The science behind the numbers

Aerobic index

The fitness a single run reveals, from the pace you held and the heart rate it cost.

VO2 max

Your standing aerobic ceiling, and where it sits against the population for your age and sex.

Race predictions

Predicted race times from your fitness and how long you can hold a fraction of your VO2 max.

Training paces

Every training pace, from Easy to Repetition, derived from one number: your VDOT.

Grade adjusted pace

What your hilly pace would have been on the flat, from the energy cost of running on a gradient.

Aerobic decoupling

Whether your heart rate stayed coupled to your pace, or drifted as fatigue set in.

Efficiency factor

Speed per heartbeat, read as a trend: the same pace costing fewer beats means a stronger engine.

Fitness, fatigue & form

Three running averages of your daily load that show whether you are building, buried, or fresh.

Training stress (rTSS)

A single score for how much a run cost you, from its intensity and duration.

Heart rate zones

Five effort bands, set from the room between your resting and maximum heart rate.

Critical speed

The fastest pace you can sustain in a steady state, a physiological threshold anchor.

Durability

How well your efficiency resists fatigue late in long runs.

Injury risk & load progression

The intuition (build gradually) is sound; the popular thresholds behind it are not. Here is the honest picture.