Original episode & show notes | Raw transcript
The debate over whether to pace endurance rides by power, heart rate, or feel is more than just a matter of preference. As the podcast explains, the choice you make fundamentally alters the training stimulus you receive. Understanding the “why” behind this requires a look at how our muscles function, fatigue, and adapt over time.
The foundational workout for any endurance cyclist is the “endurance” or “base” ride. The traditional wisdom is that these long, slow rides build a wide “aerobic base,” which is necessary to support higher-intensity work later on. With modern tools, the question becomes: what is the best way to measure and control the intensity of these rides?
The three main methods are:
Power (Watts): An objective measure of the work you are performing on the bike.
Heart Rate (BPM): A measure of your body’s internal physiological response to the work.
Rate of Perceived Exertion (RPE): Your subjective feeling of the effort.
A core concept that the podcast challenges is the idea of a physiological “steady state.” Old-school exercise physiology textbooks often suggest that below a certain intensity (like your FTP or lactate threshold), your body can operate like a machine in a stable, unchanging state for hours.
However, anyone who has done a long ride knows this isn’t true. The podcast provides a clear example: an athlete holds a steady power output for six hours, but his heart rate drifts upwards by 16 beats per minute from the first hour to the last. This phenomenon is known as cardiac drift or decoupling. This simple observation proves that even at a constant external workload, your internal state is anything but steady. The key to understanding how to pace is understanding why this drift happens.
The primary driver of cardiac drift during a long endurance ride is a concept called Henneman’s Size Principle.
Motor Units: Your muscles aren’t one big sheet of tissue. They are organized into motor units, each consisting of a single nerve and the bundle of muscle fibers it controls. When the nerve fires, all the fibers in that unit contract in an “all-or-none” fashion.
Recruitment by Size: Your brain recruits these motor units in a specific order: from smallest to largest.
Small Motor Units: These are recruited first for low-intensity efforts. They are typically composed of slow-twitch (Type I) muscle fibers, which are highly fatigue-resistant and very efficient.
Large Motor Units: These are recruited as the demand for force increases. They are composed of more fast-twitch (Type II) muscle fibers, which are powerful but fatigue quickly and are less efficient.
The Process During an Endurance Ride:
Start of the Ride: You begin your ride at an endurance pace. Your brain recruits the smallest, most efficient slow-twitch motor units to do the work. Everything feels easy.
Fatigue Sets In: After an hour or more, these initially recruited motor units begin to fatigue. The primary cause of this fatigue is the depletion of their internal fuel source, glycogen.
Recruitment Continues: To maintain the same power output (e.g., 200 watts), your brain must compensate for the fatiguing fibers. It sends a stronger signal and begins to recruit larger, less-efficient, fast-twitch motor units that were previously dormant.
The Efficiency Cost: These newly recruited fast-twitch fibers are less efficient. Efficiency is the measure of how much work (watts) you get for a given amount of energy input (oxygen). Because fast-twitch fibers require more oxygen to produce the same amount of power as slow-twitch fibers, your body’s overall oxygen demand increases.
The Result is Cardiac Drift: To deliver this extra oxygen to the working muscles, your heart has to beat faster. Your breathing rate also increases. This is the physiological explanation for why your heart rate drifts upwards even when your power is constant.
Key Evidence: The podcast references a classic study by Dr. Ed Coyle where cyclists with a high percentage of slow-twitch fibers (e.g., 83%) produced significantly more power (around 9% more) for the exact same level of oxygen consumption as athletes with a low percentage of slow-twitch fibers (e.g., 46%). This directly links fiber type to cycling efficiency.
Now, let’s look at the pacing methods through the lens of the size principle.
If You Pace by Power: You hold your watts steady for the entire ride. As your initial motor units fatigue, you force your body to recruit those larger, less-aerobically trained fast-twitch motor units to help out. In doing so, you are actively training them to become more fatigue-resistant and efficient. This provides a progressive training stimulus within the ride itself.
If You Pace by Heart Rate: You hold your heart rate steady. As your initial motor units fatigue and your body’s oxygen demand starts to rise, the only way to keep your heart rate from increasing is to decrease your power output. You are essentially telling your body, “Don’t recruit those larger, less-efficient motor units.” You are only training the most fatigue-resistant fibers and not challenging the others to improve. The training stimulus actually decreases over the course of the ride.
While pacing by power appears superior to pacing by heart rate, the podcast argues for a third, more nuanced approach: pacing by feel (RPE).
Riding to power is objective, but it doesn’t account for your body’s day-to-day readiness. Stress, poor sleep, or inadequate nutrition can mean that a “Zone 2” power target is excessively hard on a given day. Conversely, on a day you feel great, you might be able to handle more.
Riding by RPE allows for autoregulation. The guideline is to ride at a pace that “feels like you’re working, but you could do it all day.”
Benefits of RPE:
It’s Holistic: It integrates all factors—muscle fatigue, mental state, daily stress—into one feeling.
It Prevents Burnout: It avoids the “neurosis” of chasing specific numbers, which can be mentally draining and lead to overtraining.
It Develops Body Awareness: It is a skill that makes you a better, more intuitive athlete. You learn to listen to your body’s signals.
The ultimate goal is to use your power meter and heart rate monitor as tools to calibrate your RPE. You learn what a sustainable effort feels like, and you can trust that feeling, whether the numbers are slightly higher or lower than usual on a particular day.
Based on the physiology, we can create a hierarchy for pacing endurance rides:
Gold Standard (Best): Pace by Perceived Exertion. This method allows for autoregulation, develops crucial athletic intuition, and provides a sustainable, holistic training stimulus. Use power and heart rate as background data to learn and confirm what you are feeling.
Silver Standard (Good): Pace by Power. This ensures a consistent external workload and forces the recruitment of a wider range of muscle fibers, providing an excellent training stimulus. This is far superior to pacing by heart rate.
Bronze Standard (Acceptable): Pace by Heart Rate. While better than nothing, this method can lead to a decreasing power output and a less effective training stimulus over the course of a long ride as you fatigue.
By understanding the interplay between muscle fatigue, motor unit recruitment, and efficiency, you can move beyond simple zones and make more intelligent decisions about your training.