The Science of Cycling

↓ The Cycling Training Zone Model
↓ Understanding Each Training Zone
↓ Fat vs. Carb Burning While Cycling
↓ Exercise Physiology - A Very Brief Overview
↓ Type I Muscle Fibers and Type II Muscle Fibers
↓ What Happens At Each Cycling Training Zone?
↓ Cycling Training Zones – Application to Training
↓ Why Polarized Training Works
↓ Conclusion

When you look at a workout in your training calendar, you might see something like “Ride in Zone 2” or “Do 4x8 minutes in Zone 4.” From a practical standpoint, it’s easy to see how riding endurance miles can help you get faster, or why high intensity training can raise your FTP, but what actually happens within the body when you ride in these zones?

We sometimes have a tendency to get caught up in a numbers game with training, but it’s important to understand the physiologic “why” behind your workouts– what changes in your body when you cross from Zone 2 into Zone 3? What exactly causes you to reach your limit when riding above FTP?

I like an analogy that I heard once from Sebastian Weber (who has coached top level professional cyclists like Peter Sagan and Tony Martin), likening a cycling workout to a prescription from a doctor.

When a doctor prescribes a medicine, they understand exactly how the medicine works, what system it affects in the body, possible side effects and why it will help the patient.

It’s the same with training. Knowing what changes a workout causes in your body and being aware of the side effects can help you better prescribe your training to become a faster cyclist. Equally as important, knowing the purpose behind your plan can help give you confidence in your approach, and in cycling, confidence is everything.

In the first part of this blog, I will describe what happens when you cycle through each training zone. Then we will look at some practical applications to your training in the second part.

The Cycling Training Zone Model

There are many different zone models you can use for cycling, but most of the ones used by cyclists will look something like this. *Bear in mind that there are endless different percentages of FTP which people use to define these zones, but they are all centered around certain physiologic benchmarks.*

Zone 1 - Typically the “cycling recovery” zone <50% FTP

Zone 2 - The endurance cycling zone. This ranges usually between 55-75% FTP (this is a huge range and I will discuss more on this later.

Zone 3 - This is where the often referred to “tempo” and “sweet spot” zones occur. Again, this is a huge range, but it’s often going to be around 80-93% FTP.

Zone 4 - This zone is right around where your FTP or lactate threshold is and the range where commonly prescribed “threshold” intervals are done perhaps between 93-105% FTP.

Zone 5 - At >105% FTP, this is where cyclists will do “VO2max intervals.” They are referred to as such because if you ride in Zone 5, you won’t sustain this pace for very long before reaching VO2max.

Zone 6 - Zone 6 is a bit of an oddball relative to the other training zones since it is considered an anaerobic training zone (meaning you produce energy without oxygen) These are short, all out efforts of <2 minutes in duration.

Understanding Each Training Zone

So… why’s it like that? How’d we come up with these arbitrary percentages and why do they matter?

While it is true that the plethora of different zone models are arbitrary to some extent, they are all rooted in the framework of physiologic changes that occur when you reach a certain threshold within the body.

I think we often have a tendency to overcomplicate training by worrying too much about TSS or different percentages without paying much regard to the underlying reasons behind why we make certain decisions about our training. Understanding this can help simplify your decisions.

Before I discuss more about training zones, I want to provide a brief overview of some exercise physiology basics so you can have a greater understanding of what’s going on behind the scenes.

Fat vs. Carb Burning While Cycling

When you are cycling, you burn a combination of both carbohydrates and fats. Fats come from your stored body fat or intramuscular triglycerides and can only be used to power your aerobic energy system. Everyone has a practically unlimited amount of energy they can use from fat, however, the ability to tap into your fat stores must be trained with zone 2 miles.

Carbohydrates on the other hand can be used to fuel both your aerobic and anaerobic energy systems. Any high intensity at or above lactate threshold will burn almost entirely carbohydrates.

Unlike fats, you have a very limited amount of carbohydrates stored within your body in the form of muscle glycogen. We want to preserve muscle glycogen at all costs so we can use it when needed most. The longer you can maintain glycogen levels, the longer you can put out peak power numbers. This is absolutely essential for racing or hard training sessions.

Thus, it is very important to train your body’s ability to use fat as a fuel source in order to spare your precious muscle glycogen.

Why do I bring this up now? Well, carb vs. fat burning is one of the primary changes that occurs while riding at different intensities and this phenomenon is connected to many of the other bodily responses that happen as intensity increases.

Exercise Physiology - A Very Brief Overview

Envision that you have entered into a lab and are asked to complete a VO2max ramp test where the intensity increases until you reach volitional fatigue. As the intensity increases, your body must also increase its metabolic rate to meet demand.

There are several primary systems which work together to allow your body to meet energy demand:

Your Cardiovascular System – When you begin to exercise more intensely, your heart must deliver more oxygenated blood to the working muscles. To accomplish this, your heart will pump faster and more powerfully to increase cardiac output. This blood is then delivered to the muscles and the oxygen within is extracted out of the blood to be used for energy production.
Your Pulmonary System – As a greater amount of blood is being circulated throughout your body, your lungs must also increase ventilation to meet demand and replenish the now deoxygenated blood with oxygen before it is recirculated throughout the body.
Your Skeletal Muscles – When this oxygen rich blood is delivered to the working muscles it is then used by the mitochondria to produce energy aerobically. When energy demand increases beyond what your body can create aerobically, it must then rely on anaerobic glycolysis to meet this demand.

Type I Muscle Fibers and Type II Muscle Fibers

Another thing that changes when you are cycling through each zone is the types of muscle fibers you recruit. Within your skeletal muscle, you have both Type I (slow-twitch) and Type II (fast-twitch) muscle fibers.

Your Type I muscle fibers are very aerobically efficient and house large and numerous mitochondria. These fibers are fatigue resistant and can sustain low-to-moderate intensity for hours on end. When you are riding at your zone 2 endurance pace, you are training your Type I muscle fibers to become stronger.

Your Type II muscle fibers, on the other hand, are more anaerobic and useful for shorter, more intense efforts. Within the Type II umbrella, there are Type IIx and Type IIa fibers.

Type IIx fibers are extremely powerful and useful for short sprint efforts but they also fatigue very quickly. However, with training, these Type IIx fibers can convert to more fatigue resistant Type IIa fibers which are much more useful for cycling.

Type IIa fibers can take on both aerobic and anaerobic characteristics. During an intense set of intervals or in a race, you will rely heavily on these muscle fibers.

Type IIx to Type IIa conversion occurs naturally with consistent endurance training. During long slow distance riding, if your Type I fibers become fatigued, you will begin to rely upon Type II fibers to help out, which can promote fiber type conversion. High intensity intervals and strength training also promote fiber type conversion.

There is also some research that suggests you can convert Type II muscle fibers to Type I and vice versa, with Howald et al. finding 12% increase in Type I fibers following 6 weeks of endurance training in sedentary participants (granted, these individuals were not trained, but this does demonstrate that is possible)

At low intensity, you initially only need to recruit your Type I slow twitch muscle fibers. The harder you ride the more you begin to recruit your Type II fibers to help keep the power down

What Happens At Each Cycling Training Zone?

Now that we have the basics down, we can discuss more specifically about how all these things work together while you are cycling in each zone.

Zone 1

While you are riding in zone 1, you burn mostly fat for fuel and recruit your Type I muscle fibers. With unlimited fat for fuel and fatigue resistant Type I fibers, you can sustain this pace ALL DAY without getting tired. Your heart rate and ventilation remain under control because you are easily able to supply the needed oxygen to meet energy demand.

Zone 2

Your physiologic response riding in zone 2 is very similar to what it will be in zone 1. You are still burning mostly fat for fuel and training your Type I fibers. The only real difference here is that you are expending more energy and your Type I will fatigue more quickly if you ride in zone 2 for long enough.

Additionally, this is a point of very high metabolic efficiency where you can extract the most oxygen for each breath you take in. Training in this zone is an excellent way to build a massive aerobic engine and build endurance for your events.

If zone 2 and 1 are so similar, then does it really matter which one you train in? There is some debate on this topic.. More on that later!

Zone 3

As you cross over into zone 3, often referred to as “tempo cycling zone,” some big changes happen. Sandwiched between zone 2 and 3 is your aerobic threshold (sometimes referred to as VT1 in the three zone model).

As you cross over this threshold, you switch from burning predominantly fat to predominantly carb burning and begin to recruit more of your Type II muscle fibers. As a result, you will begin to rely more upon glycolysis to supply energy.

The main byproduct of glycolysis is lactate. This lactate can be recycled and used for fuel by your Type I aerobic fibers. However, when you cross over your aerobic threshold, lactate production exceeds what can be recycled by your aerobic system and it begins to appear in the blood.

At this stage, though, you still have not reached your lactate threshold because your body has a safeguard in place. The bicarbonate buffering system scavenges the excess hydrogen ions that come as a byproduct of lactate accumulation. Thus, you can sustain zone 3 power for extended periods without reaching exhaustion.

Zone 4

Up until this stage, you can still ride fairly comfortably for a long time, however, right around zone 4 is when everything changes. This zone is associated with the terms Functional Threshold Power (FTP), lactate threshold, anaerobic threshold etc.. Once you go above zone 4, you will begin to fatigue very quickly.

Zone 5

Physiologic terminology aside, zone 5 is where lactate accumulation becomes too great for your body to handle. At this point, you will rely heavily on your Type II fibers to produce power and consume almost entirely carbohydrates for fuel.

As a result, your aerobic system becomes backlogged and can no longer clear lactate fast enough to prevent you from fatiguing. The appearance of lactate in the blood and resulting hydrogen ions becomes too great for your bicarbonate buffering system to handle.

The accumulation of hydrogen ions increases the acidity of your blood and causes the “burning” sensation you feel when you ride above FTP. It’s only a matter of time before you either have to slow down or stop altogether.

At this stage you will also notice a disproportionate increase in your breathing and begin to hyperventilate. The overloading of the bicarbonate buffering system causes an excess of CO2 to accumulate in the blood and your body will drastically increase ventilation in an attempt to get rid of it.

Your heart rate will be nearing maximum as your heart tries to simultaneously deliver the maximum amount of blood to your muscles while also getting rid of waste products caused by glycolysis.

The end result of riding in zone 5? Max heart rate, hyperventilation and a lot of pain. Sound familiar?

Zone 6

Purely anaerobic efforts consist of exercise bouts <2 minutes in length. With anaerobic efforts, unlike the prior training zones, you can produce energy without the presence of oxygen. These efforts utilize the glycolytic system, which uses glucose (carbs) to create energy.

While this system is very useful for creating quick energy needed for short spurts, the end result of glycolysis is the formation of lactate, which ultimately causes muscle fatigue.

If you have ever done an all out 30 second effort, you know that when you begin to get to around 20 seconds in, your legs begin to feel like they are seizing up and your power will begin to decline.

Unlike aerobic efforts, energy is not created in the mitochondria, but outside in the watery matrix called the cytosol. However, the aerobic and anaerobic energy systems are inextricably linked when it comes to cycling performance.

The lactate that is produced by the glycolytic system is cleared aerobically by the mitochondria and can be used for fuel. This is why those with well trained aerobic systems can repeat anaerobic efforts with greater consistency. For racing, where you can expect to repeat anaerobic efforts constantly, this is paramount.

Above Image: The lactate threshold (AnT) can be clearly seen here. VE (ventilation) disproportionately increases in an attempt to get rid of excess CO2.

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Cycling Training Zones – Application to Training

Now that we know exactly what happens at each training zone, how can we use this information to better inform why we want to train in each zone? Let’s now dive into the benefits or potential drawbacks of training in each zone.

Zone 1 Training

Zone 1 training would typically be considered the “recovery zone” by most and would lie below 60% of FTP. At this zone, you are burning almost entirely fats for fuel and training your Type I fibers.

In theory, this would be a great zone to train in to help build your aerobic base, however, this zone might not actually be “challenging” enough on the body to elicit the most adaptations, particularly if you train <15 hours per week.

There does seem to be some contention on this topic. Many professional riders still do long endurance miles below 60% of FTP. However, they also must train 25+ hours per week to build the resilience for WorldTour racing, so this might come out of necessity due to the large amounts of volume they are riding.

Zone 2 Training

This should be the bread and butter of your training. Zone 2 training will help you improve practically every aspect of your aerobic fitness with very little cost. You can continue to stack zone 2 rides regularly without accumulating too much fatigue, leaving you fresh to smash your intervals and big races.

Here is a short summary of the benefits of zone 2 training, if you want a more in-depth explanation of the benefits of endurance riding, invest in our training pack here.

Improved Endurance – Zone 2 training will enhance your fat burning capabilities so you can ride for hours without fatigue

Increased FTP and VO2Max – Zone 2 training will cause structural changes in your body that will allow you to consume more oxygen (raising your VO2Max), produce less lactate, and clear more lactate.

Greater Repeatability – In racing, it’s usually not the cyclist who has the best one-off effort who wins, but the one who can repeat peak power outputs many times during a race. With zone 2 training, you will become more durable and fatigue resistant, and you will clear more lactate so you can recover faster from efforts.

Better Recovery – Since you will be more proficient at burning fat for fuel, your glycogen stores will become less depleted after big rides and it will become easier to refuel for the next bout of training. Better recovery means you can stack harder and harder workouts to build your fitness.

Zone 3 Training

As described in the previous section, once you cross over into zone 3, you will begin to recruit more of your Type II fibers and burn more carbohydrates than fat. As a result you will lose a lot of the benefits of zone 2 training when you begin to ride in zone 3.

This is why replacing your zone 2 training with tempo or sweet spot cycling is a big mistake! You can’t replace the benefits of riding in zone 2 by riding harder. This is also why it is paramount to ride steady on your endurance rides and avoid going up into zone 3 as this will disturb what we are trying to do with zone 2 endurance riding.

However, while zone 3 training is not a replacement for zone 2 training, it is still useful when prescribed properly. Outlined below are some benefits of zone 3:

Enhanced Muscular Endurance: Tempo riding is a great way to train your muscles to become stronger and more resilient. By riding in tempo for extended periods, you can recruit your Type II fibers to become stronger aerobically. Doing these at low cadence (high torque) can add an extra challenge for your muscles.

A Good Mid-Level Session: If you’re in the middle of a heavy race block and don’t really need to do full-on high intensity in the middle of the week, tempo training is a good way to get in some stimulus to keep the legs firing without being too taxing.

Race Specificity: If your event will include lots of time riding at tempo, incorporating a few tempo cycling sessions in the weeks leading up can be a great way to prepare. For example, I would often do 1-2 hour tempo blocks to prepare for the big hill climbs we have in Colorado. However, I did these sparingly, usually a week or two out from the event.

Zone 4 Training

Zone 4 intervals are analogous to “FTP Intervals” or “Threshold Intervals” for the purpose of this discussion and are performed right around your FTP. The classic 2x20 minute FTP session was pretty popular a few years ago.

As the polarized training methodology has gained momentum, we’ve begun to realize that the best way to increase your FTP is to actually train above your FTP (in zone 5) rather than do extensive intervals at FTP.

While lactate threshold training might not be the best way to increase your FTP, these intervals are still a valuable piece of many cyclists training plans for the following reasons:

Enhanced Muscular Endurance: Along similar lines to zone 3 training, long FTP intervals can help train your muscle to become more fatigue resistant. They are more taxing mentally and physically, but might provide greater benefit. If you are a naturally explosive rider but struggle on long, sustained climbs, longer FTP intervals might be what you need.

Race Specificity: Just like tempo training, FTP intervals are useful for race specific training leading up to an event. If your event features sustained bouts at FTP, like a hill climb. Gran Fondo or time trial, FTP intervals are a great way to prepare.

Zone 5 Training

Zone 5 training will refer to any aerobic based interval above FTP for the purpose of this discussion. This could include VO2Max intervals, Over/Unders, Seiler’s 4x8 session, 30/15s, and the like.

It stands to reason that, if you want to increase your FTP, you must train above your FTP to raise your ceiling. If your FTP is 300 Watts but you want to increase it to 310, then spend lots of time at 310 Watts!

The reason zone 5 training is so effective is because, as mentioned in the prior section, you will begin to accumulate high levels of lactate and approach your VO2Max. By accumulating lactate while riding above your FTP, you can also train yourself to clear which will increase your FTP.

Lactate is also an important signaling molecule for aerobic adaptations, so workouts that accumulate high levels of lactate in the blood can lead to greater aerobic gains then workouts that do not reach this level of intensity.

By training your VO2Max, you can raise your ceiling for peak aerobic power. This will give you an added kick at the end of a race and also give your FTP more room to grow.

Put simply, if you want to get faster, you need to ride hard and push the limits– zone 5 training is the best way to do that!

Zone 6 Training

Where does anaerobic cycling fit into your training plan? Since cycling is a primarily aerobic sport, anaerobic training will comprise a very small part of your training, if at all depending on your goals.

Too much anaerobic training can actually decrease your FTP. This occurs because when we do anaerobic training, we are actually training ourselves to produce lactate rather than clear it. The signaling mechanisms that govern the glycolytic and aerobic systems can compete with one another and too much anaerobic training can impair your aerobic gains.

However, for some riders, anaerobic training has its place and is worthwhile if it fits into your goals. You can read more about that in our anaerobic training blog.

Why Polarized Training Works

So there are five main aerobic zones to train in, and each has their own benefits and drawbacks. What is the best intensity distribution and where should you spend the bulk of your time?

Most likely you will need to train in every zone at some point throughout the year, however, if your main goal is to build as much aerobic fitness as possible, we would recommend polarized training for cycling.

The polarized training approach consists of spending the majority of your time training in zone 2, and most of your high intensity sessions would be performed in zone 5. Just how is this so effective?

We can think of this as a pyramid, where zone 2 training lays a wider foundation for which to build your top-end fitness. The bigger your foundation, the higher peak fitness you can attain.

Zone 2 training will cause structural changes in your body, such as increased mitochondrial density, muscle capillarization, type II fiber type conversion and improved fat burning capabilities. All of these factors will then contribute to greater performance when high intensity is performed.

With more mitochondria and muscle capillarization, you can deliver and consume more oxygen when doing a VO2max interval. With type II fiber conversion, you can achieve a higher power output and sustain it for longer to drive your aerobic system further. With improved fat burning capabilities, you will become less reliant on carbs for energy and decrease contributions from the glycolytic system to raise FTP further.

This is not as complicated as it seems if we look at this from a practical standpoint. Consider two different scenarios in which you do the exact same interval sessions for the entire year but change the amount of volume you ride in zone 2.

In the first scenario, you train a total of 400 hours for the year and aside from your interval sessions, you train exclusively in zone 2. In the second scenario, you do the same exact interval sessions as in the first, but you train 600 hours for the year.

In which case do you think you can get faster?

Whilst zone 2 training is a great means of overloading your training, zone 5 training is likely the most effective for translating your base fitness into actual race speed. Zone 5 intervals will push your aerobic capacity, muscular endurance, and lactate clearing capabilities in ways that zone 3 and 4 simply don’t.

A lot of riders tend to avoid zone 5 training because it is out of their comfort zone. We often see riders who like to do a lot of “feel good” sweet spot rides or mid-level microburst workouts.

The reality is that cycling is a hard sport, and to get really good at it, you need to seek out the uncomfortable and embrace it. Over time, you will not only improve your physical capabilities with zone 5 training, but your mental strength as well. Ultimately, when you begin to see the benefits of doing hard workouts, you will begin to love suffering (in a weird way!).

Conclusion

Well you’ve made it to the end of the blog post and you are now an expert on all things training zones! If you still have questions and want to investigate further, you can check out our cycling training blog and our YouTube channel.

If you would like to seek out expert guidance on your cycling training, we offer one-on-one personalized coaching from professional cycling coaches. Check out our training programs here.

The Science of Cycling: Understand Exercise Physiology to Get Faster!