SEGLY
Fatigue is one of the most revealing metrics for analyzing performance in ultra-trail running. Over 171 km and 10,000 m of elevation gain—as in the UTMB—the way an athlete manages their energy over the long haul is often what separates the winners from the rest.
By analyzing the official GPX tracks, we calculated the fatigue factor for the Kilian Jornet vs. Mathieu Blanchard showdown (UTMB 2022) and for the winners from 2022 to 2025. The results are sometimes counterintuitive.
What is the fatigue factor?
The fatigue factor expresses the percentage decrease in speed between the first half and the second half of a course:
Fatigue factor (%) = (1 − Speed in the second half / Speed in the first half) × 100
- 0%: perfectly constant pace—theoretical ideal
- 0 → 10%: excellent consistency—very controlled pacing
- 10 → 15%: moderate slowdown, often due to a more ambitious first half
- > 15%: significant drop in pace in the second half
As we’ll see, a low fatigue factor doesn’t always guarantee victory—starting pace matters just as much as consistency.
Kilian Jornet vs. Mathieu Blanchard — UTMB 2022
The 2022 UTMB delivered a legendary duel. Kilian Jornet crossed the finish line in 19:49, followed by Mathieu Blanchard in 19:54—a difference of just 5 minutes over 171 km of racing.
Analysis of their GPX tracks reveals a counterintuitive result: the “loser” had the better fatigue factor.
The paradox of the 2022 race: Mathieu Blanchard posted the best fatigue factor (9%), a sign of remarkably consistent pacing. Kilian Jornet, on the other hand, slowed down more in the second half (14%). Yet it was Kilian who claimed victory.
- Kilian Jornet: starts at a significantly higher speed and builds a decisive lead in the first half → Fatigue factor: 14%
- Mathieu Blanchard: more consistent pacing, much less pronounced slowdown—but insufficient starting speed → Fatigue factor: 9%
The key lesson: the fatigue factor must always be interpreted in relation to absolute speed. The lead Kilian built in the first half was so substantial that it held up despite a more difficult second half. Mathieu catches up and closes the gap, but it’s not enough to overtake Kilian.
Fatigue Factor of UTMB Winners — 2022 to 2025
By expanding the analysis to the last four editions, we discover a wide variety of strategies among the winners—and an underlying trend toward consistency.
What the data reveals
- 2022 — Kilian Jornet (14%): the highest fatigue factor in the series. An explosive approach offset by an exceptional starting pace. Proof that a high fatigue factor can coexist with a resounding victory.
- 2023 — Jim Walmsley (9.1%): After several failed attempts at the UTMB due to overly fast starts, Jim adopted a much more consistent strategy and finally secured victory with one of the lowest factors in the series.
- 2024 — Vincent (11%): A middle-of-the-road approach, between the explosiveness of 2022 and the consistency of 2023. An effective balance.
- 2025 — Tom (8.9%): the lowest pacing factor in the series—proof that an ultra-consistent strategy is fully effective at the UTMB and that the trend is taking hold.
The underlying trend: recent victories are shifting toward more conservative pacing strategies (below 10%). The 2022 Kilian model remains a shining exception—but requires an extraordinary physiological level to be replicated.
Analyze your own fatigue factor with Segly
The fatigue factor isn’t just for elite runners. Whether you’re running a race or a long run, it helps you understand how you manage your effort and adjust your pacing strategy.
How to use Segly:
- Import your GPX track into Segly
- Choose your preferred segmentation mode
- Enter the duration of your last trail run in hours and run the analysis
- Adjust the fatigue factor to get the times for each segment
Tracking this ratio race after race allows you to measure your progress in pacing and identify whether you tend to start too fast or too slow.
Key takeaways
- The fatigue factor measures the percentage slowdown between the first and second halves of a course
- Mathieu Blanchard (9%) was more consistent than Kilian Jornet (14%) in 2022—but Kilian won thanks to his explosive first half
- The 2022–2025 UTMB winners had fatigue factors ranging from 8.9% (Tom, 2025) to 14% (Kilian, 2022)
- The recent trend is toward more consistent strategies: Jim Walmsley 9.1%, Tom 8.9%
- Calculate your own fatigue factor on any GPX track with Segly
Conclusion
The fatigue factor reveals two major racing philosophies at the UTMB: Kilian Jornet’s explosive approach in 2022 (14%, victory thanks to a dominant first half) and the consistency embodied by Jim Walmsley in 2023 and Tom in 2025 (under 10%, victory through consistency). Neither is universally superior—it all depends on your physiological profile and your ability to manage the effort over the long haul.
What about you—what’s your race strategy? Upload the GPX track from your latest ultra to Segly, compare your split times with the times predicted by Segly using different fatigue factors to determine your fatigue factor. Find out whether you’re more like Kilian or more like Tom.