Original episode & show notes | Raw transcript
This document provides a detailed exploration of the physiological concepts underlying endurance performance, focusing on the phenomenon known as the VO2 slow component. Drawing from the principles of thermodynamics and landmark studies in exercise physiology, we will dissect how muscular efficiency, fiber type composition, and neural recruitment strategies intersect to define an athlete’s performance capacity.
At its core, human metabolism is a process of energy conversion, governed by the laws of thermodynamics. As the podcast notes, “metabolism is therefore combustion.” To understand muscular work, we must first appreciate the relationship between the energy we consume and the work we produce.
Work & Heat: When our muscles contract to produce force (i.e., pedal a bicycle), they are performing work. However, this process is not perfectly efficient. A significant portion of the energy released from breaking down fuels (carbohydrates and fats) is lost as heat. This is why our body temperature rises during exercise.
Enthalpy (
ΔH
): In thermodynamics, enthalpy represents the total energy content of a system. For our purposes, it is the sum of the useful work performed and the heat generated.
Efficiency = Work Out / (Work Out + Heat Out)
Gibbs Free Energy (
ΔG
) & Entropy (
ΔS
): The Second Law of Thermodynamics provides a more nuanced view. Enthalpy (
ΔH
) is composed of two parts:
ΔH=ΔG+TΔS
Gibbs Free Energy (
ΔG
): This is the portion of total energy that is actually available to do mechanical work.
Entropy (
ΔS
): This represents the energy that is unavoidably lost as disordered heat, often described as a “tax” on any energy conversion. It reflects the universe’s tendency towards dispersal and randomness. In this house, we obey the laws of thermodynamics; perpetual motion is impossible because this entropy “tax” must always be paid.
In exercise science, we quantify efficiency to understand how effectively an athlete converts chemical energy into mechanical power.
Gross Efficiency: The most common measure. It’s the ratio of the total mechanical work produced to the total energy expended.
Gross Efficiency = (Total Work / Total Energy Expended) * 100 A typical range for trained cyclists is 18-23%. As the podcast calculates, an athlete producing 325 watts might be expending over 1400 watts of total energy, with the remaining ~1100 watts dissipated as heat.
Net Efficiency: This is similar to gross efficiency but subtracts the energy cost of resting metabolism, isolating the efficiency of the exercise itself.
Delta Efficiency: This measures the change in energy expenditure for a given change in work rate, providing insight into how efficiency changes as intensity increases.
The podcast highlights a pivotal study by Horowitz, Sidosis, and Coyle that directly links muscular efficiency to muscle fiber composition.
Design: The researchers took a homogenous group of highly trained male cyclists with very similar VO2max values. They performed muscle biopsies on the vastus lateralis to determine each cyclist’s percentage of Type I (slow-twitch) and Type II (fast-twitch) muscle fibers. The group was then divided into a “high Type I” group (>56%) and a “normal Type I” group (<56%).
The Critical Finding: The researchers matched pairs of cyclists—one from each group—who consumed virtually the same amount of oxygen during a one-hour maximal effort. In every single case, the athlete with the higher percentage of Type I fibers produced significantly more power (20-40 watts more) for the same oxygen cost.
Conclusion: A higher proportion of Type I muscle fibers confers a greater gross efficiency. These athletes can do more work with the same metabolic “fuel” (oxygen). The study found a strong correlation (R² ≈ 0.7-0.8) between the percentage of Type I fibers and gross efficiency. This is partly because the myosin ATPase—the enzyme that breaks down ATP to cause contraction—in Type I fibers operates more slowly and economically than in Type II fibers.
This brings us to the central topic: the VO2 slow component. This is the gradual, steady increase in oxygen consumption (
VO2
) that occurs when exercising at a constant, high-intensity work rate (typically above the lactate threshold).
If power output is constant, why does the oxygen cost increase over time? The answer lies in the Size Principle of motor unit recruitment.
Your nervous system recruits muscle fibers in an orderly fashion, from smallest to largest.
Small motor units, which innervate the fatigue-resistant, highly efficient Type I fibers, are recruited first for low-force tasks.
As the demand for force increases, or as the initial fibers begin to fatigue, the brain sends a stronger signal to recruit larger motor units. These larger units innervate the powerful, but far less efficient, Type II fibers.
The podcast discusses a second study that perfectly illustrates this process:
Condition 1: Constant Work Rate Test (e.g., a 3-minute max effort at a fixed power).
VO2 Response: Oxygen uptake starts relatively low and climbs steadily, only reaching VO2max near the very end of the effort. This gradual climb is the VO2 slow component.
EMG Response: Electromyography (EMG), which measures electrical activity in the muscle, shows a progressive increase throughout the trial. This is direct evidence that the brain is recruiting more and more motor units just to maintain the same power output as the initial fibers fatigue.
Condition 2: All-Out Test (e.g., a 3-minute effort starting with a maximal sprint).
VO2 Response: Because the initial sprint demands maximal force, the brain immediately recruits all available motor units (both Type I and Type II). This creates a massive initial oxygen demand, and VO2max is reached very quickly (within ~60-70 seconds) and then sustained.
EMG Response: The EMG signal is highest at the start and decreases over time as the athlete fatigues and power output drops.
The Mechanism of the Slow Component: During a sustained, hard effort, your efficient Type I fibers begin to fatigue. To maintain the required power output, your brain follows the Size Principle and recruits additional, larger motor units (Type IIa and IIx). These fibers are less efficient; they require more ATP and thus more oxygen to produce the same amount of force. This recruitment of less efficient fibers to compensate for fatiguing efficient ones is what drives the slow, upward drift in your total oxygen consumption.
Understanding this physiology directly impacts how we train and interpret performance data.
Heart Rate Decoupling (Cardiac Drift): The gradual rise in heart rate during a long, steady-state effort, even when power is constant, is often a proxy for the VO2 slow component. As your body requires more oxygen to do the same work, your heart must beat faster to deliver it. This is not, as is often misunderstood, a sign that you are “going anaerobic.” Rather, it’s a sign that you are fatiguing and recruiting less efficient fibers.
Training to Improve Endurance: Shying away from efforts where heart rate decouples means you are never fully stressing and stimulating adaptation in those larger, less-efficient motor units. To improve your fatigue resistance, you must perform work that induces the slow component, forcing those Type II fibers to become more efficient and fatigue-resistant over time.
The Nature of FTP and Threshold: Even at your Functional Threshold Power (FTP), your metabolic state is not truly “steady.” The podcast cites a study showing that during a 60-minute effort at MLSS (Maximal Lactate Steady State), VO2 increased by a substantial ~11%. This demonstrates that even at “threshold,” the slow component is active as you continuously recruit more fibers to sustain the effort.
In summary, the VO2 slow component is a window into the dynamic process of muscular fatigue and recruitment. It is not an anomaly but a fundamental feature of sustained, high-intensity exercise, driven by the necessary trade-off between the body’s highly efficient Type I fibers and its powerful but costly Type II fibers.