Chapter 01The science behind your riding, one principle at a time.
Every ride you've ever done has a price.
Your head unit already knows it. Most riders never look. It's called TSS, and it is the single number coaching science is built on.
01 · The unit
Training Stress Score is one formula.
Hours ridden, times intensity squared, times a hundred. Intensity is your power relative to your Functional Threshold Power. That's it.
Squaring intensity is the point. Twice as hard is not twice the cost. It's four times. A short, high-intensity ride can carry the same metabolic stress as a long, easy cruise.
02 · Why it matters
Three kinds of damage. One number.
TSS is designed to mirror the physical fatigue your body experiences during a ride. A ride costs you in three different places, and TSS is the only metric that adds them up.
I
Glycogen
The fuel you burned. What has to be replaced before you can go again.
II
Muscle
The metabolic stress on the fibres that did the work.
III
Cardiovascular
The load on the heart and lungs that carried it.
And you never have to calculate it. Every head unit does it automatically, on every ride. It's already on your device. It's already in your history.
TSS is the currency of training. Everything else is built on it.
03 · What it buys
Fitness, Fatigue and Form are all made of TSS.
Add up the cost of your rides over different windows of time and three numbers fall out. These are the three every coach in the world watches. FORM calculates them all from your rides, automatically.
Fitness · CTL
Your capacity
How much training stress your body has learned to carry. Built slowly, lost slowly.
If you want to geek out: 42-day exponentially weighted average of daily TSS.
Fatigue · ATL
Your recent cost
What the last week has taken out of you. Rises fast, clears fast.
Same maths, 7-day window. Reacts six times faster than Fitness.
Form · TSB
The difference
Fitness minus Fatigue. How ready you are to perform today.
Form = Fitness − Fatigue. Negative means the last week outran your base.
Fitness climbs slowly as Fatigue spikes and clears. Form is the gap between them. Below the line, you're building.
04 · Now the fun part
To get fitter, stronger, faster, you have to go into the red.
Fitness only grows when you load your system with more than it's used to carrying. That means Fatigue has to outrun Fitness, which means negative Form is not a warning. It's the point.
The further below zero, the bigger the dose. But there's a floor. Past −25, you're no longer training. You're digging.
Below −25
Overtraining
The dose has become damage. Back off.
−25 to −10
Building
The productive zone. This is where fitness is made.
−10 to 0
Absorbing
Load still landing, body catching up.
0 to +10
Fresh
Recovered and sharp. Race here.
Above +10
Detraining
Too fresh for too long. Fitness is leaving.
This model tells you how ready your engine is to train, to ride hard, to race. You could read it yourself. You don't have to. The coach reads it for you.
Form −28 · Tuesday
"You're too cooked for that VO2max session. Not today."
Form −19 · Thursday
"You've dug yourself a hole before Saturday's race. Two easy days, now."
Form +6 · Race day
"Clean taper. You're fresh. Go crush it."
Form +14 · Two weeks off
"You're losing fitness. Time to load again."
05 · How FORM uses TSS
TSS is the foundation. FORM builds on it, with no touch.
FORM uses TSS to coach how much you're ready to absorb today, and how to build more capacity over time. All of it happens automatically. No typing. No interpreting. No dashboards. You ride; the coach reads.
Connect your head unit
Garmin, Wahoo, Strava. Your TSS history imports on day one.
Read today's Form
Fitness, Fatigue and Form on the home screen, with the coach's call.
Train with the coach's read
Go into the red on purpose. Come out on schedule.
Help make FORM better
What worked, what didn't. Your feedback shapes what's next.
You already own a coach. It's in your Garmin. It just doesn't talk yet.
This chapter gave you the engine: what a ride costs, and how Fitness, Fatigue and Form are built from it. That's half of what a coach asks every morning. The other half is whether your body is absorbing the work, and no power meter can tell you that.
Next chapter: your body files a report every morning. HRV, resting heart rate and sleep, and why a raw number means nothing.
Field Notes · Chapter 01 · The currency of trainingNext · Chapter 02 · The body files a report →