Four numbers sit at the heart of every data-driven training plan: TSS, CTL, ATL, and TSB. Together, they measure training stress, fitness, fatigue, and race readiness — and they were developed by coaches Andrew Coggan and Hunter Allen specifically to give endurance athletes a precise, objective way to manage their workload. Today, these metrics are built into TrainingPeaks, Intervals.icu, and most modern coaching platforms.
If you train with power or pace data, understanding how these four figures interact is not optional — it is the difference between guessing and knowing.
What Is TSS and How Is It Calculated?
TSS, or Training Stress Score, is a single number capturing how much physiological stress a workout placed on your body. It combines two variables: how hard you rode relative to your FTP, and how long you sustained that effort.
The formula, defined by Coggan and Allen in Training and Racing with a Power Meter, is:
TSS = (duration in seconds × Normalised Power × Intensity Factor) ÷ (FTP × 3600) × 100
The benchmarks are straightforward. A one-hour ride at exactly your FTP scores 100 TSS. An easy 90-minute endurance spin comes in around 60–70 TSS, while a hard five-hour sportive can exceed 350 TSS.
One critical detail: every TSS figure is anchored to your FTP. Set that number too high or too low, and every stress score you accumulate is proportionally wrong — along with everything built on top of it. Re-test your FTP every 4–6 weeks, or after any meaningful fitness change, to keep the whole system honest.
What Are CTL and ATL — Fitness and Fatigue Explained?
CTL, or Chronic Training Load, is your fitness number — a 42-day exponentially weighted rolling average of your daily TSS. The longer time window means it moves slowly, which is precisely the point. CTL reflects the aerobic base built over weeks and months, not yesterday's hard session.
ATL, or Acute Training Load, uses a 7-day exponentially weighted average of daily TSS. Because the window is six times shorter, ATL reacts roughly six times faster than CTL to any shift in training load.
Consider a training camp week. Five hard days push ATL sharply upward — you feel wrecked. CTL barely moves, because seven days is a small fraction of a 42-day average. Fitness builds slowly; fatigue arrives quickly. That asymmetry is exactly what these metrics are designed to expose.
Coggan and Allen's CTL/ATL model mirrors the supercompensation theory first described by Nikolai Yakovlev between 1949 and 1959. Hard training depresses performance; adequate recovery allows it to rebound above the original baseline. The model gives that biological process a quantifiable shape.
What Is TSB and What Does 'Form' Actually Mean?
TSB, or Training Stress Balance, is CTL minus ATL. Also called "form," it answers one question: are you more fit than you are fatigued, or the other way around?
The zones below, based on Joe Friel's periodization guidance, offer a practical read on what each TSB range means:
TSB Range | What It Means | Recommended Action |
|---|---|---|
Below −20 | High fatigue, overreaching risk | Prioritise recovery |
−10 to +5 | Productive training range | Continue structured build |
+5 to +25 | Race-ready sweet spot | Taper and race |
Above +25 | Detraining risk | Return to training stimulus |
A negative TSB is not a problem — it is a normal and necessary feature of any build block. The goal is timing: lift TSB into positive territory just before race day while keeping CTL as high as possible.
How Does the Performance Management Chart Bring It All Together?
The Performance Management Chart, or PMC, plots CTL, ATL, and TSB on a single time-series graph. CTL typically appears in blue, ATL in pink or red, and TSB in yellow or gold. Months of training history become readable at a glance.
A typical season arc looks like this. During base-building, CTL rises steadily while TSB stays chronically negative — fitness accumulating faster than recovery can occur. In a peak training block, ATL spikes and TSB dips further. Then the taper begins: volume drops, ATL falls faster than CTL, and TSB climbs into positive territory. On race day, CTL is still high and TSB is positive. That is peak form.
The PMC is, in effect, a quantified representation of Yakovlev's stress–recovery–supercompensation cycle, and it is available in TrainingPeaks, Intervals.icu, and PlanWatts, where it also informs automatic workout scheduling.
How Do You Use TSB to Peak for a Race?
Peaking for a target event is a structured process. Here is how to approach it:
- Identify your race date.
- Note your current CTL and TSB.
- Plan a taper of 7–14 days — shorter for sprint or Olympic-distance triathlon, longer for an Ironman or gran fondo.
- Reduce volume by 40–60% while keeping some intensity sessions to prevent CTL from crashing.
- Monitor TSB daily and aim for +5 to +20 on race morning.
The most common taper mistake is going too far. If TSB climbs above +25, CTL has likely dropped too much and fitness is being lost rather than consolidated. Joe Friel's periodization framework underpins this taper design. Stephen Seiler's polarized model adds a useful note: even during reduced-volume tapers, maintain your intensity distribution rather than eliminating hard efforts entirely.
One practical rule applies throughout: log every workout consistently. Gaps in data distort CTL and ATL calculations, which makes TSB unreliable precisely when you need it most.
What TSS should I aim for per week?
Rough weekly TSS targets by level: beginners, 300–500; intermediate athletes, 500–800; advanced athletes, 800–1,200 or more. At any level, avoid jumping more than roughly 10% per week — rapid load increases raise both injury and overtraining risk.
How quickly does CTL build and how quickly does it drop?
CTL builds slowly, over weeks to months, and decays at roughly the same 42-day constant. Even two weeks completely off the bike will produce a noticeable drop. Fitness takes time to earn and less time to lose than most athletes expect.
What TSB should I have on race morning?
The evidence-based target, per Friel's guidance, is +5 to +20. Individual variation exists — some athletes perform best at the lower end, others toward the higher end. Tracking your own race-day TSB across multiple events is the most reliable way to find your personal range.
Can I use these metrics without a power meter?
Yes. Heart-rate-based TSS, or hrTSS, is available in both TrainingPeaks and Intervals.icu. It is less precise than power-based TSS because heart rate responds to heat, hydration, and stress in ways power does not — but it remains a workable alternative.
Why does my TSB go very negative after a training camp?
ATL spikes sharply over the camp's 7-day window while CTL rises only modestly. A deeply negative TSB after a camp is expected and normal. With a return to regular training volume, TSB typically recovers within 5–10 days.
How often should I re-test FTP to keep TSS accurate?
Every 4–6 weeks, or after any significant fitness change, as Coggan and Allen recommend. FTP is the anchor for every TSS figure, so an outdated number quietly corrupts your entire training load picture. --- TSS, CTL, ATL, and TSB are four interconnected numbers that, read together on the Performance Management Chart, give you a clear and objective picture of where you stand. TSS quantifies each workout. CTL tracks the fitness you have built. ATL tracks the fatigue you are carrying. TSB tells you the net balance between the two. Use them consistently, keep your FTP current, and the guesswork in your training largely disappears.
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