ATP: the only fuel muscles can use
ATP (adenosine triphosphate) is the energy molecule in every cell. When a muscle contracts it breaks ATP into ADP + phosphate, and energy is released.
The muscle holds only a tiny store of ATP: enough for about 2โ3 seconds of all-out effort. So the body must keep remaking ATP from ADP. It has three ways to do this, called the energy systems.
The three energy systems
1. ATP-PC (phosphocreatine) system
Uses creatine phosphate (PC) stored in the muscle. PC gives its phosphate to ADP to make ATP. No oxygen needed. Very fast and very powerful, but lasts only about 8โ10 seconds. No tiring by-products. Examples: 100 m sprint, shot put, a jump, a tennis serve.
2. Anaerobic glycolytic (lactic acid) system
Breaks glucose (from glycogen) without oxygen. Fast, high power, lasts about 10 seconds to 2โ3 minutes. Gives only 2 ATP per glucose. By-product: lactic acid, which turns into lactate and hydrogen ions. These make muscles feel burning and tired. Examples: 400 m run, 100 m swim.
3. Aerobic system
Uses oxygen to break down glucose and fat (and a little protein) in the mitochondria. Slow to start, lower power, but lasts for hours. Gives a lot of ATP (about 36โ38 per glucose). By-products: carbon dioxide and water, which are easy to remove. Examples: marathon, long-distance cycling, walking.
Word equation: glucose + oxygen โ carbon dioxide + water + energy.
Aerobic and anaerobic exercise
Aerobic exercise means 'with oxygen'. It is steady, lower-intensity work where the heart and lungs can supply all the oxygen needed, for example jogging.
Anaerobic exercise means 'without oxygen'. It is short, very high-intensity work where the body cannot supply oxygen fast enough, for example sprinting or heavy lifting.
The energy continuum
The systems do not take turns like a switch. All three work all the time; the intensity and duration decide which one gives most of the energy. Team games such as football, hockey and basketball use all three in turn: sprints use ATP-PC, a long run up the field uses glycolysis, and jogging between plays is aerobic.
EPOC and recovery
At the start of hard exercise the body cannot get enough oxygen yet. This shortfall is the oxygen deficit. After exercise you keep breathing fast. The extra oxygen used during recovery is EPOC: excess post-exercise oxygen consumption (sometimes called the oxygen debt).
EPOC has two parts:
- Fast (alactacid) part, first 2โ3 minutes: refills ATP and PC stores (about 50% in 30 s, nearly all in 3 minutes) and reloads myoglobin with oxygen.
- Slow (lactacid) part, up to an hour or more: removes lactate (mostly turned back to energy or glucose in the liver), keeps breathing and heart rate high, and cools the body down.
Good recovery: an active cool-down (light jogging) keeps blood flowing and clears lactate faster; refuel with carbohydrate and drink water; rest and sleep.
Altitude and heat
High altitude (for example 2,500 m in the mountains) has lower air pressure, so each breath carries less oxygen into the blood. The aerobic system cannot supply as much, so the athlete uses anaerobic energy sooner, makes more lactate and tires faster. Endurance times get slower. After a few weeks the body makes more red blood cells, which is why some athletes train at altitude.
Heat: blood is sent to the skin to lose heat, and sweating can cause dehydration, so less blood carries oxygen to the muscles. Heart rate rises, and the athlete tires sooner. Drink regularly, wear light clothes and allow time to get used to the heat.
Try it: feel the systems change
Run on the spot as fast as you can for 10 seconds, then rest. Next, run as hard as you can for 60 seconds, then rest. Count your breaths per minute straight after each one and again after 3 minutes. Which one left you breathing harder for longer? (The 60-second effort used more glycolysis, so the slow part of EPOC lasts longer.) Then set the same times on the slider in the 3D and check which bar was biggest.
Key formulas and definitions
- ATP โ ADP + phosphate + energy
- PC + ADP โ ATP + creatine (ATP-PC system)
- glucose โ lactic acid + energy (2 ATP, no oxygen)
- glucose + oxygen โ carbon dioxide + water + energy (about 36โ38 ATP)
- EPOC = extra oxygen used after exercise to recover
Worked examples
1. Which energy system mainly powers a 100 m sprint? Why?
The ATP-PC system. The race lasts about 10โ12 seconds at maximum intensity, and ATP-PC gives the fastest, most powerful energy for around 10 seconds without oxygen.
2. Why do your legs burn at the end of a 400 m race?
The race lasts about 50โ60 seconds at high intensity, so anaerobic glycolysis gives most of the energy. It makes lactic acid; the hydrogen ions make the muscles feel burning and tired.
3. A footballer jogs, then sprints for the ball, then jogs again. Which systems are used?
Jogging is aerobic. The sprint uses ATP-PC. If the sprint and running go on for longer, glycolysis helps. During the jog afterwards, the aerobic system refills the PC stores.
4. Why do you keep breathing hard after a race even though you have stopped?
This is EPOC. Extra oxygen is needed to refill ATP and PC stores, reload myoglobin, remove lactate and bring body temperature and heart rate back down.
Common mistakes
- Thinking the systems switch on one at a time. All three work together; one just gives most of the energy.
- Saying lactic acid causes soreness the next day. The burning is during exercise; next-day soreness is mostly tiny muscle damage.
- Mixing up oxygen deficit (shortfall at the start) and EPOC (extra oxygen after).
- Saying the aerobic system uses only glucose. It also burns fat, especially in long, steady exercise.