The three energy systems.

A working guide to the three engines the body runs on, how they layer during exercise, what each one wears down, and how to train them on purpose.

● SYSTEMM Editorial~ 11 MIN READ

Every contraction in your body, sprinting, lifting, walking, breathing, is paid for in ATP. The cell stores almost none of it. The three energy systems are the three ways the body manufactures ATP on demand, and they have wildly different speeds, capacities, and recovery characteristics. Understanding them is the difference between training that works and training that hurts.

The three systems.

● SYSTEM 01

Phosphocreatine (ATP, PCr).

FUELStored creatine phosphate in muscle SPEEDFastest. ATP available in milliseconds. CAPACITYTiny. Roughly 8, 12 seconds of all-out effort before stores deplete. RECOVERYFast. 2, 3 minutes rest restores most stores. Full at ~5 min. POWERS1RM lifts, vertical jump, the first 60 m of a sprint, a short Olympic lift. TRAIN WITHHeavy strength work, jumps, max-effort sprints, throws. Long rest intervals.

● SYSTEM 02

Glycolytic (anaerobic lactic).

FUELMuscle glycogen, blood glucose. Breaks down without oxygen. SPEEDFast. ATP available in seconds. CAPACITYLimited. 10 seconds to ~2 minutes of hard effort before lactate and H+ buildup forces a slowdown. RECOVERYSlow. 5, 15 minutes for full repeat-ability. Hours for full glycogen replenishment. POWERSAn 800 m, a 90-second hard cycling effort, a 20-rep set of squats, the second through fifth rounds of a CrossFit metcon. TRAIN WITH30s to 2 min intervals at very hard intensity, lactate-threshold work, hard sled pushes, hill repeats.

● SYSTEM 03

Oxidative (aerobic).

FUELFats, carbohydrates, and a small amount of protein, all burned in the mitochondria with oxygen. SPEEDSlow to spin up, sustainable. CAPACITYEssentially unlimited at submax intensities. Hours. RECOVERYContinuous during exercise; deep aerobic recovery within hours between sessions. POWERS5K to marathon, long bike rides, the recovery between hard sets, daily life. TRAIN WITHZone 2 work, long steady efforts, VO₂ max intervals (4 × 4 protocol), tempo runs.

How they layer.

The systems don't switch on one at a time. They overlap, with the dominant contributor shifting as effort lengthens and intensity changes. A maximum sprint pulls almost entirely from the phosphocreatine system in the first 6 seconds, but the glycolytic system is already firing by second 4 and dominant by second 15. Oxidative metabolism is running quietly under all of it and becomes the main contributor past 2 minutes.

The practical version: even a "pure" sprint workout is partly aerobic, and even a "pure" aerobic workout has anaerobic moments. The question is which system is contributing the most, and which one a given training session is built to develop.

● Approximate share by effort duration

0, 10 seconds PCr ~80%, glycolytic ~15%, aerobic ~5%
10, 30 seconds PCr ~30%, glycolytic ~60%, aerobic ~10%
30 seconds, 2 minutes PCr ~5%, glycolytic ~60%, aerobic ~35%
2, 10 minutes Aerobic dominant, but glycolytic still meaningful, especially near threshold
10+ minutes Aerobic ~90%+, glycolytic ~5, 10% near efforts at threshold

The metrics that actually matter.

VO₂ max. The ceiling of the aerobic engine. Maximum rate of oxygen consumption. Strongest single predictor of all-cause mortality in adults. Most people can move it 5, 10 ml/kg/min with twelve weeks of structured work.
Lactate threshold (LT1 and LT2). The points at which blood lactate begins to rise above baseline (LT1, the floor of Zone 2) and then accelerates rapidly (LT2, the top of sustainable hard work). Training raises both ceilings without raising max heart rate.
FTP (Functional Threshold Power). Cycling, specific proxy for LT2: roughly the highest sustainable power output for an hour. The number every cycling protocol references.
Critical power / critical speed. The asymptote of the power-duration curve, the boundary between sustainable and unsustainable effort. Within a few percent of LT2 for trained athletes.
Resting heart rate and HRV. Aerobic conditioning lowers RHR and raises HRV over weeks. The signal lives in the trend, not the daily value.

How we train each one.

PCr. Heavy strength work (1, 5 reps, long rest), max-effort sprints under 8 seconds, jumps and throws, Olympic lifting variations. The work is short, the rest is generous, the recovery between sets is non-negotiable. Hurried PCr work turns into glycolytic work, which defeats the purpose.

Glycolytic. Intervals of 30 seconds to 2 minutes at near-max effort with incomplete recovery. Hill repeats, hard sled, 400 m repeats, 90-second bike efforts. The work is brutal, the rest is brief, the burn is the point. Don't do this every day; it's the most taxing system to recover from.

Oxidative. Two distinct flavors:

Zone 2. Conversational pace, nose breathing if you can, 30 minutes to 2 hours, several times a week. Builds mitochondrial density, lowers RHR, raises LT1, almost no acute fatigue cost.
VO₂ max intervals. The Norwegian 4 × 4 protocol is the cleanest version: 4 minutes near maximum heart rate, 3 minutes easy, four rounds, one or two days a week. Moves the aerobic ceiling faster than any other protocol.

A common mistake.

The most common training error in the population that reads this magazine is doing all aerobic work at medium intensity, hard enough to feel tired, not hard enough to drive VO₂ adaptation, hard enough to interfere with strength work but not hard enough to make it count as conditioning. The 80/20 polarization rule exists for a reason: 80% of aerobic work at Zone 2 (truly easy), 20% at near-max (truly hard), almost nothing in the muddy middle.

● The protocolThree sessions a week: one strength (PCr-dominant), one VO₂ max interval (oxidative ceiling), one or two Zone 2 (aerobic base). Add one glycolytic session every 7, 10 days if sport demands it. Rest is part of the protocol.

How this maps to fat loss and longevity.

Fat oxidation peaks at low intensities, the upper end of Zone 1 into Zone 2. The "fat-burning zone" on cardio machines is real for fuel mix, but irrelevant for total fat loss, which is governed by caloric balance. Zone 2 is for mitochondrial health, not for "burning fat."

For longevity outcomes, the relevant metric is VO₂ max. People in the top quartile for VO₂ max have ~5× lower all-cause mortality than the bottom quartile, an effect size larger than any drug. The relevant protocol is 4 × 4 once or twice a week on top of an aerobic base.

Next steps.

Pick the system you've trained least. For most readers, that's the aerobic base. Spend eight weeks adding two Zone 2 sessions plus one 4 × 4 to whatever you're already doing. Re-test VO₂ max (via a Cooper test if no lab access) at the start and the end. The number will move.

● Last updated 2026 · SYSTEMM Editorial · This guide updates as the protocols evolve