What is aerobic vs anaerobic exercise?
Like stalactites and stalagmites, most of us have heard of ‘aerobic’ and ‘anaerobic’ but might struggle to remember which is which. Here we’ll help you to clarify the terms and understand how exercising in both ways can benefit your running. We’ll also clear up some common misconceptions, and teach you how to mix different intensities in your training – which all adds up to increased running ‘might’ – or is it ‘tight’?
Defining intensity
Put most simply, the terms aerobic and anaerobic refer to the different ways in which your body produces energy while you’re running. We use the words to distinguish between different running intensities, and to define the different training zones and thresholds.
Adenosine triphosphate (ATP) is the ‘currency’ your body uses to produce the energy required to run. In order to sustain longer periods of running – or multiple bouts of running (e.g. during an interval session) – your body needs to replenish this ATP and it will do this via both aerobic and anaerobic means.
What is aerobic exercise?
Aerobic exercise is a term commonly used to refer to activities that primarily use oxygen to generate energy. During aerobic exercise, your body relies on a continuous supply of oxygen to break down carbohydrates, proteins and fats, providing the fuel needed for sustained running. Even the most leisurely stroll or conversational jog with pals counts as an aerobic session, up to the kind of run where you have picked up the pace but still have more in the tank – say a 6-7/10 effort. The key to understanding training at an aerobic intensity is that it involves the ability to sustain effort for extended periods without feeling excessively fatigued.
What is anaerobic exercise?
Your body uses anaerobic production of energy when the oxygen supply is unable to provide enough. It looks for extra energy stored in the muscles. You need energy produced anaerobically when you’re doing shorter, high intensity blasts of running or sprinting – the kind where your effort levels get higher quickly, which will be obvious when your heart beats a lot faster and your breathing becomes more rapid. We’re talking 8-9/10 or even greater, a pace that can only be kept up for a short period of time before everything is screaming at you to slow down. To complicate things a little further, anaerobic energy production is typically split into two systems:
The glycolytic system will provide a significant amount of the energy in hard efforts lasting from about 10 seconds through to about two to three minutes. Your body uses its stored glycogen to create energy rapidly, but it doesn't last long and is associated with a rapid increase in fatigue and markers of fatigue (such as blood lactate).
The Creatine Phosphate (CP) system provides energy using creatine phosphate stored in the muscles. While this energy is provided very quickly, it also depletes very quickly – typically lasting only for around 8-10 seconds and then requiring a significant period of time to replenish. This system is relied upon when sprinting but also to ‘get going’ at the start of races.
How to apply the aerobic and anaerobic systems to your training?
Training that relies on aerobic energy production can be everything from short or long easy runs, to what is commonly known as tempo or threshold training. If you want to increase the contribution of anaerobic energy production in any run, you will likely need to do this with some form of interval based training where you work hard in short bursts, then recover before repeating the hard effort. The key to getting your sessions right is that the harder you work, the greater the percentage of anaerobic energy you will require. As a result, you will both reduce the amount of time you can run at that effort and increase the recovery you will need between efforts.
Making smart training decisions
When structuring your training there are a range of things you need to consider which will influence what will be the optimum balance of aerobic and anaerobic energy production:
Race demands: The majority of the races you’re likely to run will rely on a very high percentage of aerobic energy. Even 1500m races will typically see around 85% of energy contribution coming from aerobic sources. And as you go longer, the aerobic contribution increases.
Tip tip: Even in longer races if you are racing on hilly or uneven terrain be aware that your body may have short periods of increased anaerobic energy production. The same is also true in races where you might have short surges of pace. Consider interval sessions or fartlek runs which mix short fast efforts with longer steadier intensity to prepare for this.
Intensity: At lower intensities your body can use a greater percentage of aerobic energy production, at higher intensities – particularly beyond what is commonly called ‘lactate turnpoint’ – your body will be relying on an increasing the amount of anaerobic energy product to sustain your pace.
Top tip: There are a range of ways you can monitor your running intensity. Some, such as heart rate, can be more useful for putting a cap on your aerobically focused sessions, while others such as perceived effort, timed splits or power might be more useful for your anaerobically focused work.
Duration and volume: After a short period of high intensity running your body will have rapidly depleted its creatine phosphate and glycogen stores, so the longer you try to run, or the more repetitions you aim for at a high intensity, the more that depletion of fuel stores will mean your body is forced to rely on aerobic energy production.
Top tip: If you want a greater percentage of anaerobic contribution in a session, you need to use interval based training with efforts of typically two minutes or less for the glycolytic system, and eight seconds or less for the creatine phosphate system.
Recovery: During an interval session the greater the percentage of anaerobic energy you use, the longer recovery you will need between efforts to hold the session together.
Top tip: A simple way to ensure that an interval or fartlek session has more of an aerobic focus is to stick with very short recoveries or keep jogging or easy running on your recoveries. Conversely, if you want to run at intensities which rely on anaerobic energy production give yourself long recoveries. I will often set hill sprints of eight seconds with 2-3 minutes rest.
Fitness level: Runners new to the sport may have much less of an aerobic buffer before they start to rely on anaerobic energy production.
Top tip: Newer runners focusing on longer distances will generally benefit from spending a good period of time focused on easy running and more aerobically focused sessions before adding more anaerobic focused sessions later in their training.
It’s not quite either/or
Got all that? Now let’s make things a little more complicated. It’s important to understand that your body does not neatly separate aerobic and anaerobic energy systems. Instead, they exist on a continuum and work in tandem to meet the demands of running. Both aerobic and anaerobic processes are simultaneously active, albeit to varying degrees. Even during low-intensity running, anaerobic energy pathways contribute to overall energy production. Similarly, during high-intensity anaerobic activities, the aerobic system continues to play a role in supporting energy production.
This means that we shouldn’t neglect one type of exercise in favour of the other. It’s much more beneficial to create an integrated training regimen that incorporates both elements. A 2023 study in the International Journal of Physiology, Nutrition and Physical Education found that mixing the order of aerobic and anaerobic training in male 3,000m hurdlers led to a significant improvement in performance.
One key reason why aerobic and anaerobic training should be seen as complementary rather than mutually exclusive is their mutual influence on physiological adaptations. Aerobic exercise, with its emphasis on sustained effort, enhances cardiovascular fitness, improves endurance, and increases the efficiency of oxygen utilisation. These adaptations, in turn, benefit anaerobic performance by facilitating faster recovery between bouts of intense exercise. Conversely, anaerobic training, which focuses on power and strength, can enhance the body's ability to generate force, leading to improved performance in both aerobic and anaerobic activities.
Embracing a more holistic approach to your running allows for greater versatility and flexibility in training. By incorporating a mix of aerobic and anaerobic dominant intensities, you can enjoy a more diverse and engaging plan. This variety not only prevents boredom, but also helps prevent overuse injuries that can occur from repetitive movements associated with exclusively focusing on one type of training. It will also give you more physical tools in your armory.
Seeing the bigger picture
My mentor at Stirling University, Dr Andrew Kirkland, was a big influence in my thinking about training and coaching. Building on an article by Mark Hargreaves in the Journal of Applied Physiology in 2008, Andrew argues that not only is there a false dichotomy but thinking about your running in simple aerobic/anaerobic or ‘metabolic’ terms is oversimplifying things.
He argues that there are a far greater ranger of factors we need to consider such as muscle activation, psychology, fuelling and even the social environment you run and race in that will have as much if not more of an impact than the training zone you use and the split of aerobic and anaerobic energy linked to it.
So now you understand the different definitions, you can focus on what’s really important: keeping consistency in your training, working at a mix of intensities, keeping injuries at bay and having fun.
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