Fair warning: it is about to get slightly sciency in here.

Stick with it. Understanding how your body produces energy is one of the most practically useful things you can know as an athlete. It changes how you read a training program, evaluate your own performance, and spot the holes in your fitness — because every hole in military or tactical fitness can usually be traced back to an under-trained energy system.

What bioenergetics means

Bioenergetics is the study of how the body produces and uses energy. The food you eat is chemical energy. Your body converts it into glycogen, fat, and protein, then uses those stores to produce adenosine triphosphate — ATP — the molecule that powers muscular contraction and almost every other metabolic process.

When ATP is broken down, energy is released. That energy moves the muscles. The question worth understanding: how does your body make ATP, and which process does it use when?

The answer depends on what you are doing and for how long.

The three energy systems

Your body has three primary systems for producing ATP, each suited to a different intensity and duration of effort.

The phosphocreatine system (0–10 seconds)

Your emergency generator. When you need energy right now — a maximal power clean, a 40-yard sprint, an explosive jump — the phosphocreatine system kicks in. One chemical step makes it extremely fast. It also burns through its fuel almost immediately.

Key characteristics:

• One chemical reaction — fastest of the three
• Fueled by stored creatine phosphate
• Lasts roughly 5–10 seconds
• Anaerobic (no oxygen required)
• Primary system for maximal speed and explosive power

This is why creatine supplementation has a legitimate evidence base — it replenishes the substrate for this system.

The glycolytic system (10 seconds – 2 minutes)

When the phosphocreatine system runs dry and you are still going, glycolysis takes over. It breaks down carbohydrates — glucose from the bloodstream or glycogen from muscles and liver — to produce ATP without oxygen.

Slower than the phosphocreatine system, but produces more total ATP. This is the system being hammered during a 400-meter sprint, a heavy set of squats taken to failure, or a two-minute all-out effort on a rower.

Key characteristics:

• 18 chemical reactions
• Produces 2–3 ATPs per glucose molecule
• Anaerobic
• Lasts roughly 30 seconds to 2 minutes at high intensity
• Primary system for sustained high-intensity work

The oxidative system (2+ minutes and everything at rest)

Your aerobic engine — slow, complex, and nearly limitless at lower intensities. The oxidative system powers everything from a long ruck to a casual run to sitting at a desk. It requires oxygen, involves 124 chemical reactions, and produces 36–37 ATP per glucose molecule — far more than the other two systems combined.

Key characteristics:

• Most complex of the three
• Requires oxygen
• 36–37 ATPs produced per glucose molecule
• Slow to ramp up, sustainable for hours
• Fatigues primarily when glycogen stores are depleted
• Primary system for endurance events and all low-intensity activity

Why this matters for how you train

Here is the practical point: most people default to training one system while neglecting the others.

The runner who can go 18 minutes on the 3-mile but folds under a ruck? Heavy oxidative investment with an underdeveloped phosphocreatine and glycolytic base — and usually a weak strength foundation. The gym athlete who is strong as hell but gasses out after two minutes of hard conditioning? Heavy investment in the first two systems, with an undertrained aerobic engine.

To perform across the full range of physical demands — which is exactly what military and tactical work requires — all three systems need deliberate development. Your training cannot be only lifting, only running, or only intervals. It has to work across all three energy domains.

For a look at how different cardio approaches develop these systems differently, the HIIT vs. steady-state cardio breakdown covers the practical implications.

The bottom line

You do not need to memorize the Krebs cycle. But understanding that your body has three distinct energy systems — and that each one needs training — helps you evaluate any program and identify what is missing from your current approach.

If your programming does not include work across all three systems, it is leaving performance on the table.

The Strategic Foundations Training Team is built to develop all three — strength, conditioning, and aerobic capacity — within a program that fits a real schedule. Free 14-day trial.