Trail running glossary

A single number combining a trail race's distance and elevation into a flat-equivalent workload, so courses of different shapes can be compared. TrailMath uses an enhanced form that also charges for descent, because downhill running carries a real eccentric cost. ITRA's public course-effort formula counts ascent only.

Units:

Distance in kilometres, gain and loss in metres. The result is unitless but reads as "flat kilometres".

Source:

ITRA course-effort concept; descent term is TrailMath's addition.

Used by: Race Effort Estimator , ITRA Score & UTMB Index Estimator , GPX Explorer , Aid Station Planner

The pace you would be running on flat ground for the same metabolic cost as your actual pace on a slope. It lets an uphill kilometre and a flat kilometre be compared on equal terms. Running 8:00/km up a 10% grade is roughly a 4:50/km flat effort.

Units:

Pace in minutes per kilometre or per mile; grade as a percentage.

Used by: GAP Calculator , Incline Calculator , Race Time Predictor

How much more energy running on a given slope costs compared with flat ground, where flat equals 1.0. It comes from a fifth-order polynomial fitted to treadmill measurements of the energy cost of gradient running. Flat running costs about 3.6 J/kg/m; at +10% the cost roughly doubles.

Units:

Energy cost in joules per kilogram per metre travelled (J/kg/m). The multiplier is unitless.

Source:

Minetti et al. 2002. Valid between -25% and +30% grade; steeper values are extrapolation.

Used by: GAP Calculator , Incline Calculator

The gradient above which hiking becomes as fast as or faster than running for most trail runners, because the energy cost of running climbs faster than the speed gained. It sits near 15% for the majority of runners, though the exact crossover is individual and shifts with fatigue.

Units:

Gradient as a percentage.

Source:

Koop 2021; consistent with the Minetti cost curve.

Used by: GAP Calculator , Incline Calculator

A power-law that predicts a race time at one distance from a known time at another. The exponent controls how much you slow as distance grows: 1.06 is the classic road value. Trail ultras need a larger exponent, because technical ground, night running and nutrition compound the decline, and TrailMath scales it from 1.06 up to about 1.15 with distance.

Units:

Times in the same unit on both sides; distances as km-effort rather than raw kilometres for trail.

Source:

Riegel 1981, derived from road-race data.

Used by: Race Time Predictor

A 0-1000 score rating a trail runner's performance level, published by the International Trail Running Association. The official formula is proprietary. TrailMath estimates it with a transparent log-linear model fitted by ordinary least squares to publicly readable anchor results, and always returns a range rather than a single authoritative number.

Units:

Score is unitless, 0-1000. Speed v is km-effort per hour.

Source:

Official index is ITRA's. TrailMath's approximation is fitted to public per-category UTMB Index values and reports a ±25 point band, covering the largest held-out error in its anchor set.

Used by: ITRA Score & UTMB Index Estimator

A 0-1000 performance score maintained by UTMB World, calculated per race category rather than as one overall number. A runner's category index is a weighted mean of up to their five best results in that category over the previous 36 months. Races need at least 20 km-effort to be scored.

Units:

Unitless, 0-1000.

Source:

utmb.world/utmb-index. Categories are set by km-effort: 20K below 50, 50K from 50, 100K from 100, 100M from 170.

Used by: ITRA Score & UTMB Index Estimator

Qualifying credits earned at UTMB World Series races, awarded on a tier scale set by the race's km-effort. They apply only to designated World Series events, and are used to enter the lottery for UTMB Mont-Blanc.

Units:

Km-effort thresholds; stones are a count.

Source:

UTMB 2023 rules. Indicative - always confirm against the current official race listing.

Used by: Race Effort Estimator

The concentration of dissolved particles in a sports drink, which governs how fast it empties from the stomach. Blood sits at 275-295 mOsm/kg. Drinks below 275 mOsm/L are hypotonic, 275-300 is isotonic, and above 300 mOsm/L they are hypertonic and can slow gastric emptying and cause GI distress during a race. Hypotonic drinks deliver fluid best during exercise, with 200-260 mOsm/L reported as optimal (Rowlands et al., 2022).

Units:

Milliosmoles per litre (mOsm/L). Ingredients in grams, volume in litres.

Used by: Drink Mix Builder

The maximum rate the gut can absorb carbohydrate during exercise, set by separate intestinal transporters. Glucose and maltodextrin share the SGLT1 transporter and saturate near 60 g/hour; fructose uses GLUT5 and adds roughly 30 g/hour on top. Mixing the two is what allows intakes above 60 g/hour: a 2:1 glucose:fructose blend supports up to about 90 g/hour, while a 1:0.8 blend trades some efficiency for a higher ceiling of 90-120 g/hour in gut-trained athletes.

Units:

Grams of carbohydrate per hour.

Source:

Jeukendrup 2004; Podlogar et al. 2022 for the 1:0.8 high-rate protocol.

Used by: Nutrition Calculator , Drink Mix Builder

Rate of climbing, measured as metres of elevation gained per hour, ignoring horizontal distance. It is the cleanest way to compare climbing fitness across different gradients. Recreational trail runners typically sustain 500-800 m/h on a long climb; elite mountain runners exceed 1500 m/h.

Units:

Metres per hour (m/h).

Used by: Incline Calculator , GPX Explorer