Endurance 101: Zone 2 - Zone 2 (Endurance) Made Simple

Why should I care what "zone2" is?

It is very useful for the description of exercise in exercise physiology or exercise science, as well as the prescription of exercise by coaches, to have a common language for the different conditions that our body might be in. In laymen's terms we might differ only between conditions of "rest" and "exercise", or have some sense that there is a "cardio" form of exercise as intensity increases, but these terms are imprecise.

When we ride, we have a type of "power-duration curve" (PDC), in that there is an inverse relation between intensity and duration: we can sprint for 5 seconds or walk all day. We cannot sprint all day. Training therefore involves some combination of these two factors-intensity and duration-effective training optimizes this balance.

Some scientists do try to describe intensity using "exercise domains", but in qualitative terms, such as "light, moderate, heavy and extreme", but again these are somewhat imprecise. What is needed is a way of describing exercise intensity-for both prescription and description-in a precise way. This is where the idea of numbered "zones" comes in, and in particular attempting to anchor the boundaries of these zones with quantitative physiological parameters that can be observed and measured.

What is "zone 2"?

Zone 2 is a kind of low-intensity, high-duration training in which you ride at a pace that you can sustain indefinitely-in theory-or at least for a long time. It is also sometimes known as "endurance pace", or-more historically-as "LSD" (long, slow distance). It is training that contrasts to high-intensity, shorter-duration training that you may have encountered, and which is typically delivered in "intervals" of high intensity, interspersed with periods of rest, in a "work-rest-work-rest-work-rest" structure. Examples of interval training include SIT - sprint intervals, HIIT - high intensity interval training etc. To explain "zone 2" more precisely we do need to dig a little deeper into the science.

Why is it called "zone 2"?

The human body, while exercising, can be in one of a number of physiological states along an axis of intensity-from complete rest to all-out maximal effort, such as sprinting. As our effort levels increase from rest, our body tries to make quick adjustments in order to meet the demands being placed on it. This mostly involves the work to get what the components need to your muscles in order to deliver that effort, such as oxygen and ATP.

Your body does it automatically, and it is quite good at it. This can be anything from your body changing parameters such as vasodilation, converting stored substrates into energy, or increasing breathing rate and heart rate. The rate at which these increase varies as exercise intensity increases, and some new biological systems are activated or increased - for example glycolysis (more later on this). If we can recognise discrete states, then we can describe these different states the body goes through as (numbered) "zones".

How many zones are there?

Much confusion can come in here because there is no unanimous agreement on how many exercise zones there are. Different models exist, each having a different number of zones. This is so because different scientists may only be interested in certain specific markers of physiological conditions within the body as defining physiologically significant boundaries on either side of which the body can be considered to be in an observably different state.

What is the 3-Zone Model?

One generic zone model in widespread use is the 3-zone model, which most directly relates to the production and clearance of lactate within the muscles. Lactate is both a by-product of exercise but also a fuel. It's always produced and consumed, even at rest, albeit in very small quantities.

When there is just enough lactate for our muscles, they can convert it into energy in specialized cells. But there is such a thing as "too much of a good thing"-if we drown the muscles in lactate, it does its best to clear that excess out of those muscles and shuttle it elsewhere in the body in an attempt to prevent lactic acidosis. This lactate is then transported, optimally, to other cells that are not flooded and which can still use it, therefore, as fuel to make ATP-energy. It can also be transported to the liver where it can be converted back, via pyruvate, into glucose in a process known as the "Cori cycle".

Two other conditions regarding lactate concentration in muscles concerning exercise can be seen as exercise increases. This can be observed through muscle biopsy (ouch) or more commonly by taking a blood sample and extrapolating from that. What makes lactate a great marker of intensity boundaries is that it does not simply increase linearly as intensity increases, but rather in shifts "suddenly" (ish) a couple of times at what are called "inflection points". This makes it a great divisor for different exercise intensity boundaries.

This 3-zone model is quite simple, comprising 3 states. At low intensity, lactate is produced and consumed at a low and (relatively) constant rate. If we have gradually increased the intensity, say by 5 watts per minute, on the bike while continuing to sample our blood, we would find that at a certain point our body begins to produce-and use-more lactate, yet the lactate levels of the blood begin to increase. If we made a note at the intensity of exercise at which this rise in blood lactate occurred in an individual, we have just found the first "lactate inflection point" (a.k.a LT1) and found the boundary between zones 1 and 2, in the 3 zone model.

If we were to continue increasing the intensity of exercise, the lactate levels in working muscles would continue to increase in a relative steady linear fashion until suddenly our system lacked capacity to clear certain localised muscles under load of lactate and excess lactate enters the blood with levels spiralling up out of control. This is the second inflection point, LT2, which closely correlates to our FTP power and whose name "lactate threshold" gives rise to the term "threshold training".

Great, so I know what zone2 is now. right?

No. The 3-zone model is not generally used in cycling, outside of triathlon. Instead we typically use a 5- or 7-zone model, with further subdivisions. But the above science still applies insofar as "zone 2" is an upper half of that first - lowest-zone from the 3-zone model as we approach our LT1 inflection point. That is we are not "noodling" at a super-easy pace - strolling around town is rarely "zone 2" - but neither are we under significant strain, and out blood lactate is just barely stable.

If LT1 is a "point" why is it called a "zone" - range?

This is because despite the LT1 being referred to as an "inflection point", very rarely does our body suddenly snap between two completely different states. Rather, there is somewhat of a grey area of transition. Besides, we have millions of muscle fibers and some may well be still able to clear and shuttle lactate at first while others which are more highly loaded may be swamped. So this whole transition range is therefore considered to be the "zone 2".

Is lactate the only way I can determine zone 2?

Whereas zone 2 is typically set from lactate samples in the lab via blood draw-e.g., via a lancet-this hasn't widely been implemented in the home exercise environment, nor is continuous lactate monitoring a thing-right now, although, as with advances such as CGM sampling, it might be at some point in the next few years, conceivably.

However, for now we can consider "proxies" for lactate concentration - other things we can easily observe that might mirror blood lactate increases and so serve as a marker of our zone 2 range. It isn't 100% precise, but it involves scientists in a lab - having found lactate turning point LT1 - observing other physiological parameters which the human subject is in at or around the LT1 inflection point. We'll be able to make use of these secondary markers in the future as "proxy indications"-metrics that can more easily be observed "in the field" (e.g., when training at home) and without specialized equipment-which should provide a useful estimation of zone 2 conditions in any individual subject.

What are these "in the field" tests" for zone 2?

There are three main field tests that have been observed to map almost universally to conditions that can be observed when we exercise in zone 2. First, and most simply, is whether we feel like we are in zone 2 when it is described to us. This may sound overly simplistic, but in fact studies have shown that this is as good as any other field measure.

How should zone2 "feel"?

In other words, it should feel like you are exercising at an intensity that is enough to take some conscious effort but not feel unduly uncomfortable or unpleasant, and you feel you could maintain indefinitely. Attempts at defining this more precisely can be done via simple tests such as "can you breathe through your nose only?" or "can you talk - albeit not easily - in complete sentences?". However, these commonly do not add much additional value over the basic sense of feel and are not universal-for example, nose-breathing at higher intensities can to some extent be trained.

Attempts to define this "feel" more precisely have led to measures such as the Borg Rating Of Perceived Exertion, a numeric scale under which you can rate your RPE - rating of perceived exertion - on a scale such as 6-20 or perhaps more intuitively between 1-10.

This all sounds wishy-washy! Just tell me what power to ride at!

Using a number rather than feel, if you feel that you need to try and ride at a power equating to zone 2, you can target around 66% of your FTP-threshold power value, as determined in an FTP test on Zwift as a pretty safe estimate. It would be an exact value, different for everybody, which also can vary within one training career or timeline of the athlete and mostly stays within a bracket of about 55-75% FTP for amateur athletes.

Similarly, it's been noticed that heart rates might come to an average of 70% of maximum when riding in zone2, but similarly, we are all different and heart rates can change day by day, level of fatigue, heat, sleep, caffeination, etc. - so again, that is not necessarily a good way to measure.

Usually, this level of specificity is not required and would only be likely to introduce undue stress or anxiety ("I was riding at 60% of my FTP but it felt too hard, am I doing it wrong?!" or "I was riding at 75% of my FTP and it felt too easy, am I doing it wrong?!"). Answer to both-if it "feels" like zone2-it probably is.

Now that we have an idea "what" zone2 is we know need to know why and when.

WHY do I need to ride in zone 2?

Up to this point, all of the above has been relatively uncontroversial. However, when we get from description (what zone 2 is) into prescription (how and when to do zone 2 training), we start internet fights (yay!). Even top coaches and exercise physiologists cannot agree on all of this. So the below is a best-effort "consensus" description at this time only and not definitive. Some may be opinionated.

To appreciate why we might want to ride in zone 2, we really need to understand the conditions that the body is in in this state.

What is happening in the body when I ride in zone 2?

As we saw above, zone 2 is measured for the purposes of exercise physiology by sampling blood lactate levels and using this to define our zone boundaries. Yet this is not, really, the most salient feature of the zone state, for either health or performance. It is just a good marker of that intensity range. What zone actually is, is a condition in which most of the energy needs of our muscles for exercise is produced aerobically-that is, in the presence of oxygen-in specialised little energy-factories called "mitochondria" in our cells. They do this utilizing a mixture of lipids, or fats, and glycogen, or carbohydrates, typically at a ratio of about 66% fat and 33% glycogen when in zone 2.

Why are the mitochondria so important?

Good mitochondrial health plays a critical role in both general well-being and exercise performance. It's implicated in everything from the prevention of disease, including certain types of cardiac events to diabetes and insulin resistance, and maybe even Alzheimer's. It has been fully demonstrated that the greater number of mitochondria and the better operating condition of each mitochondrion will produce a variety of health and performance benefits.

How can I improve mitochondrial function?

The body is almost always creating some energy aerobically-that is, in the presence of oxygen-within the mitochondria, even at rest. The rate at which this occurs maximally is at "zone 5" - otherwise termed VO2 max and which correlates to MAP or Maximal Aerobic Power on the bike. There are two further zones above this intensity, at which other energy systems come into play, and typically termed zones 6 and 7 in a 7-zone model, but we won't cover those here).

Well shouldn't I just do all my exercise in zone 5 rather than zone 2?

Good luck with that, But there are several reasons trying to ride in zone 5 all the time is not a good idea:

First of all, riding in zone 5 is really hard and just feels terrible-some people can even get sick during zone 5 training, and mentally it is very hard to keep the intensity at this level. One cycling podcast describes the feeling of "going until you see Jesus". It's hard to imagine being motivated every day for very long to do zone 5 training.

The second is that zone 5 training is hard on the body, and very high levels of fatigue of both the central and peripheral nervous system result. This generally makes it very difficult to do more than 20 minutes of zone 5 work in any single session, even when you break it into 5 minute blocks. You cannot ride all of the time in zone 5.

This would also mean that if you tried to train in this manner, over time you would be training in an increasingly depressive state and never actually reach true VO2 max.

Fourthly, zone 5 training is "stressful", in that it prompts the release of stress hormones like cortisol, which are not good for you if chronically high. In our modern lives, our lifestyle is such that our cortisol levels are often chronically raised anyway - adding to that with additional stress via training may not be a good idea. Chronically raised cortisol is a "disease state": not good.

Fifth, while the benefits from zone 5 VO2 training can be realized in very short time, they also do not "stack" for long. They cap relatively quickly and cannot be improved further. Also, the adaptations from VO2 training are the fastest to be subject to reversal, or loss, by the body.

Sixth, and least proven, there is some belief that high levels of lactate in the body, eg from zone5 training, may in fact inhibit mitochondrial function - acutely, that is, within a workout, at least. This is controversial somewhat and not entirely agreed on.

OK, so I should do all my training in zone 2 then. right?

You're probably expecting me to say no. But you'd certainly be a lot better off doing zone 2 training all of the time than doing nothing, and actually, riding only zone 2 all of your life would put you in great stead. Zone 2 can be considered your bread and butter, and where you should spend the most of your riding time, not just this week, but week in and week out for your life. In fact, in Dr Seiler's "pyramid of training needs", he rates volume as the biggest single determinant of performance, above other factors such as intensity. This is sometimes stated in prescription terms as "go slow to go fast".

But there are a few caveats to this approach:

First, zone 2 alone will not get you "race fast". If you race and care about being the absolutely fastest you can be on a given date, then you generally want to "peak" (maximize your fitness and performance) on that date. You can do this a number of ways. The typical approach to planning a training season in terms of "macro-periodisation" might be to ride for weeks or even months primarily, but not exclusively, in zone 2; then, as the key target race approaches, you would reduce the volume- that is, the number of miles you ride-but increase the intensity of each ride.

You would only do this for a short term because you cannot be "perma-fit.". You can be fast and fit all of the time. You cannot, however, be at your fittest and fastest all the time. If you try to be your fastest all of the time, then you will never be your fastest. You can peak a few times a year perhaps but you cannot stay in this state permanently. This can often seem counter-intuitive to new riders.

OK, but if I'm not interested in racing, should it all be zone 2?

No, variety is important. You need to enjoy your riding and you also don't want to grow bored. You also don't want your body to adapt to one type of training and then make no further adaptations. You can prevent such stagnation by mixing in "other" training types at higher intensity. Until you reach a quite high level of fitness, it doesn't overly matter exactly what form this high-intensity riding takes. It can be loose-form "fartleks" (unstructured intensity, such as sprinting up your favourite hill, or every time you see a blue van), all-out racing in Zwift, or structured interval training. General advice is to follow an approximate "80/20" rule: you go hard no more than 1 ride in every 5 - 20% - or twice a week, whichever you hit first - and go easy on the remainder of your rides - 80%. This has been observed in some studies by Dr Seiler etc to map more-or-less to the training distribution seen in the world's top athletes, too - although this does vary, has been disputed, and intensity distribution for athletes changes depending on macro-periodisation phase or season. It is sometimes known as "polarised training".

Will zone2 riding increase my FTP?

Yes. This often surprises people. There is a common misconception that to improve your VO2max, you have to ride at VO2 power, to improve your threshold-FTP power you have to ride at threshold etc. Whilst specificity is important, it is not strictly the case that training at one intensity only allows you to ride better at that self-same intensity.

Every ride you do, in every zone, delivers some form of signalling to your body that can elicit adaptations in one or more of the central or peripheral systems via gene expression, etc. And these adaptations can then benefit you when riding in other zones-as in the expression "a rising tide raises all boats". Riding at threshold may be a necessary part of optimally increasing FTP, but many of the same adaptations can also be delivered, albeit not optimally, by zone 2 riding alone.

How should I structure my zone2 rides?

There are two types of rides you can do in zone 2. The first is a strictly-by-the-book, all-zone 2 ride. You pick a power or HR target in the zone 2 range, and you just ride with little or no variation for as long as you can. In CTS terms, this is what we call "base" or "foundation" miles. It is extremely effective and very minimally-fatiguing. Unfortunately, you are also extremely likely to find it overwhelmingly dull and thus not sustainable. Most of us need a "distracted" or "extrinsic" focus for such a ride; one would either be chatting with fellow riders or watching a movie, i.e. not concentrating on the ride. Well, on Zwift, one can enable "ERG mode" to ride at a fixed percentage power, for example, and just a week ago the training platform "TrainerDay" just released a feature that dynamically adjusts a ride power to keep you in a nominated HR range.

The second type of ride you can do is what is called by CTS an "endurance" ride. You try to stay in zone2 for most of the time but aren't too concerned about going a little harder if called for, providing it isn't too hard-over LT2 or FTP power. This is generally an "intrinsic" or internal-focus ride, whereby you are invested in the ride itself. You can reproduce this on Zwift by joining a pace partner ride, for example; they may need you to surge up short climbs, but don't go too hard on these-only enough to keep with the bot-ignore people if they hammer off ahead and do not chase them.

How can I optimize my zone 2 rides?

First, try to extend the time spent riding over time. Riding 12 hrs in zone 2 is TOTALLY doable, but DON'T be afraid to start with 20 minutes at first, and build over time. You are you, don't worry what anyone else is doing or where they are on their own journey.

My reading of the literature is that there is some indication number of contractions in muscle are important, so consider maximizing cadence in zone 2.

I find zone2 rides hard, do you have any tips?

There are loads you can do, naturally not technical, but just fun rides-make them a pleasure! The best ride is the one you finish. You can ride on Zwift with "RoboPacers", ride outside, watch Netflix, chat with friends, buy yourself new kit-do what works for you and makes it enjoyable, rewarding, but above all, comfy.

Second, make it comfortable: Eat, drink, stand up every now and again, ensure your bibs fit and your saddle is comfortable. Consider a bike fit. Consider a saddle fitted to you with a pressure relief channel. Stand up in the saddle regularly, particularly when riding indoors.

Third, make the place you do your rides a nice place to be. We're all familiar with the term "paincave", but why not make your ride space nice, treat yourself to a new PC or TV for Zwift, put some motivation posters up, decorate!

Fourth, if you find it easier to do your zone 2 rides outside than on Zwift, then don't be afraid to do them outside. If you are hilly you might want to make sure that you have a large cassette range so that you don't need to go too deep when you're riding up hills.

Fifth, look into techniques for motivation on longer tasks, it is out of the scope for this article, but methods include "chunking".

Finally, remember that zone2 is not life, nor is it cycling. Zone2 is extremely time-consuming, so don't get sucked into zone2 or Zwift riding in general at the expense of life, or your family etc. Keep the big picture in mind!

Should I go harder if it feels too easy?

Normally, no, if you feel you are in the right zone. You want maximum time in zone; you should ride as long as you can. You want to try to spend a majority of your zone2 rides "out" not "up". In other words, this is called "time to exhaustion (TTE)". Never push the power to a pace that makes you doubt if you can continue at that pace indefinitely or cuts a ride short.

Is one long, 2-hour ride better than two shorter, 1-hour rides?

This is sort of a tough one to call because both the duration of ride and frequency of ride are important from a health standpoint. In general, one would want to try and ride a minimum of around 4 times a week, but if this frequency has already been reached, then it is around this point where riding each one of those 4/5 rides longer-distance/duration is likely to be more beneficial than adding in that 5th/6th ride-increasing the frequency.

There are some specific, additional benefits from a performance, rather than health, perspective to extending rides to 2 hours upwards - 4, 6, 8. - in that as our slow-twitch muscles fatigue we recruit - and therefore train - additional muscle fibres.

Should I do zone2 riding even if I only ride for N hours a week?

If you're severely time-limited, then you're probably better off riding at a higher intensity, such as tempo/SST pace (out of scope for this), which gives you much of the same value. The extra fatigue from this can't be tolerated by riders doing more total hours per week, but for severely time-constrained athletes this might be a more-optimal approach.

How can I tell if my zone 2 rides are "long enough"?

From a health perspective, all zone 2 riding is 'good'. However from a performance perspective, then even though zone 2 rides are "easy" in intensity, they still need to stress your body in some way in order to deliver adaptation - its still training, even if you're watching Netflix ;-) The method to use is duration extension, riding longer than your body is used to doing previously.

Typically, a zone 2 ride will be in a physiological "steady state" in that metrics relating to internal state should remain fairly constant-after initial warmup-until the point at which adaptations are occurring. A good example of a measure of internal state is heart rate ("HR"). Assuming you have adequate cooling, hydration and fueling then your heart rate, once the initial warm up has subsided, will remain at a given steady value for a given zone 2 power. You can measure your heart rate most conveniently using a chest strap such as Polar HRM.

When you see "HR drift" or "HR decoupling") - that is, your HR going up even though your power output is not - then you are causing stress to your body. You want to stay in this state for some period of time in order to cause stress to your body and initiate adaptation.

Since this point of decoupling is an individual one, general answers to questions such as "how long should a zone2 ride be?" are impossible to give, other than for you as an individual. Two hours may be far too short for one person with a very long training history, whereas 10 minutes may be much too long for someone else who is morbidly obese and bedridden.

Is it possible for you to stay in an individual zone 2 ride too long?

Yes. You're looking for a minimal effective dose per training session. Once you think you've delivered this-by example, having ridden longer than in the past, or by feeling subjectively fatigued, or by noting that you're experiencing HR decoupling-then from a training standpoint you may want to call it a day, in order to devote time to recovering to ride again, rather than continuing to ride and delivering additional fatigue.

Is it possible to do too much zone2 training overall?

Yes. Zone 2 training is still training and, if done properly, still provides a stress. As with all training, the gains are reaped during recovery from the training-the exercise is just the trigger. To say it another way, you have to balance the damage from doing the training with the benefit gained from it. Riding all day every day is still likely better than being completely sedentary but isn't optimal for health, either. But unless you're doing more than 30 hours a week training, this isn't something that you'll likely have to start considering.

When training, it is always important to not only monitor the total training stress, but also cap the ramp rate - how fast you would increase training from week to week or month to month. Increased in training load must be gradual over time.

How quickly will I see gains?

It will take time. The adaptations you see from low-intensity zone 2 riding are the longest to materialise and are not instant. This isn't something you can do for two weeks and see the progress. Effective zone 2 riding will, however, continue to deliver benefits months and even years after it is first introduced, while high-intensity riding may only deliver new adaptations for as little as 4-8 weeks.

Is cycling in zone2 enough to give me optimal health?

Not on its own, no. Although metabolic health is very important, so are other forms of exercise. In particular, cycling is a concentric sport and is low stress on our joints. This is good in some ways but does mean that we can be prone to weak bones and other issues. The human body does need its joints loading and this is normally done via some form of strength training such as weight lifting. This delivers eccentric loading that is critical and not available via cycling. You should look to combine this with your riding.

Zone 2 training-are there any gender-specific recommendations?

This is a bit of an under-studied area at the moment, but to my knowledge, at the moment, there is no difference in prescribing zone2 in relation to gender.

No. This was briefly fashionable but is a horrible, horrible idea. You don't need to over-eat but you should aim to fuel at least 33% of the calories that you burn riding while on the bike-the proportion typically fuelled at zone2 intensity from glycogen-especially if riding for more than 90 minutes. This should be mostly or exclusively carbohydrates such as sugars, bread, etc.

Won't I get diabetes from all the sugars and carbs on the bike?

No, insulin does not spike from simple carb consumption during exercise-your body has no need to clear excess glycogen from the blood, since it is busy shuttling it into your liver and muscle tissue to replenish the stores you are burning. These are the simple carbs, like sugars, that get digested easily and are very quickly converted into energy and optimal, but we're human, too, not machines, so by all means experiment to find out what works best for you because of taste, satiation, or for whatever mental reason you might have.

Do not over-feed (attempt to feed at approximately 33% of energy expenditure during riding in zone 2 and Zwift will be able to provide this data, but will likely vary between approximately 400 and 1000 calories an hour, depending on what you produce).

You can still must monitor your dietary intake whilst off the bike.

But zone 2 is "fat burning", so why do I need carbs?

Your brain needs carbohydrate for fuel.

Your muscles need carbohydrate as fuel as well. If you do not do this during the training, your body will start trying to break down the proteins in your body to meet these demands. It literally eats your muscles for fuel, besides putting you into a catabolic state, which might bring along hormonal problems, brittle bones, and much more. You can also "bonk"-feel dizzy-and end your ride, failing to reap the advantages of longer zone2 riding. When you do "hit the wall," it is normal to feel absolutely horrible.

There is even a condition now referred to as "RED-S" (relative energy deficiency in sports) where athletes starve themselves before exercise too much and cause themselves, what is sometimes permanent, damage. Please, please, please - eat while riding and stay the hell away from anyone that advises you otherwise.

The reason for eating carbs and not fats while exercising is because the body has around 30,000-50,000 calories of stores in fat that it can use for fuel, but only about 2,000 calories of available glycogen; hence, this needs replenishing, whereas fats do not.

But I want to lose weight!

YOU WILL NOT GET FAT EATING WHILE RIDING YOUR BIKE. Worse still, if you put your body into a "starvation state" through fasted riding it will commonly trigger hormones to cause you to eat more immediately you get off bike - and hence put on more weight. Fasted riding is not a good idea even if your main goal is weight loss.

Nonsense, I lost weight riding fasted!

Yes you did lose weight. But that mass will have been predominantly muscle and water. This is how people get "skinny fat," with a lower overall scale weight but a higher percentage of body fat. Your body weight drops because you are catabolizing muscle. Since muscle is heavier than fat, your scale weight goes down. Low weight is not a good indicator of either body fat percentage or of metabolic health - it can be a "false flag" metric.

But someone told me zone 2 is "fat burning"

Well, this is a misunderstanding. The mitochondria do meet their energy needs primarily from fats ​(66%) vs carbs ​(33%) when riding at low intensity, but you need to understand that:

1. The total amount burned in grams of fat is minimal,

2. Total calorie burn is higher at higher intensity

3. You lose weight from being in a total caloric deficit

You should aim to lose weight via a healthy diet. On the bike, "fuel the work" simply makes your rides more enjoyable, allows you to ride longer, prevents catabolism, and increases your ability to recover.