Load & readiness

Training stress (rTSS)

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

Coaching heuristic

A coaching heuristic with a proprietary lineage (McGregor). It depends entirely on an accurate threshold pace and handles heat and hills poorly, so read it as a personal relative measure.

Your Running Training Stress (rTSS) puts a single number on how much a run took out of you. It is scaled so that one hour at your threshold pace equals 100: an easy hour lands near 50, and a hard hour climbs well past 100.

The idea in one line

Take how hard the run was, square it, and multiply by how long it lasted. Squaring the intensity is the whole trick: going harder raises the score steeply, so intensity matters more than duration.

rTSS = duration_hours × IF² × 100

Step 1: how hard the run was

The intensity factor (IF) compares the pace you actually held to your threshold pace, the pace you could hold for about an hour. We compute it in speed (metres per second) so that a harder, faster effort gives a higher IF:

IF = speed_{grade-adjusted} ÷ speed_threshold

The grade-adjusted speed is a hill-adjusted version of your speed: it estimates the flat-ground speed equivalent to the effort you actually put in on the hills. We compute it with our own grade adjustment, the same one behind grade-adjusted pace, so a hard climb scores like the hard effort it was.

Step 2: the squared relationship

For a fixed one-hour run, the score is just IF squared times 100. Doubling the length of a run doubles its score, but running harder bends the score upward far faster:

For a one-hour run the score grows with the square of intensity. The dot marks a full threshold hour: IF of 1.0 scores exactly 100.

Worked example

A 45-minute run at an intensity factor of 0.85

A 45-minute run is 0.75 hours, held at an intensity factor of 0.85:

Square the intensity: 0.85² = 0.7225
Multiply by duration and 100: 0.75 × 0.7225 × 100 = 54.2

So this run scores an rTSS of about 54.2. For reference, an easy hour might be around 50, while a hard hour runs well over 100.

What it is not

rTSS rests entirely on an accurate threshold pace. If your threshold is wrong, every score built on it is wrong too, so it is worth revisiting as your fitness changes. The model also handles heat, hills, and stop-start runs poorly: it flattens conditions that genuinely change how hard a run feels.

Treat it as a personal, relative measure of how much a run cost you, not an exact physiological quantity. The running training-stress method was devised by Dr Stephen McGregor. It is a coaching heuristic with a proprietary lineage, useful for comparing your own runs to each other, and best read that way.