The science behind training zones

The idea behind training zones is they help you to target a specific aspect of fitness and these zones should form the basis of every training session you do on the bike.

Before we look at each zone specifically, it helps to understand why you body reacts the way it does when working at certain intensities.

We’ve used the seven-zone model used by two popular ride-logging and power analysis tools, Strava and Training Peaks, though you may also see different models with either more or less zones. This is because, from a physiological standpoint, there are only three zones. We call them exercise intensity domains – moderate, severe and heavy – and they are separated by measurable thresholds.

The moderate domain is an exercise intensity that doesn’t raise lactic acid levels in your blood. Contrary to popular belief, lactic acid, which is produced when you exercise, is not a waste product, but a fuel which can be processed to produce energy, providing there is sufficient oxygen.

However, your muscles only have a certain amount of processing capacity and as the intensity of exercise increases, so does the amount of lactic acid produced. Once the exercising muscles reach capacity, the excess lactic acid is transferred elsewhere in your body for processing, subsequently raising lactic acid levels in your blood.

The moderate zone consists of exercise intensities where your muscles are processing all the lactic acid product, so the amount of lactic acid in your blood isn’t raised above baseline levels. Once you begin exercising at an intensity where the amount of lactic acid becomes greater than your muscles’ capacity, the excess enters the blood system and you reach your aerobic threshold or, in other words, the first lactic threshold (LT1 for short). This defines the border between the moderate and heavy domains.

So why do we have more than three zones?

If, physiologically speaking, we only work at three exercise intensities, why do we have more than three zones? Simply put, it’s so, as coaches and athletes, we can set training to a higher degree of accuracy.

For example, zone one is there to ensure you don’t go too hard in your recovery rides. If a coach were to say ‘one hour in the moderate domain’ an athlete with a FTP of 300w might go and ride for one hour at 225 watts – too much for a recovery ride. By having zone one, with a maximum of 165 watts for a 300-watt FTP, both you and your coach know you will be riding easily enough so as to not induce any more fatigue.

You can see the severe domain is split up into a number of different zones. These relate to the three different ways (also known as pathways) to produce power anaerobically and allow us to target specific anaerobic pathways.

How and when to use each power zone

Zone one: Recovery – maximum two hour

Training at this intensity means you can stay active without adding extra fatigue. After training hard, your body will often go into shut down mode to try and recover as quickly as possible, but this can leave you feeling sluggish. Training in zone one allows you to keep your legs turning over without adding to your levels of fatigue. Perfect for when you need to recover from a hard session but don’t want to feel sluggish the next day.

Zone three: Tempo – one to three hours

Zone three works on your ability to hold a consistently high pace and represents an effort that will feel hard but comfortable. Most people really enjoy zone three training as they feel they are riding quickly, without riding too hard. Training in zone three has some of the advantages of riding in zone two, however it is a lot more tiring, so the number of sessions completed in a row, or the length of each individual session, needs to be limited.

Zone three work also stimulates some of the same adaptations as zone four – specifically your body’s ability to process lactic acid – but the primary reason to train in zone three remains to increase the amount of glycogen it can store (in zone three glycogen is starting to overtake fat as the primary fuel source).

As you can see, zone three is a very varied zone – at the bottom you won’t notice a real difference over zone two work and at the very top end it will feel like a threshold effort. Upper zone three work is often described as ‘sweetspot training’- the level at which you are getting a high return for your time on the bike and something I have covered in depth before. Zone three is also typically the zone in which Ironman athletes will ride during the bike leg.

Zone four: Threshold – ten minutes to one hour

Zone four works on increasing your anaerobic threshold, so it starts just below a rider’s anaerobic threshold (LT2) and stretches to just above. This way, it allows a rider to push up their anaerobic threshold by using longer intervals just below LT2 and pull up their threshold by training using shorter intervals just above.

Training in zone four produces a great deal of lactic acid and, therefore, gives your body a strong stimulus to increase the number of mitochondria in the muscles, as it is the mitochondria which process the lactic acid. However, zone four training is also very fatiguing and can only be done for limited periods, so more often than not you will ride in zone four in intervals with a period of recovery between efforts.

You should be able to maintain zone four for up to one hour at lower zone four and around 10 minutes at upper zone four – the middle represents LT2/ FTP power and the crossover between the heavy and severe domains. If you were riding a 25-mile time trial, you’d look to do it in zone four.

Zone five: VO2 Max – three to ten minutes

Zone five is the intensity you will be riding at when going as hard as you can up a small climb, no longer than ten minutes long. Training in zone five is very fatiguing and these are typically leg-burning efforts (your legs are burning not due to lactic acid but because of an increase in a by-product of anaerobic metabolism – hydrogen ions). During shorter efforts, your heart rate may not have time to respond to the effort and your max heart rate comes after you have finished (this is one of the benefits of training with a power meter – you get an instant measure on how hard you are working).  

You are working anaerobically in zone five and training at this intensity works on the anaerobic glycolysis pathway (remember, there are three anaerobic pathways) – this is where glycogen gets broken down to produce energy but this process is happening without oxygen. Zone five produces a large amount of lactic acid – more than your body can process, leading to high levels of blood lactate. To process this lactic acid, your body needs oxygen but in zone five your body is unable to provide enough, so eventually the exercise will become unsustainable. As a result, training in zone five works on your cardiac output and VO2 Max – how much and how quickly your heart can pump blood to where it is needed, and how much oxygen that blood can carry, therefore providing the muscles with as much oxygen as possible. It also works on the number of mitochondria in the muscles and, consequently, the amount of oxygen your muscles can process. Zone five is the zone in which a rider will complete an individual pursuit.

Zone seven: Neuromuscular – less than 30 seconds

Training in zone seven works on your sprint power. It also causes the most muscle size increase (hypertrophy), so if you want bigger, more powerful legs you need to be working in zone seven.

Alongside the phosphocreatine stores, your body has an even smaller store of an energy source known as ATP, which can be thought of as your body’s energy currency. ATP is used to drive muscle contractions and in doing so is broken down (it then becomes ADP), but before it can be re-used it needs to be put back together – again, this requires energy. All the metabolism that takes place in your body during exercise is used to put ATP molecules back together but sprinting in zone seven requires power so quickly that there is hardly any time for any type of metabolism. Therefore, you body needs to use the ATP stores it has to provide that instant energy to allow you to sprint.

Zone seven also works on the way in which your muscles fire. When you go to sprint a message is sent from your brain down to the muscle fibers in your legs to contract and the way that message is sent is very important – the order in which the muscle fibers fire and how many fibers are firing at the same time dictates the amount of power that goes through the pedals. Repeated sprinting improves the signals from your brain to your muscles and improves your sprint, which is why this zone is known as the neuromuscular zone.