Your body has three energy systems that power every workout. Understanding how they work and how to train them is the difference between randomly suffering and strategically building your engine.
Think of these systems like gears in a car. You don’t use fifth gear to accelerate from a stoplight, and you don’t redline first gear on the highway. Your body shifts between energy systems the same way, seamlessly adjusting based on how hard you’re working and for how long.
All three systems work simultaneously, but their contribution shifts based on intensity and duration. Here’s how they actually work.
The Three Energy Systems
1. ATP-PC System (Phosphagen System)
Time domain: 0-10 seconds of maximal effort
Fuel source: Phosphocreatine stored in muscles
What it powers: Max lifts, short sprints, explosive jumps. Anything that requires immediate, explosive power.
How it works: ATP (adenosine triphosphate) is the actual currency your muscles spend to contract. You have a tiny reserve stored directly in muscle cells, enough for about 2-3 seconds of all-out effort. Phosphocreatine (PCr) sits in reserve, rapidly regenerating ATP to extend that window to roughly 10 seconds before it runs dry.
This is why you can sprint full speed for 40 meters but not 400 meters. By the 10-second mark, you’re tapped out and your body shifts gears.
Examples: 1RM back squat, 40m sprint, max height box jump, heavy clean and jerk.
Recovery: 3-5 minutes for full PCr restoration. This is why you rest several minutes between max effort lifts. Cut that rest short and you’re not training maximum strength anymore.
How to train it: Heavy lifts (1-3 reps), short explosive sprints (10-30m), Olympic lifts, plyometrics. The key is quality over volume and full rest between efforts.
2. Glycolytic System (Lactic Acid System)
Time domain: 10 seconds to 2-3 minutes of high intensity
Fuel source: Glucose and glycogen (stored carbohydrate)
What it powers: The brutal middle ground. Hard efforts that last longer than a sprint but shorter than steady cardio. This is where most people suffer.
How it works: When you’re working too hard for the aerobic system to keep up but your ATP-PC system is already spent, glycolysis kicks in. It breaks down glucose without oxygen (anaerobic), producing ATP fast enough to sustain high-intensity work. The problem? It also generates lactate and hydrogen ions as byproducts. That burning sensation in your legs during a hard 400m run or halfway through Fran? That’s your glycolytic system at work.
Your body can buffer these metabolites to some degree, but there’s a limit. Push past it and you’re forced to slow down or stop.
Examples: 400-800m runs at hard pace, 90-second max calorie row, Fran (21-15-9 thrusters and pull-ups), any metcon in the 2-5 minute range done at high intensity.
Recovery: Partial recovery in 30-90 seconds with active rest, full recovery takes 10-30 minutes depending on how deep you went.
How to train it: Intervals 30 seconds to 3 minutes at high intensity with short rest. Think 8x400m with 90 seconds rest, 5×2 minute max effort row with 2 minutes rest, or classic CrossFit benchmarks. The goal is to accumulate lactate, learn to work through the discomfort, and improve your body’s ability to buffer it.
3. Oxidative System (Aerobic System)
Time domain: Anything longer than 2-3 minutes
Fuel source: Carbohydrates and fats, using oxygen
What it powers: Long sustained efforts, recovery between hard intervals, baseline energy production during daily life.
How it works: This is your endurance engine. The oxidative system uses oxygen to break down carbs and fats into ATP. It’s slower than the other two systems, but it has nearly unlimited capacity as long as you have oxygen and fuel available. You can run a marathon because your aerobic system can churn out energy for hours.
But here’s what most people miss: the aerobic system isn’t just for endurance athletes. It’s the foundation for everything.
Why it matters for everyone: Even if you never run a marathon, the aerobic system determines:
- How quickly you recover between hard efforts in a metcon
- How long you can sustain work in 15-20 minute workouts
- Your ability to repeat high-intensity efforts without completely falling apart
- How fast you recover between training sessions
A weak aerobic base means every workout feels harder than it should, you gas out early in longer efforts, and you need excessive rest between rounds.
Examples: Long runs (30+ minutes), 2k+ row at moderate pace, 20+ minute AMRAPs at controlled pace, recovery jogging between sprint intervals.
How to train it:
- Long slow distance: 30-90 minute efforts at conversational pace. Builds mitochondrial density and teaches your body to burn fat.
- Tempo work: 20-40 minutes at comfortably hard pace. Improves your lactate threshold.
- Aerobic intervals: Longer efforts (3-8 minutes) at moderate-high intensity with short rest. Example: 5x1000m row with 90 seconds rest.
How Energy Systems Work Together
Here’s the critical part most people miss: these systems don’t operate in isolation like separate compartments. They overlap and contribute simultaneously, with the dominant system shifting based on what you’re doing.
Example: Fran (21-15-9 Thrusters and Pull-Ups)
First 30 seconds: ATP-PC system is dominant. You’re moving fast, reps feel relatively smooth.
30 seconds to 2 minutes: Glycolytic system takes over. The burn intensifies. Your breathing gets ragged. This is the pain cave.
2+ minutes (if you’re slower): Aerobic system starts contributing more. You’re forced to pace, break up sets differently.
Throughout the entire workout: Your aerobic system is working in the background, clearing lactate and providing baseline energy even during the hardest moments.
Example 2: 10x400m Repeats with 90 Seconds Rest
Each 400m effort: Glycolytic system dominant with aerobic support. You’re accumulating lactate with each round.
90-second rest periods: Aerobic system working overtime to clear lactate and partially restore ATP-PC stores.
As reps accumulate: Your glycolytic capacity decreases (you can’t hit the same pace), aerobic contribution increases (you’re relying more on oxygen), and mental toughness becomes the deciding factor.
Understanding this interplay lets you pace intelligently instead of blowing up halfway through.
Training All Three Systems
Most people make one of two mistakes: only training what they’re good at (aerobic athletes avoid intensity, CrossFitters avoid long slow work), or training randomly without understanding which system they’re actually developing.
Weekly training distribution for well-rounded fitness:
- 60-70% aerobic work (long slow efforts, tempo runs, aerobic intervals)
- 20-30% glycolytic work (hard intervals, classic metcons)
- 10% ATP-PC work (heavy lifts, short sprints, max efforts)
Practical examples by system:
ATP-PC development:
- Heavy back squats: 5×3 with 3-4 minutes rest
- Sprint intervals: 6x40m with full recovery (walk back)
- Max effort sled pushes: 8x20m, rest 2-3 minutes between
Glycolytic development:
- 8x400m run at hard pace, 90 seconds rest
- 5 rounds: 500m row at very hard pace, 2 minutes rest
- EMOM 10: 12 cal assault bike (should take 30-40 seconds, leaving 20-30 seconds rest)
- Classic metcons: Fran, Grace, Isabel
Aerobic development:
- 45-60 minute easy run at conversational pace
- 30 minute continuous row at moderate effort
- 3x2000m row at threshold pace, 3 minutes rest
- 40-minute AMRAP at sustainable pace
Common Training Mistakes
Mistake 1: No true aerobic work
Going hard all the time trains your glycolytic system but neglects your aerobic base. You become good at suffering for 5-10 minutes but fall apart in longer efforts and recover poorly between sessions. Your engine has no foundation.
Fix: Add 2-3 true aerobic sessions per week. Long, slow, boring work at conversational pace.
Mistake 2: Never going truly hard
Always working at moderate intensity (the “grey zone”) doesn’t fully stress any system. You’re too hard to build aerobic capacity efficiently and too easy to develop glycolytic power. You end up mediocre at everything.
Fix: Polarize your training. Easy days truly easy (can hold a conversation). Hard days truly hard (can barely breathe).
Mistake 3: Insufficient rest for ATP-PC work
Doing max effort lifts or sprints with inadequate rest trains the wrong system. If you’re only resting 60-90 seconds between heavy squats, you’re not training max strength anymore. You’re training muscular endurance with heavy weights.
Fix: If the goal is maximum strength or power, rest 3-5 minutes. If that feels too long, you’re ego lifting.
Mistake 4: Random workout selection
Picking workouts based on how you feel or what looks fun creates imbalanced development. You end up with gaps in your fitness that show up when it matters.
Fix: Program with intention. Track which energy systems you’re training each week and make sure nothing gets neglected.
Simple Guide: What You’re Training
Use effort duration and intensity to know what system you’re developing:
- 1-10 seconds, max effort = ATP-PC
- 30 seconds to 3 minutes, very hard = Glycolytic
- 3-10 minutes, hard to moderate = Glycolytic + Aerobic mix
- 10+ minutes, moderate = Aerobic dominant
- 30+ minutes, easy to moderate = Pure aerobic base
The Bottom Line
All three energy systems matter. Your body uses them in combination during nearly every workout you do. The best athletes aren’t the ones who only train what they’re good at. They’re the ones who strategically develop all three systems.
Build your aerobic base with long, easy work. It’s boring but it’s the foundation everything else sits on. Develop your glycolytic capacity with hard intervals and classic metcons where you learn to suffer productively. Maintain your ATP-PC system with heavy lifts and short sprints so you can generate power when it counts.
Stop picking workouts randomly. Track what you’re doing. Make sure you’re not neglecting any system.
Want structured programming that develops all three energy systems with intelligent progression? The Complete Endurance Program provides 12 weeks of aerobic development across running, rowing, biking, and skiing.
