When you sprint for a matatu, carry a heavy jerrycan up the stairs or jog 3 kilometres around Kololo, your muscles need fuel in a form they can use instantly. That fuel is a molecule called ATP, and understanding how your body makes it explains why a 100 metre dash, a set of squats and a long walk feel so different.
ATP: the only fuel muscles actually burn
Adenosine triphosphate, or ATP, is the energy currency of every cell. When a muscle contracts, it splits ATP and releases energy. The catch is that your muscles store only a tiny amount, enough for a few seconds of hard effort. After that, the body has to rebuild ATP as fast as it uses it.
Food, whether matooke, rice, beans or fish, is not burned directly by muscles. It is broken down into fuels like glucose and fatty acids, and these are used to remake ATP. Your body has 3 main ways of doing that, called energy systems. They overlap all the time, but one usually dominates depending on how hard and how long you are working.
System 1: the phosphagen system (0 to about 10 seconds)
This is your instant fuel. Muscles hold a store of creatine phosphate, which can donate a phosphate to rebuild ATP almost immediately and without oxygen.
It powers:
- A heavy single or triple on the squat or deadlift.
- A 20 to 60 metre sprint.
- A jump, a throw or the early seconds of a push.
It is powerful but short. The store runs low in around 10 seconds, and then it takes roughly 2 to 5 minutes of rest to refill most of it. That is why strength coaches ask you to rest several minutes between heavy sets. It is not laziness, it is chemistry.
System 2: glycolysis (about 10 seconds to 2 minutes)
When the instant store runs low, your body breaks down glucose, from the blood or from glycogen stored in muscle, to make ATP quickly. This process is called glycolysis. It does not need oxygen to run, so it is fast, but it is not very efficient and it produces by-products.
You feel it in:
- A set of 12 to 20 squats or lunges.
- A 400 metre run at full effort.
- A hard 60 second burst on the rowing machine or battle ropes.
That burning, heavy sensation in the legs is related to the build-up of hydrogen ions and other by-products, not a simple "lactic acid" poison. Lactate itself is actually a useful fuel that muscles, the heart and the brain can use again. People often blame it for soreness the next day, which is wrong. Next-day soreness comes from the stress on the muscle fibres, not from lactate.
System 3: the oxidative (aerobic) system (minutes to hours)
This is the long-haul engine. With oxygen available, your cells burn carbohydrate and fat inside structures called mitochondria, producing a lot more ATP from each unit of fuel. It is slower to ramp up, but it can run for hours.
It dominates during:
- Walking, easy jogging and cycling.
- Long football or netball matches between the bursts.
- Recovery between hard sets and between sprints.
The harder you work, the more you lean on carbohydrate. At low intensity, a larger share of the fuel is fat. That is why a brisk walk is a good fat-burning activity in the narrow sense, but it does not mean it burns the most calories overall. A hard effort uses more total energy per minute.
How the systems work together
No system ever switches off completely. Think of them as 3 taps feeding the same bucket. At the start of any effort, all 3 open, but the fast taps give the most early on and the slow tap takes over as time passes.
Take a 10 minute game of football with friends:
- The phosphagen system fuels each short sprint and kick.
- Glycolysis helps when you chase the ball repeatedly without a break.
- The aerobic system refills the fast stores in the pauses and carries you through the rest of the game.
This is why a strong aerobic base helps even athletes whose sport is mostly explosive. The better your aerobic system, the faster you recover between hard efforts.
Training each system
You can train your energy systems on purpose. Here is a simple guide.
Phosphagen (power)
- 5 to 10 second all-out efforts such as sprints, jumps or heavy singles.
- Rest 2 to 3 minutes between efforts, longer for very heavy lifts.
- 6 to 10 efforts is plenty. Quality drops fast when you are tired.
Glycolytic (hard conditioning)
- 30 to 90 seconds hard, with rest of about the same length or double long.
- Examples: rowing intervals, hill sprints, kettlebell swings, sled pushes.
- 4 to 8 rounds is a tough session. Do it once or 2 times a week at most.
Aerobic (the base)
- 20 to 60 minutes of steady work at a pace where you can talk in short sentences.
- Walking quickly to work, easy cycling, swimming or light jogging all count.
- The WHO guideline of at least 150 minutes of moderate activity a week is a good starting target.
Most people do better with a bit more of the base than they expect. Easy aerobic work builds more mitochondria, improves blood flow in muscles and speeds up recovery, and it is not very stressful on the body.
Fuel and breathing in practice
A few practical notes follow from the science.
- Do not skip all carbohydrate before hard training. Glycolysis runs on glucose, so a small meal of rice, bread, sweet potato or a banana an hour or 2 before a hard session helps.
- Rest properly between heavy sets. 2 to 3 minutes is not wasted time. You are refilling the phosphagen store.
- Breathe out of rhythm if you must, but do not hold your breath for long. Holding briefly during a heavy rep is normal, but long breath holds can spike blood pressure. If you have high blood pressure or heart disease, ask your doctor before heavy lifting or all-out intervals.
- Hydrate in Kampala's heat. Sweating reduces blood volume, which makes your heart work harder and weakens the aerobic system.
Why you feel out of breath after a sprint
After a hard effort, you keep breathing heavily for a while. This is sometimes called the oxygen debt, and more precisely, extra oxygen use after exercise. Your body uses that oxygen to restore creatine phosphate, clear by-products and return the heart rate and temperature to normal. The more intense the effort, the larger the bill, and the longer it takes to pay.
Key takeaways
- ATP is the actual fuel for muscle, and your body remakes it through 3 overlapping energy systems.
- The phosphagen system powers the initial 10 seconds, glycolysis powers the next minute or 2, and the aerobic system carries long efforts and recovery.
- Lactate is a fuel, not the cause of next-day soreness.
- Match your rest to the effort: several minutes after heavy or sprint efforts, shorter rests for conditioning work.
- A solid aerobic base improves recovery from every other kind of training, so do not neglect easy cardio.
