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Aging runners worry most about their knees – which makes sense, given that knees are the most common site for running-related aches and pains. But if you’re hoping to keep your speed up, it turns out that there’s another key joint you should be focusing on.

“As people get older, the joint where we see the biggest decline in function is the ankle,” says Zoey Kearns, a biomechanics researcher at the University of Memphis. “We don’t entirely understand why, but we do know it happens, regardless of your activity level.”

At this year’s American College of Sports Medicine conference in June, Ms. Kearns and colleagues at the University of Memphis presented new data on the downsides of weak or fatigued calf muscles, and explored the best ways to strengthen them.

When you run, the muscles around the knee and ankle joints typically generate similar levels of force applied to the ground, equivalent to a little less than 10 times your body weight. According to a Finnish study published in 2016, the difference between the two joints is that the ankles are nearly maxed out while running. Even during explosive jumps, they can only generate 10 bodyweights, whereas the knees have a higher ceiling of about 14 bodyweights.

That means any loss of muscle as you get older will eat into your ability to generate those 10 bodyweights with your ankles, weakening your stride. Sure enough, other studies have found that the forces produced by your ankles while running declines steadily from your 20s to your 70s, while those produced by your knees and hips stay roughly constant.

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One final puzzle piece: a German study in 2018 found that ankle forces declined steadily during a single hard 10-kilometre run, forcing the knees and hips to take on more work and making the runners less and less efficient. Interestingly, this pattern was most pronounced in slower runners. The fastest runners in the study were better able to keep generating force with their ankles even as they fatigued.

These results combine to make a compelling case for maintaining strong ankles – but they don’t say anything about the best way to accomplish that goal. That’s the question Ms. Kearns set out to answer, working with her graduate adviser Max Paquette, an associate professor in sports science and biomechanics at the University of Memphis and a former elite runner from Canada.

Working with 26 runners with an average age of 51, Ms. Kearns compared three strength-training approaches.

One group did heavy resistance training, focusing on exercises such as front squats and heel raises with heavy weights that they lifted for four sets of up to eight reps each.

Another group did heavy resistance training with fewer sets, but added explosive “plyometric” exercises such as box jumps, forward hops, and alternate leg bounds.

The third group used lighter weights for single sets of 15 to 20 repetitions to focus on strength endurance, and also included plyometrics.

After 10 weeks, all three groups had improved their ankle-flexing strength, with no significant differences between the protocols. All three groups also improved their running economy, which is a measure of how much energy it takes to sustain a given pace.

This latter finding suggests that there’s more going on in the ankles than just how hard they press into the ground. The calf muscles connect to the Achilles tendon, Dr. Paquette points out, which stretches like an elastic band to store and then return energy with each step. In addition to strengthening muscles, resistance training – and heavy weights in particular – are thought to increase the stiffness of tendons, allowing them to store even more energy as they’re stretched.

There may also be other ways of boosting ankle strength, such as short, intense hill sprints. But the main takeaway from the new study is simple, according to Dr. Paquette.

“Just do something focusing on your calves,” he says. “If you can handle heavy loading, that’s potentially a bit better, but I don’t think you need to overcomplicate things.”

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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