What is aerobic exercise

Aerobic exercise takes its name from the way your body produces energy while you do it.

Aerobic exercise is exercise in which energy comes from burning fuel with oxygen.

What does that mean?

Without going deep into metabolism: during aerobic exercise your blood can deliver enough oxygen to your muscles to keep energy production going. Because the muscles get the oxygen they need, they can release energy from fat as well as from carbohydrate. That is why this kind of exercise is so often recommended to people who want to lose weight.

The opposite is anaerobic training, where the muscles work so hard that your blood cannot keep up with the oxygen demand, so they produce energy without it. Fat cannot be used that way, so to cover the shortfall the body burns something else - mainly stored carbohydrate.

Aerobic training

For training to stay aerobic, the muscles cannot be loaded too heavily, because then the blood cannot supply enough oxygen. Aerobic training is therefore steady effort at a constant but moderate level of muscular work.

Aerobic sessions are usually built so that they do not exhaust one particular muscle but instead engage large muscle groups: the legs, the back, the arms. Done evenly and continuously, they do not wear the muscles out, yet they are intense enough to get your circulation going and push your metabolism up.

Intensity of aerobic training

For training to be aerobic, your heart rate must not be too high. A very fast heart rate means the heart is working hard to pump oxygenated blood to muscles that are short of oxygen. On the other hand, the session still has to be demanding enough to make your body work.

The effective range for aerobic exercise is 55% to 85% of maximum heart rate. Where you sit in it depends on your fitness and your goal.

• Beginners should start at the bottom of the range and train harder as their fitness improves.
• If your goal is losing weight, stay near the lower end - work without great effort, but for long stretches (40-90 minutes).
• If you mainly want to raise your endurance and strengthen your cardiovascular system, you can train harder. Just never overdo it.

Which fuel your body burns

Your body always burns fat and carbohydrate at the same time. What changes is the proportion, and the thing that changes it most is how hard you are working:

• Carbohydrate - circulating blood sugar and the glycogen stored in your muscles and liver. The harder the effort, the larger its share, because carbohydrate can be broken down quickly, with or without oxygen. Your glycogen stores are limited, and running them down over a long hard session is what makes you "hit the wall" - it is not the moment fat burning switches on.
• Fat - broken down through beta-oxidation, which needs oxygen. Fat is being used from the first minute of exercise, and it supplies the greater share at easy intensities, which is exactly why easy aerobic work is prescribed for fat loss. But what decides whether you actually lose fat is the total energy you spend over weeks against what you eat - not the ratio inside any single session, and not its length crossing some threshold.