Original episode & show notes | Raw transcript
This document provides an in-depth exploration of the concepts discussed in the Empirical Cycling podcast regarding “Zone 2” training. The central theme is a critical examination of what makes this type of training special—or perhaps, what doesn’t. We will move beyond simple definitions to understand the underlying physiology and its practical application for athletes.
The first critical concept is to redefine what we mean by “Zone 2.” The term itself, popularized by power-based training models, can be a trap.
The Problem with Prescriptive Zones: When training zones are presented as rigid, prescriptive targets (e.g., 56-75% of FTP), many athletes make a critical error: they ride at the very top of that range. They treat it as a target to hit, believing that harder is always better. This leads to several problems:
Excessive Fatigue: Riding at the upper end of Zone 2 for hours is significantly more taxing than riding in the middle or lower end. This accumulated fatigue can compromise recovery and subsequent high-intensity workouts.
Burnout: An athlete who constantly pushes their endurance rides can feel wrecked, wondering how professionals manage 20-30 hour weeks when their own 4-hour “Zone 2” ride requires two days of recovery.
The “Endurance Riding” Mindset: The podcast advocates for shifting from thinking in terms of “Zone 2” to a broader philosophy of “endurance riding.” This is less about hitting a number and more about the intent of the ride. The primary metric is not a power number, but rather the Rate of Perceived Exertion (RPE).
The “Talk Test” as the Gold Standard: The most effective and practical way to gauge correct endurance pace is the talk test. You should be able to hold a full, comfortable conversation without gasping for breath in the middle of a sentence. Someone next to you should barely be able to tell you’re exercising.
What it is: A simple, qualitative measure of intensity.
Why it works: It naturally corresponds to a physiological state below the first lactate threshold (LT1), where the body is in a sustainable, aerobic state and fatigue accumulates very slowly.
Historical Context: This approach aligns with the traditional concept of “Long Slow Distance” (LSD), where “slow” is relative to race pace or interval intensity, not necessarily a slow speed.
The central question the podcast tackles is whether Zone 2 training provides unique physiological adaptations that cannot be achieved at other intensities. The answer, decisively, is no.
The Continuum of Aerobic Adaptation: Physiology does not operate like a series of light switches. There isn’t a magical point where you cross from Zone 1 to Zone 2 and a new set of adaptations suddenly turns on, only to be turned off when you enter Zone 3.
Instead, aerobic adaptations exist on a continuum. The same fundamental adaptations occur across a wide range of intensities, from easy endurance pace up to and including threshold (FTP). These adaptations include:
Increased mitochondrial density and function
Increased capillary density (improving oxygen delivery)
Increased plasma volume
Enhanced glycogen storage
Conversion of fast-twitch muscle fibers toward more aerobic, fatigue-resistant characteristics
The difference between riding at an endurance pace versus a tempo or sweet spot pace is not the type of adaptation, but the intensity of the adaptive signal and the rate of fatigue accumulation. A higher intensity provides a stronger signal per unit of time, but at a much greater physiological cost.
If Zone 2 doesn’t offer unique adaptations, what makes it so special and foundational to training? The answer lies in its exceptional stimulus-to-fatigue ratio.
The Superpower: Low Fatigue Accumulation The single most important characteristic of correctly-paced endurance riding is that it generates a valuable aerobic stimulus with very little associated fatigue. This is its superpower.
This low fatigue cost allows for the single most powerful driver of long-term fitness: high training volume.
The Formula for Profound Adaptation: The relationship can be understood as follows:
Low Fatigue → Ability to Sustain High Volume → Massive Cumulative Stimulus → Profound and Durable Aerobic Adaptation
You are physically limited in how many high-intensity (threshold, VO2 max) sessions you can perform and recover from in a week. However, because endurance riding is not very fatiguing, you can accumulate many hours of it. A 25-hour training week will drive adaptations that a 10-hour, high-intensity week simply cannot replicate, especially over the long term.
Understanding the “why” allows us to correctly apply endurance riding in a training plan.
For High-Volume and Developing Athletes: Endurance riding should be the bedrock of the training plan. The ability to handle large volumes of work is what builds a deep and resilient aerobic base, which in turn supports higher-intensity training.
For Time-Crunched Athletes: If an athlete is limited to a few short rides per week (e.g., three 2-hour sessions), they are not missing out by prioritizing intensity (e.g., threshold, sweet spot) over pure endurance riding. In this scenario, the total volume is too low for endurance riding to provide a sufficient stimulus. Intensity becomes necessary to achieve an adequate training dose. The aerobic adaptations are still being triggered.
For Recovery and Fitness Maintenance: Endurance riding is an invaluable tool for:
Active Recovery: On days when you are too fatigued for structured intervals but feel good enough to ride, an easy endurance ride promotes blood flow and recovery without adding significant stress.
Fitness Maintenance: During intense racing blocks or periods of high fatigue, endurance riding is remarkably effective at maintaining threshold and VO2 max fitness. It allows the body to recover from high-intensity stress while preventing a decline in aerobic capacity. Taking the time off entirely would lead to detraining, whereas endurance riding maintains the foundation.
Pitfall: Riding Your “Zone 2” Too Hard.
The Cause: Misinterpreting training zones or relying on flawed physiological testing (e.g., defining LT1 as a fixed 2.0 mmol/L blood lactate level, which is often too high).
The Solution: Prioritize the talk test and RPE. If in doubt, ride easier. The podcast hosts state they would congratulate an athlete for riding 20-30 watts easier than they thought they should, as there is no harm in it.
Pitfall: The Fear of “Going Too Easy.”
The Cause: A belief that training below a certain intensity threshold yields zero benefit.
The Solution: Understand that the lower limit for beneficial aerobic adaptation is much lower than most athletes assume. Consistent pedaling time is key. An hour of very easy riding is vastly superior to an hour on the couch.
Pitfall: Over-quantifying with the Wrong Metrics.
The Cause: Focusing on metrics like “time-in-zone” or “total weekly kilojoules from Zone 2.”
The Solution: Use total weekly riding time (volume) as the primary benchmark for training load. Kilojoules can be misleading because they increase disproportionately with intensity. Total hours provides a more holistic and accurate view of the work being done to build the aerobic engine.
Henneman’s Size Principle: This principle states that motor units are recruited in order of their size, from small (slow-twitch, endurance fibers) to large (fast-twitch, power fibers), as the demand for force increases. This means that when you are riding at threshold, you are still recruiting and training all of the slow-twitch fibers used during an endurance ride. You are not “missing” them by going harder.
Fatigue-Induced Recruitment: During a very long endurance ride, as your highly efficient slow-twitch fibers begin to fatigue, your nervous system must recruit larger, less efficient fast-twitch fibers to maintain the same power output. This is a key mechanism by which long-duration, low-intensity exercise can provide a training stimulus to a wider range of muscle fibers.
The special nature of endurance riding is not found in a unique, magical adaptation that only occurs within a specific power range. Its special quality is practical, not mystical.
Endurance training is special because its low fatigue cost allows for the accumulation of high training volume. It is this volume that serves as the most powerful long-term driver of aerobic fitness. It builds the foundation upon which high-intensity performance is built and provides a versatile tool for recovery and fitness maintenance. The key is to embrace the philosophy: ride by feel, prioritize consistency, and when in doubt, ride easier to allow for more volume tomorrow.