Running Toolkit

Runner Profile Assessment

Enter your best recent races and see your VDOT plotted across every distance. The shape of that curve is your skew — whether you're a speed runner with an endurance deficit, a balanced athlete, or an endurance motor — and it tells you which lever moves your goal race fastest.

Part of the Running Pace Calculators toolkit.

Enter at least two races and press Assess my profile to see your VDOT curve and skew.
How this works — the VDOT curve & what skew means

Jack Daniels' VDOT is a single fitness score worked out from a race you've actually run. In theory it's distance-invariant — a "normal" endurance runner scores about the same VDOT at every race distance. Real athletes don't: the score drifts as distance changes, and the direction of that drift is your skew.

Aerobic decay (the headline)

We compute VDOT at each race you enter, then report VDOT(shortest race) − VDOT(longest race). A big positive number means your fitness leaks as races get longer — a speed skew with an endurance deficit. Near zero means you keep your whole engine to the long stuff — an endurance motor.

Skew slope (the maths behind it)

A line of best fit of VDOT against log2(distance) gives a slope in VDOT per doubling of distance. That slope drives the label:

  • > +0.5 — endurance-skewed ("motor"): lever is top-end speed / economy (R & I reps, strides, hills).
  • −0.5 … +0.5 — balanced: train the specifics of your goal race.
  • −1.5 … −0.5 — mild speed lean: a modest aerobic emphasis.
  • < −1.5 — strong speed-skew (endurance deficit): lever is aerobic durability — mileage, threshold, long runs.

Deviation table

Your longest race is the aerobic anchor. For each shorter race we show your actual time minus the time that anchor predicts — e.g. "34 s faster at the mile than your 8 k predicts" is the fingerprint of a speed skew.

Training paces pinned to your profile

Standard Daniels sets every training pace from one VDOT. But each zone targets a different system that peaks at a different race duration, so we read the VDOT for each zone off your curve at the distance that drives it: Reps from your 800 m–mile speed, Intervals from your 3–5 k (VO₂max), Threshold from ~15 k, Marathon from the marathon, Easy from your aerobic end. A flat curve reproduces normal Daniels exactly; a skewed curve keeps your fast zones fast while holding the slow zones honest to your endurance.

This is a BFS method — grounded in, not quoted from, the literature: Daniels' zone definitions and %vVO₂max; the anaerobic speed reserve framework (MSS − vVO₂max; Buchheit & Laursen 2013, Sandford et al. 2021), where speed- and endurance-types need different rep structures; and durability/resilience research showing a single VDOT over-predicts the long end by an athlete-specific amount (Joyner 1991; Jones 2023). Sub-800 m sprints stay out of the aerobic VDOT but set a neuromuscular strides ceiling.

This measures aerobic skew only. VDOT is invalid below ~800 m, so the curve cannot see pure top-end sprint speed (a 400 m). A flat or high 800 m VDOT does not mean good raw speed. Sub-800 m times inflate VDOT as an artifact and are rejected. Planning estimates, not promises — run by feel on the day.