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Aerobic capacity, adaptation and RPE

By April 20, 2021May 4th, 2021Athletic Performance

Authored By Pete Jacobs, World Champion and Performance Expert

Pete shares his world renowned knowledge on MAF (Maximum Aerobic Function) + RPE (Rate of Perceived Exertion), explaining their relationship to aerobic exercise. He couldn’t resist touching on other topics too….enjoy.

*This is in no way meant to be a complete description of energy pathways or list of factors that influence our health and performance. This is a simplified discussion about some key elements the athletes I coach could currently benefit from, as well as my personal experiences.

I hope my written words give you pause for thought, even if they are doubtful ones. 😉 PJ.

Maximum Aerobic Function (MAF)

MAF is based on the formula of 180 minus your age. It was developed by Dr Phil Maffetone in the late 70’s when he was coaching world class and everyday athletes via heart rate (HR) monitoring. It was a tool to help them be the healthiest, most efficient, and therefore highest performing athletes they could possibly be.

By monitoring his athletes (and patients) Dr Maffetone deduced that to build aerobic capacity it was a requirement to spend the majority of time at a HR that was equal to the athletes highest level of fat burning possible (as a percentage of fuel used over glucose).

We are always burning fat and glucose, however, the more fat we burn, the less glucose we burn, and the less stress (oxidative stress) we induce on the system – therefore longer periods of exercise at higher intensity is possible when more aerobically and fat-adapted. Burning glucose creates more stress on the system, leading to lower ability long term to produce energy within the mitochondria within the muscle.

The MAF formula works for most people as a starting point. But maybe not for you.

Dr Maffetone has written many books and articles about MAF. I suggest you do some googling and reading about all the nuances that Phil has written about regarding exercising to your MAF HR. He is an amazing author, and all of his books and blogs about health and fitness are worth reading.

But nothing stays the same… including the threshold at which fat burning is optimal.

Some days our heart rate will be way down when perceived effort is up, and other days perceived effort will be down and heart rate way up. This is due to the many, almost infinite factors that fluctuate within the human body daily – which is what makes us a miracle of chaos.

There are a few predictable key factors that will influence energy production and the ability to burn fat preferentially to glucose. These also can influence how we feel and perform day to day.

Stress & Inflammation Impact on Fuel & Energy

As someone who was prone to inflammation for most of my life, to the point of it being crippling both mentally and physically at times, I am well versed in what it feels like to be unable to produce energy as I would like (or as any human would like).

Since I was about 15years old when unexplained (at that time) fatigue first affected me, I have been aware of, and continually developed, the ways I can measure my “energy”, both subjectively (how I “feel”) and objectively (external data).

Stress and inflammation affect your aerobic capacity through many pathways, including but not limited to – the food you eat, mental state, sleep, nervous system, pollutants, viruses, both singularly or in combination. One of the detrimental effects from stress is a lower ability for oxygen to be used in the energy production cycle, meaning you will burn more glucose by default due to lower oxygen levels.

In this scenario you may be exercising at a low HR, but due to limited oxygen in the cells you will be using more glucose for fuel and training your muscles to burn more glucose. Continuing to push exercise in this state of high RPE and low HR will continue the cycle of increasing stress due to energy production with low oxygen levels, whilst delaying recovery from whatever created the stress in the first place. Very easy exercise or complete rest is recommended.

Knowing the feeling of increased oxidative stress and less oxygen getting into the muscles/brain/body is familiar for most people. It feels sluggish, physically, often mentally, and any effort to push even moderately hard will result in a slight to painful “burn” in the muscles commonly known as “lactic burn” (even though technically it is not lactic acid that causes the pain). When you feel this way, it is imperative not to go beyond super easy relaxed exercise. Pushing into the pain threshold when the heart rate is low is basically detraining your adaptation to exercise by training in a low oxygen environment, at low levels of activation, in a highly stressed cellular environment. The cycle of stress will snowball and become a bigger problem.

Mindset Effect & Approach

Emotional stress is both a symptom and a cause of your system running sub optimally.

Asking yourself if the “effort” needed to complete the task is going to add to your current state of emotional stress, or help relax it.

Depending on your current state of mind, going ahead with the session/task will require different approaches accordingly. Only you will be able to choose when and how they are used.

For example, on a day where you are emotionally exhausted or anxious, the approach should be far from “toughen up and get this done you weakling”, and much closer to “move gently, breathe gently, and only do what feels relaxing”.

A high emotional stress level may not change heart rate, or physical RPE, but ignoring your state of mind will have negative consequences on your nervous system, health, and recurring mental behaviours.

Tip: Do not push yourself physically (at or above MAF HR) when feeling mentally stressed with sympathetic nervous system dominance.

Adaptation to Exercise: Oxygen, Energy & Nerves

Yes, there are more factors for energy production and adaptations that occur due to exercise, but I’m only going to talk about capillaries and oxygen, mitochondria and energy, and nerves and signals. Yes, very simplified I know.

The most common questions about MAF are:

“Why is it so hard to get up to my MAF HR on the bike?”


“Should I ride at MAF if my RPE is really high?”

For some people, holding an effort at MAF on the bike is not easy, and impossible for many…….I’ll try my best to give you an explanation in my words.

My theory is that the muscles aren’t able to demand enough energy (not enough mitochondria) or deliver enough oxygen (not enough capillaries) to require a large blood-flow, and therefore heart rate increase is difficult…a bit like trying to increase your heart rate by moving your fingers rapidly. You can try and try to the point of exhausting your fingers but your heart rate will barely change. However the mental effort to move the fingers is high, but there simply is not enough mitochondria to demand enough energy, or capillaries to deliver enough oxygen, or nerve signals to trigger enough muscle mass to increase the demand for more blood from the heart and increase heart rate. [Yes, fingers don’t have enough total muscle mass anyway but you can picture my point, hopefully].

Part of the process and goal for adaptation to training is an increase in mitochondria and capillary density in the muscles so more energy can be produced at a faster rate within the muscle, and more oxygen can be delivered to allow for more aerobic energy production. Another key adaptation is the ability to activate the muscles via nerve signals to move the muscles.

More neural connections = more muscle activation = more demand for energy & oxygen.

We have several seconds of stored ATP (Adenosine Triphosphate) which allows the muscles to move. ATP is the energy currency the muscles use for movement. When the muscles are told to move (by our brain via nerves) we use our stored ATP, while starting production on more ATP.

People with low aerobic capacity are still able to produce relatively high power for a very short burst around 7 seconds due to the ATP being readily available and not requiring production that has limitations.

Neural Connectivity

One of the limits to the power able to be exerted during this “stored ATP” sprint system are the nerves and their ability to fire the muscles to move. This can be increased in a very short timeframe through training, particularly through maximum efforts which trigger and build stronger connections to a larger number of muscle fibres. Heavy weights and max effort sprints which force a larger amount of muscle activation “build” these neural connections quickly. As these neural pathways already exist, the increased communication along these pathways is a very quick adaptation, as it is just making the connections more “useable”.

As a neural pathway is used, it gets easier and easier to use. Like the connections in your brain of a habit, a reactive response of emotion, or a physical trait, the same can be said for the connections that transport signals for your muscles to move. If you only ever use 50% of your muscle, the signalling pathways to the other 50% will be slower (weaker).

ATP Limitations

The other limit is the ability for the mitochondria within the muscles to produce energy as demanded, which can take longer to grow adaptation to.

Short bursts of energy are possible at high intensity due to stored ATP. However, sustained moderate efforts requiring constant production of relatively high amounts of ATP may be harder for athletes who have not done many months of training on the bike to build up their mitochondria and capillary density in their muscles. These adaptations support greater production of ATP to the level which is needed for sustained effort to a greater muscle mass, which would mean an increase in heart rate.

Day To Day Fluctuations

As we train, and alter our training through different length of efforts, at different intensities, there will be days which feel much better than other days throughout the process of adaptation and recovery. We have to induce stress through physical effort to instigate the adaptations needed for improvements, and some days the stress induced will inhibit the following sessions for a day or more. The goal is to induce enough stress for adaptation without going into a zone of detraining through excess stress, and to be consistent week after week, month after month for constant growth and adaptation.

Days to Push

On days when your heart rate is moderately high, around MAF or slightly above, at an RPE slightly lower than usual, these are the days to work sustained solid aerobic efforts. These are the days where you can put more time into the legs at a higher intensity yet remain very aerobic – an efficient time for adaptation. On the bike, these sessions may range anywhere from 10 second efforts with short rest repeated many times, or 5-minute efforts with 2 minutes rest, right up to many hours at MAF HR.

No matter the session, the point is that you are either pushing more power than usual at an equal perceived effort, or a higher heart rate than normal at an equal perceived effort, and are able to do so for a total accumulative time longer than you can normally hold for said power, heart rate, or perceived effort.

,On those days where you can push and increase your heart rate you have a situation where more muscle mass is capable of doing more work than usual – mitochondria are in a healthy low stress state, the body has low inflammation levels, and all necessary elements are present in optimal amounts (e.g. electrolytes). When you can push your muscles harder in an aerobic state that is the time to take advantage and put more workload into the muscles so every part of your mind and body can adapt to this increased state of activation and connection. Strengthening the mind/body connection when more energy is flowing through more cells than usual is key for peak performance on race day, or just for health, wellbeing, and performance in general.

The cellular memory and subconscious/autonomic nervous system impact on your performance (in life, or on race day) cannot be underestimated. This must be trained. Training with a relaxed calm mind in the present moment is paramount to being able to push on race day (or in a stressful situation like that important board meeting or keynote speech) without overly exciting and fatiguing your mind and body.

Keeping emotionally calm and in the moment on key training days is a crucial part of your training. The opportunity for higher adaptation on these days when you can push your system physically through HR, power, or RPE, is not just physical. Equally important is adapting your nervous system, mindset, and muscle memory by training in a relaxedstate while the stress load is greater.

Factors for Fuel Preference

The fuel your muscles are choosing to use is not solely based on your heart rate.

It is a known fact that generally less healthy people (those with chronic illness/disease) are less aerobic people (have very little aerobic capacity), and will generally be burning more glucose than fat at all times……more than a “healthy” person.

I put “healthy” in quotations because generally this is true, that unhealthy people will burn glucose predominately, however fat optimized people can also have acute or chronic health problems. It is likely you could see a change in the fuel percentages of fat adapted people when they are not healthy.

The fuel you burn can be so closely tied with your current state of health that Dr Maffetone has even developed a product to breath test people in hospital to determine their health status at that moment based on measurements showing what percentage of glucose and fat they are burning and exhaling.

The preference for which fuel is being used relies on many factors, but for fat optimized athletes on a day where they feel great, they will continue to burn fat at a higher heart rate than on days where RPE is higher and they have higher levels of free radicals or other inflammation prohibiting optimal transfer of oxygen into the energy production processes. Limited oxygen will result in more glucose being burnt.


On days you feel strong and fit, your RPE is an excellent measurement. Tracking HR is also important. Power is a bonus to have and helps with objective feedback and confidence, and also useful for limiting an effort, or keeping motivation and the effort high.

As with tracking any average such as speed/watts, it is important not to push for a personal best average. An average means very little as every day has different conditions, and quite easily become a trap for the ego that makes you push much too hard and disconnect from the self-awareness you are trying to build. Save those best efforts for race day. In any race there needs to be a certain level of control and awareness over ego. Practicing this in training will make you a smarter racer.

Feeling good and going hard and fast with a low RPE is an opportune session, but could end in disaster with long lasting elevated stress levels if not aware of ego and its’ influence.

Sustainable Effort

A sustainable effort is a basis for good training. Being able to sustain an effort, or perform repeated intervals, is key to targeting the adaptations required for consistent training.

Not easy to explain, a sustainable effort requires the instinctive, intuitive pacing of each individual to decide on a given day for a given session. HR and power should be used as a guide, as past data combined with experience can help influence the effort and awareness positively. Intuition, being aware, and feeling the effort is key to allow you to constantly micro-adjust your effort within the session, and on a more macro level from day to day.

For example, on the bike, how hard can you go if aiming for a best ever average power on the windtrainer if given 10 x 30 seconds, or 10 x 5 mins, or 50mins non-stop? You could not answer me exactly. There are too many variables within the session, day to day, and the sessions themselves are not ones you may have even attempted before. Likewise, a coach cannot tell you exactly how hard you should go to get the most out of the session. If the coach sets a power target for 50mins that you have to work really hard to achieve for the first 20mins, over which time your heart rate continually climbed until you could no longer hold the power, and in the last 30mins you felt rubbish with little oxygen getting to your muscles, burning more glucose than you should, and producing more oxidative stress than you should, you would have ended up detraining your system.

Yet had you held the power for the first few minutes, and then hit a perceived effort, and a heart rate – which you knew from experience cannot go much higher and be sustainable – you would then have lowered the power slightly using your intuition, RPE, and HR. Subsequently holding an effort which was sustainable for the duration of the session and achieved the adaptation goal of the session.

Likewise, if your heart rate or RPE dropped throughout the effort, you are able to make changes throughout the session to increase it, or hold steady until the end of the session and maybe push harder, depending on the goal.

Getting to know what it feels like at different intensities is key to being able to moderate your efforts in training, and in a race. And learning how to hone your instinct for going as hard as possible without going over the edge, or inducing too much stress too early, is an invaluable skill that takes time to develop.

Athletes that grew up pushing themselves, competing, and training at different intensities, have a clear advantage when it comes to getting the most out of themselves in training and racing, without overdoing it.

Is RPE the best measurement?

Personally I don’t think any one single measurement is going to be the best measurement every single day. We perform best when chaos is present. With so many pathways for energy, such as perception, hormones, consciousness, and subconsciousness interacting, it would be impossible to keep track of what factors and data was most relevant on a single day. Imagine trying to track all points of influence into the energy system to see which was having an impact and when, from the many pathways of fuel and energy (many more than simply fat or glucose and ATP), extending to oxygen, carbon dioxide, nervous system, hormones, thoughts, feelings, mindset, and electrolytes. the list is almost endless and far beyond my knowledge. That is why many recommendations based on “research” need to be questioned with these uncontrollable factors in mind. That is why what an individual experiences personally, what results they can reproduce consistently, and feel are some of the best measures for an individual to determine what factors helped on any given day for them to perform better, or worse. And also what condition the body was in prior to undergoing any given test is also an unknown, as tests for oxidative stress, electrolytes, nervous system, plasma volume, and other factors previously mentioned (and all those not mentioned) are all playing their part in the outcome of a performance.

Without Chaos..

The opposite of chaos is seen in low heart rate variability and chronic health conditions. Embrace chaos, don’t let your mind and body get too stuck in habitual behaviours.

What makes us feel great one day may not work the next. Additionally we don’t necessarily want to do the same thing repeatably for many adaptive/habitual/addictive reasons. Some routine is good, especially when it promotes being grateful, calm, and present, but beware of becoming inflexible, dogmatic, or stubborn to “different”.

Your mind and body is a chaotic experiment of life, so don’t treat it like a self-piloted robot and let it mindlessly go where it wants to… towards the actions and thoughts of least resistance, to the ones you use most often (just like the pathways mentioned above for muscle activation) the ones that trigger faster, activate with less effort or intention.

Imagine if your most active and strongest pathways, those reinforced with routine and reward (hormonal), were negative thoughts and unwanted or unhealthy habits…


Intuition, feeling, instinct, mindset, subjective and objective feedback – these are the most important factors to gauge your efforts every single day.

Use all the tools at your disposal to motivate you, push your limits, and to improve your state of mind or body.

The goals for each session may change from what the coach wrote down, or what you set yourself, and might not become apparent until the session is a half-way through.

Your goal is always to become more aware, while at the same time thinking less.

A quiet mind is more aware of the present.

A relaxed mind and body becomes more aware of the feedback from one another, and can be altered in tiny increments easily and constantly over and over as feedback is felt, such is the control and connection that comes through practicing being present.

Many different ways to achieve the same goal.

Every day may require a different approach from you personally. Your coach should have provided you with a variety of tools to trigger you to think about this connection in ways you had not before, but it is up to you to use them, and then talk about the ones that worked so you can refine them. It is imperative you learn to draw on those tools and skills with self-awareness that will allow you to find the right approach at the right time.


I set out to write a paragraph about MAF and RPE, and how to use them in combination to get the most out of your exercise…

Many pages, days, and brain dumps later I hope I have explained some of the factors that influence our ability to adapt, improve, and perform.

The bottom line is you need to get to know your mind and body. You need to practice being connected, relaxed, aware and present, while building intuition and instinct of your physical output & accumulative stress that is sustainable over a given time, in that moment.

Tip: Practice how to alter your perception of that effort, either through exogenous or endogenous sources. That is, for example, what you eat, drink, listen to etc. Or by your thoughts, breath, hormones etc.

There are so many ways to alter perception of energy, and therefore energy demand, which leads to more production of energy. I suggest finding one that works for you, actually finding several because every day could be different.

Final note
I did not mention when heart rate is unusually high and RPE is also high. Clearly this is not a time to have a high RPE. High heart rate is a sign of inflammation, and could be caused by illness – this is not a good time to push the heart rate and nervous system.

Pete Jacobs
World Champion Ironman

Read more from Pete Jacobs on his website.