What is Training Stress Score (TSS)?
TSS is the number most endurance coaches use to quantify how hard a session was. What it measures, how it's calculated, and why it matters if you want a training plan that doesn't grind you into the ground.
By PulseLab ·
Training Stress Score (TSS) is a single number that describes how much physiological stress a session created, accounting for both intensity and duration. A 90-minute easy run might score 60. A 45-minute hard interval session might score the same. Duration without intensity, or intensity without duration, does not tell you much on its own.
The formula was developed by Andrew Coggan and Hunter Allen for cycling power analysis, then extended to running and swimming. For cycling, it uses normalised power relative to your functional threshold power (FTP). For running, it uses pace or heart rate relative to your threshold. The exact numbers vary by tool, but the principle is the same.
Why it matters
When you stack TSS scores across days and weeks, patterns emerge. Chronic Training Load (CTL) is a rolling 42-day average (roughly your fitness). Acute Training Load (ATL) is a 7-day average (your current fatigue). The difference between them is Training Stress Balance (TSB), sometimes called form.
A very negative TSB means you are carrying a lot of fatigue relative to your fitness. That is fine during a hard build block, but it is a warning sign if it drags on for weeks. Most athletes perform best at races when TSB is between 0 and +25, fit but rested.
The number has limits
TSS does not capture sleep quality, nutrition, life stress, or how your legs actually felt. A session that should score 70 might feel like 100 if you were dehydrated and had three bad nights. That subjective layer matters as much as the objective score.
This is why PulseLab combines training load (which feeds into the TSS model) with HRV, sleep quality, energy, and soreness to produce the PULSE Score. The TSS tells you what your body did. The PULSE Score tries to account for how your body is handling it.
Step-by-step guide
- 1.
Establish your threshold (FTP or threshold pace)
TSS is meaningless without an accurate threshold. Do a 20-minute all-out effort and take 95% of average power (cycling) or pace (running). This is your FTP or threshold pace. Re-test every 6-8 weeks as fitness changes.
- 2.
Aim for weekly TSS targets by phase
Recreational athletes training 8-10 hours per week typically accumulate 400-600 TSS weekly in base phase. Build phase pushes toward 600-800. Peak phase may exceed 800. Recovery weeks should drop to 250-350. These are rough guidelines, not rules.
- 3.
Watch your TSB, not just your weekly TSS
TSB = CTL minus ATL. A TSB between -10 and -30 is a normal training state. Below -30, injury and illness risk increases. Above +15, you are either rested or detrained. Aim for TSB between 0 and +25 heading into a goal race.
- 4.
Do not chase TSS for its own sake
More TSS is not always better. A junk 3-hour easy ride adds TSS but little fitness stimulus. Quality sessions at the right intensity contribute more to adaptation. Use TSS to measure load, not to justify volume.
- 5.
Layer in subjective data
TSS captures effort but not recovery. A week where TSS looks fine can still leave you depleted if sleep was poor and stress was high. Track HRV and morning energy alongside TSS to get the full picture.