Training Stress Score (TSS) is a number that quantifies how much physiological stress a single workout placed on your body. Developed by Dr. Andrew Coggan for cycling, it has since been adopted across running, swimming, triathlon, and strength training.
Why TSS matters
Before TSS, athletes had no reliable way to compare the stress of different workouts. A 2-hour easy run and a 45-minute threshold session can leave you equally tired — TSS makes that comparison objective and comparable over time.
When you accumulate TSS across days and weeks, you can track three critical metrics:
- CTL (Chronic Training Load): Your fitness. The rolling 42-day average of daily TSS.
- ATL (Acute Training Load): Your fatigue. The rolling 7-day average of daily TSS.
- TSB (Training Stress Balance): Your form. CTL minus ATL. Positive means rested; negative means fatigued.
This is the basis of the Performance Management Chart (PMC) — the same tool used by professional cyclists, triathletes, and running coaches worldwide.
How TSS is calculated
For cycling (power-based):
TSS = (Duration in seconds × NP × IF) ÷ (FTP × 3600) × 100Where NP = Normalized Power, IF = Intensity Factor (NP ÷ FTP), and FTP = your 60-minute max sustainable power.
For running, TSS uses pace and threshold pace instead of power. For heart rate-based sports, it uses LTHR.
What is a good TSS per week?
| Weekly TSS | Training Level |
|---|---|
| Under 150 | Beginner / maintenance |
| 150–300 | Recreational athlete |
| 300–500 | Committed amateur |
| 500–800 | Advanced amateur / club athlete |
| 800+ | Elite / full-time athlete |
A sudden spike in weekly TSS is one of the biggest predictors of injury. Most coaches recommend increasing no more than 10% per week.
TSS for strength training
PROTR also calculates TSS for strength sessions using volume load (sets × reps × weight) and session RPE. This means your PMC reflects your full training picture — not just cardio.
How to use TSS in PROTR
PROTR calculates TSS automatically for every workout. For endurance sessions, it uses your threshold data from Settings. For strength, it uses volume load and session RPE. All TSS values feed into your PMC chart under the Analysis tab.
On first enabling endurance analytics, PROTR infers your FTP, threshold pace, and LTHR from Apple Health history — so you get a populated PMC from day one, not a blank chart.