Energy for exercise

Why is a muscle like a motor bike?

Although muscles and engines work in different ways, they both convert chemical energy into energy of motion.

• A motorbike engine uses the stored energy of petrol and converts it to heat and energy of motion (kinetic energy).
• Muscles use the stored chemical energy of food we eat and convert that to heat and energy of motion (kinetic energy).

Where does the energy for muscle contraction come from?

The source of energy that is used to power the movement of contraction in working muscles is adenosine triphosphate (ATP) – the body’s biochemical way to store and transport energy. However, ATP is not stored to a great extent in cells. So once muscle contraction starts, the making of more ATP must start quickly.

Since ATP is so important, the muscle cells have several different ways to make it. These systems work together in phases. The three biochemical systems for producing ATP are, in order:

• using creatine phosphate
• using glycogen
• aerobic respiration.

Using creatine phosphate

All muscle cells have a little ATP within them that they can use immediately – but only enough to last for about 3 seconds! So all muscle cells contain a high-energy compound called creatine phosphate which is broken down to make more ATP quickly. Creatine phosphate can supply the energy needs of a working muscle at a very high rate, but only for about 8–10 seconds.

Using glycogen (and no oxygen)

Fortunately, muscles also have large stores of a carbohydrate, called glycogen, which can be used to make ATP from glucose. But this takes about 12 chemical reactions so it supplies energy more slowly than from creatine phosphate. It’s still pretty rapid, though, and will produce enough energy to last about 90 seconds. Oxygen is not needed – this is great, because it takes the heart and lungs some time to get increased oxygen supply to the muscles. A byproduct of making ATP without using oxygen is lactic acid. You know when your muscles are building up lactic acid because it causes tiredness and soreness – the stitch.

Using aerobic respiration (using oxygen again)

Within two minutes of exercise, the body starts to supply working muscles with oxygen. When oxygen is present, aerobic respiration can take place to break down the glucose for ATP. This glucose can come from several places:

• remaining glucose supply in the muscle cells
• glucose from food in the intestine
• glycogen in the liver
• fat reserves in the muscles
• in extreme cases (like starvation), the body’s protein.

Aerobic respiration takes even more chemical reactions to produce ATP than either of the above two systems. It is the slowest of all three systems – but it can supply ATP for several hours or longer, as long as the supply of fuel lasts.

Here’s how it works

You have missed the bus and start running to college for a 9.00am exam:

• For the first 3 seconds of your run to college, your muscle cells use the ATP they have within them.
• For the next 8–10 seconds, your muscles use creatine phosphate stores to provide ATP.
• Since you haven’t made it to college yet, the glycogen system (which doesn’t need any oxygen) kicks in.
• Still not there, so finally aerobic respiration (that’s ATP using oxygen) takes over.

Different forms of exercise use different systems to produce ATP

A sprinter is getting ATP in a very different way to a marathon runner.

• Using creatine phosphate – This would be the major system used for short bursts (weightlifters or short distance sprinters) because it is fast but lasts for only 8–10 seconds.
• Using glycogen (no oxygen) – This lasts for 1.3–1.6 minutes, so it would be the system used in events like the 100 metre swim or the 200 m or 400 m run.
• Using aerobic respiration – This lasts for an unlimited time, so it’s the system used in endurance events like marathon running, rowing, distance skating and so on.

Explore this further in the article Marathon versus sprint.