PHYS-07 · Principle

What is critical power, and why do reps above it end so quickly?

It approximates the top of the heavy domain. Below it the body can settle; above it VO2 climbs to its maximum and a limited reserve runs down, so the effort soon ends.

How the body works, agreed across the reviews below; a study entry shows what one experiment measured.

established physiologyApplies to continuous and interval efforts in every endurance sportEstablished — the moderate, heavy and severe domains and their different responses (REF-024, REF-168, REF-281, one group; a second group's critical review, REF-173), and that fixed percentages of HRmax, VO2max…

Practical Application

What is critical power, and why do reps above it end so quickly? It approximates the top of the heavy domain. Below it the body can settle; above it VO2 climbs to its maximum and a limited reserve runs down, so the effort soon ends.

What it means for training: %FTP and %HRmax zones are proxies for the domains; anchor them to a measured first threshold and CP or critical speed (SYNTH-27, SYNTH-51), and expect the first threshold to fall late in long sessions (SYNTH-32).

What it does not mean: that one percentage puts everyone in the same domain, or that CP is a fixed-duration power: it is a boundary, not a duration (SYNTH-27).

Mechanism

Above the lactate threshold a VO2 slow component lowers efficiency and draws on glycogen; above CP acid-base status and gas exchange cannot settle and VO2 rises to VO2max (REF-281); there W′ and VO2max set tolerance, while below CP responses can stabilise (REF-024).

A second group finds support for LT1, GET or VT at the lower boundary and for CP or critical speed at the upper, and little merit in fixed percentages (REF-173); in 100 healthy adults MLSS fell anywhere from 69 to 96% of VO2max and 75 to 97% of HRmax (REF-038, B+); across 10 small crossovers, CP-MLSS limits of agreement ran from 19 W below to 44 W above (REF-1316).

Riding at CP, nothing was steady in 7 trained men (REF-750, B). Profiles measured rested shift over long efforts (REF-195, REF-141); in 14 trained riders, power at the first threshold was about 10% lower after 2 h (REF-179, B).

Common misreading

"Threshold is FTP, or 4 mmol/L, for everyone"; "anything below threshold is steady": the heavy domain drifts (PHYS-11).

Backs

SYNTH-27 "Use CP (critical power), not FTP, as the heavy/severe boundary when prescribing intervals" · SYNTH-27 "CP is a boundary, not a duration" · SYNTH-27 "Never mix markers" · SYNTH-51 "keep easy work comfortably below the first threshold"