You may hear runners or coaches talking about ‘aerobic’ or ‘anaerobic’ training and wonder exactly what it means and how it might be relevant to your training. Below, we explore these terms and challenge some misconceptions to help you better understand the various intensities which can impact on your fitness – and how you can apply that understanding to make your training more effective.

Defining intensity

The terms aerobic and anaerobic have long been used to make a distinction between different running intensities and they play a role in the development of the various training zones and thresholds.

Essentially these terms refer to energy. 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. This covers a wide range of intensities, from a walk or chatty-paced jog at a 1-2/10 effort, through to moderate efforts where you feel like you’re working harder but are in control at 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?

Anaerobic energy production relies on stored energy sources in the muscles at times when the body cannot produce adequate energy through aerobic means. Anaerobic energy production is required for short, higher intensity periods of running or sprinting. Typically you will feel a rapid rise in perceived effort (up to 8-9/10 or higher), heart rate and breathing rate and you will be aware you can only sustain the effort for a short period before slowing 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.

What do these systems mean for your running?

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.

A false dichotomy?

All seem relatively clear up to here? Well, we're about to muddy the waters. 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.

The false dichotomy between aerobic and anaerobic training often leads to a narrow focus on one type of exercise at the expense of the other. This approach overlooks the benefits that can be gained from a more integrated training regimen.

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.

It can be helpful to understand the terms, but focus on what's important – good consistent training with a mix of different intensities, staying injury free and enjoying your running.

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