Kilometres, hours, and "hard sessions" are approximate proxies for training load. They don't distinguish between a 20km easy run and a 20km tempo run. They can't be meaningfully summed across a week of mixed efforts. Training Stress Score (TSS) solves this problem by quantifying every session on a single, comparable scale — and combining them into metrics that tell you whether you're building fitness, accumulating fatigue, or approaching your race in peak condition.
What TSS Measures
TSS is a dimensionless number that captures the training stress of a single session, accounting for both intensity and duration. The reference point: a one-hour session at exactly your threshold intensity = 100 TSS.
- An easy 2-hour run at 65% threshold might generate 80 TSS
- A 45-minute interval session at 110% threshold might generate 95 TSS
- A 5-hour easy bike ride at 70% FTP might generate 200 TSS
This allows direct comparison — and summation — across sessions of different types, durations, and intensities.
How TSS Is Calculated by Sport
Cycling (Power Meter)
TSS = (Duration in seconds × Normalised Power × Intensity Factor) / (FTP × 3600) × 100
Where Intensity Factor = Normalised Power / FTP. This is the most precise TSS calculation because power is measured directly.
Running (rTSS)
Running TSS uses Normalised Graded Pace (NGP) relative to threshold pace, following the same mathematical structure. A pace 10% faster than threshold = IF of 1.10. A pace 20% slower than threshold = IF of 0.80.
Swimming (sTSS)
Swimming TSS uses pace relative to threshold pace in the water. Note that an hour of hard swimming generates significantly less TSS than an equivalent hour of running — partly because swimming exerts far less musculoskeletal load. This is appropriate: swimming doesn't injure you like running does, so its TSS impact on total load should be lower.
The Three Derived Metrics: ATL, CTL, and TSB
TSS per session combines into three summary metrics that form the Performance Management Chart (PMC):
Chronic Training Load (CTL) — "Fitness"
A 42-day exponentially weighted average of daily TSS. CTL rises slowly with sustained training and falls slowly with rest. It represents your long-term fitness base — the engine you've built over months.
A recreational runner averaging 400 TSS/week might have a CTL of ~57. An elite marathon runner training 1,000+ TSS/week might have a CTL of 120+.
Acute Training Load (ATL) — "Fatigue"
A 7-day exponentially weighted average of daily TSS. ATL responds quickly to training spikes and drops quickly with rest. It represents current accumulated fatigue. After a hard training week, ATL exceeds CTL — you're carrying fatigue above your fitness level.
Training Stress Balance (TSB) — "Form"
TSB = CTL − ATL
- Negative TSB: Fatigue exceeds fitness expression. You're "in training" — fitter than you feel.
- Zero TSB: Balanced.
- Positive TSB: Fresh — fitness can be fully expressed. The state you want on race day.
Research and practitioner experience suggest optimal race-day TSB is approximately +5 to +25. Below 0: undertapered. Above +30: likely detrained from overtapering.
Using TSS to Plan Your Training
Building Fitness Safely
CTL builds at a rate determined by weekly TSS. Increasing CTL too rapidly increases injury risk. A common guidance: increase weekly TSS by no more than 5–10% per week. Aggressive CTL ramp rates (more than 5–7 TSS/day per week) consistently correlate with overuse injury risk across endurance sports.
Timing the Taper
The taper's purpose is to reduce ATL (fatigue) while preserving CTL (fitness). A 2–3 week taper reducing weekly TSS by 30–50% achieves this for most athletes. The taper length needed depends on how negative your TSB was at peak training — the deeper the hole, the longer the taper required.
Diagnosing Stale Legs
If you feel flat despite light training, your TSB is probably positive but CTL has dropped too far — overtapering. If sessions feel unsustainably hard despite consistent training, ATL is high and TSB is deeply negative — you need recovery, not more load.
Calculating Your TSS and Tracking Form
Use the NorthLine Training Load Calculator to input your weekly training data — duration, intensity, and sport — and calculate your TSS, ATL, CTL, and TSB. Monitor these metrics weekly during your build to stay in the ACWR safe zone and arrive at your target race with a positive TSB and peak-season CTL.
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