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Energy for Action

Sunlight – your body’s energy source

When you bend your elbow, take a step or wink your eye, you use muscle power. But what exactly is muscle power and how does the muscle get its power?

It seems remarkable – but would you believe that your muscle power comes from sunlight!  It’s the same kind of power that charges the batteries of a space probe, or starts a fire when the sun’s rays are focused through a magnifying glass. It is the radiant energy from the sun (which in turn is produced by the conversion of hydrogen atoms into helium atoms).

That solar energy gets into your body through the foods you eat. A process known as photosynthesis locks the energy from sunlight into the starches and sugars of plants. The sunlight is trapped by the green chlorophyll and used in the manufacture of carbohydrates from carbon dioxide and water.

These carbohydrates can be stored in your muscles and liver (in the form a substance called ‘glycogen’) to provide fuel for muscle power.

As your muscles use this locked energy, the carbohydrate molecule breaks down into its original components – carbon dioxide and water, which are removed from your body as waste products.

Your body can also get energy from its fat stores and, to a lesser extent, from protein.

The fact that energy is released during physical activity is apparent as you feel your body heating up, causing you to sweat. In fact around 75% of all the energy released is in the form of heat, whilst approximately 25% is harnessed for muscle power.

Muscle Fibres – your body’s ‘action kit’

All of this chemical activity – molecules of sugar and fat breaking down to release energy to power your muscles – goes on in tiny muscles fibres, about the size of a human hair!

Your body has more than 600 muscles containing over six billion muscle fibres – varying in length from about one millimetre to several centimetres.

The smaller muscles of your eyes and hands contain fewer and smaller fibres than the larger muscles of your arms and legs.

Although muscle fibres are extremely small, they are capable of supporting over 1,000 times their own weight.

When groups of fibres act together, they can move your limbs in a powerful manner; the more fibres recruited, the stronger your actions.

Aerobic v Anaerobic

Food provides energy for action, just as fuel provides energy for a car engine. But the engine has only one way of converting fuel, whilst your body can shift from one energy system to another.

Anaerobic metabolism – instant energy

Your muscles can work for limited periods of time without oxygen. This is termed ‘anaerobic’ work. During the few minutes of intense effort or lifting & carrying heavy objects, your body uses an energy store located within the fibres.

First, your muscles tap a substance called ATP (adenosine triphosphate). This is the only fuel that can power your muscles. But your store of ATP is tiny – and it burns out about one second. So, your body then uses another substance stored in your muscles, CP (creatine phosphate) to rebuild a fresh supply of ATP. This system supplies enough energy for around 10 seconds of intensive exertion. After that, your muscles begin to use a third energy source, glycogen (a form of carbohydrate stored in your muscles – with extra supplies stored in your liver) to rebuild the ATP. Unfortunately, this energy system also generates a substance called ‘lactic acid’ – which builds up in your muscles causing pain and fatigue. When you stop exercising this lactic acid is flushed out of your muscles into the blood stream, enabling your muscles recover.

Lactic acid therefore, is the substance that causes your legs to ache – for example, when you’ve climbed a few flights of stairs.

Anaerobic energy is vital to us and it provides the instant energy required for the first few seconds of intensive physical activity and for the first few minutes of muscle endurance work.

Aerobic metabolism – the key to stamina & endurance performance

For activity to last more than a few minutes, we have to get our energy from a much longer-lasting source. This is termed aerobic energy’ and relies on oxygen being supplied to your muscles by your lungs, heart and blood circulation. This is why aerobic fitness (‘aero’ means with oxygen) is often called cardiorespiratory (heart-lung) fitness.

The oxygen you breathe in and transport to your muscles then combines with molecules of fat or carbohydrate to provides the energy for aerobic exercise – such as walking, cycling and swimming.  Aerobic metabolism is around 16 times more efficient at generating energy than anaerobic metabolism. That’s why after a few minutes of exercise you settle into your ‘second wind’. Your body has gradually changed from supplying your energy anaerobically during the first few minutes to a more efficient aerobic energy system as you continue to exercise.

Carbs or Fats

At rest your body is using a mixture of carbohydrates (glycogen) and fats to supply its energy needs. During anaerobic exercise, your body uses its glycogen stores to generate the bulk of its energy needs (along with CP).

When you exercise aerobically, at a fairly vigorously intensity of around 75-85% maximum heart rate, carbohydrate (glycogen) is your body’s preferred fuel.

If you ‘go for the burn’ with very high intensity workouts, then your body has to use glycogen and has to work more anaerobically as you near exhaustion.

However, when you exercise at a moderate pace – such as a brisk walk – your body will prefer to use fat as the fuel to rebuild fresh supplies of ATP. This is why L-S-D (‘long-slow-distance’) activities, such as walking and cycling are excellent ‘fat-burning activities’. However, it takes around 15 minutes for your fat metabolism to swing into action, so a half hour brisk walk is ideal to get your fat-burning enzymes moving!

Fibre Types and Colours  

Are you a Fast-Twitch or a Slow-Twitch person?

All muscle fibres are connected by nerves to your brain and they respond to nerve impulses. A nerve impulse is rather like a tiny electrical current travelling at speeds of up to 100 metres a second.

Some of our fibres appear quite white in colour and are connected to your central nervous system by high-speed nerves. They are called ‘fast-twitch fibres’ and provide speed and high power movements. A top class sprinter would have a very high proportion of fast twitch fibres.

Other fibres appear quite red and are connected by less-fast nerves to your brain. These fibres are termed ‘slow-twitch fibres’ and are better suited to more prolonged, endurance work. A top level marathon runner may have muscles containing 80-90% of these red, fatigue-resistant, slow-twitch fibres.


Most of us have a good mix of red and white – and various shades of ‘pink’ in between. Some of us have a higher proportion of white fibres and are better at strength and power activities whilst others are ‘red-fibre folks’ and are better suited to stamina-type, endurance work. Different types of fitness programmes will train these different fibre types. We can train for strength & power or for endurance. Each aspect of fitness will be improved best by training which is specific to it. So strength training is quite different to stamina training.

We can see these different fibre types in the sprinters and endurance performers of the animal world. For example, a breast of chicken consists of white meat – chickens are adapted for short, fast movements. Most game birds, such as grouse & pheasant, have pale pinkish-brown pectoral (breast) muscles, as they are flying sprinters.

On the other hand, pigeons, ducks & migrating birds have much darker, redder pectoral muscles and are well adapted to flying hundreds of miles non-stop.

There are also many examples where the two types of fibres are present in one animal – as in fish. If a grilled trout is laid on its side and the skin carefully removed, a thin line of red muscle (brown after cooking) can be clearly seen, which contrasts to the paler colour of the bulk of the muscle. This red muscle is involved in normal swimming, whilst the white muscle only comes into operation for more violent & powerful movements of short duration, such as when leaping rapids or caught on an angler’s hook.

World-Class Sprinters & Endurance Performers

  • The cheetah is the fastest sprinter – it can accelerate to 45mph in 2 seconds and reaches 70mph at full speed!! Its muscles consist predominantly of white, fast-twitch fibres. However it has little stamina and can sustain such speed for only a relatively short chase.
  • A flea can propel itself over 200 times its own body length in a single jump!! (compare this to a Carl Lewis Long Jump of just over 5 times his body length).
  • The monarch butterfly flies over 2,000 miles at an average speed of 75 miles per day to reach its wintering site!!
  • The humming-bird flies over 1,500 miles across the Gulf of Mexico in 24 hours!!

Humans – all-purpose beings!

Whilst there are some superlative performances from animals, insects, birds & fish, we humans are designed as wonderful all-purpose beings. Individuals of our species can sprint 100 metres in less than 10 seconds, run 26 miles in just over 2 hours, cover 5,000 miles in less than 107 days (Max Telford ran from Anchorage, Alaska to Halifax, Nova Scotia in 1977, a distance of 5,110 miles in 106 days 22 hours), fly for one mile with the aid of artificial wings and swim the English Channel (21 miles) in less than eight hours.

In addition, we can produce movements of great precision, such as playing musical instruments, writing & typing, or striking a moving ball – all of which require extraordinary co-ordination. No other species is capable of this versatility!


The human body has astonishing endurance capabilities. In the animal kingdom only camels have been found to surpass humans in terms of endurance capacity. Australian Aborigines can wear down and catch kangaroos. Mexican Indians run great distances through mountainous terrain to catch deer. Navajo Indians can catch longhorn antelope – one of the fastest animals on earth – simply by wearing it down over long distances. A top ultra-distance runner (male or female) will easily beat a horse over a 100-mile race!!


Your body is capable of generating energy for action in remarkable and elegant ways.  But remember, in order for your body to work efficiently, it must be kept in good shape with regular physical activity and healthy eating habits.



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