Stop Training Like You Have One Speed: Energy Systems Explained
You know that person at your gym who does everything at maximum intensity? Sprints through their warmup, crushes themselves during squats, then immediately jumps into a burpee heavy metcon and wonders why they feel like garbage after 30 seconds?
That’s what happens when you don’t understand how your body actually produces energy.
Meanwhile, the person next to them is systematically working through different intensities, looking fresh throughout their entire workout, and somehow getting better results with what appears to be less effort.
The difference? They understand that your body has three distinct energy systems, each designed for specific types of work. And mixing them randomly isn’t training smart. It’s training yourself into the ground.
The Problem: Everyone Trains Like They Have Two Settings
Walk into any gym and you’ll see the same mistake repeated endlessly: people treating their body like it only has two settings: barely moving and completely redlined.
They either cruise through their warmup sets at 20% effort or attack every single exercise like it’s a one minute all-out sprint. There’s no in between. No strategy. No understanding that different intensities serve different purposes.
It’s like watching someone drive across the country using only first gear and fifth gear. Sure, you’ll eventually get there, but you’re going to burn out your engine (and yourself) in the process.
Here’s what they don’t understand: your body operates on a spectrum of intensities, each designed for specific types of work. Each intensity targets different energy systems, creates different adaptations, and requires different recovery strategies.
Mixing them randomly isn’t smart training. It’s a recipe for burnout.
Energy System 1: The ATP-PC System (Your Sprint Speed)
This is your body’s top end speed. It provides immediate, explosive energy for maximum intensity efforts lasting up to 10 to 15 seconds.
The ATP-PC system provides energy for maximal intensity, short duration exercise for between 10 to 15 seconds before it fatigues, using stored adenosine triphosphate (ATP) and phosphocreatine (PC) in your muscles.
You’re using this system when:
- Sprinting 10 to 50 meters at maximum speed
- Performing 1 to 3 rep max lifts
- Explosive jumps or throws
- Short bike or rowing sprints (10 to 15 seconds)
- Heavy medicine ball slams
Key characteristics:
- Immediate energy production (no delay)
- No oxygen required
- No lactate production (no burning sensation)
- Burns out quickly and completely
How to train it properly: Use maximum effort with complete recovery. For the ATP-PC system the rest ratio is 1:10 to 12, meaning for every second of work you need 10 to 12 seconds for recovery.
Example session: 8 x 15 second bike sprints with 2.5 to 3 minutes complete rest between efforts.
What most people do wrong: They don’t rest long enough between efforts, turning ATP-PC training into glycolytic torture that defeats the purpose.
Energy System 2: The Glycolytic System (Your High Intensity Gear)
This is your sustained high speed setting. It kicks in when your efforts last from roughly 15 seconds up to 2 minutes at high intensity.
Maximum ATP regeneration capacity from glycolysis is achieved when a rate of work requiring an energy load greater than an individual’s maximum oxygen uptake is performed for as long as possible, which for the average trained athlete is between 2 to 3 minutes.
You’re using this system during:
- 200 to 800 meter runs at high intensity
- Barbell complexes lasting 60 to 90 seconds
- High rep lifting sets (15 to 25 reps)
- Sustained bodyweight circuits
- Those brutal one minute burpee tests
Key characteristics:
- Doesn’t require oxygen (anaerobic)
- Produces lactate as a byproduct (the burn)
- Can sustain high power output for 1 to 2 minutes
- Creates significant fatigue
How to train it properly: Embrace the discomfort. Work at intensities that create lactate buildup with incomplete recovery that keeps the system under stress.
Example session: 5 x 90 second bike intervals at 85 to 90% effort with 90 seconds active recovery.
What most people do wrong: They either go too easy (turning it into aerobic work) or too hard (burning out after 30 seconds and missing the glycolytic training zone).
Energy System 3: The Aerobic System (Your Cruise Control)
This is your sustainable pace setting. The aerobic system is the primary source of energy for activities lasting longer than 2 minutes, using oxygen to burn carbohydrates, fats, and sometimes proteins.
You’re using this system during:
- Steady runs, bikes, or rows lasting 20 plus minutes
- Zone 2 heart rate training
- Long, paced workouts
- Recovery between high intensity efforts
- Most of your daily activities
Key characteristics:
- Requires oxygen to function
- Can use multiple fuel sources (carbs, fats, proteins)
- Sustainable for hours when paced properly
- Supports recovery of the other two systems
How to train it properly: Most aerobic training should feel conversational. You could talk but wouldn’t want to sing. Continuous exercise at an intensity just below your lactate threshold, or roughly 70 to 75% maximum heart rate, is a great way to train this system.
Example session: 30 to 45 minute steady bike ride at 70% max heart rate.
What most people do wrong: They go too hard during easy sessions, turning aerobic work into glycolytic suffering that defeats the recovery purpose.
The Reality: All Systems Work Together (But One Dominates)
Here’s where most fitness advice gets it wrong: these systems do not operate in isolation. Rather, they work simultaneously and synergistically to meet the energy demands of various physical activities at any given time.
Think of it as a sliding scale rather than distinct categories:
0 to 15 seconds: ATP-PC dominates (roughly 95%), others contribute minimally
15 seconds to 2 minutes: Glycolytic dominates (roughly 70%), ATP-PC and aerobic support
2 plus minutes: Aerobic dominates (roughly 85%), others provide power for surges
The same exercise can train different systems depending on how you execute it:
5 minute row as all out sprint: Primarily glycolytic (and you’ll die)
5 minute row at steady pace: Primarily aerobic (sustainable)
15 second rowing sprints with full rest: Primarily ATP-PC (explosive)
Why Most People’s Training Is Backwards
The biggest mistake? Trying to train all systems simultaneously in every workout.
You see this everywhere:
CrossFit workouts that start with heavy lifting, then immediately jump into high intensity circuits
HIIT classes that never allow full recovery (so they’re really just glycolytic punishment)
People who think every workout needs to leave them gasping on the floor
The problem with this approach:
You can’t develop any system optimally when you’re randomly mixing intensities
You accumulate fatigue faster than you can recover from it
You miss the specific adaptations each system provides when trained properly
You burn out mentally and physically from constant high intensity chaos
How Energy Systems Apply to Different Training Goals
Here’s the truth: energy systems matter dramatically for some types of training and barely at all for others.
Where energy systems are critical:
Functional fitness and CrossFit: Energy system development is absolutely essential. These workouts constantly mix time domains and intensities. Understanding when you’re training power (ATP-PC), when you’re building work capacity (glycolytic), and when you’re developing an aerobic base determines whether you improve or just get really good at suffering.
Sports performance: Every sport has specific energy system demands. A 100 meter sprinter needs massive ATP-PC power. A 400 meter runner lives in the glycolytic pain cave. A marathon runner is all about aerobic efficiency. Train the wrong system and you’re preparing for the wrong sport.
General fitness and health: If your goal is to feel good, move well, and live long, energy system balance matters enormously. Most people need more aerobic base work and less random high intensity chaos.
Conditioning work: This is literally energy system training. Whether you’re improving VO2 max, lactate threshold, or recovery capacity, you’re targeting specific energy pathways.
Where energy systems matter less:
Pure hypertrophy and bodybuilding: If you’re trying to get jacked, energy systems are secondary at best. You care about mechanical tension, training volume, and proximity to failure. Whether your bicep curls are powered by ATP-PC or glycolytic energy is irrelevant to muscle growth. The only consideration is allowing enough rest between sets to maintain training quality.
Pure strength training: Powerlifters and strength athletes care about one thing: moving maximum weight for 1 to 3 reps. This is almost exclusively ATP-PC work, but the focus is on neuromuscular adaptation, not energy system development. Rest periods are determined by strength recovery (3 to 5 minutes), not energy system training goals.
The Smart Approach: Train Systems Systematically (When It Matters)
For training that involves mixed time domains and intensities, structure your approach to develop each system with purpose instead of random intensity mixing.
For general fitness and longevity:
70% Aerobic: 3 to 4 sessions focused on steady, sustainable effort
20% Glycolytic: 1 to 2 sessions with sustained high intensity intervals
10% ATP-PC: 1 session with explosive, fully recovered efforts
For athletic performance:
Adjust percentages based on sport demands
Maintain the systematic approach rather than random mixing
Sample Weekly Structure
Monday: ATP-PC Development
- 8 x 15 second bike sprints
- 3 minute complete rest between efforts
- Total time: 30 minutes
Tuesday: Aerobic Base
- 40 minute steady rowing at conversational pace
- 70 to 75% max heart rate
Wednesday: Glycolytic Power
- 6 x 90 second high intensity intervals
- 90 second active recovery between efforts
Thursday: Aerobic Recovery
- 30 minute easy movement
- Walking, light cycling, or yoga
Friday: Mixed Glycolytic and ATP-PC
- 5 x (15 second sprint + 45 second moderate + 60 second easy)
- Teaches system transitions
This is exactly how complete programs like Flex Program structure conditioning work. You get dedicated aerobic base building, glycolytic capacity development, and explosive ATP-PC power work, all organized so each system gets proper attention without everything competing.
The Three Critical Questions
Before any workout, ask yourself:
- What energy system am I trying to develop?
- What intensity and duration will target that system?
- What rest periods will support that goal?
If you can’t answer these questions, you’re probably training randomly.
Stop Doing This
Random intensity mixing within workouts
Going hard on every single training day
Incomplete rest during power and speed work
Pushing aerobic sessions into the glycolytic zone
Start Doing This
Plan each session with a primary energy system focus
Use appropriate intensities and rest periods for your goal
Include true recovery sessions that stay aerobic
Track your pacing to ensure you’re hitting the right zones
The Bottom Line: Your Body Is Smarter Than Your Ego
Your body has three distinct energy systems that have evolved over millions of years to handle different types of physical demands. Each system has optimal training methods, recovery needs, and adaptation timelines.
Trying to train all of them simultaneously in every workout isn’t functional or intense. It’s just ignorant.
The athletes and lifters who understand energy systems train with purpose. They know when to go hard, when to go easy, and when to go explosive. They develop each system systematically rather than randomly.
The result? Better performance, faster recovery, and sustainable progress over years rather than burnout over months.
Your choice: keep training like you have one engine and wonder why you’re always tired, or start training like you understand how energy actually works and watch your performance take off.
The science has been clear for decades. The only question is whether you’re ready to train smarter instead of just harder.
