Empirical Cycling Podcast The Watts Doc #8: FTP vs Critical Power
Jun 4, 2019
Exploring the discrepancies between Critical Power (CP) and Functional Threshold Power (FTP) models, their measurement accuracy, and practical applications in training. The importance of understanding the difference between FTP and CP, and their limitations for longer endurance efforts. Highlighting the physiological impossibility of maintaining critical power indefinitely and a humorous anecdote about a misapplication of the CP model.
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FTP Is A Physiological State Not A 60 Minute Number
- FTP represents the highest power you can sustain at maximal lactate steady state (MLSS) rather than simply a 60‑minute power.
- Kyle and Colleen stress FTP is a physiological state tied to steady lactate, not an arbitrary duration-based number.
CP Can Overestimate Sustainable Power For Some Riders
- CP often comes out higher than FTP, especially when fitted from short anaerobic and moderate aerobic tests, producing a practical overestimate.
- Kyle gives his 2016 example: CP=304 W vs FTP≈285–290 W, a ~7% higher CP that makes intervals much harder if used as FTP.
How Critical Power Is Calculated From Work Versus Time
- Critical Power (CP) is derived by fitting a linear model to total work (kJ) vs time, so only two or three maximal efforts between ~30s and 20min are required.
- That linear fit yields a slope (CP) that interpolates well inside the tested duration range but can mislead if extrapolated beyond it.