If you’ve ever clicked through the race photos that get sent to you after a marathon, you may have been shocked by the transformation. Little by little, the bright-eyed and bouncy-striding runner who smiled at the camera for the early photos is replaced by a grimacing ogre hobbling awkwardly toward the finish.
Or maybe that’s just me. But the question of how much your running stride changes during a race is one that researchers are increasingly interested in. Most biomechanical research has typically looked at fresh, unfatigued runners who spend a few minutes on a treadmill—but it’s pointless to say that one shoe or running style is “better” than another if its benefits disappear when your legs get tired.
The latest study to tackle this question comes from Carl Jewell, Katherine Boyer, and Joseph Hamill at the University of Massachusetts Amherst, published in the Journal of Sports Sciences.
They focused on forefoot runners, the relatively rare subgroup who land first on the front part of the foot instead of the heel. These runners typically use their flexed ankles to absorb some of the shock of landing, producing a smoother transfer of force between foot and ground. But are the ankles still effective shock absorbers when they get tired?
The study involved 14 habitual forefoot runners who typically ran about 48 kilometres per week. They ran a time-to-exhaustion test that took an average of about 15 minutes, and the researchers took detailed biomechanical measurements of their stride at the beginning and end of the run.
Even after just 15 minutes, there were significant stride changes. Eight of the 14 runners were landing further back toward the heel by the end of the run, and overall joint angles reflected similar changes. For example, the ankle was more dorsiflexed (toes up) when the foot hit the ground, and the knees were more flexed throughout the stride cycle. That may be a strategy that gives a little more passive shock absorption as the ankle muscles get too tired to provide active shock absorption.
The impact forces also reflect a shift from the forefoot pattern toward the typical heel-striking pattern. Here’s sample data showing the vertical ground reaction force at the start (dashed line) and end (solid line) of the run for one of the subjects:
Graph of vertical force.
IMAGE COURTESY OF JOURNAL OF SPORTS SCIENCES
The extra mini-peak in the solid line is the impact transient that’s usually associated with heel striking (and which disappears when a flexed ankle is able to absorb the initial forces instead). This is what happens after 15 minutes, so it’s no surprise that previous studies have found that there are fewer and fewer forefoot strikers in the later stages of long races like marathons.
Is this “good” or “bad”? Well, in a perfect world, it would be nice to have enough fatigue resistance to keep your stride always the same. In reality, your stride will gradually change as you fatigue, whether you’re a heel striker, midfoot striker, or forefoot striker. Whether that causes problems depends on the specifics of your stride and context.
One potential area of concern is if you’re running (and forefoot striking) in very minimalist shoes under the assumption that your ankles can absorb the impact, but you don’t have enough muscular endurance to maintain that form throughout a long race. Then you may be crashing down on your heels late in the race, without the benefit of a more cushioned heel. That’s part of the rationale that has made ultra-cushioned shoes like Hokas popular in the ultra world.
For now, though, linking stride changes to injury risk is just speculative—which is why it’s good to see data being collected. Some of the apps and stride monitoring devices that are now becoming available offer a way of doing some self-monitoring, allowing you to see how much (and in what ways) your stride changes during long runs. Knowing that you see big fatigue-induced stride changes might be a good argument for opting for a more cushioned shoe than you’d otherwise choose in a long race.
Plan A, of course, is to keep accumulating more mileage so that you build up fatigue resistance, no matter what you’re wearing!