Entry #009: Why functional threshold power may not fully capture durability in cycling

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The Scientist’s NotebookThomas Mortelmans

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It is well established that Functional Threshold Power (FTP) can function as a pedagogical construct for intensity domains. Now, we need to address the metabolic elephant in the room: the decay of performance over time. For decades, the endurance community has operated under the reductionist assumption that a 20-minute field test scales linearly to performance in the fourth, fifth, or sixth hour of competition. This assumption is physiologically flawed.

We often conflate "potential" (what you can do when fresh) with "resilience" (what you can do when homeostatic perturbations accumulate). Emerging data suggests that two athletes with identical FTP values can have vastly different physiological responses after 2000 kilojoules of work.

If your training architecture focuses solely on raising the ceiling (FTP) without reinforcing the foundation (Durability), you are building a "glass cannon"—impressive on the dyno, but fragile in the field. This briefing dissects the dissociation between threshold power and fatigue resistance.

Executive Summary

• FTP is a Snapshot, Not a Movie: Standard FTP testing measures power in a rested, glycogen-loaded state. It fails to account for the "durability" parameter, which dictates how much that power declines after significant energy expenditure.
• The TTE Variable: Time to Exhaustion (TTE) at FTP is not a fixed 60 minutes. It varies wildly (from 35 to 70+ minutes) based on training history and muscle fiber typology, meaning FTP is a poor predictor of endurance capacity in isolation.
• The Fourth Pillar: Modern physiology now identifies Durability (or fatigue resistance) as an independent determinant of performance, alongside VO2max, Threshold, and Economy.
• Decoupling of Internal Load: In athletes with poor durability, internal stress (heart rate, lactate accumulation) rises significantly relative to external output (watts) as duration increases, creating a "drift" that standard zones do not capture.

The Science at a Glance

The following table contrasts the physiological profile of a "High FTP" phenotype against a "High Durability" phenotype. Note that in shorter events, the former wins; in extended events, the latter dominates.