Running on empty

All the events in the Olympic Games have one thing in common: the athletes have to move their bodies. This movement is achieved
by muscles. Muscles are special parts of the body which are able to do only one thing: they can contract, which means they can get
shorter when instructed to by the brain. As a muscle contracts, it pulls on the bones it is attached to, and that moves the athlete's
body. Easy peasy.

But there's a snag. To contract, a muscle needs a supply of energy. And it must get that energy in a special form: a magic molecule
called ATP. As a molecule of ATP splits apart in muscle it releases the energy that makes the muscle contract. Muscles run on ATP

The shot put

When the strong man of the Olympic world, the male shot putter, launches his 7.25 kg missile, his arm muscles contract for less than a second to do the job. And that's fine, because rested muscle contains enough ATP to contract flat out for a full second. That's why his arm doesn't get tired half way through the throw, and why he doesn't have to sit down panting afterwards. An easy life, eh?

The 100m sprint

But things are very different for the poor old sprinters. It will take them about 10 seconds to reach the end of the track, and
after the first second all of the ATP in their leg muscles will be used up. Ooops. Fortunately, muscle contains far more of another
chemical called creatine phosphate. This is able to regenerate the ATP using the chemicals left over when it is broken down.

Muscles can use this ATP to keep working for longer. Leg muscles contain enough creatine phosphate to contract flat out for
about 6 seconds, which gets the runner to about the 60m mark. But then it runs out.


You don't see 100m runners slowing down or stopping at 60m, because they have a way to make more ATP to keep their muscles contracting. Muscles also contain a large store of the chemical glycogen. Glycogen can be broken down to the sugar glucose, and when a glucose molecule splits in half it releases more ATP.

The sprinter's leg muscles contain enough glycogen to keep contracting for a long time, so he or she can easily run flat out for the full 100m.

Biologists call this breakdown of glucose glycolysis: "lysis" for "splitting" and "glyc" for "sugar".

The 400 metres

However there's a problem with glycolysis: it also makes lactic acid. After about 20 seconds sprinting flat out the muscles gets quite acidic. This stops them contracting so efficiently so, try as they might, the sprinters can-
not keep running at full speed. It also hurts! You know the feeling - when your muscles feel on fire because you have worked them
too hard for too long.

For this reason, the Olympic record for the men's 400m (43.5 seconds) is more than twice the time for the 200m (19.3 seconds).
The sprinter just has to slow up over the second half of the race.

Once the unlucky sprinters stops running, they have to get rid of the lactic acid from their muscles. This needs extra oxygen, so
the runners will breathe hard for quite a time to get the extra oxygen needed into their blood. They are "paying back the oxygen
debt". You will have noticed the same problem after you've run fast!

The Marathon

There's another snag about glycolysis too: it only extracts about 5% of the energy from each glucose molecule in the form of ATP and wastes the rest. This is fine when running 400 metres, but in the Marathon (42.2 kilometres) this depletes the athlete's glycogen reserves far too fast.

So Marathon runners extract ATP from their glycogen using aerobic respiration, so-called because it makes use of oxygen from the air. This breaks down the glucose completely to carbon dioxide and water with the help of oxygen.

The glycogen stored in their muscles and liver is enough to get them to about the 32km mark like this. But they have to be careful. The speed they can run at is limited by how fast they can get oxygen from the air to their muscles, and that speed is a lot slower than a sprint. Go too fast and they start using glycolysis, which burns up their precious fuel a frightening twenty times too quickly!

Running on empty

For the last 10km of their run the Marathon runners are clean out of glycogen. They have to start burning fat instead, using the
fat reserves under their skin. This may be a useful slimming aid, but fat is a much slower fuel to burn than glycogen, so the
runners are forced to slow up. They also feel dreadful.

They describe the effect of running out of glycogen as "hitting the wall". Marathon runners really earn their medals!

Record time

The average speed of a male Olympic gold medallist, when running different events, is given below. Can you now describe and
explain this pattern?