion (ROM) is another possible cause that makes intuitive sense. When ROM at any joint is poor, the muscles must work harder to move the joint, much like it takes more force to twist a tight screw than it does to twist a loose one. Poor ankle ROM can have several causes: an acute injury like an ankle sprain, tight shin muscles, and tight calf muscles. It's easy to see how tight calf muscles or tight shins can reduce your ankle's range of motion and increase strain on the Achilles. In the case of a tight shin muscle (tibialis anterior for the scientifically minded), the calf has to work harder when contracting to plantar flex the foot. Likewise, if the calf muscles are tight, there is also increased strain on the Achilles because the baseline tension is greater. So, for a given impact or push-off, the Achilles will experience a higher stress.
So, all these various causes, perhaps combined with some more run-of-the-mill methods of overloading the Achilles (overzealous hill running or a block of higher mileage, for example) can set off an Achilles tendon injury. But what actually happens on the cellular level when the Achilles (or any tendon) is injured? This brings me to John's First Maxim of Injury: injury-related pain is the result of real, physical damage to the musculoskeletal system. It may seem like a silly maxim, but you'd be surprised how many people fool themselves into think that the only reason something hurts is because there's "inflammation in the area"-as if their chronically injured Achilles was perfectly intact and they'd be good to go if not for that pesky inflammation. So they'll pop a few advil or ice a bit and head out the door. This brings me to John's Second Maxim of Injury: Your recovery plan should be geared at repairing the damage and addressing the root cause of your injury. This is not to say that tackling inflammation with icebaths or icecups is a bad idea.
Without getting too far off-track (because we know how often that can happen on this blog), inflammation is the body's response to an injury. Anyone who's seen his or her ankle swell up to the size of a grapefruit following an ankle sprain can tell you that. Inflammation rushes fluids to an injured area, and also prevents fluids from draining out of an injured area. Inflammation is usually demonized as an out-of-control overreaction to an injury, but it does have some redeeming qualities: it stimulates nerve endings and increases the pain coming from an injury, strongly discourages you from doing additional damage from continuing to run (this is why avoiding painful activities while injured is usually a good idea). Despite this, it is an overreaction, and should be dealt with accordingly. I usually recommend icing 3-5 times a day (10 minutes each if using an ice cup, 20 minutes if using an ice bag) for 2-3 days following minor muscle and tendon injuries. But this is just hocus-pocus magic.
Moving back to the Achilles tendon, let's take a hypothetical college runner, Sam. He's running 10-12 miles a day during the summer, preparing for his sophomore college cross country season. One day after finishing his long run, he notices his left Achilles tendon feels tight and sore. The next day, it start to tighten up in the last mile or so of his easy run. In the following days, the pain increases to the point where he feels an ache in his Achilles with virtually every step he takes. Being the hard-headed type, Sam ices it a bit and goes ahead with his training, but after nearly a week of this, he feels pain even walking, and his Achilles tendon is sore to the touch. Even with several days' rest, his Achilles aches if he tries to run.
If we could have looked at a sample of Sam's Achilles tendon before he got injured and put it under a microscope, it'd look like the pink sample above--with collagen fibers neatly arranged in a wave-like pattern (this is illustrated even more beautifully in the image to the left, which I couldn't resist including in this post. It was created using a tendon from a rat and a non-linear optical microscope at Texas A&M). However, if we took a sample of Sam's tendon one or two days after it had started to hurt, it'd look like a war zone--there would be frayed collagen fibers, inflammatory cells, swelling, and amino acids everywhere. Why the difference? When Sam injured his Achilles, probably during his high-volume training in the past week or so, he actually caused a microsized rupture of the tendon. While Sam is probably not in danger of a true Achilles tendon rupture, his body is responding to the damage nonetheless. Gaping holes in the collagen structure in his Achilles are patched frantically as his body struggles to keep pace with the damage. Every time Sam runs on his injury, he is tearing apart some of the new collagen that is attempting to shore up the tiny rupture in his Achilles, magnifying the problem. Once Sam finally does take enough time off to allow his body to repair the damage by laying down new collagen, the result is nothing like what he started with. If we took a biopsy of Sam's Achilles tendon after he's been cross training in the pool for a week or two, it'd look something like this:
Perhaps it wouldn't be quite this bad--this is from a woman who actually ruptured her Achilles tendon, but the same mechanism is at work in Sam's Achilles. This looks more like a plate of spaghetti than the flowing waves of collagen we saw earlier. The continual damage/repair/damage cycle that Sam has put his body through while trying to "run through" his injury has caused the new collagen to grow in a disorganized, random fashion, effectively forming scar tissue. This cycle of injury and repair does not necessarily have to take place in rapid succession; several injuries to the same Achilles tendon over a period of months or years can also bring about the disordered collagen arrangement characteristic of chronic Achilles tendonitis. Either way, the consequence of this is that the disruption in the natural collagen orientation has altered the mechanical properties of the Achilles tendon.
In this study, runners with Achilles injuries had measurably less-flexible Achilles tendons compared to a group of healthy volunteers. When the calves were isometrically contracted, an injured Achilles is pulled tighter than a healthy one. Why? Because of the disrupted collagen arrangement. In a healthy tendon, the orderly, wave-like pattern of collagen allows the proteins to stretch slightly, granting more "give" to the tendon. But the disordered collagen pattern in a chronically injured tendon is not flexible. Like pulling on a knotted rope, strain on a injured Achilles tendon disproportionally affects the injured (knotted) area. The tighter you pull, the tighter the knot becomes. This, along with the poor blood supply to large tendons like the Achilles which slows the recovery and rebuilding process, makes recovering from chronic tendon injuries difficult.
We've established that Sam, our hypothetical runner, has done quite a number on the collagen fibers in his Achilles tendon. All Sam cares is that his Achilles hurts and he can't run on it. What should his rehabilitation program look like? Most sports doctors and podiatrist would refer Sam to a physical therapist, who would give Sam a set of stretches and exercises to perform daily. These programs vary from therapist to therapist, but mostly involve exercises similar to those in the graphic below:
Here's the logic behind these exercises:
- Calf Stretches: if the calf muscle is tight, as covered earlier, it will increase strain on the Achilles. Stretching the calf, both with a straight knee and bent knee, should loosen up the muscle and take some strain off the Achilles.
- Calf strength (heel raises): strengthening the calves should also take some strain off the Achilles.
- Upper-leg strength and balance: Improving overall strength and balance should reduce the total amount of strain the Achilles has to take during running--better balance and more coordination should create less of a shock on the Achilles during impact.
Unfortunately, few of these "theoretically sound" exercises actually have evidence-based proof supporting their claims. I contend that fully half of the exercises above are inefficient and three of them--the calf stretches--might even be harmful.
Why do I think calf stretching a bad idea for Achilles tendonitis? I've already established that tight calves are bad and predispose you to Achilles problems, and stretching loosens tight muscles, so why not stretch them out? Because stretching increases strain on the Achilles. Remember the knotted rope analogy? Every time you stretch, it's like tugging on both ends of that knotted rope. Additionally, there is no substantial evidence supporting its use. Especially in the "acute" phase of an Achilles injury, I recommend avoiding stretching, because it will only tug on the Achilles more and disrupt the collagen healing process. Once the collagen has healed (which probably takes anywhere from a few days to a few weeks depending on severity of the injury, but I'm just guessing here), gentle stretching can be helpful, but runners are far too eager to crank away, thinking stretching more forcefully is better. Fortunately, there are ways to loosen up the calves in the acute phase without putting any strain on the Achilles.
Foam rolling and heat (applied to the CALVES, not the Achilles tendon!) can accomplish nearly the same thing as calf stretching without the drawbacks. Foam rollers are an inexpensive and highly versatile recovery aid, and I would wager a large sum of money that you can find one underneath nearly every professional runner's bed. You can get one at your local running shop. You might find that a foam roller is too soft for some areas, including your calves. In this case, you can move up to something harder--an 18" section of 3" PVC pipe from a hardware store is ideal. A few minutes twice a day on the foam roller will go a long ways towards loosening your calves and is helpful for other injuries to. Heating up your calves with a hot pack, especially before rolling, will loosen your calves too--just avoid applying heat to the Achilles tendon, unless you're about to leave for a run.
Sam might also receive a heel lift or a custom orthotic insert for his shoes from his doctor, though there is surprisingly scant evidence for their use. One very small study (only three participants) found that a heel lift actually increased maximum Achilles tendon force, while a somewhat larger study (seven participants) found a highly variable effect with no statistically significant change overall. In some individuals, peak Achilles tendon force decreased; in others, it increased. This should not be surprising to regular readers of this blog, as we've seen the same effect with orthotic and shoe changes meant to control pronation. This doesn't mean heel lifts or orthotics are to be avoided at all costs--you're probably just as likely to be helped as you are to be hurt by them. However, there are concerns about the long-term effects of wearing heel lifts. The deleterious effects of high-heeled women's shoes are well documented: a modified gait, increased pressure on the forefoot, and possibly shortening of the Achilles tendon itself. A standard running shoe with a heel elevation of 12mm combined with a 10mm heel wedge to treat Achilles tendonitis brings the total heel-to-toe drop to 22mm--nearly an inch. While most studies on the effects of high heels involve larger drops of 45 or 80 mm, it would not be outrageous to propose that even more moderate heel elevation would be harmful long-term. Alas, this is another topic for another day.
Eccentric heel drops: a superior treatment method
In the past decade or so, using targeted eccentric exercise on injured areas has become a very important part of rehabilitation for several chronic overuse injuries. Although there was evidence that eccentric calf strengthening was useful in Achilles tendonitis treatment since as early as 1992, it was not until this 1998 study by a top-notch team of Swedish doctors and therapists that the value of eccentric calf strengthening was realized. The study used thirty recreational runners who had been suffering from chronic Achilles tendonitis for several years on average (one subject had been injured for over eight years!), all of whom had tried rest, nonsteroidal anti-inflammatory drugs, physical therapy, and footwear/orthotic intervention to no avail. Fifteen were assigned a rigorous eccentric calf strengthening routine and fifteen were assigned surgical debridement of the Achilles tendon.
The eccentric strength protocol consisted of two exercises: straight-knee heel drops and bent-knee heel drops. These exercises were performed twice a day, every day for twelve weeks, and done slowly. Alfredson et al. describe the exercises as follows:
From an upright body position and standing with all body weight on the forefoot and the ankle joint in plantar flexion, the calf muscle was loaded by having the patient lower the heel beneath the forefoot (Fig. 1). They were only loading the calf muscle eccentrically, no following concentric loading was done. Instead, the noninjured leg was used to get back to the start position. The patients were told to go ahead with the exercise even if they experienced pain. However, they were told to stop the exercise if the pain became disabling. When they could perform the eccentric loading exercise without experiencing any minor pain or discomfort, they were instructed to increase the load by adding weight. This could easily be done by using a backpack that was successively loaded with weight.
A straight-knee heel drop, as the name suggests, involves keeping the knee locked throughout the exercise. A bent-knee heel drop involves bending the knee upon descent (thereby strengthening the soleus muscle), as illustrated below.
After 12 weeks of eccentric exercise, the results were astonishing. Fully 100% of the subjects had been able to return to their pre-injury running level. Their reported pain during activity, reported on a 100-point scale (the visual analogue scale, or VAS, for the medically-minded), decreased from 81.2 to 4.8! They also showed a marked increase in strength on their injured side. But perhaps even more impressive is how the eccentric exercise group compared to the control group who underwent surgery. All 15 of the surgery group members were eventually allowed to return to running, but their VAS pain scores, which averaged 71.8 pre-surgery, only dropped to 21.2 ±11.4 after 24 weeks post-surgery (recall that the eccentric exercise group was scored after 12 weeks). Thus, the eccentric exercise protocol was superior to surgery in every respect--it had better results in less time without any risks that surgery entail.
What is it about the eccentric exercise protocol that enabled such drastic improvements, even in subjects who had been injured for many years and not responded to any conventional treatment? A later study by the same group ruled out a simple increase in strength--in that study, 44 subjects were assigned either a concentric (calf raise, going "up" only) or eccentric (heel drop, going "down" only) protocol. 18 of the 22 members in the eccentric exercise group were able to return to their previous activity level, while only 8 of the 22 members in the concentric strength group were able to do so. So there is more at play than just muscular strength.
Three factors were crucial to the success of the Alfredson et. al 1998 rehabilitation program: the emphasis on eccentric strength, the progressive nature of the exercise (accomplished by adding weight), and the requirement to continue the exercise even when in moderate pain. Eccentric overload on muscles and tendons is actually blamed for causing many injuries, so it seems odd that the same type of activity encourages recovery.
Recall that the root cause of chronic Achilles tendonitis is the disproportionate strain on the disordered collagen at the injury site--the knot in the rope, per our analogy earlier. Uncontrolled eccentric exercise does more damage, which causes more disordered collagen (scar tissue) to build up, magnifying the problem. Additionally, eccentric movements in regular activity (like running) are paired with concentric movements, which don't damage a healthy tendon, but do put strain on an injured one. But controlled and targeted eccentric exercises--like eccentric heel drops---actually reverses the problem. Here's how:
- A moderate amount of eccentric exercise (into moderate pain) damages and breaks down the disordered collagen at the injury site in the Achilles tendon
- The body responds by increasing collagen synthesis and laying down new fibers
- Because all of the exercise is eccentric, the disordered collagen fibers are disproportionately broken down. If a new collagen fiber happens to be laid down in the proper direction (in line with the original, healthy fibers), it will not break down when exposed to additional eccentric exercise. But if it is a coiled mess like many of the fibers in the microscope image above, it will be broken down again
- Over time, as the eccentric load increases, more and more disordered fibers are broken down and replaced with properly-ordered fibers. Eventually, the tendon has been repaired enough to handle a return to high-level training
Scientific evidence for the actual restructuring of the collagen fibers in the Achilles tendon is only circumstantial, probably because a tendon biopsy is the last thing on the mind of anyone who's just recovered from a multi-year bout with Achilles tendonitis. But ultrasound imaging and studies of vascular flow in the Achilles have shown strong evidence that the mechanism by which eccentric exercise works is a fundamental rearrangement of the tendon's structure. Additionally, microdialysis of the injured area has confirmed that eccentric exercise increases collagen synthesis.
Conclusion: treating and preventing Achilles tendonitis
Although I've broken my promise to keep this post short, I'll try to wrap things up briefly. As you've seen, the focus on eccentric movements, progressive addition of weight, and exercising into moderate pain all contribute to the ability of eccentric heel drops to heal chronic Achilles tendonitis. There are ancillary benefits too: the heel drops also stretch the calf muscles and strengthen the lower leg. Obviously, a progressive rehab protocol of eccentric heel drops should be the central part of your recovery plan if you have Achilles tendonitis. In short, the eccentric heel drop protocol is as follows:
- 3 sets of 15 eccentric straight-knee heel drops and 3x15 bent-knee heel drops over a step or ledge, starting "up" on one leg, descending slowly, and returning to the "up" position uisng the uninjured leg.
- The two exercises are to be performed twice a day, every day for twelve weeks.
- Continue the exercise even into moderate pain, but stop if the pain becomes debilitating.
- When you are able to do all three sets without any pain, add weight using a backpack.
- Once you have recovered, continue to do eccentric heel drops several times a week as a preventative measure.
As mentioned earlier, there are other things you can do to hasten your recovery from an Achilles injury: Stretching your shins, foam rolling your calves and shins, and applying heat to your upper calves (not the tendon itself) will loosen those muscles up and improve your ankle's range of motion. A doctor, podiatrist, or physical therapist can get you orthotics or heel lifts, which may help your condition, and can also prescribe additional rehab exercises to improve general strength and balance. Heat up the tendon with a heat pack or warm water bath before you run, and ice it afterwards.
If you're lucky enough to have never suffered Achilles tendonitis, you can still apply many of these same principles to prevent it. Incorporating eccentric heel drops and calf and shin stretching & rolling into your daily or weekly routine might help prevent future problems. Doing a moderate amount of running in low-heeled shoes like racing flats or spikes (or even running barefoot) will allow your calf and Achilles to work through their entire range of motion. This will reduce the shock to the area at the beginning of the racing season. Far too many runners go the entire summer without running a step in anything other than their trainers, which have a 10 or 12 mm heel-to-toe drop. In the fall, they slip on racing spikes (which, because of the metal spikes in the forefoot, effectively have a negative heel-to-toe drop) for their first race and risk calf soreness and Achilles tendon injury due to the sudden overload on the lower leg. Doing a few miles of barefoot running and a few strides in spikes every week will go a long ways towards preventing these sorts of injury.
Next time, we'll look at a similar eccentric rehab protocol for another common chronic injury: patellar tendonitis.