Chapter 02The science behind your riding, one principle at a time.
Your body files a report every morning.
TSS measures the training you did. It says nothing about how well your body recovered from it. For that, you need to read the body itself, and it reports in three numbers, before you've touched the bike.
01 · The blind spot
TSS is built from power. Recovery isn't.
Chapter 01's model, Fitness, Fatigue and Form, is made entirely of what your power meter saw. That's its strength and its limit. A 200 TSS ride is 200 TSS whether you slept eight hours or four, rode in 25° or 38°, at sea level or at altitude, on a calm week or a brutal one at work. The dose is the same. The absorption is not.
What TSS sees
- Durationhours
- Intensity relative to your FTPIF
- The accumulation over 7 and 42 daysATL · CTL
What TSS cannot see
- How your body respondedrecovery
- How you sleptsleep
- Heat and altitudeenvironment
- Stress, travel, illnesslife
- Whether you ate enoughfuel
Adaptation doesn't happen during the ride. It happens in the 20 hours after it, while you sleep, eat and sit still. TSS measures the stimulus. The response shows up somewhere TSS never looks: your heart, at rest, overnight.
02 · The report
Three numbers, taken before you move.
Your wearable collects them overnight: a Garmin or Apple Watch, a CIRQA, a Whoop band, an Oura ring. Together they describe one thing: is the nervous system that runs your engine rested enough to be loaded again?
HRV · Heart rate variability
The lead signal
The tiny variation in time between heartbeats. High and steady means the parasympathetic system, rest and rebuild, is in charge. Suppressed means the body is still fighting yesterday.
Measured overnight in milliseconds. The single best non-invasive read of recovery status we have.
RHR · Resting heart rate
The cross-check
A heart that beats faster at rest is working harder to do the same job. It moves after HRV does, but when both agree, they're rarely wrong.
Rises 3–5 bpm above your normal is meaningful. Illness often shows here first.
Sleep
The repair window
Where glycogen is restocked, muscle is rebuilt and the hormonal work of adaptation actually happens. Short it, and the other two signals will tell on you within a day.
Tracked as sleep debt against your need, not a single night's total.
TSS is the dose. These three tell you how it landed.
03 · The principle
A raw HRV number means nothing.
Your 45 ms and another rider's 45 ms are unrelated facts. HRV is personal. It varies with age, genetics, fitness and the device on your wrist. What matters is where today sits inside your own recent history. Sports science calls this the smallest worthwhile change, and it is the method FORM is built on.
Same athlete, thirty mornings. The band is theirs alone. Three amber mornings and one red are what the coach acts on.
Rider A · normally 68 ms
A high-HRV athlete, ten below their own normal, well outside their band. Red.
Rider B · normally 41 ms
A naturally lower-HRV athlete, three above their own normal. Green, and a better recovery day than Rider A's 58.
If you want to geek out: "meaningfully low" is half a standard deviation below your 30-day mean; "clearly suppressed" is a full one. Until FORM has about a week of your mornings, it will mention HRV but never act on it. You don't get a veto from a baseline you haven't earned.
04 · What the coach does with it
The report sets today's ceiling.
HRV, sleep and resting heart rate each get judged against your own baseline, then combined into one body verdict. That verdict doesn't pick your workout. It sets the hardest thing you're allowed to do.
Green
Unconstrained
Body recovered, nervous system rested. Whatever the plan says, you're cleared to do it.
HRV inside your band · sleep debt clear · RHR normal
Amber
Capped at tempo
Something is off but not broken. Ride, but nothing above tempo. The VO2max session moves.
HRV meaningfully low, or sleep debt with any other flag
Red
Rest or easy spin
The body is still paying for what you already did. Loading it again buys nothing and costs a lot.
HRV clearly suppressed, regardless of anything else
Form can be +6. Sleep can be fine. If HRV is clearly below your normal, the answer is rest, because the one signal that reads the nervous system directly has said it isn't ready, and the others are still catching up.
05 · In practice
One low morning is noise. Three is a pattern.
Trend beats any single morning. One low reading is noise. Three in a row is a pattern. Five is a call.
Amber · Tuesday
"Two mornings under your band. Today's ceiling is tempo. The intervals move to Thursday if tomorrow's reading recovers."
Red · Friday
"HRV clearly suppressed, third day running. Form says +6. Your nervous system disagrees, and it gets the vote. Rest."
Green · Sunday
"Back inside your band. Streak's over. Tuesday's VO2max stands. Go get it."
Green, but · Any day
"HRV is fine. You say you feel wrecked. You win. HRV tells us the probability, not the answer."
Your wearable already writes the report. FORM is the coach who reads it.
Chapter 01 gave you the engine: what training costs and how fitness is built. This chapter gave you the body: whether you're absorbing it. Together they're the two questions a coach asks every morning, and FORM asks them for you, before you've had coffee.
Next chapter: you don't have one engine. You have six. What each system runs on, and which ones your rides actually trained.
Field Notes · Chapter 02 · The body files a report← 01Next · Chapter 03 · Six engines →