Body Reps · Physiology

Energy systems

Every rep you do is paid for in one currency: ATP. Your muscles only hold a tiny amount of it, enough for a few seconds. So the body has three ways of making more, fast or slow.

This page names the three energy systems, shows which one leads in which kind of effort, and clears up one big gym myth. Then it explains heart-rate zones, and why 220 minus your age is only a rough guess.

One rule before you start: all three systems are working all the time. None of them switches off when another takes over. What changes is which one is doing most of the work.

Start training Try the interactives

Sources for this intro: [S8], [S9], [S10], [S11]

Currency

The currency

ATP

Say it: AY-tee-PEE. In full: adenosine triphosphate (uh-DEN-uh-seen try-FOS-fayt)
Word parts: tri: three. phosphate: a phosphorus group (phosphorus is from Greek for light-bringing). ATP has three phosphate groups. Adenosine comes from adenine, named from Greek adēn, gland, because it was first taken from an ox’s pancreas.
Everyday: the body’s energy currency
What it is: The molecule that powers muscle contraction. Breaking off its third phosphate releases the energy.
In the gym: Every contraction in every set uses ATP. Your muscles store only enough for a few seconds of work, so the three energy systems exist to keep remaking it.

ATP is cash. It’s the only money your muscles accept. The three energy systems are cash machines that keep refilling the wallet.

Sources: [S8], [S9]. Word parts: [S18].

Two words you need first

Anaerobic

Say it: an-uh-ROH-bik
Word parts: an: without. aer: air. bios: life.
Everyday: without oxygen
What it is: Not using oxygen.
In the gym: The two fast systems (ATP-PC and glycolytic) are anaerobic. They make ATP quickly without waiting for oxygen to arrive, but they can’t keep it up for long.

An- means without. An-aerobic: without air.

Aerobic

Say it: air-ROH-bik
Word parts: aer: air. bios: life.
Everyday: with oxygen
What it is: Using oxygen.
In the gym: The aerobic system needs a steady supply of oxygen from your heart and lungs. It’s slower, but it can keep going for a very long time.

Aerobic: with air. The aerobics class takes its name from it.

Sources: [S8], [S10]. Word parts: [S18].

System 1: ATP-PC (phosphocreatine)

Phosphocreatine

Say it: FOS-foh-KREE-uh-teen. Shortened to PC (or PCr, or creatine phosphate).
Word parts: phospho: phosphate. creatine: from Greek kreas, flesh.
Everyday: creatine phosphate
What it is: An energy store held in muscle. It hands its phosphate to ADP to make ATP again, very quickly.
In the gym: It is the first and fastest back-up when you go all-out. It powers the start of every explosive effort.

Creatine, from the Greek for flesh: it sits right in the muscle, ready to go.

ATP-PC system

Say it: AY-tee-PEE PEE-SEE SIS-tum
Word parts: ATP + PC (phosphocreatine).
Everyday: the phosphagen system
What it is: The energy system that remakes ATP from phosphocreatine. No oxygen needed.
In the gym: It leads in heavy, explosive, very short efforts. It runs low fast. Oxygen is needed afterwards to rebuild the store, which is one reason you’re still breathing hard after a heavy single.

The turbo button: instant, huge, and gone in seconds.

	ATP-PC system
Fuel	Phosphocreatine stored in the muscle (plus the small store of ATP itself)
Oxygen?	No
Speed of energy supply	Fastest
Leads for roughly	The first 10 to 15 seconds or so of an all-out effort (approximate)
Gym examples	A heavy single or 1RM attempt. A short all-out sprint. A max box jump.

ATP-PC

Fuel: Phosphocreatine stored in the muscle (plus the small store of ATP itself)
Oxygen?: No
Speed: Fastest
Leads for roughly: First 10 to 15 seconds or so of an all-out effort (approximate)
Gym examples: Heavy single or 1RM. Short all-out sprint. Max box jump.

Sources: [S8], [S10], [S11]. Word parts: [S18].

System 2: glycolytic

Also called anaerobic glycolysis, the lactate system.

Glycolysis

Say it: gly-KOL-uh-sis
Word parts: glyco: sweet, sugar. lysis: loosening, breaking apart.
Everyday: sugar-splitting
What it is: Breaking down glucose (sugar) to make ATP. The first stage needs no oxygen.
In the gym: It supplies ATP quickly in hard efforts that last longer than a few seconds. The glucose comes from your blood or from glycogen stored in the muscle.

Glyco-lysis: sugar-splitting. Glyco, sweet. Lysis, breaking apart.

Glycolytic system

Say it: gly-kuh-LIT-ik SIS-tum
Word parts: glycolytic: of glycolysis.
Everyday: the lactate system, the lactic acid system, anaerobic glycolysis
What it is: The energy system that makes ATP fast from glucose without oxygen. It produces lactate as it goes.
In the gym: It starts almost at once in a hard effort and hits its top speed at around 10 to 15 seconds. It does a lot of the work in a hard set or sprint lasting up to about a minute. It can’t keep up that pace for long.

The sugar-burning sprint engine: fast, strong, and it can only hold that pace for about a minute.

Lactate

Say it: LAK-tayt
Word parts: lact: from Latin lac, milk. Lactic acid got its name because it was first found in sour milk.
Everyday: lactic acid
What it is: What the glycolytic system produces when it’s running fast. Your body can clear it and reuse it, including in the liver, where it can be turned back into glucose.
In the gym: It is not the cause of next-day soreness (DOMS). Lactate in the blood is back to normal within about an hour of a hard session. DOMS peaks a day or two later. The burn and fatigue during a hard set is a different thing. Rising acidity in the muscle may play a part, but the exact causes of fatigue are still not fully known.

Lactate shares its root with lactose, the sugar in milk. It’s gone within the hour, so it can’t be what makes you sore two days later.

DOMS

Say it: DOMZ. In full: delayed onset muscle soreness.
Word parts: plain English. Delayed onset: starts late.
Everyday: next-day soreness, “feeling it the day after”
What it is: Muscle soreness that starts 12 to 24 hours after unaccustomed exercise and is often worst between 24 and 72 hours.
In the gym: It’s linked to tiny damage in the muscle fibres and the repair that follows, not to lactate. It is most common after new exercises and lowering (eccentric) work, such as the way down in a curl or walking downhill.

Delayed is in the name: it turns up a day or two later, long after the lactate has gone.

	Glycolytic system
Fuel	Carbohydrate: glucose from the blood and glycogen stored in the muscle
Oxygen?	No
Speed of energy supply	Fast, but slower than ATP-PC
Leads for roughly	From about 10 to 15 seconds of hard effort up to about a minute (approximate)
Gym examples	A hard 30 to 60 second set. A fast 400 m for a trained runner. An all-out 30 second bike sprint.

Glycolytic

Fuel: Glucose and glycogen
Oxygen?: No
Speed: Fast, slower than ATP-PC
Leads for roughly: About 10–15 s up to about a minute (approximate)
Gym examples: Hard 30–60 s set. Fast 400 m for a trained runner. All-out 30 s bike sprint.

A hard 60 to 90 second effort sits on the crossover. By about 75 to 80 seconds of all-out work, the aerobic system is supplying roughly half the energy. So a hard 60 to 90 second set, or a 400 m that takes you well over a minute, is shared about evenly between the fast systems and the aerobic system.

Sources: [S8], [S10], [S11], [S12], [S13]. Word parts: [S18].

System 3: aerobic

Aerobic system

Say it: air-ROH-bik SIS-tum
Word parts: aer: air. bios: life.
Everyday: the oxidative system
What it is: The energy system that breaks down carbohydrate and fat using oxygen to make ATP, in the mitochondria inside your muscle cells.
In the gym: It makes far more ATP from each glucose than glycolysis does, and it can use fat. It is the main system at rest and in any steady effort, and it takes over most of the work in longer all-out efforts too. It needs your heart and lungs to keep oxygen coming, which is where the heart and lungs pages meet this one.

Your aerobic system is the diesel engine: never the fastest, but it runs for hours.

	Aerobic system
Fuel	Carbohydrate (glucose and glycogen) and fat (fatty acids)
Oxygen?	Yes
Speed of energy supply	Slowest, but by far the biggest capacity
Leads for roughly	At rest, in steady effort, and in all-out efforts lasting longer than about 75 to 90 seconds (approximate)
Gym examples	Steady-state cycling. A long class. A steady row or jog.

Aerobic

Fuel: Carbohydrate and fat
Oxygen?: Yes
Speed: Slowest, biggest capacity
Leads for roughly: Rest, steady work, and all-out efforts over about 75–90 seconds (approximate)
Gym examples: Steady cycling. Long class. Steady row or jog.

Sources: [S8], [S9], [S10]. Word parts: [S18].

Together

How the three work together

All three systems work at once, from the first second. Think of a relay race where the runners overlap for a long stretch instead of handing over a baton. Even in a 10 second sprint, the aerobic system is chipping in a little. Even in a long, steady class, the fast systems kick in the moment you speed up for a hill or a burst.

Two things decide which system leads:

How hard you’re working. The harder the effort, the more the fast systems lead.
How long it lasts. The longer the effort, the more the aerobic system leads.

The durations on this page are for all-out efforts. They’re approximate. Research studies don’t agree exactly, and they vary with the person, their training, the type of exercise and how they pace it. In easier work, the aerobic system leads from very early on.

Why you’re still puffing after a heavy set. After a hard effort your body needs extra oxygen to rebuild ATP and phosphocreatine and to process lactate. That’s why your breathing stays high after you’ve racked the bar.

Summary table

System	Fuel	Oxygen?	Speed	Leads for roughly (all-out)	Gym examples
ATP-PC	Phosphocreatine	No	Fastest	First 10 to 15 seconds or so	1RM attempt, short sprint
Glycolytic	Glucose and glycogen	No	Fast	About 10 to 15 seconds up to about 1 minute	Hard 30 to 60 second set, fast 400 m
Aerobic	Carbohydrate and fat	Yes	Slowest, biggest capacity	Rest, steady work, and all-out efforts over about 75 to 90 seconds	Steady cycling, long class

ATP-PC

Fuel: Phosphocreatine
Oxygen?: No
Speed: Fastest
Leads for roughly: First 10 to 15 seconds or so
Gym examples: 1RM attempt, short sprint

Glycolytic

Fuel: Glucose and glycogen
Oxygen?: No
Speed: Fast
Leads for roughly: About 10–15 s up to about 1 minute
Gym examples: Hard 30–60 s set, fast 400 m

Aerobic

Fuel: Carbohydrate and fat
Oxygen?: Yes
Speed: Slowest, biggest capacity
Leads for roughly: Rest, steady work, all-out over about 75–90 s
Gym examples: Steady cycling, long class

All ranges are approximate. The systems overlap and always work together.

Sources: [S8], [S10], [S11]

Heart-rate zones

Maximum heart rate

Say it: MAK-sih-mum HART rayt. Shortened to max HR or MHR.
Word parts: plain English.
Everyday: max heart rate
What it is: The highest your heart rate will go in an all-out effort.
In the gym: Heart-rate zones are worked out as percentages of it. Measuring it properly takes a lab exercise test, so most people use an estimate.

220 minus your age is a starting guess, not a measurement.

Talk test

Say it: TAWK test
Word parts: plain English.
Everyday: the talk test
What it is: Judging how hard you’re working by how easily you can talk.
In the gym: It needs no kit and works on anyone. NHS guidance: if you can talk but not sing, that’s moderate. If you can only say a few words without pausing for breath, that’s vigorous.

Talk but can’t sing: moderate. A few words, then gasp: vigorous.

RPE

Say it: AR-pee-EE. In full: rating of perceived exertion (per-SEEVD eg-ZUR-shun).
Word parts: perceived: how it feels to you. exertion: effort.
Everyday: “how hard, out of ten?”
What it is: A score for how hard an effort feels. One common version runs from 0 (sitting still) to 10 (maximal effort).
In the gym: On that 0 to 10 scale, moderate is about 5 or 6 and vigorous starts at about 7 or 8. It adjusts to the person: the same pace can feel moderate to a very fit client and vigorous to a new one.

Out of ten, how hard does this feel? That number is your RPE.

The 220 minus age formula

Estimated maximum heart rate = 220 − age

Example: a 40-year-old would have an estimated max of 220 − 40 = 180 beats per minute.

This is only a rough estimate. It can be well off for any one person:

It uses age alone. Two 40-year-olds can have very different real maximums.
It was built from testing on men and tends to overestimate max heart rate in women.
A Harvard cardiologist says estimates for individuals can be off by as much as 15 beats per minute.
Some medicines, such as beta blockers, slow the heart. For anyone taking them, the formula doesn’t fit.

Smart watches usually use the same age-based estimate, and they can also misread your heart rate.

Sources: [S2], [S4], [S15], [S16]

Zone calculator (optional)

Enter an age to see estimated max and bpm bands for a common teaching model. Not a prescription.

Age (years)

Enter an age to see estimated max HR and zone bpm.

Zone	% of estimated max	Example bpm	What it feels like
Easy	Below about 50%	—	Breathing a little faster than at rest. You could still sing.
Moderate	About 50 to 70%	—	Warmer, breathing faster. You can talk, but not sing.
Hard (vigorous)	About 70 to 85%	—	Breathing hard and fast. Only a few words before you need a breath.
Very hard	Above about 85%	—	Short bursts of near-maximum effort, with rest between.

Easy

Below about 50% of estimated max

—

Breathing a little faster than at rest. You could still sing.

Moderate

About 50 to 70%

—

Warmer, breathing faster. You can talk, but not sing.

Hard (vigorous)

About 70 to 85%

—

Breathing hard and fast. Only a few words before you need a breath.

Very hard

Above about 85%

—

Short bursts of near-maximum effort, with rest between.

The moderate (50 to 70%) and hard (70 to 85%) bands are the ones given by the British Heart Foundation and the American Heart Association. The easy and very hard rows simply sit either side of them. No source on this page gives them as separate bands. Different organisations draw the lines in different places — for example, Harvard Health uses 64 to 76% for moderate and 77 to 93% for vigorous. This is a common teaching model, not a prescription.

Talk test and RPE picker

Select how a set feels. No prescriptions — just the matching description from the content.

Pick one Choose how the set feels or how easily you can talk.

Standard guidance, not medical advice. Anyone with a heart condition, anyone taking medication that affects heart rate, and anyone with symptoms such as chest pain, dizziness or palpitations when exercising should follow their GP’s advice on exercise, including what heart rate (if any) to aim for. The NHS also advises speaking to your GP first if you haven’t exercised for some time or have medical conditions or concerns.

Sources: [S4], [S14], [S15], [S16], [S17]

Try it

Effort timer and name the system

Two ways to feel how the three systems overlap. All three work together; none switches off.

The effort timer

Press start on an imagined all-out effort and watch which system leads as the seconds pass. Bars show which system leads, not exact shares. Timings are approximate and for all-out efforts.

0.0 s

Ready. Start an all-out effort.

ATP-PC

leads

Glycolytic

Aerobic

No bar ever drops to zero — that is the lesson. The bands overlap. Durations are approximate.

Steady pace (aerobic leads almost from the start)

Name the system

Type which system leads for the gym scenario. Sensible name variants count. After each answer, remember the other two systems were working too.

Scenario Loading…

Recall

Quick check

Type each answer from memory. No list.

Sources

[S2] OpenStax, 19.4 Cardiac Physiology — openstax.org
[S4] British Heart Foundation, What should my heart rate be when I exercise? — bhf.org.uk
[S8] OpenStax, 10.3 Muscle Fiber Contraction and Relaxation — openstax.org
[S9] OpenStax, 24.1 Overview of Metabolic Reactions — openstax.org
[S10] Gastin & Suppiah, Anaerobic and Aerobic Energy System Contribution During Maximal Exercise (Sports Medicine, 2026) — Springer
[S11] Baker et al., Interaction among Skeletal Muscle Metabolic Energy Systems (2010) — PMC
[S12] ACSM, Delayed Onset Muscle Soreness (DOMS) brochure (via Semantic Scholar PDF mirror; original ACSM link 404) — semanticscholar.org
[S13] Maes & Kravitz, Treating and Preventing DOMS — unm.edu
[S14] NHS, Physical activity guidelines for adults aged 19 to 64 — nhs.uk
[S15] American Heart Association, Target Heart Rates — heart.org
[S16] Harvard Health, All about your heart rate — health.harvard.edu
[S17] CDC, How to Measure Physical Activity Intensity — cdc.gov
[S18] Online Etymology Dictionary (Etymonline) — etymonline.com

All accessed 6 October 2026.

The currency

ATP

Two words you need first

Anaerobic

Aerobic

System 1: ATP-PC (phosphocreatine)

Phosphocreatine

ATP-PC system

ATP-PC

System 2: glycolytic

Glycolysis

Glycolytic system

Lactate

DOMS

Glycolytic

System 3: aerobic

Aerobic system

Aerobic

How the three work together

Summary table

ATP-PC

Glycolytic

Aerobic

Heart-rate zones

Maximum heart rate

Talk test

RPE

The 220 minus age formula

Zone calculator (optional)

Easy

Moderate

Hard (vigorous)

Very hard

Talk test and RPE picker

Effort timer and name the system

The effort timer

Name the system

Quick check

Sources

Train the names