Same Pace, Different Stride: What Happens to Your Form on Long Runs

You know the feeling when the pace on your watch looks fine, but your stride feels different from what it was at the start? Twelve trained trail runners—six women and six men—spent more than three and a half hours on a treadmill helping researchers put a number on that feeling.¹Cartón-Llorente, A., García-Pinillos, F., & Jaén-Carrillo, D. (2026). Power-based running durability in trained runners: Preliminary evidence of speed-adjusted mechanical drift as a marker of fatigue. European Journal of Sport Science, 26(11), e70275. PubMed Central. https://doi.org/10.1002/ejsc.70275

They ran three blocks, each consisting of an hour at a steady speed followed by a 12-minute uphill time trial. A foot pod recorded their mechanics throughout. The repeated steady hours are what make this study interesting. Out on the trail, the obvious explanation for a change in stride is often that the grade or your speed just changed. Here, the researchers could compare runners at the same speed as the miles accumulated.

During the steady running, cadence, step length, estimated power, and the energy cost of running all held fairly steady. The one wearable measure that shifted significantly was duty factor, the share of time spent in contact with the ground. It rose from 71.3% at the first-hour measurement to 72.7% at the third. The runners could still hold the pace at a similar energy cost, but their feet were spending a slightly larger fraction of the run on the belt.

What this means for runners

If your foot pod reports duty factor, try comparing the same flat stretch early and late in a familiar long run, at roughly the same speed. Does that number drift on the days your stride starts to feel heavy? I’d want to see that pattern across several runs before drawing much from it. A 1.4-percentage-point shift in a dozen runners is interesting enough to look for. It’s a thin basis for changing your stride mid-run to make the number go back down.

References

• 1
  Cartón-Llorente, A., García-Pinillos, F., & Jaén-Carrillo, D. (2026). Power-based running durability in trained runners: Preliminary evidence of speed-adjusted mechanical drift as a marker of fatigue. European Journal of Sport Science, 26(11), e70275. PubMed Central. https://doi.org/10.1002/ejsc.70275 ↩︎