Build
Fitness rising steadily, form controlled. The productive zone — the weeks when work actually accumulates as adaptation.
Training Load · CTL · ATL · TSB on Apple Watch
Your training load,
back in your hands.
You used to read CTL, ATL, and TSB to know what your body could hold. Then a chronic illness rewrote the rules. Body Insights reads your Apple Watch and rebuilds the picture — tuned for a body whose baselines move.
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Free for core features.
You remember what fresh felt like.
There was a year — maybe more than one — when you knew exactly what your legs could do on a Saturday. You read your form number on Monday. You planned the week around it.
Then something changed. A long illness. A diagnosis. A perimenopausal year that broke every recovery rule you'd ever trusted. The training-load tools you used were built for healthy athletes ramping toward a race. They didn't know what to do with a body that needs rest as the work, some weeks.
Body Insights reads your Apple Watch the way your old coach used to. Fitness. Fatigue. Form. The Banister Impulse Response triad — published in 1991 and still the cleanest way to picture how training accumulates — sitting next to your sleep, your HRV, and (if you wear one) your overnight glucose.
Three lines, read together.
Fitness rises slowly. Fatigue spikes and recovers. Form is the gap between them — and it's the line that tells you when to push and when to rest.
Read the three lines together — fitness building, fatigue spiking, form drifting below zero — and the week your body asked for a break stops being a guess.
The part nobody else does: metabolic-mTSS.
Classic training stress reads heart rate and duration. That's a clean number when your physiology is steady. It's a misleading one when you're recovering from illness, when perimenopause has shifted your glucose response, or when last night's sleep gave you a different body to train with.
Metabolic-mTSS folds your overnight and intra-workout glucose stability into the stress cost of each session. A run on a calm metabolic day registers as the work it was. The same run after a poor night, with glucose drifting all afternoon, registers as more — because it cost more. The CTL, ATL, and TSB built on top of that score reflect the body you actually trained with, not the body the textbook assumed.
If you don't wear a CGM, the metabolic layer drops out and standard mTSS carries the read.
Get your form number back
Free for core features. No card needed.
For people who want the science.
If you used to read training stress charts on a regular basis, the framework below will feel familiar — built on the Banister Impulse Response Model, which has anchored coached training since 1991.
The phases your body moves through.
Form and fitness don't sit still — they cycle through phases. Body Insights names the phase you're in so you don't have to interpret the chart yourself.
Peak
Load coming down, form fresh, base intact. The phase classic coaching calls a taper — the body sharpest just as the work eases.
Overreaching
Fatigue running well above fitness. A short stretch of this is part of training. A long stretch is the early room of overtraining.
Overtrained
Deep negative form, ramp rate rising fast. The label exists so you have something to point at when your body says enough.
Maintenance
Steady ramp, neutral form. Not building, not losing — the holding pattern. Often the right one during a flare or a busy season.
Detraining
Load tapering away, fitness drifting down. Named explicitly so a long rest period reads as recovery, not as failure.
What we read from your Apple Watch.
Reads workout, heart rate, and (optionally) glucose data from Apple Health — the same data your Apple Watch and your CGM already write there.
Workout stress
Duration, heart rate, intensity zones — translated into a per-session stress score for every activity Apple Health captures.
Fitness (CTL)
Your chronic training load — the work your body has gotten used to, weighted toward steady accumulation rather than recent spikes.
Fatigue (ATL)
Your acute training load — the work still in your legs, weighted toward the most recent sessions.
Form (TSB)
Training stress balance — the gap between fitness and fatigue. Positive is fresh, negative is fatigued, drifting back to zero is the rhythm of training.
Ramp rate
How fast fitness is changing week over week. The signal that separates productive build from the climb into overreaching.
Metabolic-mTSS
If you wear a CGM. Adjusts the stress cost of a workout for glucose stability — the layer no other training-load tool reads.
About the Banister model.
Eric Banister and colleagues published the Impulse Response Model in 1991, describing performance as the difference between a slow fitness response to training and a fast fatigue response — the framework every CTL, ATL, TSB tool since has built on.
It's an honest model. It doesn't claim to know everything about your body — it claims that, on average, training accumulates as a long-running curve while fatigue burns off faster. That insight is what makes form readable.
Body Insights uses the same triad, calculated on the workouts your Apple Watch already records, and shown next to the sleep and metabolic context that determines how much of that load your body could actually absorb.
Apple Watch training load vs Strava, TrainingPeaks, Whoop.
You may have used one of these before. Here's the honest comparison.
| Strava F&F | TrainingPeaks | Whoop | Body Insights | |
|---|---|---|---|---|
| CTL / ATL / TSB | Yes | Yes | Strain only | Yes |
| Reads all Apple Health workouts | Partial | Partial | Strap-only | Yes |
| Overnight glucose layer | No | No | No | Yes |
| Tuned for variable baselines | No | No | No | Yes |
| Subscription | $12 / month | $20 / month | $30 / month | Free for core |
| Extra hardware | None | None | Strap | None |
Strava, TrainingPeaks, and Whoop are real products built by careful people. They were built for athletes whose physiology cooperates. If yours doesn't always — chronic illness, hormonal shifts, post-illness recovery — this was built for you.
Questions you're probably asking.
Can Apple Watch show training load?
Apple Watch records workouts and heart rate. Body Insights reads them and builds the classic CTL, ATL, TSB triad on top — the picture Strava and TrainingPeaks made famous, now sitting next to the rest of your health data.
What is CTL, ATL, and TSB?
CTL is fitness — what your body has sustained. ATL is fatigue — what's still in your legs. TSB is form — the gap between them. Positive is fresh, negative is fatigued, the rhythm matters more than any single day.
Is this only for athletes?
No. We built it for ex-athletes managing chronic illness who miss having form numbers to plan a week. The math is the Banister framework; the lens is a body whose baselines move.
How is this different from Strava?
Strava reads cycling and running. Body Insights reads anything Apple Health captures — strength, walks, yoga, swims — and folds overnight glucose stability into the stress cost. No subscription. No platform lock-in.
Do I need a CGM?
No. Without a CGM you get standard CTL, ATL, TSB read off your Apple Watch workouts. With one, metabolic-mTSS adjusts the cost of each session for glucose stability.
How much does it cost?
Free for core training-load features. A premium tier unlocks the metabolic-mTSS layer and deeper phase analytics. No card required to start.
Your training data stays yours.
On-device. No account. No subscription required. No data sold. Ever.
- Reads from Apple Health
- Stays on your iPhone
- You choose what to share
Get your form number back.
Free for core training-load features. Premium adds metabolic-mTSS.
