#007 – Iron Heart

The three energy systems that dictated the outcome of our survival.

It might not seem like it these days, with the rise of weightlifting, bodybuilding, and other strength-dominant sports. But what if I told you that humans were built to run?

Our primary evolutionary drivers, beyond intelligence—were our lungs, cardiovascular system, and sweat glands. These systems made us endurance specialists. We used persistence to rundown and outlast faster prey over long distances (essentially making them over heat). Our aerobic system, working along with the anaerobic system, became one of our greatest survival tools. This allowed us to recover, regulate, and keep moving when other species couldn’t.

What separates most athletes from good to great? Well that usually comes down to endurance, stamina, or repeatability. True performance is built on a foundation of energy system development: the ability to produce force fast, sustain effort under fatigue, and recover quickly under pressure.

At the core of all human movement are three primary energy systems:

• ATP-PC: short bursts, heavy lifts, explosive output

• Anaerobic Glycolysis: moderate-duration, high-intensity effort

• Aerobic Oxidative: long-duration endurance, recovery, and repeatability

These systems work together to fuel everything—from a single heavy lift to going the distance in the ring.

And just so we’re clear… it didn’t start in the gym.

Elite performance today means reclaiming and refining the foundation we were built on. It means layering speed, power, and repeatability on top of what evolution gave us. When you train all three energy systems with intent, you're not just increasing capacity.

You're building resilience.
You're building efficiency.
You're building an Iron Heart.

Speaking of survival….

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THE ENERGY SYSTEMS:

SECTION Ⅰ - ATP-PC / Alactic:

Your ignition switch; built for short, brutal bursts of power.
This is the system that fuels your very first move: the opening hit, the first punch, that burst of power that sets the tone. It runs entirely off stored ATP and phosphocreatine in your muscles, so it kicks in instantly and shuts off just as fast. Usually within a 10 second window. When you need maximal force with zero buildup, this is where you live.

Training Examples:
Sprint clusters- 6 × 20 m all-out with full recovery, medicine-ball throws/slams, and heavy compound lifts performed as contrast sets (heavy squat to vertical jump) are your go-to drills.

These keep the neural drive and PCr stores primed, so every rep feels just as explosive as the first.

0-10 sec explosive power (sprint clusters, contrast lifts).

SECTION ⅠⅠ - Glycolytic / Anaerobic: 

Where power goes beyond the first few seconds.
This system breaks down muscle glycogen (carb source stored in muscles) into lactate, creating the familiar “burn” as you push through mid-round scrambles or on that 8th rep of squats. It’s the bridge between explosive bursts and steady endurance.

Training Examples:
Tabata-style intervals 20 s work/10 s rest, power circuits (max reps in fixed time), weight lifting routines (gym), and EMOM grinders 10 burpees on the minute.

Teach your body to tolerate and clear lactate. Over time, that burn becomes less of a limiter and giving yourself a higher threshold.

10-120 sec high intensity work (Tabata, power circuits, weight lifting, and EMOM)

SECTION ⅠⅠⅠ - Aerobic / Oxidative:

The system that keeps on moving. Powering recovery, repeatability, and long-end durability.
From minute three onward, of continuous work, you’re in this territory. This oxidative engine uses oxygen to convert carbs and fats into energy, letting you recover between bursts, sustain long efforts, and keep grinding. It’s the reason humans evolved as endurance hunters. And why you can push back on fatigue in round five.

Training Examples:
Zone-2 cardio sessions 30–45 min at conversational pace, sled drags for time, and mixed aerobic circuits 10 min row + 10 min body-weight EMOM at low intensity.

Build capillary density and mitochondrial function. The more efficient your aerobic system, the faster your ATP-PC and engine recover. Giving you the ability to come back quicker and hit harder.

2 min+ endurance & recovery (zone-2, sled drags, and aerobic circuits)

THE TAKEAWAYS: 

Each energy system plays a role: explosive output, sustained effort, and long-haul recovery. If you don’t train each, your performance has a ceiling.

Most athletes live in the burn zone, chasing fatigue instead of progress. But true conditioning comes from all three systems. Develop each one intentionally throughout your mesocycles. So when it comes to the fight, you can begin and finish strong.

Train with intent. Recover with purpose. Build your capacity from the inside out.

That’s how you forge an Iron Heart.

 See You in Issue #008!

If you’ve made it this far, I’m assuming you’re finding some value in this week’s newsletter; and that’s exactly my goal, so I’m glad to hear it.

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It would mean a lot. Thank you.

Thank you,

Stephen Holmes Founder, Linking Performance & The Weekly Standard

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