Training load is a single number for how much stress a session put on you, built from how long you went and how hard. CTL (fitness) is a 42-day weighted average of that daily load, ATL (fatigue) is a 7-day one, and TSB (form) is CTL minus ATL. When form is negative you are carrying fatigue; when it is positive you are fresh.
That is the whole model, and it is useful. It also has a blind spot that matters for HYROX: it only counts the load your heart rate, pace or power recorded. After a heavy sled and lunge session your form can read fresh while your legs are anything but. This guide explains what the three numbers mean, how to read them in intervals.icu, what the research on acute and chronic workload actually found, and where the maths breaks for hybrid training.
Key Takeaways
- CTL is a 42-day exponentially weighted average of daily training load, ATL a 7-day one, and TSB (form) is CTL minus ATL.
- Negative form during a training block is normal. Joe Friel likes his athletes at about +15 to +25 on the day of an A race.
- The form zones in intervals.icu come from Friel, and the platform’s developer calls them a rule of thumb rather than science.
- The “sweet spot” of 0.8 to 1.3 for the acute to chronic workload ratio is contested. In a cohort of 435 Dutch runners, a low ratio carried more injury risk than a high one.
- Load from heart rate measures what your heart did, not what your legs did, so station work and strength sessions are undercounted.
- Read the trend over weeks, not one day’s number, and check it against how your legs and your sleep feel.
What Training Load Is, and Where the Number Comes From
Training load scores one session by combining its duration and its intensity, so an easy hour and a hard 30 minutes can land on similar numbers. The scale comes from Andrew Coggan’s Training Stress Score for cycling, where one hour at threshold scores 100, and most platforms keep that anchor for every sport.
In intervals.icu, the load for each activity comes from one of three sources: power, heart rate or pace. You set the order of priority per activity type in your settings, and heart rate is the default outside cycling, as the developer explains in the HRSS thread . HRSS is a heart-rate training load scaled to the same range as power-based load.
Each source has its own weak spot. Power needs a power meter and a correct threshold. Pace load suits steady runs but means little on a sled. Heart-rate load needs a correct maximum and resting heart rate, and it lags on short intervals, because your heart rate does not climb far enough in a 40-second rep before the rest begins.
This is also why your load in intervals.icu rarely matches the figure Garmin or Strava shows for the same run. They use different formulas on different scales, so compare a number only with itself, inside one platform.
CTL, ATL and TSB in Plain Numbers
CTL, ATL and TSB are three readings of the same daily load. CTL averages it over about six weeks, ATL over about one week, and TSB is the gap between the two. intervals.icu labels them Fitness, Fatigue and Form.
The table below holds the three side by side.
| Metric | intervals.icu label | What it averages | What it tells you |
|---|---|---|---|
| CTL, chronic training load | Fitness | Daily load, weighted over 42 days | How much training you have absorbed recently |
| ATL, acute training load | Fatigue | Daily load, weighted over 7 days | How much you have done this week |
| TSB, training stress balance | Form | CTL minus ATL | Whether this week is above or below your usual |
Notice that “fitness” here is a label for a load average, not a measure of how fast you are. Two athletes with the same CTL can race very differently.
CTL: Your Chronic Training Load
CTL is an exponentially weighted average, so recent days count more than older ones, but six weeks of training all leave a trace. The intervals.icu developer describes fitness as decaying over 42 days by default. It rises slowly when you train more and falls slowly when you stop, which is why a week off barely dents it.
ATL: Your Acute Training Load
ATL uses the same method over seven days, so it reacts fast. One big weekend moves it a long way, and three easy days bring it back down.
TSB: Your Training Stress Balance, or Form
TSB is CTL minus ATL. If this week’s load is above your six-week norm, form goes negative. If you have backed off, it goes positive. The idea goes back to Eric Banister’s fitness and fatigue model from 1975: every session adds a little fitness that lasts and a lot of fatigue that fades, and performance is the difference between them.
How to Read Your Form Number
Form is most useful at its extremes. A reading a little below zero during a build is normal and means you are training. A deep negative reading for weeks means fatigue is piling up faster than you absorb it. A high positive one means you are fresh, and if it lasts, losing fitness.
intervals.icu colours its form chart in bands, from High Risk at the bottom through Optimal and a Grey zone to Fresh at the top. You can show form as an absolute number or as a percentage of your fitness. The in-app tooltip, quoted on the TrainerRoad forum , says the percentage option scales the zones to your current fitness and “is probably the better choice”. Users there read the High Risk band as starting at about -30%.
Treat the bands as a guide. Asked whether they had scientific backing, the developer answered on the intervals.icu forum that “the zones come from Joe Friel and are for absolute TSB”, and that he kept them for the percentage view because they seemed to work for his own training. A rule of thumb, in other words, and a sensible one.
On race day, the number to know is Friel’s. In his TrainingPeaks article on training stress balance , he likes athletes at about +15 to +25 for an A race, and notes that some race best at +5 to +10. Below -10, he says, you are probably too tired to race well. Getting there is the job of the taper, and the guide to why you feel worse during a taper covers what that fortnight feels like.
The Acute:Chronic Workload Ratio, and Why the Evidence Is Messy
The acute to chronic workload ratio (ACWR) divides this week’s load by your load over the last four weeks or so. It is the same idea as form, written as a ratio instead of a difference: 1.0 means this week matches your norm, and 1.5 means you did half as much again.
The ratio was popularised by Tim Gabbett’s 2016 paper in the British Journal of Sports Medicine , which argued that athletes are protected by high chronic load and hurt by sudden spikes. It put the lowest risk at a ratio of about 0.8 to 1.3, the “sweet spot”, with risk climbing above 1.5. Most training-load explainers stop there.
The evidence did not stay that tidy. Three findings are worth knowing:
- The maths flatters itself. The recent week sits in both halves of the ratio. In simulated data, Lolli and colleagues showed that this alone creates a correlation of about 0.50 between acute and chronic load, before any athlete gets hurt.
- The studies cannot set rules. A 2020 commentary by Impellizzeri and colleagues in the Journal of Athletic Training concluded that the load and injury studies lack the framework to say how the ratio should be used to reduce injuries.
- In runners, it ran the other way. In a prospective cohort of 435 Dutch runners (Nakaoka et al., Sports Medicine, 2021), runners with a ratio under 0.70 had a predicted injury risk of 9.6%, against 1.3% for those above 1.38. The weighted-average method, the one closest to CTL and ATL, gave sparse and non-significant results.
So the honest reading is this: big, sudden jumps in load are worth avoiding on common sense, and a ratio is a reasonable way to spot them. No number on your chart is a measured injury threshold.
Why Training Load Misreads HYROX and Hybrid Training
Training load from heart rate measures what your heart did. It cannot see what your legs did. For a runner that is a fair trade. For a HYROX athlete it is a real gap, because much of the hardest work in the sport is muscular: sled push and pull, sandbag lunges, wall balls, and the strength sessions behind them.
A set of heavy lunges or a 50m sled push raises your heart rate for well under a minute, then you rest. HRSS sees a short spike and a long recovery, and scores the session lightly. Your quads see something else. The same applies to a leg day with long rests between sets: a big muscular cost, a modest heart rate, a small load.
The result is a form number that is too optimistic after exactly the sessions that leave you sore. Take Callum, training for his second HYROX. On Saturday he does a 75-minute station session: sled push, sandbag lunges and 100 wall balls, logged with a chest strap. With all the rest between sets, intervals.icu scores it about the same as a 45-minute easy run. By Monday his form reads fresh, so he runs his threshold session hard, and his legs fall apart on the third rep. The chart had his heart right. His legs were never in it.
There are three ways to close the gap:
- Choose the load source per activity type. Pace load is often better for interval runs, where heart rate lags. Leave heart rate as the source for easy aerobic work.
- Correct the load on strength and station sessions. intervals.icu lets you edit an activity’s load, and users on the forum suggest adding an estimate for training the chart does not capture. A rough number beats a zero.
- Read the chart next to your legs. If form says fresh and your legs say otherwise, believe your legs, and use the guide on when to adjust your HYROX training to decide what to change. Your HRV and resting heart rate against your own usual range are a second check that does not depend on the load formula.
This is also why compromised running is so hard to plan from a chart: the run after a station feels nothing like its heart-rate load suggests.
How to Use CTL, ATL and TSB Without Being Ruled by Them
Use the three numbers to see the shape of your training over weeks, not to decide today’s session. A rising CTL tells you the block is building. A form line that never comes back above zero tells you there is no recovery in the plan. That is the information worth having.
A few habits make the chart useful:
- Watch the ramp rate, not the peak. intervals.icu shows how fast your fitness is changing each week. A steady climb is a build; a sudden jump is the spike the workload research worries about.
- Plan the dips. Form should come up before the races that matter and go down again after. Niels Post, Kracey’s founder, put it plainly after racing three times in one hot Berlin weekend: “Next season, I have to book in much more peaking moments and then going down, chill, deload … instead of, like, peaking every single race, it’s just way too much.”
- Do not chase a CTL number. Load built from junk miles raises CTL as much as load built from good sessions. The number counts what you did, and it has no opinion on whether it was the right thing.
- Keep your zones right. Heart-rate load is only as good as the maximum and resting heart rate behind it. The training zone calculator shows your zones by three methods, so you can see how far apart they sit.
A good coach reads these numbers and rarely says them out loud. That is how Kracey’s coach is built: it reads your fitness, fatigue and form from intervals.icu every day, and it is told to translate them into plain words (“you’re fresh”, “carrying some fatigue”) rather than recite the acronyms. It also never saves today’s form as something to remember, because a number that changes daily goes stale by tomorrow. If you would rather ask Claude or ChatGPT, the free Kracey connector reads the same fitness, fatigue, form and ramp rate from intervals.icu, once you tick the box that shares your heart and recovery data.
FAQ
There is no good CTL in general. It depends on your history, your sport mix and how intervals.icu scores your sessions, so compare your CTL only with your own past numbers. A CTL that climbs steadily through a build and peaks a few weeks before your race is what matters.
No. Negative form during a training block means this week is harder than your six-week norm, which is how a block works. Worry when it stays deeply negative for weeks, or when your legs, sleep and resting heart rate agree that you are worn out.
Joe Friel aims for about +15 to +25 on the day of an A race, and notes that some athletes race best at +5 to +10. Below -10, he says, you are probably too tired to race well, which may be fine for a low-priority race.
They use different formulas on different scales. intervals.icu scores load from power, heart rate or pace in the order you set, while Garmin has its own load model, so the figures will not match. Compare each platform only with itself.
Only as far as its heart rate or power shows it. A heavy strength or station session usually scores low, because heart rate rises briefly and you rest a lot. You can edit the load on those activities in intervals.icu so your form reflects them better. ---
Read the Trend, Then Trust Your Legs
Training load is a useful model. CTL is your six-week load, ATL your one-week load, and TSB the gap that says whether you are fresh or tired. Negative form in a build is normal, positive form belongs before a race, and the zones and the workload ratios are rules of thumb, not measured thresholds. For HYROX and hybrid athletes, the biggest error is the one the chart cannot see: station and strength work that the heart rate barely records.
Your Kracey coach reads your training load, fitness, fatigue and form every day through intervals.icu, which syncs from Garmin, COROS, Polar, Suunto, WHOOP and Oura. It reads them next to your sleep, HRV and resting heart rate, and tells you what they mean in words. It lives in Telegram, so it messages you first. Meet your hybrid coach : 14 free days, no card, and they start when your coach can see your training, because a coach that cannot see you is not coaching you.