Trail Race Time Predictor

The classic Riegel formula (T2 = T1 × (D2/D1)^1.06) was derived from road race data. It assumes distance is the only factor in fatigue — ignoring elevation, terrain friction, and ultra-specific physiological degradation. For mountain trail races, these factors dominate.

ITRA km-effort normalises trail races by effort rather than distance. A 50km race with 3,000m of gain has a km-effort of 80 — equivalent in effort to an 80km flat race. Our version also accounts for descent cost (loss_m / 150), because steep descents cause significant eccentric muscle damage (Minetti, 2002) that isn't free.

Ultra fatigue: Millet et al. (2011) documented progressive neuromuscular degradation in ultra-marathons — central fatigue, GI distress, and eccentric muscle damage compound over time. The fatigue exponent in our prediction scales from 1.06 (≤marathon) to 1.15 (>100km), producing increasingly conservative predictions as distance grows.

Terrain multipliers are practical heuristics, not peer-reviewed constants. Technical trail is approximately 1.30× slower than road pace for the same flat effort — accounting for footplacement, rocks, roots, and lateral stability demands. These multipliers are applied relative to your known race's terrain, so comparing trail-to-trail or road-to-road eliminates the terrain effect and leaves only effort differences.

How to use this: Choose a known result you're proud of and that reflects your current fitness. A race from 6 months ago still works if your training hasn't changed significantly. The prediction gives a range — aim for somewhere between optimistic and conservative based on your training specificity for the target race.

ITRA. km-effort formula. International Trail Running Association. Riegel PS. (1977). Athletic records and human endurance. American Scientist. Millet GY et al. (2011). Neuromuscular consequences of an extreme mountain ultra-marathon. PLoS ONE. Minetti AE et al. (2002). J Appl Physiol. 93(3):1039–46.