Aerobic vs. Anaerobic Training for Coaches (Simplified With ATP)
Frameworks with Carl Hardwick | CoachRx Podcast Network
Energy systems training gets messy fast. One coach says "aerobic base," another says "intervals," and suddenly we're arguing about zones, charts, and what counts as conditioning.
I keep it simpler by anchoring everything to ATP, the energy currency of the body. If I can keep ATP available for the task, I can coach with more intention and less guesswork. From there, I sort training into three buckets I use all the time: Gain, Sustain, and Pain.
Why energy systems training matters (and why it gets confusing)
Most coaches don't struggle with the idea that energy matters. The struggle is the language. People toss around terms that sound precise, yet they often describe completely different efforts in real life.
Here are a few that commonly get blended together:
Aerobic
Anaerobic
Intervals
Conditioning
Metabolic work
The fix is not memorizing more labels. The fix is understanding what the body is trying to do in real time. That's why I like the ATP lens. Every movement, whether it's a heavy squat, a sprint, a 30-minute bike, or just walking up stairs, comes back to the same question: how am I making ATP fast enough to keep going?
Once I see energy systems as "how I regenerate ATP for this effort," the rest gets clearer. I can stop asking "Is this aerobic?" and start asking "Which system is dominant right now, and why?"
That "why" matters because the dominant system is not decided by the workout title or my intent. The body doesn't care that I called something "engine work." It responds to the actual intensity and duration of what I'm doing.
ATP in plain English: the one fuel you always spend
ATP stands for adenosine triphosphate. It's the only usable form of energy in the human body. That sounds like textbook talk, but it's practical when I coach.
Every contraction costs ATP:
speaking and gesturing
breathing
rowing strokes
reps in a bench press
a max-effort sprint
ATP does its job when it breaks down into ADP plus a phosphate. That breakdown releases energy, which is what makes work possible.
A simple gym example is a bench press set that gets grindy around rep five or six. If rep six is the limit today, and rep seven won't happen, that's not just "mental toughness." A big part of it is that I'm running low on the quick energy needed to keep producing force at that level.
Here's the catch: I store almost no ATP. I've got enough for only a few seconds of hard work. So the body has one constant job during training: rebuild ATP fast enough to keep me moving.
This is where relativity matters. Fitness level, training age, skill, and efficiency change the cost of work. A pace that's "easy aerobic" for one person might be a redline for another. Even the same person can shift day to day depending on recovery.
So when I plan energy systems training, I try to keep this as my central idea:
The real question isn't which system is "on." It's which system is dominant at the effort I'm actually producing.
From there, I use three labels:
Gain: creatine phosphate system
Sustain: aerobic system
Pain: anaerobic system (glycolysis)
All three work together all the time, but one will lead depending on intensity and duration.
To make that difference easy to scan, here's the quick comparison I keep in my head.
BucketDominant systemWhat it feels likeWhat drives dominanceGainCreatine phosphateExplosive, snappy, powerfulVery high effort, very short duration, lots of restSustainAerobicSteady, repeatable, you can keep goingLower to moderate effort, longer duration, controlled breathingPainAnaerobic (glycolysis)Burning, heavy fatigue, you need long restHigh effort, longer than a few seconds, big byproducts
The takeaway is simple: the body decides based on the work. Intensity and duration win every time.
Gain training: power needs full intent and full rest
What Gain is really using (and why it fades fast)
Gain is the fastest way to regenerate ATP. The body uses creatine phosphate stored in muscle to resupply ATP almost instantly. That's also why creatine monohydrate got so popular in performance settings. If those stores are topped off, I have more immediate fuel for short, hard efforts.
Gain dominates during short, explosive work like:
a 1-rep max, 3-rep max, 5-rep max
many hard sets in the 1 to 8 range (sometimes out to 10)
jumps, throws, and accelerations
short sprints (think 5 to 7 seconds of true pop)
This system produces almost no fatigue byproducts, but it depletes quickly. Once creatine phosphate is gone, power drops. That drop shows up fast in training.
A back squat set of eight is a good example. Early reps feel crisp, then reps six through eight climb in effort. Some of that is local fatigue, but a big piece is that quick ATP supply getting drained.
How I program Gain so it actually builds power
If I want Gain adaptations, I can't pretend fatigue is the goal. Power work needs three things: full intent, full rest, and clean execution.
Full intent inside the rep
Even when I prescribe tempo, that explosive intent matters. A tempo like 20X1 makes the point: control down, then drive up hard. That "X" is not optional if power is the goal.Full rest between sets
Rest is not laziness here. Rest is the requirement that lets me repeat high output. If I remove rest, I stop training power and start training how to survive.Clean execution
Speed plus bad positions is how people get hurt. If someone can't move well, they can't safely go full intent.
A "what not to do" example is brutal but clear. If I tell someone to do eight tough reps at 75 percent every minute on the minute, power will drop across sets. They might finish, but the adaptation shifts. At best, they're practicing moving load while tired. More likely, they're building strength endurance, not true power.
The same rule applies outside the weight room. If I want sprint speed, I need enough rest. Otherwise, I'm teaching someone to sprint slower each rep.
In short, Gain equals power, but power is hard to sustain. Rest is what makes it repeatable.
Sustain training: the base that supports everything else
What makes the aerobic system different
When I think Sustain, two words come to mind: sustainability and longevity.
Sustain (the aerobic system) is the slowest and least powerful way to make ATP, but it produces a massive amount of ATP with minimal fatigue byproducts when I dose it right. It uses oxygen and fuel substrates (carbs, fats, and sometimes protein, especially when carbs are low).
That "minimal byproducts" part is a practical separator from Pain. With Pain work, the burn and aftermath are the point. With Sustain, I want output I can repeat without a meltdown.
Exercise choice changes what "aerobic" even looks like, because contractions have costs. Rowing is a good example. An intermediate client with limited rowing time might build fatigue just from posture demands, grip, and repeated pulls. Biking can do the same thing, especially if someone isn't used to local quad fatigue or cranks the damper or resistance too high.
Mixed-modal circuit work adds even more friction. Burpees, kettlebell swings, push presses, box step-downs, then hop on a machine, it's fun, but it's hard for most people to keep those contractions truly aerobic in a short time window. Heart rate shifts, position changes, bracing demands, and elevation changes all make sustainability harder.
None of that means mixed work is "bad." It just means I need to be honest about what system ends up dominant for that person.
How I keep Sustain work sustainable (even when it gets hard)
Sustain includes long duration, low to moderate intensity, and also daily movement. Walking, hiking, climbing stairs, easy rides, those are all part of the aerobic story.
Still, Sustain can get tough, and I like that. In the OPEX method, the MAP continuum (maximum aerobic power) is a good example of progressive aerobic work. The idea is to start with longer intervals, then over time move toward shorter intervals with higher turnover. A common progression mentioned is working from 30-minute intervals down toward 30-second intervals.
The key is that even at 30 seconds, it's still sustainable. For example, 30 seconds on, 30 seconds off for 20 rounds can stay repeatable across the whole session. The rest is short, so the effort has to match that reality.
This is where people get tripped up because both Sustain and Pain can show up in the same time window.
If I do 30 seconds of work, it could be aerobic or anaerobic. The difference is the effort, and the easiest way to police effort is the work-to-rest ratio.
30 seconds on, 30 seconds off (1:1) suggests a sustainable effort.
30 seconds on, 3 minutes off suggests a much higher effort.
If I hit a Pain effort but rest like Sustain, I'll blow up fast. If I hit a Sustain effort but rest like Pain, I'll feel like I'm wasting time.
Work-to-rest ratios don't just organize the session, they tell the athlete what effort makes sense.
Why Sustain supports Gain and Pain, not just endurance
Sustain is more than "cardio." When I dose it properly, it supports recovery between tough sets, between sprints, and between sessions. It can help me repeat high-quality work because it improves my ability to recover.
That synergy cuts both ways, though. If I overdo it, I can interfere with strength and recovery. The win is in the dosage, not in trying to live on a bike forever.
Pain training: high intensity, big byproducts, real recovery cost
What Pain is and why it feels so rough
Pain is anaerobic work, mostly powered by glycolysis. Intensity stays high, time pushes past a few seconds, and the body leans hard on carbohydrates without oxygen.
It produces ATP fairly quickly, but it's inefficient. The byproducts build up and that's what people recognize as the burn, the heavy fatigue, the "I need to lie down" feeling. Some athletes describe a copper taste in the mouth when they've really pushed deep.
Pain tends to dominate hard efforts lasting roughly 10 seconds to 90 seconds for many people, especially those with decent power. Past that, power often slides and the aerobic system starts to carry more of the load while the athlete hangs on.
A clear example is a bike interval at roughly 95 to 97 percent effort for 30 seconds, followed by three minutes of rest. Do that for five sets, maybe rest longer, then repeat in blocks. That rest is long on purpose because the effort is supposed to sting.
How I think about "progression" in Pain work
Pain programming is less about fancy progressions and more about creating a specific metabolic effect for a certain duration. One simple way to progress it is to push time to the right. In other words, extend the interval length over time.
This is also the one place where I accept a reality that bothers people at first: as time increases, power decreases. That's normal. I can produce far more power for 10 seconds than for 2 minutes, even when both efforts hurt.
For exercise selection, I prefer cyclical options for Pain work, like biking, rowing, or sprinting. Turnover matters. An assault bike doesn't limit cadence, my body does.
On the other hand, mixed movements often cap intensity because turnover slows. A set of burpees, kettlebell swings, then box step-downs can feel hard, but step-downs alone can bottleneck speed. Once speed drops, I'm not really getting the pure Pain stimulus I think I'm getting.
Pain comes with real costs:
Recovery cost: it can be hard to bounce back, especially as someone gets stronger and can produce more output.
Hormonal and emotional cost: people often need to psych themselves up for this work.
Coaching cost: beginners may not even have the power base to do true Pain work. They cannot generate enough output to justify the rest.
Because of those costs, Pain is not a system to live in, and it's not a system everyone needs. Many people can get the metabolic benefits they're chasing by building toward tougher aerobic work instead.
The coaching lens that keeps it all straight
Energy systems training gets simple when I stop treating it like a label and start treating it like a response to real effort.
Here's what I keep on repeat:
All systems are always active, but one is dominant.
Dominance is driven by intensity and duration, not the name of the workout.
Individualization matters, because the same pace can be different systems for different people.
Gain builds power and force, but it demands intent and rest.
Sustain builds capacity, recovery, and longevity, and it supports everything else.
Pain creates a strong metabolic stress, but it has a cost, so I dose it carefully.
A lot of programs overuse Pain because it feels like a workout. People leave sweaty and crushed, so they assume it was effective. I'd rather reframe what "good work" looks like. Plenty of hard, effective training lives in Gain and Sustain when the intent is clear.
Where I take this next (program design and tools)
In the extended episode, I walk through applying Gain, Sustain, and Pain to real program design.
If you want to follow that deeper programming side, I use these resources:
Conclusion: keep ATP available, then coach the effort
When I zoom out, energy systems training is just the art of keeping ATP available for the job in front of me. Gain, Sustain, and Pain give me simple language, but the real driver is still intensity plus duration. If I coach effort honestly and match rest to the goal, the "what system is this?" question usually answers itself.
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Have questions? DM Carl on Instagram @hardwickcarl
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