Critical Power & Functional Threshold Power (FTP)

Functional Threshold Power (FTP) is the highest average power output, in watts, that can be sustained for approximately 45–60 minutes before fatigue forces a slowdown. It is "functional" because it is a field measure rather than a lab value, "threshold" because it sits near the lactate threshold (≈ LT2; see Lactate Threshold (LT1 and LT2)), and "power" because it is measured in watts (requiring a power meter; see Power and FTP). Because an all-out 60-minute test is brutal and hard to repeat, most riders perform a 20-minute maximal test and take ~95% of that average power as their FTP estimate.

FTP is the single anchor for power-based training zones: a common system (Coggan) defines seven zones as percentages of FTP, so getting FTP right keeps easy rides easy and hard intervals correctly placed (see Heart-Rate Zones (5-Zone Model) for the heart-rate analog). The concept generalizes beyond cycling — running has an analogous threshold pace, and swimming a threshold pace per distance.

Critical Power (CP) is the underlying physiological model FTP draws on: it represents a roughly metabolically steady-state power output, derived from the hyperbolic relationship between power and how long it can be sustained across several maximal efforts. FTP is the practical, single-test approximation of this idea. Power-based thresholds are valued because watts are an objective, instantaneous output measure unaffected by the lag, drift, and day-to-day variability of heart rate.

• FTP = highest power sustainable for ~45–60 min; near the lactate threshold (≈ LT2).
• Common estimate: 95% of a 20-minute maximal-effort average power.
• Anchors power-based zones (e.g., Coggan's seven zones as %FTP).
• Critical Power (CP) is the physiological model (power–duration hyperbola) FTP approximates.
• Watts are objective and instantaneous, unlike the lag/drift of heart rate.