The Energetics of Endurance: ATP Mastery for the Ultrarunner
At its core, endurance is not simply a trait—it's a metabolic capacity. When we speak of performance in ultrarunning, we’re talking about the efficiency and resilience of the body’s energy systems, measured not by how “fit” you look, but by how effectively your physiology converts stored fuel into mechanical work. This is the true meaning of energetics—and it all comes down to ATP mastery.
Metabolism: Fueling Physical Work
Metabolism is the biological engine that transfers energy from fuel (fat, carbohydrate, sometimes protein) into usable force. That usable energy is adenosine triphosphate (ATP)—the universal energy currency of human movement. Whether you're jogging a gentle downhill or grinding up the final climb of a mountain ultra, your ability to sustain output is governed by how efficiently your body regenerates ATP.
There are two principal metabolic pathways in endurance:
In ultrarunning, success depends overwhelmingly on your aerobic system. Why? Because the aerobic engine is your big motor, capable of powering you for hours, even days, without faltering--if you train it correctly.
The Triad of Endurance Performance
Three key physiological factors determine endurance performance:
These three domains interact, but all of them rely on ATP supply. No matter how good your form is or how tough you are mentally, if you can’t generate ATP efficiently, performance will collapse.
Lactate Dynamics: The Key to Resilience
Lactate is not the villain. It’s a marker—a signal of energy stress. In fact, trained athletes recycle lactate as a high-octane fuel. What matters isn’t the presence of lactate, but how well your system buffers, clears, and reuses it. That’s why lactate threshold training—done intelligently—is so critical. It teaches the body to operate at higher intensities without falling off the metabolic cliff.
The better your lactate clearance, the more aerobic power you can bring to the table. You become metabolically resilient—capable of holding higher intensities for longer, with less cost.
ATP Mastery: The Real Game of Endurance
Whether we’re talking about fat adaptation, zone 2 training, long steady-state runs, or high-intensity interval blocks—it's all part of the same story: increasing your ability to produce and regenerate ATP.
More ATP = more power, more endurance, and more durability.
This is not just about physiology. It’s strategy. At Energetics.studio, we train athletes to master their ATP systems. That means developing aerobic capacity as the foundation, tuning lactate dynamics as the bridge, and then layering in event-specific demands like terrain, altitude, and heat.
Technique and Movement Economy: The Hidden Multiplier
Efficiency is free speed. Running economy—how much oxygen you need to run at a given pace—can be the difference between finishing strong and breaking down. Good technique minimizes energy leaks. It aligns biomechanics, reduces muscular fatigue, and improves oxygen utilization.
Drills, strength training, barefoot work, and gait retraining all feed this domain. You don’t have to look pretty to be efficient—but you do have to be economical to endure.
VO₂ Max and the Breath: Lessons from the Freediving World
We now know VO₂ Max isn’t just genetics. You can train it. One of the most underused tools? Breathwork.
Adaptations from the freediving community—such as CO₂ tolerance tables, apnea walks, and slow exhalation training—can improve breathing efficiency, respiratory muscle strength, and even hemoglobin oxygen saturation. These methods can subtly but powerfully shift your performance ceiling upward, especially when paired with targeted high-intensity aerobic work.
Fueling the Fire: Fat Adaptation and Flexibility
An ultrarunner’s body must know how to burn fat. Fat stores are nearly unlimited compared to carbohydrates, but they burn slower and require a well-trained aerobic system. Strategic low-glycogen training, fasted long runs, and careful dietary periodization can increase metabolic flexibility—the ability to switch efficiently between fuel sources depending on demand.
But beware: fat adaptation is not a silver bullet. It works only when built on a foundation of robust aerobic capacity.
The Foundational Truth: Aerobic First, Always
You can’t cheat physiology. The single most limiting factor for most ultrarunners isn’t grit, mindset, or race-day nutrition—it’s an underdeveloped aerobic base.
No athlete will ever reach their endurance potential without maximising basic aerobic capacity first. It is the cornerstone of all performance. Build the big engine. Then refine the system. That’s the roadmap.
At energetics.studio, I train ultrarunners to understand and own their physiology. This isn’t just about logging miles—it’s about strategic adaptation, grounded in science, and focused on the only metric that really matters: usable energy over time.
Endurance is earned. ATP is the currency. Let’s master it.
- JD Hixson
Metabolism: Fueling Physical Work
Metabolism is the biological engine that transfers energy from fuel (fat, carbohydrate, sometimes protein) into usable force. That usable energy is adenosine triphosphate (ATP)—the universal energy currency of human movement. Whether you're jogging a gentle downhill or grinding up the final climb of a mountain ultra, your ability to sustain output is governed by how efficiently your body regenerates ATP.
There are two principal metabolic pathways in endurance:
- Aerobic metabolism, which relies on oxygen to generate a steady, sustainable supply of ATP, primarily from fat and carbohydrate.
- Anaerobic metabolism, which kicks in when energy demands exceed oxygen availability—generating ATP rapidly, but inefficiently, with lactate as a byproduct.
In ultrarunning, success depends overwhelmingly on your aerobic system. Why? Because the aerobic engine is your big motor, capable of powering you for hours, even days, without faltering--if you train it correctly.
The Triad of Endurance Performance
Three key physiological factors determine endurance performance:
- VO₂ Max – your maximal oxygen uptake. Think of this as the size of your engine.
- Lactate Threshold – how much of that engine you can use before fatiguing. This is your sustainable output limit.
- Movement Economy – how efficiently you turn energy into forward motion. In other words: form, biomechanics, and technique.
These three domains interact, but all of them rely on ATP supply. No matter how good your form is or how tough you are mentally, if you can’t generate ATP efficiently, performance will collapse.
Lactate Dynamics: The Key to Resilience
Lactate is not the villain. It’s a marker—a signal of energy stress. In fact, trained athletes recycle lactate as a high-octane fuel. What matters isn’t the presence of lactate, but how well your system buffers, clears, and reuses it. That’s why lactate threshold training—done intelligently—is so critical. It teaches the body to operate at higher intensities without falling off the metabolic cliff.
The better your lactate clearance, the more aerobic power you can bring to the table. You become metabolically resilient—capable of holding higher intensities for longer, with less cost.
ATP Mastery: The Real Game of Endurance
Whether we’re talking about fat adaptation, zone 2 training, long steady-state runs, or high-intensity interval blocks—it's all part of the same story: increasing your ability to produce and regenerate ATP.
More ATP = more power, more endurance, and more durability.
This is not just about physiology. It’s strategy. At Energetics.studio, we train athletes to master their ATP systems. That means developing aerobic capacity as the foundation, tuning lactate dynamics as the bridge, and then layering in event-specific demands like terrain, altitude, and heat.
Technique and Movement Economy: The Hidden Multiplier
Efficiency is free speed. Running economy—how much oxygen you need to run at a given pace—can be the difference between finishing strong and breaking down. Good technique minimizes energy leaks. It aligns biomechanics, reduces muscular fatigue, and improves oxygen utilization.
Drills, strength training, barefoot work, and gait retraining all feed this domain. You don’t have to look pretty to be efficient—but you do have to be economical to endure.
VO₂ Max and the Breath: Lessons from the Freediving World
We now know VO₂ Max isn’t just genetics. You can train it. One of the most underused tools? Breathwork.
Adaptations from the freediving community—such as CO₂ tolerance tables, apnea walks, and slow exhalation training—can improve breathing efficiency, respiratory muscle strength, and even hemoglobin oxygen saturation. These methods can subtly but powerfully shift your performance ceiling upward, especially when paired with targeted high-intensity aerobic work.
Fueling the Fire: Fat Adaptation and Flexibility
An ultrarunner’s body must know how to burn fat. Fat stores are nearly unlimited compared to carbohydrates, but they burn slower and require a well-trained aerobic system. Strategic low-glycogen training, fasted long runs, and careful dietary periodization can increase metabolic flexibility—the ability to switch efficiently between fuel sources depending on demand.
But beware: fat adaptation is not a silver bullet. It works only when built on a foundation of robust aerobic capacity.
The Foundational Truth: Aerobic First, Always
You can’t cheat physiology. The single most limiting factor for most ultrarunners isn’t grit, mindset, or race-day nutrition—it’s an underdeveloped aerobic base.
No athlete will ever reach their endurance potential without maximising basic aerobic capacity first. It is the cornerstone of all performance. Build the big engine. Then refine the system. That’s the roadmap.
At energetics.studio, I train ultrarunners to understand and own their physiology. This isn’t just about logging miles—it’s about strategic adaptation, grounded in science, and focused on the only metric that really matters: usable energy over time.
Endurance is earned. ATP is the currency. Let’s master it.
- JD Hixson