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It’s no coincidence that the tendon connecting your calf muscles to your heel bones is named for the fatal weak spot of the Greek warrior Achilles. For athletes of all levels, this tendon is such a common and troublesome site of pain that it’s more or less proverbial.

Achilles tendon injuries are also notoriously difficult to rehabilitate, which is what a new study in the journal Medicine & Science in Sports & Exercise hopes to address. It doesn’t offer any easy solutions; instead it quantifies for the first time exactly how hard various rehab exercises are on the tendon.

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“What we find really exciting,” says Josh Baxter, a researcher in the University of Pennsylvania’s department of orthopedic surgery and the study’s lead author, “is that clinicians can use [these results] to develop patient-specific rehabilitation protocols based on their patients’ symptoms and response to loading.”

Achilles injuries generally fall into two categories. Some people rupture the tendon completely, while others suffer chronic discomfort from a degenerative condition known as Achilles tendinopathy. Either way, the core treatment is an exercise program that progressively increases the load on the tendon to promote tissue regeneration and healing.

But getting the balance right is tricky, Baxter says. If the exercises are too easy, healing isn’t triggered; if they’re too hard, you risk stretching the tendon beyond its optimal length or even rerupturing it.

Working with colleagues from two other U.S. schools, Northeastern University and the University of Delaware, Baxter enlisted eight volunteers to perform sets of more than two dozen tendon rehab exercises. Using three-dimensional motion capture cameras and force-measuring plates in the floor, they calculated the load (in multiples of body weight) on the Achilles tendon.

The range of results was considerable. Sitting in a chair and raising your heel, even with a 15-kilogram weight, puts just a 0.5-body-weight load on your tendon. At the opposite extreme, hopping forward on one foot applies a load of 7.3 body weights. For context, walking applies a peak load of 3.3 body weights, and running applies 5.2 body weights.

With those numbers in mind, the researchers suggest two four-stage progressions to gradually increase tendon loading until it can handle the loads encountered in running and other sports.

For exercises that focus on just the ankle, they suggest starting with a seated heel raise, then progressing to single-leg standing heel raise, double-leg hop, and finally single-leg forward hop.

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For multijoint movements, which are crucial preparation for returning to sport, they suggest body-weight squats; step-downs from a 12-centimetre step; single-leg jumps starting from an upright position then squatting down before jumping; and single-leg drop jumps stepping down from a 12-centimetre step.

The results will be particularly helpful for clinicians to guide their patients’ exercise progressions, says Chris Napier, a sports physiotherapist and researcher at the University of British Columbia who was not involved in the study. “It also reinforces the fact that many of the exercises we do in the clinic don’t load the Achilles tendon enough if running is the ultimate goal,” he says.

However, the study doesn’t address the trickiest question of all: How do you know when it’s time to move up to the next stage?

For Achilles tendinopathy, pain can be a useful guide, says University of Delaware tendon researcher Karin Silbernagel, another co-author of the new study. On a 10-point scale where 0 is no pain and 10 is the worst pain imaginable, pain shouldn’t exceed 5 during or after the exercise or the next morning, and pain shouldn’t increase from week to week.

Another factor is how well you’re able to do the exercises, which in some cases are quite challenging. “We tend to progress when the exercise gets too easy and pain decreases,” Silbernagel says.

Pain isn’t a reliable guide after a fully ruptured tendon, so in that case it’s best to consult a therapist or doctor to determine an appropriate exercise progression – hopefully one that’s based on, or at least consistent with, the new results.

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Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.

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