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- Normalized Power (NP) accounts for variable efforts — a hard interval session with easy recoveries has a higher NP than a steady ride at the same average power, because NP weights the hard efforts more heavily
- Intensity Factor (IF = NP / FTP) tells you how hard the ride was relative to your threshold. IF <0.75 is recovery; 0.75–0.85 endurance; 0.85–0.95 tempo/threshold; >1.0 is above threshold (VO2/anaerobic)
- Variability Index (VI = NP / Average Power) measures how punchy a ride was. VI 1.0 = perfectly steady (TT); VI 1.1–1.3 = variable (intervals, racing); VI >1.3 = very punchy (crits, attacks)
- These three metrics are how TSS is calculated (TSS = duration × NP × IF / FTP × 100). Understanding them lets you read any ride file and know exactly what happened
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Why average power lies
Average power is the mean of every second's wattage over the entire ride. It treats a 200W steady effort and a ride alternating between 300W and 100W (same average) as identical. They are not — the variable ride is much harder, because the 300W efforts produce more physiological stress (the body's response to power is nonlinear, following a 4th-power curve for lactate accumulation).
Normalized Power (NP) fixes this. It uses a 30-second rolling average (to smooth micro-variations) and then raises it to the 4th power (to weight hard efforts), takes the average, and takes the 4th root. The result: a number that reflects the true metabolic cost of the ride. A steady TT has NP almost equal to average power; a punchy criterium has NP well above average power.
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The three metrics
The power-analysis vocabulary. These three numbers decode any ride file.
| Metric | Formula | What it tells you |
|---|---|---|
| Normalized Power (NP) | 4th-root of mean of (30s rolling avg)^4 | The physiological cost of the ride — harder efforts weighted more |
| Intensity Factor (IF) | NP / FTP | How hard the ride was relative to your threshold |
| Variability Index (VI) | NP / Average Power | How punchy/steady the ride was |
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Reading Intensity Factor
IF is the single most useful number for understanding a ride. It normalizes effort against your FTP, so you can compare rides of different durations and types:
IF < 0.75: recovery ride. Very easy, active recovery. You should be able to do this every day without accumulating fatigue.
IF 0.75–0.85: endurance (Zone 2). Sustainable for many hours. The bread and butter of base training.
IF 0.85–0.95: tempo to threshold. Hard but sustainable for 1–3 hours. The intensity of a fast group ride or a rolling road race.
IF 0.95–1.05: threshold. A sustained effort at or just above FTP. Typical of a 20–40 km time trial or a hard climb.
IF > 1.05: above threshold. VO2 or anaerobic efforts. Sustainable for minutes, not hours. Interval sessions and race-winning moves.
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Reading Variability Index
VI tells you the character of the ride:
VI = 1.0: perfectly steady. The only real-world example is a flat time trial on a calm day. NP equals average power.
VI 1.0–1.1: fairly steady. Endurance rides on rolling terrain, or a tempo group ride. Some natural variation.
VI 1.1–1.3: variable. Interval sessions (hard efforts with easy recoveries), rolling road races, or hilly rides. The higher VI reflects the punchy nature.
VI > 1.3: very punchy. Criteriums, races with repeated attacks, or a very hard interval session. These rides have high NP relative to average power because the hard efforts dominate the metabolic cost.
Use VI to validate your interval sessions: if you did 5×5 min at threshold with easy recoveries, your VI should be ~1.15–1.25. If it's 1.05, your 'recovery' intervals were too hard; if it's 1.35, your work intervals were too short or too hard.
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