How the body uses energy

Why bother with a rest between sets when you're exercising? 

Ever wondered why you're made to rest between sets of exercise when lifting weights? Obviously you might feel you need a breather but you're also doing it for the body to resynthesise ATP, the body's fuel.

The body has three energy systems to use depending on how intense you're working. All of them use ATP to power the movement.

It's a bit like having three pans on a hob. Each one is on but the one that's best for the activity will have the heat turned up while the other two are on a low simmer.

Phosphate creatine system
Power, sprinting, fast movements are using the phosphate creatine system. You only have a very short-lived supply of this, 8-10 seconds before it runs out. That's why you come to a grinding halt when you're doing intense exercise.

Power lifting, 100m sprint, explosive movements use this system.

Lactic acid system
This is burning glycogen (stored carbs) for energy but there's no (or not enough) oxygen coming in. That's the burning sensation as the lactic acid begins to build. You can't keep this up for long, 2 to 3 minutes.

Playing football, 400m run, weight lifting will use this system most.

Aerobic system
It's using glycogen and fat but with oxygen. The only bi-product is CO2 which you breathe out so you can keep going and going. It takes the body about 3 minutes to get going in full aerobic mode. 

That first mile you run feels hard? The body starts with burning oxygen in the lactic acid system but as you keep going at a steady pace and you can breathe enough oxygen in, the body can tick over. If you decide 15 minutes in you're going to sprint to the finish line you're turning up the heat on the lactic acid pan (as you can't get enough oxygen in to meet demand) and the lactic acid starts to build.

Long distance cycling or marathon running are using this system.

The aerobic system can keep going forever in the circular Krebs cycle. If it's a couple of years since you did GCSE Biology here's an easy watch video if you fancy a refresh.