Energy Systems: How to Train the Right Energy System

Conditioning can seem complicated when we start talking about the ATP-PC system, glycolysis, aerobic metabolism, lactate, VO₂ max, and all the other terminology that comes with energy systems.

But you don't need to overcomplicate it.

At its core, conditioning is about manipulating intensity and duration to emphasize the energy system you want to train.

And before we go any further, remember:

All of your energy systems are always working.

You don't completely shut one system off and turn another one on. They work together continuously. What changes is which system is contributing the most based on the intensity and duration of the effort.

So, for simplicity, let's break conditioning into three major buckets:

1. ATP-PC / phosphagen

2. Glycolytic / anaerobic

3. Aerobic

1. ATP-PC: Short, Explosive Efforts

Think:

Sprint. Jump. Throw. Max-effort effort.

The ATP-PC system is primarily responsible for producing energy during very short, high-intensity efforts.

For example, let's say we're doing a 5-second sprint.

That's extremely high intensity, but it's very short in duration.

This is where work-to-rest ratios become really important.

If we're training maximal sprint speed, we want enough recovery between efforts that we can continue producing high-quality, explosive outputs.

If I sprint for 5 seconds and then immediately sprint again with very little rest, the goal starts to change.

I'm no longer able to produce the same maximal output, and I'm increasingly relying on other energy systems to help meet the demands of the effort.

So, if the goal is speed and power, we need to respect the recovery.

This is why you might see something like:

5 seconds work → 30 + sec recovery

The exact ratio depends on the goal, athlete, exercise, and training level, but the principle is simple:

If you want to train explosiveness, recover enough to actually be explosive.

2. Glycolytic: Hard and Sustained

Next is the glycolytic system, which becomes increasingly important as high-intensity efforts last longer.

Think:

Hard intervals. Hills. Repeated sprints. Tough circuits.

This system allows us to produce energy quickly when the demand is too high to be met primarily through aerobic metabolism.

For example, imagine a hard 30–60 second effort.

It's too long to rely primarily on the ATP-PC system, but it's also far too intense to be something you could sustain for several minutes at the same output.

This is where glycolytic energy production becomes particularly important.

And again, intensity and duration matter.

If I perform a 30-second all-out effort and take three minutes to recover, I'm emphasizing something different than if I perform 30 seconds of work followed by only 15 seconds of rest and repeat that for 10 rounds.

The latter creates a much greater accumulation of fatigue and requires greater contribution from the aerobic system as the workout continues.

So, when programming glycolytic conditioning, we're not just choosing an exercise.

We're manipulating:

How hard?

How long?

How many rounds?

How much rest?

3. Aerobic: The Long Game

Then we have the aerobic energy system.

This is the system that becomes increasingly important as exercise duration increases and intensity decreases.

Think:

Walking. Easy running. Cycling. Hiking. Longer continuous efforts.

This is where Zone 2 training comes in.

Zone 2 is an intensity that you can sustain for a relatively long period of time. It's useful for developing aerobic fitness and can be particularly valuable for people preparing for longer-duration activities.

For example, if you're training for a marathon, ultra, long hike, or mountaineering objective, you're going to need the ability to produce energy aerobically for hours.

That's why you can't build your conditioning entirely around short, hard intervals.

You need the aerobic base to support the work.

Aerobic training is also associated with improvements in cardiovascular fitness and cardiometabolic health, and it contributes to total energy expenditure, which can support body-composition goals when combined with appropriate nutrition.

But one important clarification:

Zone 2 isn't magically a fat-loss zone.

Your body is using fat and carbohydrates at different proportions across different intensities. Total energy expenditure, nutrition, and your overall training program matter much more for changes in body composition than simply staying in a particular heart-rate zone.

So How Do We Actually Use This?

This is where I think energy systems become much easier to understand.

You don't need to memorize a bunch of complicated physiology.

Start with two questions:

1. How hard is the effort?

Very high intensity → greater anaerobic contribution

Lower intensity → greater aerobic contribution

2. How long is the effort?

Very short → ATP-PC contribution is high

Short-to-moderate and hard → glycolytic contribution increases

Longer duration → aerobic contribution becomes increasingly important

Then add your work-to-rest ratio.

That's where the programming really starts to change.

For example:

5-second sprint + 2-minute rest

→ prioritize speed and power

30-second hard effort + 60-second rest

→ greater glycolytic demand

30-minute continuous run

→ primarily aerobic

None of these completely isolates an energy system.

We're simply manipulating the variables to emphasize the adaptation we're after.

Stop Chasing Energy Systems. Start Training the Goal.

This is ultimately how I want you to think about conditioning.

Don't start with:

“Which energy system should I train today?”

Start with:

“What am I trying to get better at?”

Are you trying to sprint faster?

Improve repeat-sprint ability?

Build your aerobic base?

Prepare for a marathon?

Improve your ability to climb for hours?

Increase your overall work capacity?

Your goal determines the demand.

Then intensity, duration, volume, and recovery help you build the session around that demand.

Energy systems aren't separate switches.

They're overlapping systems that work together all the time.

Our job as coaches is simply to manipulate the training variables so we can emphasize the system and adaptation that best matches the goal.

And that's what makes conditioning effective, not just doing something that makes you tired.

References

Gastin, P. B. (2001). Energy system interaction and relative contribution during maximal exercise. Sports Medicine, 31(10), 725–741

Buchheit, M., & Laursen, P. B. (2013). High-intensity interval training, solutions to the programming puzzle: Part I & II. Sports Medicine, 43, 313–338, 927–954

Kodama, S., et al. (2009). Cardiorespiratory fitness as a quantitative predictor of all-cause mortality and cardiovascular events in healthy men and women: A meta-analysis. JAMA, 301(19), 2024–2035.