Achilles experts ponder effects of heel elevation
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.”
The range of findings reported by studies of heel-elevating interventions—most of which have been conducted in healthy people— underscores the challenges involved.
Since the publication of its initial study, the QUT team has published further research that sheds some light on the situation.5Concurrently, 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