Fitness, fatigue and form explained

Fitness, fatigue, form. Or CTL, ATL and TSB. Three curves that appear in every training tool and are rarely explained. The idea behind them is simple.

The foundation: training load

All three build on a single number per ride: training load, usually called TSS (Training Stress Score). It captures how hard a session was — duration times intensity.

For orientation: one hour at your threshold, the pace you can just about hold for an hour, is 100 points by definition. Two easy hours might be 90. Twenty minutes of intervals might be 60.

The three curves are nothing more than differently sluggish averages of that one number.

Fitness (CTL) — your base

CTL is your training load averaged over the last 42 days.

It moves slowly, because it averages over six weeks. A good week barely lifts it; a missed week barely drops it. That is the point: CTL describes the base you have built over months — and will not lose in a week either.

If CTL rises over weeks, you are becoming more durable. If it falls over weeks, you are detraining.

Fatigue (ATL) — the fresh load

ATL is the same training load averaged over the last 7 days.

It reacts fast. A hard Sunday is clearly visible on Monday and largely gone after a few quiet days. ATL does not describe how fit you are, but how much you are currently carrying around.

Form (TSB) — the difference

TSB = CTL − ATL. Fitness minus fatigue.

This is the number that matters day to day, because it answers the real question: is any of my base available right now, or is it buried under fresh fatigue?

clearly negative — you are in a loading block. Normal in hard weeks, but no state to race in.
around zero — balanced. Load and recovery are level.
slightly positive — rested with the base intact. Where most people produce their best performances.
strongly positive for a long time — you are rested, but fitness is eroding. Normal after a holiday or illness.

The exact thresholds are individual. What matters is not the absolute value but where you were when things were going well for you.

And without a power meter?

TSS was originally designed for power meters: Normalized Power relative to FTP, times duration. Without a power meter that calculation is not available.

The alternative is hrTSS — the same idea from heart rate instead of watts. Time in heart-rate zones is weighted rather than the power distribution.

That is coarser, and it is worth knowing why:

Heart rate lags the effort by a minute or two. Short, hard intervals are systematically underrated as a result.
Heat, poor sleep, caffeine and stress raise your pulse without raising your output.
On very long rides heart rate drifts upward even when the pace stays the same.

For long and moderately hard rides hrTSS is still useful, and the multi-week trends hold. What matters is that you know which source is behind a number — which is why Ride Streak marks every training load with a source badge: from power, or estimated from heart rate.

And do not mix the two sources carelessly. If half your rides have power and half do not, the curve jumps for reasons that have nothing to do with your training.

What to actually do with it

Look at weeks, not days. A single value is noise.
Raise CTL slowly. Big jumps in a short time are the classic route to overreaching.
Plan your form. To be fresh on a given date, cut the load in the final week or two without stopping entirely — ATL falls fast, CTL barely moves. That gap is exactly what the difference is for.
Trust how you feel when it disagrees. The numbers know nothing about your sleep or your job.

In Ride Streak

Fitness & Form is part of Ride Streak Pro and requires RideStreak Cloud. Training load is calculated on your device; only the finished daily number is synced — never your GPS routes, never raw heart-rate data.

You get the three curves over 6 weeks, 3 months or a year, plus a card on the stats tab showing today’s form and your fitness trend at a glance. Rides need heart-rate or power data to count.