After an Australian study raised questions about the conventional practice of elevating the heel to decrease Achilles tendon load, researchers and clinicians have been revisiting what is known about the many factors—including pain—that can affect tendon load and function.
by Cary Groner
Two years ago, researchers at the Queensland University of Technology (QUT) in Brisbane, Australia, published a study that startled the sports world.1 Using a custom ultrasound device to measure the propagation of acoustic waves in tendon tissue, the team found that walking in standard running shoes with a typical cushioned heel appeared to increase loads on the Achilles tendon compared with barefoot walking.
The findings produced consternation among other researchers and clinicians, given the long-held belief that modest heel elevation decreases Achilles loading by slightly plantar flexing the ankle and shortening the muscle-tendon unit during gait.2
The stakes are fairly high: In athletes, Achilles tendinopathy has a prevalence of 19% and a cumulative lifetime incidence of 24%.3 Achilles tendon pathology accounts for 8% to 15% of injuries in recreational runners, in fact.4 But the condition isn’t limited to athletic populations; a third of people diagnosed are sedentary.3 The crucial question is what to do about it, and if heel raises may actually increase tensile loading, there’s some important rethinking to be done.
But other researchers and clinicians say wide variability among tendinopathy patients, and the long list of factors other than heel elevation that can affect tendon loading, are also important to consider.
“If you think of the Achilles as a spring, we know that if you use your spring at a shorter length, you may put less load on it,” said Jill Cook, PhD, a professor and deputy director of the Australian Centre for Research in Injury in Sport and its Prevention at La Trobe University in Melbourne, Australia. “In theory, a heel raise should take some load off the Achilles, but it may not be effective, depending on a range of other things such as how strong your calf is. The heterogeneity of the population you’re investigating is going to complicate your outcomes, and in practice it’s really variable.”
Since the publication of its initial study, the QUT team has published further research that sheds some light on the situation.5 Concurrently, other experts have had time to gather their wits, write papers of their own, and try to make sense of it all.
Heel lifts and heterogeneity
An important aspect of the QUT study was that the shoes were associated with changes in several spatiotemporal gait parameters, including lower cadence (five fewer steps/min), greater stride and step length (5% each), and longer step duration (12%), even though both conditions involved walking at identical speeds. Such changes could themselves affect tendon loading, of course, and lead author Scott Wearing, PhD, acknowledged this. Wearing, who is a professor of clinical sciences in the Institute of Health and Biomedical Innovation at QUT, told LER after that study’s publication that it was unclear which aspects of the shoes contributed to the increased loading effects—the heel raise itself or some other variable.6
The study participants, moreover, were healthy—without tendon pain or pathology—and, as noted, the test involved walking rather than running. Research has shown that gait in runners with chronic tendon complaints often differs from healthy controls,7 however, and it’s such patients that primarily concern clinicians.
Complicating matters further, the concept of load has different clinical implications depending on whether one’s objective involves alleviating painful symptoms or treating a patient’s underlying issues.
“I might tell a patient, ok you can put in a heel lift, but that’s only for symptomatic relief,” said Karin Silbernagel, PT, PhD, ATC, an assistant professor in the Department of Physical Therapy at the University of Delaware in Newark. “But to treat the problem, we want to load the tendon.”
Cook agreed, noting that tendon loading can’t necessarily be quantified in a clinically relevant way.
“Tendons are very sensitive to how you load them, and it’s difficult to evaluate pathology or structural change and make inferences about pain; they don’t necessarily correlate,” she continued. “We’re getting better at measuring load, but it’s hard to do, and I’m not sure that absolute load is a factor. If you run two hundred kilometers a week, your tendon is going to be much more resilient than that of a couch potato, so you can’t say, ‘This much load makes a difference.’ What you can say is that the tendon has to adapt to a certain percentage above the [usual] load. That’s where the heterogeneity in populations becomes tricky. As soon as you get people with pathology and pain—different ages, genders, loading environments, biomechanics, strengths, kinetic chain function—it just becomes a minefield to get anything out of it.”
The range of findings reported by studies of heel-elevating interventions—most of which have been conducted in healthy individuals—underscores the challenges involved. As far back as 1995, Canadian researchers found that heel lifts didn’t decrease Achilles tendon loading at a statistically significant level across their study population—but in two of the five runners studied, it did.8 Then in 2002, British researchers reported in the Journal of Applied Biomechanics that heel lifts were associated with both increases and decreases in peak Achilles tendon force, depending on the runner being tested.9 They also noted that treatment success might be due to heel raises causing a later tendon force peak, which reduced the average rate of loading. A 2005 paper with the same lead author reported similar findings in soccer players: A 10-mm heel lift had no significant effect on peak Achilles tendon force, peak plantar flexion moment, or corresponding loading rates across the group; but loading did, in fact, significantly increase with the heel lift in some players and decline in others.10 Another 2015 paper found that walking with heel wedges didn’t reduce Achilles tendon load, but did redistribute it from the medial to the lateral triceps surae during inclined walking.11
“There are very individual responses to shoes and heel lifts, and some people respond totally the opposite of what you expect,” Silbernagel said.
Reconnaissance and strategy
Cook and Silbernagel point out that the Achilles tendon isn’t a monolith, and the part of the tendon where pain or pathology occurs affects diagnosis and treatment strategies.
“Sliding and rotation give the tendon its springlike behavior,” Cook said. “But there’s the insertion point, there’s the midtendon, and there’s the peritendon, which is more about friction loading. So there are actually three pathologies producing different signs and symptoms, caused by different loads, that require different interventions.”
Silbernagel described how such complexities affect her therapeutic decisions.
“If there’s insertional injury, I won’t have patients exercise barefoot,” she said. “In those cases, I want to start them in various degrees of plantar flexion to minimize the pain. You get compression on the tendon, over the bursa, and onto the bone, when you go into more dorsiflexion. If you put in a heel lift to unload that, you get a great effect. Someone with midportion tendinopathy, but no insertional pain, can often start exercising barefoot, though never on the stairs.”
Cook agreed that effective therapy typically entails increasing the loading capacity of the tendon.
“If change in load is a critical factor that induces pain, then you have to adapt the musculotendinous unit to tolerate higher levels of load,” she said. “Loading is always critical—tensile loading; heavy, slow resistance training; and then training the tendon to do the energy storage loading that’s by far the best thing for your mid-Achilles and insertional Achilles. The difference between those is that with insertional Achilles, you must keep people in a heel lift until symptoms start to resolve. For the peritendon, it’s about reducing movement, so heel lifts can help there for a different reason, because you stop the friction between the tendon and the underlying tissue. The treatment can be similar but for completely different reasons.”
Cook stressed that heterogeneity in the patient population profoundly affects such decisions, as well.
“My seventy-year-old golfing lady is going to be completely different from my twenty-five-year-old sprinter, and I’ll recommend different treatments because of who they are, what factors are contributing to their pain, and what they want to achieve,” Cook said. “It’s about modifying training so that their load stays under the tendon’s capacity, but at the same time trying to increase that capacity. That means modifying how many times a week you train, what sort of drills you do, and addressing pain, strength, and endurance levels with an exercise program.”
At Silbernagel’s University of Delaware clinic, clinicians use a return-to-sport model premised on individual needs. One issue has to do with how patients experience and respond to pain.
“There are habitual overloaders, who should fear pain more than they do,” she said. “Then there are underloaders who are more fearful of the pain. If you can move people progressively over time and follow their training diaries, it solves a lot of these sensitivity issues.”
Silbernagel and her colleagues pay attention to foot-strike patterns, too, since they affect tendon loading.
“If you’re a heel striker I can allow you to run longer than if you’re a forefoot striker, which puts more load on the tendon,” she said.
Indeed, a 2013 paper reported that female runners with a forefoot or midfoot strike experienced an 11% greater Achilles tendon impulse with each step—which added up to a whopping 48 additional body weights for each mile run—compared with rearfoot-strike runners.12
Recent research has more deeply explored the levels of complexity that affect the web of biomechanics related to Achilles function—and dysfunction.
For example, in a 2015 paper, researchers at the University of Southern California in Los Angeles found that tendinotic Achilles tendons are more compliant than healthy ones, and that this affects central nervous system control, which then alters muscle activation patterns in the lower leg.13
British investigators, too, have begun to describe the complex relationship between the Achilles tendon and surrounding musculature. In a study presented in 2014 at the International Scientific Tendinopathy Symposium in Oxford, UK,14 researchers from the University of Leicester reported that runners with Achilles tendinopathy had weaker plantar flexors than healthy runners, and that the soleus muscles, rather than the gastrocnemius, appeared chiefly responsible for the deficits. They speculated that the problem could be due to central motor inhibition or preexisting weakness.
Lead author Seth O’Neill, MSc, a physiotherapy lecturer and PhD candidate at the university, told LER that he and his colleagues suspected the role of the soleus in Achilles tendinopathy partly because the condition so often affects endurance athletes.
“In any event longer than about fifteen hundred meters, the soleus is the most important force producer,15 at about eight times body weight,” he said.
He and his team designed their experiment to test isokinetic strength using a dynamometer, with the knee both extended and in 80° of flexion.
“In the extended position, both the gastrocs and soleus worked, but in the flexed position only the soleus can produce any force. The deficit in strength was very similar in both test positions, which suggests it’s the same muscle in each case—and that has to be the soleus,” O’Neill said. “Our hypothesis is that if the muscle is weak, and not producing force appropriately, it cannot absorb shock and protect your Achilles. That may be why the tendon starts to break down. We think treatment interventions need to target the muscle so your tendon can repair itself and recover.”
O’Neill believes, moreover, that therapeutic benefit may not require massive interventions.
“Altering the muscle’s motor skill, or the person’s control of the muscle, improves how it absorbs shock,” he said. “We’re finding that patients with Achilles problems tend to be uncoordinated—the tendon jitters as it eccentrically loads—and we think that’s a key component. You can do what we do, which is strength training and load monitoring on the model of Karin Silbernagel’s work. You could also change the runner’s gait with footwear, orthotics, or reeducation. It’s not just about pain; it’s about tendon structure, and whether that changes. The studies have shown how variable people’s responses are, so the key is to individualize therapy.”
Orthoses and shoes
Researchers have studied foot orthoses in this context; perhaps not surprisingly, results have been varied.
For example, a 2007 study reported that just four weeks of either physiotherapy or wearing custom foot orthoses reduced pain by half in 341 male runners with Achilles tendinopathy.16
More recently, a 2014 study reported that orthoses were associated with significant reductions in Achilles tendon load in healthy runners—though the researchers used polyurethane insoles rather than prescription devices, and hypothesized that the load reduction was related to the midsole cushioning effect.4
Jill Cook and her colleagues studied custom versus sham orthoses in 140 individuals with midportion Achilles tendinopathy, all of whom also did eccentric calf muscle exercises.3 The authors found that, after a year, the custom devices had been no more effective than the sham ones for reducing symptoms and improving function; nevertheless, all participants reported significant improvement in symptoms.
It’s easier on clinicians when everything works rather than when nothing works, of course, but it does make it harder for researchers to parse results. Other studies have shown benefits from certain types of eccentric calf-muscle training, in fact, suggesting that it’s a valuable aspect of therapy with or without orthoses.17
Not surprisingly, podiatrists and orthotists have their own take on the situation. Bruce Williams, DPM, director of gait analysis studies at the Weil Foot and Ankle Institute in Chicago, said that if he can’t stretch tendinopathy patients to an increased range of ankle joint motion, he adapts by raising the heel.
“I think Achilles issues are usually more related to tension loading than impact, though it can be both if the patients have issues with ROM and hit the end of that range,” Williams said.
He added that he rarely uses heel lifts alone because they don’t allow enough flexibility to treat the range of individual pathologies he sees.
“I use lifts in combination with a prescription for an orthotic device,” he explained. “I’ll varus or valgus post, or use a wedge under the hallux to engage the third rocker. All of these are meant to treat the whole, because heel lifts just don’t work all the time.”
Range of motion, foot type, and other variables that Williams has found to be important clinically are often not reported in Achilles studies, he said.
“Was it a high-arched foot in the subject?” he asked. “Was it low-arched? What was the resting calcaneal stance position? How was their ankle range of motion—do they have dorsiflexion issues, and to what degree?”
All such factors, he said, could affect results and partly explain the wide variation in responses.
Harvey Johnson, CO, the owner of Eno River Orthotics in Hillsborough, NC, agreed that the “to-lift-or-not-to-lift” discussion oversimplifies the problem.
“Heel lifts do reduce tensile load on the Achilles,” he said. “But when you build stability into the calcaneus with a custom orthotic, the patient isn’t sliding around as much as on a flat heel raise.”
Shoe design matters too, Johnson said.
“Tibial progression is critical, and if the shoe lacks an anterior rocker, it restricts that progression, which increases the need to dorsiflex at the ankle. That loads the Achilles as the body mass moves over the knee.”
Johnson echoed Williams’ concerns about foot type, as well.
“If studies don’t take that into account, their results are skewed,” he said. “A twelve-millimeter raise in a high-arched foot doesn’t affect Achilles/ankle dorsiflexion nearly as much as twelve millimeters in a low-arched or flat foot.”
The view from Oz
Seth O’Neill criticized the Wearing paper for failing to account for the variables in its two testing conditions.
“When they compared barefoot to shod, there were differences in cadence; they’re taking more steps [when barefoot], each stride is shorter, so effectively you’re expecting less Achilles load. It’s not a matter of heel rise, it’s a matter of absorbing shock differently.”
Interestingly, a 2016 follow-up paper by Wearing and his colleagues compared shod walking with or without an added 12-mm in-shoe heel lift, and found the added lift reduced tensile load in the Achilles.5
Wearing spoke softly from his office in Brisbane and seemed relatively calm about all the controversy.
“In the first study, what we were actually saying is that shoes modify your gait pattern, and that this change results in greater Achilles tendon loading, despite the shoes already having a heel raise,” he said. “When people put on the shoes, they had greater step lengths, lower cadences, and higher ground-reaction forces. I think all of those things, either individually or combined, could lead to higher Achilles tendon loading.”
His team wasn’t oblivious to expert blowback, as it turns out.
“We had a lot of discussions with clinicians who pointed out that heel lifts should lower forces, so then we took the same shoes and added a twelve-millimeter heel lift,” he continued. “In that study [the 2016 paper noted above], there were no changes in gait parameters, so we concluded that heel raises themselves do slightly reduce Achilles loading.”
Wearing said that all the participants in the 2014 study were rearfoot strikers whether barefoot or shod, so he didn’t think foot-strike pattern was a major variable in that case. However, his team is currently conducting a study to compare Achilles tendon loading in rearfoot-strike runners and forefoot-strike runners during barefoot running.
“It’s a little premature to discuss what we’re finding, but from early data, I will say that it looks as if foot-strike patterns do influence Achilles tendon loading,” he said.
As for the future, his team is interested in further analyzing which parts of a given shoe contribute to gait changes and the associated increase in Achilles loading.
“We know that heel elevation tends to lower those values, but not to the levels encountered during barefoot walking,” he said. “One of the great unknowns for us is how important increasing or decreasing load is in the tendon, because loading is important in homeostasis and maintaining tendon properties, but is also linked to injury. That’s the future research: What is a detrimental load?”
Cary Groner is a freelance writer based in the San Francisco Bay Area.
- Wearing SC, Reed L, Hooper SL, et al. Running shoes increase Achilles tendon loading in walking: an acoustic propagation study. Med Sci Sports Exerc 2014;46(8):1604-1609.
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- O’Neill S, Watson P, Barry S. Plantarflexor muscle power deficits in runners with Achilles tendinopathy. Br J Sports Med 2014;48(Suppl 2):A49.
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- Mayer F, Hirschmuller A, Muller S, et al. Effects of short-term treatment strategies over four weeks in Achilles tendinopathy. Br J Sports Med 2007;41(7):e6.
- Jonsson P, Alfredson H, Sunding K, et al. New regimen for eccentric calf-muscle training in patients with chronic insertional Achilles tendinopathy: results of a pilot study. Br J Sports Med 2008;42(9):746-749.