# Building the Engine: Energy Systems in CrossFit Explained

> Why your arms are on fire 90 seconds into Fran and what ATP-CP, glycolysis and the aerobic system have to do with it.

In CrossFit it is called the engine: the ability to keep producing output across the entire duration of a workout without falling apart. If you want to improve your engine, you need to understand where the energy comes from. The body runs three energy systems and every WOD taps them in a different mix.

## The three systems

**ATP-CP (alactic, 0 to 10 seconds):** For explosive efforts like a heavy clean or a sprint start, the muscle uses stored ATP and phosphocreatine directly. Huge output, tiny tank. This is exactly where creatine comes in: bigger phosphocreatine stores mean more reps at high load.

**Glycolytic (10 seconds to 2 minutes):** The muscle burns glucose without oxygen, fast but with lactate and a falling pH as side effects. The burning in your arms during thrusters? That is this system at its limit.

**Aerobic (from 2 minutes):** With oxygen, the body burns carbohydrates and fats. Less peak output but almost unlimited duration. Your aerobic base determines how quickly you recover between intervals; it is the foundation of every good engine.

## Reading metcons properly

A 21-15-9 like Fran (thrusters and pull-ups, target time under 6 minutes) lives almost entirely in the glycolytic zone. A 20-minute AMRAP, on the other hand, is an aerobic workout with glycolytic spikes. If you only ever do short sprint WODs, you never build a base. If you only train long and easy, you lose the top end. The mix is what builds the engine.

**Pacing rule:** Start every WOD longer than 5 minutes deliberately at 85 percent of your perceived maximum. The classic mistake is the full-throttle start followed by a collapse after round two. Even splits beat a fast start almost every time.

## A sample WOD for the engine

“Thirty-Thirty”, 5 rounds:

- 30 seconds of rowing or biking, brisk but controlled
- 30 seconds of rest
- 10 wall balls
- 10 kettlebell swings

Goal: identical round times. If round five is slower than round one, you started too fast.

## Fuel for long sessions

For training sessions over 60 minutes or double sessions, carbohydrate intake during training makes the difference: 30 to 60 g per hour keeps blood sugar and output stable. [WOO® Competition](/en/shop/woo-competition-can) is built exactly for that: a carbohydrate and electrolyte mix for hard sessions. For interval sessions with an endurance focus, [WOO® Fast Endurance Lemon](/en/shop/woo-fast-endurance-lemon-can) delivers quickly available energy.

**From the shop:** [WOO® Competition](/en/shop/woo-competition-can) for competition and double-session days, [WOO® Fast Endurance Lemon](/en/shop/woo-fast-endurance-lemon-can) for long metcon and interval days, [WOO® Creatin Monohydrat](/en/shop/woo-creatin-monohydrat-can) for the ATP-CP system.