Original episode & show notes | Raw transcript
Endurance performance is a complex interplay of numerous physiological factors, but at its core lies the remarkable capacity of our muscles to sustain work over long periods. This ability is inextricably linked to our mitochondria—the microscopic powerhouses within our cells. This document explores the science behind how we build endurance, focusing on the critical relationship between training volume (how much we train) and intensity (how hard we train).
Drawing from a detailed meta-analysis of scientific literature, we will dissect the mechanisms of mitochondrial adaptation. We will explore how scientists measure these changes, what the data says about the most effective training stimuli, and how these scientific principles translate into practical, actionable training advice for athletes seeking to maximize their endurance potential.
Before examining training effects, it’s essential to understand what we are trying to change. The primary target of endurance training at the muscular level is our mitochondrial network.
A common misconception, born from textbook diagrams, is that mitochondria are simply individual, bean-shaped organelles floating in the cell. In reality, they form a vast, interconnected network known as the mitochondrial reticulum. Imagine it like the root system of a tree, branching and connecting throughout the muscle cell.
This network structure is crucial. It allows for efficient sharing of energy, metabolites, and signaling molecules across the cell. When we talk about “building more mitochondria,” we are really talking about expanding and increasing the density of this entire network. This expansion enhances the muscle’s ability to:
Produce ATP (Energy): More mitochondria mean a greater capacity for aerobic respiration, the incredibly efficient process of generating ATP from fats and carbohydrates in the presence of oxygen.
Maintain Cellular Energy State: A robust mitochondrial network acts as an energy buffer, preventing the dramatic drops in ATP that lead to fatigue. This is a key factor in sparing muscle glycogen.
Improve Recovery: Mitochondria are central to biosynthesis and cellular repair, meaning a denser network can help you recover faster, both between hard efforts and between training sessions.
To study how training impacts mitochondria, scientists use several methods, each with its own strengths and weaknesses.
Transmission Electron Microscopy (TEM): This is considered the gold standard for measuring mitochondrial volume. It uses electron beams to create high-resolution, cross-sectional images of muscle tissue, allowing researchers to directly quantify the space mitochondria occupy. However, TEM is extremely expensive, time-consuming, and technically demanding, so very few studies use it.
Citrate Synthase (CS) Activity: This is the most common method. Citrate synthase is a key enzyme at the entry point of the Krebs cycle, a fundamental process within the mitochondria. Scientists take a muscle biopsy, homogenize it (blend it into a “muscle margarita”), and measure the total activity of this enzyme. This measurement is used as a proxy for overall mitochondrial content. While practical, it doesn’t distinguish between different parts of the mitochondrial network or different types of muscle fibers.
Permeabilized Muscle Fibers (Mass-Specific Respiration): This technique measures mitochondrial respiratory function. Instead of blending the muscle, scientists use a chemical (saponin) to poke holes in the muscle fiber’s outer membrane, leaving the internal structures, including the mitochondria, largely intact. They can then supply the mitochondria with unlimited fuel and oxygen to measure their maximum rate of oxygen consumption (respiration). This tells us how “powerful” the existing mitochondria are, rather than just how many there are.
The central question is: what type of training is best for expanding our mitochondrial reticulum? A landmark 2018 meta-analysis, Training-Induced Changes in Mitochondrial Content and Respiratory Function in Human Skeletal Muscle, aggregated data from dozens of studies to provide a clear answer.
To compare different studies, the researchers standardized two key variables:
Intensity: Expressed as a percentage of Wmax (the maximal power achieved in a ramp test to exhaustion).
Volume: Calculated as a single value incorporating intensity, duration, and frequency of training sessions.
When the researchers analyzed the data from 56 studies measuring citrate synthase activity, the results were striking.
A Strong Positive Correlation with Volume: There was a clear, strong relationship between higher total training volumes and greater increases in citrate synthase activity. The more athletes trained, the more mitochondrial content they built. The data showed no sign of a plateau, suggesting that for building this foundational endurance adaptation, more is always better, up to the limits of recovery.
No Correlation with Intensity: In stark contrast, when looking at training intensity (from ~45% to 90% of Wmax), there was no correlation with the change in citrate synthase. The graph was flat. This means that, for a given volume, riding at a lower intensity (e.g., 60% of Wmax) produced, on average, the same mitochondrial adaptation as riding at a much higher intensity (e.g., 85% of Wmax).
The primary driver for increasing mitochondrial content is the total amount of work done, not the intensity at which it is performed.
The story changes when looking at mitochondrial function (mass-specific respiration).
The reason for this is not definitively known, but there are two leading theories:
Motor Unit Specificity: Sprints recruit the largest, most powerful fast-twitch muscle fibers, which are less active during lower-intensity exercise. The intense stimulus may trigger a powerful adaptive response specifically within these fibers.
Measurement Bias: The podcast host proposes that the permeabilized fiber technique may favor the mitochondria located just under the cell’s surface (subsarcolemmal mitochondria). These mitochondria are critical for the rapid energy turnover needed for sprinting, and it’s plausible they adapt more robustly to that specific stimulus.
This finding demonstrates that while volume drives the quantity of mitochondria, high intensity drives the quality or specific function in certain contexts.
This science directly informs a more intelligent approach to training.
Principle 1: Volume is the Foundation. The single most important factor for building deep, resilient muscular endurance is your total training volume. The intensity of these endurance rides is a secondary concern. This supports the wisdom of building a large base of training hours.
Principle 2: Fatigue Management Dictates Intensity. If volume is king, why not do all your volume at a high intensity to “get more”? Because of fatigue. The data shows no significant extra mitochondrial benefit from riding your endurance miles at a high tempo, but the physiological cost is much greater. This added fatigue will compromise your ability to execute the high-intensity interval sessions that are essential for improving other aspects of your fitness (e.g., VO2 max, threshold power).
Principle 3: Intervals are for Specificity, Not Volume. High-intensity training is not a shortcut to building a deep endurance base. Its purpose is to provide a specific, potent stimulus that lower-intensity riding cannot. Use interval sessions to target your top-end power, threshold, and anaerobic capacity, but understand that the majority of your mitochondrial adaptation comes from the total hours you spend on the bike.
The scientific evidence presents a clear picture. The development of a robust mitochondrial network—the engine of muscular endurance—is driven primarily by total training volume. The intensity of that volume is far less important. An effective training plan, therefore, is not about finding a “magic” intensity zone, but about intelligently managing fatigue. It involves:
Maximizing training volume at a low, sustainable intensity to build the largest possible mitochondrial base.
Strategically layering in high-quality, high-intensity intervals to drive top-end adaptations and improve performance.
By focusing on volume as the foundation and using intensity as a targeted tool, you can train smarter, manage fatigue more effectively, and unlock your ultimate endurance potential.