Right, having talked about the idea of ‘supercompensation’ and the importance of getting a good rest, I’ll now go onto the body’s two main energy systems, how they work and how we can train them.

The word ‘aerobic’ means ‘with oxygen’ and describes the way our body generates and utilises energy for the vast majority of the time. As a much simplified explanation, we take energy into the body in the form of food and drink; the digestive system takes the fats and carbohydrates out of the food and the liver turns them into glycogen, which is then stored in the cells in and surrounding our muscles. Excess glycogen is turned into fat – this is why if you eat too much, you get fat because once the muscles are full, there’s nowhere else for the glycogen to go! When we exercise, looking at the process in simple terms, we take fat and carbohydrates as fuel and ‘burn’ them in the muscles using oxygen to create energy – much the same way as you would need air (oxygen) to burn wood (fuel) in a fire to make heat (energy). The oxygen, of course, comes from the air you breathe – hence, when you burn more fuel to make more energy in the muscles and run faster, you need more air and that is why you breathe harder and your heart pumps faster to get the oxygen and fuel to your muscles. This use of oxygen is where the name ‘aerobic’ comes from. See, it all links together and makes sense!

As you can see, we can burn either fat or carbohydrates in the aerobic energy system, so are there any advantages to either fuel? Well, glycogen, the primary product of digesting carbohydrates, is easy to burn, creates plenty of energy but runs out fairly quickly as the muscle cells can only store a limited amount. This is replaced by glycogen generated in the liver and transported to the muscles by the blood; unfortunately, the liver cannot replace the glycogen being used by the muscles at the same rate – it’s like trying to fill a bath with water when the plug’s out – and eventually, you run out of glycogen – the bath empties. This is usually after about 90-120 minutes of effort and corresponds to what we call ‘hitting the wall’. Now you know where the name for the Corsham Running Club newsletter comes from. When we ‘hit the wall’, we feel really tired and it becomes difficult even to shuffle along – anyone who has done a marathon has probably experienced this: the mind is willing but the body says ‘No!’. At this point, the body cannot generate enough glycogen to replace what is being used by the muscles and has to fall back exclusively onto its secondary fuel: fat. Fat is slow burning, requiring lots more oxygen to burn but makes plenty of energy; in fact, it contains twice as much energy as the same amount of carbohydrate. The problem is the slow rate at which fat burns and, unless we specifically train our bodies to burn fat at higher exercise intensities, the body will burn glycogen in preference because it’s easier. The way we ‘teach’ our bodies to burn a higher percentage of fat in preference to glycogen is to train at low intensities for long periods – this is what we do on a typical long slow distance (LSD) session, where we exercise at a low pace trying not to get to that point where the body says ‘Right, burning fat is too hard, I’m going to use all glycogen instead’. By using fat in preference to glycogen, our aerobic system becomes very efficient and we are able to maintain our speed over progressively longer distances without ‘hitting the wall’, as we are not using glycogen faster than we can replace it.

Now, you should note from the last sentence that we are still using glycogen during low intensity exercise, even when we are mainly burning fat – this is because ‘the fat burns in a glycogen flame’ (a popular way of explaining the process). So, really, we’re always using glycogen and fat, just in different proportions. By using proportionally more fat, we can make the limited amount of glycogen we have go further. But, what happens as the body works harder and harder?

As we exercise and the body makes energy using fat and glycogen, it also creates some leftovers, mainly carbon dioxide (which the blood takes back to the lungs to be breathed out), water and something called ‘lactic acid’. The lactic acid is also removed from the muscles by the blood. Going back to my ‘bath’ analogy, the production and removal of lactic acid is like running a bath with the plug in: at a steady low rate of production, the bath fills to a point and the excess drains away through the overflow; however, if we start to exercise harder and increase the flow of lactic acid into the bath, eventually we will get to a point where the overflow can’t cope, the bath will spill over and your bathroom mat will be soggy (please don’t try this at home). In the same way, the body can only remove lactic acid from the muscles at a certain rate; if we exercise too hard, the lactic acid builds up faster than we can remove it and actually starts to prevent the muscles from working properly. This is characterised by the ‘burning’ sensation of pain we feel when we’re exercising really hard. So, from this we can see that the ideal point at which to work is at, or just below, the point where we can just about remove all of the lactic acid we create, ie, water is flowing out of the bath at the same rate, or only slightly faster, than it is coming into the bath and we can increase the time taken until the bath overflows. This approximate balance point is known as the ‘Lactic Threshold’ or ‘Anaerobic Threshold’. Luckily, again, this threshold can be improved through training at or slightly below it and this is the idea of sessions like ‘tempo training’: we practise running at or just below our lactic thresholds to improve our body’s ability to get rid of lactic acid and increase our muscle resistance to fatigue. Eventually, we find that we can work harder and run faster without collapsing of exhaustion and the measure of this threshold level – subjective or objective – is a very useful guide to fitness. Raising the lactic threshold is crucial to any form of endurance training but, unfortunately, is uncomfortable especially when you approach and exceed the threshold. No pain, no gain, remember.

But what happens once we cross this threshold?

When the body reaches the point where lactic acid is not being removed from the muscles at the rate it is being produced, we are well into the anaerobic energy system, ie, we are producing energy ‘without oxygen’. Energy is still produced, using a slightly different process, but we increase the lactic acid production dramatically. The lactic acid burn prevents us from maintaining a high effort, much like the sprint at the end of a race when your legs feel like they won’t actually go any faster, and means that the anaerobic system is a relatively short-term energy supply. In an endurance sport such as running, we tend to concentrate more on tuning our aerobic system as the anaerobic system is so short-term. We shouldn’t, however, ignore the anaerobic system as we will frequently need it for high intensity bursts of energy such as hills, accelerations and sprint finishes. In particular, the ability to recover quickly after an anaerobic period of exercise is highly desirable: an example is regaining your normal race pace after getting to the top of a steep hill.

There are various ways of training the two different energy systems; for example, short high-intensity efforts above the lactic threshold, followed by long recoveries, are good for training the anaerobic system as our bodies get used to tolerating the build up of lactic acid and then flushing it from the muscles. Conversely, longer intervals force us to run at a lower intensity (below the lactic threshold) which means that we can use a shorter recovery time as there won’t be as much lactic acid in our muscles to get rid of. This trains the aerobic system and is a good heart and lung workout.

Whilst the lactate threshold is a very important value when it comes to training properly, it is also notoriously difficult to measure accurately! There are numerous methods, such as getting blood samples whilst working on a progressively harder resistance stationary bicycle machine or treadmill but this is beyond the pockets and requirements of most amateur athletes. Other ideas such as the ‘Conconi test’ mean running on a track, increasing the pace each lap, and measuring heart rate. The resulting graph of heart rate against speed will have a ‘knee’ in it which indicates the rough area of the lactic threshold. However, these methods all depend on temperature, how tired you are, what you have eaten, etc, and, ultimately, values can vary from one day to the next. To be honest, if you take the average heart rate during your last 10km race, your lactic threshold will probably be about 5 to 10 beats per minute below this value. See if you can borrow a heart rate monitor for your next race and you can get an idea. If you can’t be bothered with all of this, you can run your training sessions on ‘feel’ or ‘perceived effort’: at, or just below, your lactic threshold you will feel that you are working hard but should be able to complete the training session ahead of you. Breathing will be hard and regular but not gasping. This is a purely subjective method and relies on you being honest with yourself about how hard you are running; heart rate monitors are quite motivating as they give you a good idea of how hard you are working and take the guesswork away.

So, what have we learnt? The body has two energy systems: aerobic and anaerobic and there is a threshold between the two systems called the ‘lactic threshold’. The aerobic system is the most important to us and that’s why we spend most of our time training in the aerobic zone although we ignore the anaerobic system at our peril. The lactic threshold can be improved through training close to the threshold value. Probably most importantly, you can now see the reason behind Bleddyn’s club training sessions. They are not haphazard, random choices and they are planned to train different aspects of both of our energy systems: the aerobic system is trained through tempo sessions and longer intervals with short rests; the anaerobic system is trained through hard, high intensity sessions (such as hills, short fast intervals, etc) with longer recoveries. The aerobic and anaerobic systems complement each other and concentrating too much on one will cause a higher risk of the other system failing us during a race or training session.

I hope this was useful and I think I’ll do something on strength and flexibility for the next article. However, if there’s anything you particularly want to know about, let me know.

Also, while I’m at it, here are some good sites for training articles I have used. There are many more on the internet but these are particularly useful and are well worth a look. If you know of any other good sites, let me know and I’ll ask Brian to add them to the website. I’ll probably also list some readable books on training.

www.xtri.com - Triathlon website with stacks of articles on all sorts of things.

www.tridu.com - Same again.

www.kjerag.com - Multisport site.