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Why 20 Minutes Became Canonical
- The 20-minute interval became canonical because of a 20-minute FTP test and widespread cultural adoption among cyclists.
- Wattage forum lore credits Andy Coggin's repeated 5-minute loop sets doubled into 20-minute efforts as an origin story.
Muscle Aerobic Lag Limits Effective FTP Time
- Power meters show immediate output but muscles need minutes to reach the aerobic steady state, so the first few minutes of an interval are anaerobic.
- That lag means a 20-minute interval yields only ~15–17 minutes at true metabolic FTP, implying a minimum effective FTP interval near 10 minutes.
Progress FTP By Increasing Time Not Power
- Aim for roughly 30–35 minutes total metabolic FTP work per session as a minimum effective dose.
- Progress by extending time rather than raising power (e.g., 4x10 → 2x20 → 1x40), and retest at block end.