Empirical Cycling Podcast Watts Doc #19: The VO2max Slow Component Explained
May 6, 2020
A deep dive into why oxygen uptake can keep rising during sustained efforts. They explore thermodynamics, heat loss, and how muscle efficiency is measured. Fiber type differences and motor unit recruitment are connected to the slow component. Practical training implications and why heart rate drift may reflect changing fiber use are discussed.
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Muscle Efficiency Is Work Over Enthalpy
- Muscle efficiency equals work output divided by total enthalpy, so most energy becomes heat rather than work during exercise.
- Cole A. Moore shows a cyclist at FTP can be ~22% efficient, dumping ~1,100 watts as heat at 4.12 L·min⁻¹ and RER ~0.95.
Higher Type 1 Fiber Share Improves Efficiency
- More type 1 fibers raise gross efficiency so two riders using the same VO2 can produce different power outputs.
- Horowitz et al. matched riders by O2 use and found higher type 1% gave ~10–40 W higher 1‑hour power (avg ~20–30 W).
Matched VO2 Pair Shows Real FTP Difference
- Subject 1 (83% type 1) and Subject 8 (46% type 1) had nearly identical VO2 but produced 357 W versus 335 W for a one‑hour test.
- Cole A. Moore uses this pair to show a real ~22 W FTP advantage tied to fiber composition.