Statistics & heart-rate zones
Training load, fitness and form explained
Every workout leaves a load behind: roughly how long you trained times how hard. Added up over days and weeks, those loads answer the questions that matter in a training block. Am I getting fitter? Am I carrying too much fatigue? Am I ramping up too fast? Here is how training load is calculated, what fitness, fatigue and form mean, how to read the acute to chronic ratio and how to use all of it week to week.
Short answer
- Training load is duration times intensity, usually from heart rate, power or effort ratings.
- Fatigue (ATL) is your average load over about the last 7 days. It rises and falls quickly.
- Fitness (CTL) is your average load over about the last 42 days. It moves slowly.
- Form (TSB) is fitness minus fatigue: negative in hard weeks, rising when you ease off.
- Acute to chronic ratio compares the last week with the last month: well above 1 means a fast ramp.
How training load is calculated
A load score turns each workout into one number that puts a run, a ride and a swim on the same scale. The common ways to get it:
- Heart rate. Time in each zone, weighted by how hard the zone is (TRIMP and its variations). Works for any sport with a heart rate.
- Power. On the bike, a score based on power relative to your FTP (Training Stress Score is the best-known). Precise, but it needs a power meter.
- Effort rating. Minutes times how hard the session felt on a 1–10 scale (session RPE). Simple and surprisingly useful.
The exact scores differ between apps, so compare your numbers only within one app, never across them.
Fitness, fatigue and form
Averaging loads over two time windows gives the classic performance management model:
| Line | Also called | Window | What it tells you |
|---|---|---|---|
| Fatigue | ATL, acute load | About 7 days | How tired the last week has made you |
| Fitness | CTL, chronic load | About 42 days | How much training your body is used to |
| Form | TSB, freshness | Fitness − fatigue | Whether you are fresh or carrying fatigue |
During a build, fatigue sits above fitness and form is negative. That is normal: you get fitter by asking a little more than you are used to. Form that stays deeply negative for weeks is the pattern to watch. Before a race, a taper drops fatigue faster than fitness, so form rises towards zero or above and you start fresh.
The acute to chronic ratio
The same idea in one number: your load over the last week divided by your load over the last four weeks or so.
- Around 0.8–1.3: you are training about as much as you are used to, or building gently.
- Well above 1.3: a fast ramp, for example after a break or in a big training week. Fine once in a while, risky week after week.
- Below 0.8: an easy week, a taper or time off. Fitness slowly drops if it stays there.
These ranges are rules of thumb, not hard limits. Researchers still debate how strongly the ratio predicts injuries, but a sudden jump in load is a sensible thing to notice.
Training Load on Apple Watch
Apple Watch has its own Training Load in the Activity app on the watch: tap the Workload button to see how the intensity and duration of your workouts over the last 7 days compare with the 28 days before. It uses your effort ratings, which the watch estimates for many workouts and you can adjust after each one. From the same view you can open your overnight Vitals. It is a quick check of “more or less than usual”; it doesn’t show fitness, fatigue and form as separate lines.
Using load week to week
- Build gradually. Raise weekly load in small steps and plan an easier week every third or fourth week.
- Read it with recovery. High load with good sleep, normal HRV and a normal resting heart rate is productive training. High load with poor sleep and a rising resting heart rate is a reason to ease off.
- Mind the mix. Load says how much, not how. Check that most of it comes from easy training; see Zone 2 training.
- Taper before races. Cut volume over the last one to two weeks and keep a little intensity.
Load, fitness and form with Keho
Keho calculates a load for every workout in Apple Health, whatever sport and whatever watch, and shows your fitness, fatigue and form over time. Load is also one of the inputs of Keho’s readiness score from 0 to 10, next to sleep, HRV and resting heart rate, so a big training week and a poor night are read together. Training load and form are part of Keho’s Advanced Statistics, a one-time unlock that also comes with any other Keho purchase. Keho Coach uses them too: when load or form point to a tougher week, it offers easier options, and you decide. More on the morning score in a readiness score for Apple Watch, and on the statistics around it in workout statistics from Apple Health.
Questions
Is a negative form bad?
Not during a build. It means you are training more than you are used to. It is a problem when it stays deeply negative for weeks while sleep and HRV get worse.
Why does my fitness drop during a taper?
Fitness is a 42-day average, so it dips slightly when you train less. A short taper costs very little fitness and clears a lot of fatigue, which is the point.
Do strength sessions count?
They add load and fatigue, and a heart rate based score captures part of it. Heart rate underestimates heavy lifting, so read your load with that in mind.