When we marvel at the incredible feats and rippling muscles of the world's greatest athletes, we're not usually thinking about their ankles.
But, that may be an oversight, according to a series of recent studies. The muscles of the ankle and calf, it turns out, play a disproportionately large role in our ability to walk, run and sprint. And the progressive loss of ankle power as we age may be one of the key reasons we slow down, suggesting that skipping the biceps curls and spending that time on ankle strengthening instead might be a smart move.
The latest study, published in Medicine & Science in Sports & Exercise by researchers at the University of Jyvaskyla in Finland, compared the relative effort of muscles that straighten the ankle and knee during walking and running.
The absolute forces in the two joints were similar, equivalent to eight to nine times body weight in both cases.
But the knees were capable of much greater maximum forces – nearly 14 body weights, on average – when the subjects jumped up and down as high as they could.
In contrast, the ankles had a maximum force of less than 10 body weights, meaning that they're already working at nearly their maximum strength even during a gentle jog.
This suggests that the ankles are much more likely to hold you back if they're weak, according to Juha-Pekka Kulmala, the study's lead author. "The muscles working closest to their upper functional limits are the 'weakest link,'" he says.
The new findings help explain earlier results that Kulmala and his colleagues published in 2014. In that study, they compared the walking and running gaits of three groups of runners, with average ages of 26, 61 and 78. All three groups produced similar power from the knees and hips, but the power produced from the ankles declined steadily with increasing age.
Another study, published earlier in 2016, funded by the U.S. Army, reached similar conclusions, linking the decline in running speed between the ages of 20 and 59 to shorter strides resulting from less power in the ankles.
The muscles that power ankle extension, which are primarily located in the calves, are much smaller than the upper-leg muscles that straighten the knee. As a result, the gradual loss of muscle associated with aging may cause problems sooner in the ankle, since they have less spare capacity.
But it's not just about how much muscle you have, says Rodger Kram, the head of the University of Colorado Boulder's Locomotion Laboratory.
"Older adults still have the ability to generate a lot of force and power with their calves, they just 'forget' how to activate them," he says.
Kram and his former student Jason Franz, now at the University of North Carolina at Chapel Hill, used EMG electrodes and a force-measuring treadmill to measure the force produced and the muscle activity in the calves in older adults while walking. When the subjects were given real-time visual feedback showing their force output, they were able to adjust their stride to push off more propulsively with their calves.
Given this evidence, Kulmala believes it's worth paying more attention to ankle and calf strengthening, starting while you're young and haven't begun losing muscle yet. "However," he cautions, "it is a bit unclear what kind of training would be optimal."
Simple exercises such as calf raises are one option, but the slow motion doesn't really mimic the neuromuscular demands of walking or running.
Given the results from Kram and Franz showing that how you activate the muscle matters, a more dynamic exercise such as ankle hopping (hopping forward while staying on your toes) may be a better bet.
On the other hand, the Achilles tendon, which connects to the calf, also plays an important role in walking and running, stretching to store and return energy from one stride to the next. Isometric ankle strengthening, which involves exerting force without moving the muscle, could help enhance tendon properties, Kulmala says.
At some point, it may even become possible to use real-time EMG feedback to train yourself to use your calves more effectively.
But if all this is starting to sound too complicated, Kram has some reassuring advice: Just make sure you're using your ankle muscles vigorously and on a regular basis.
"Personally," he says, "I just run hills."
Alex Hutchinson's latest book is Which Comes First, Cardio or Weights? Follow him on Twitter @sweatscience