In our last blog post, we talked about the anaerobic lactic energy system and how it acts as a bridge between anaerobic energy production and aerobic energy production.  This time we are going to start taking a closer look at the aerobic energy pathways, the most complex and the most crucial energy system for life and sport.

The aerobic energy system is a low power output energy system that offers a sustainable flow of energy over longer durations that its anaerobic brothers.  As we discussed in the last blog post, the aerobic energy system starts to take over as the dominant Adenosine Triphosphate (ATP) producer as soon as any physical activity begins to take longer than 60seconds, regardless of intensity.   Athletes also become more and more aerobic dominant with each successive interval in interval training or interval based sports.  The aerobic energy system is ALWAYS contributing to energy production at ALL levels.  You are aerobic right now, while you are sitting and reading this blog post your body is using oxygen to produce energy (ATP).  The aerobic energy system provides the ATP necessary to support life 24/7/365 thought your entire life.  Don’t believe me? Hold your breath and see how long you survive?  Research has shown that life expectancy is directly related to levels of aerobic fitness [1,2,3,4,5].

The aerobic energy metabolism requires the presence of oxygen and glycogen (carbohydrates) or fatty acids.  Although the aerobic system is the least powerful of the three, it can function all day literally and can alternate between stored fat, carbohydrate and protein to produce energy (ATP). The higher and athletes level of aerobic fitness the faster and more effectively they will recover between sets of heavy lifts or sprint intervals because the aerobic energy system drives the recovery of the anaerobic energy pathways.   Let's say this one more time; aerobic fitness is essential for ALL athletes, not just endurance based sports.

When most of us think about aerobic training, we guess of cyclic sports like running, swimming, and cycling.  In many of these sports, we know about "training zones”, “heart rate zones”, or “aerobic zones.” These are all one in the same aerobic zone which measure the intensity at which an athletes body is using aerobic metabolism to produce energy (ATP).  In cyclic sports its easier to control the energy expenditure (zones) by watching metrics like speed, heart rate, watts and time.  Athletes can easily increase or decrease their efforts to stay within particular “zones" as not to fatigue the body, which is very useful for endurance based events.

Many endurance athletes love to know these numbers, and throughout their training cycle, they keep a close eye on the metrics to determine if aerobic fitness levels are improving.  If running speeds increase, RPE decreases, the average HR decreases or time to cover a certain distance improves, athletes, can see that their aerobic pathways are improving.  This is not the same for “functional fitness” as combining weightlifting with gymnastics, and monostructural exercise makes it impossible to use standard metrics.


The typical “functional fitness” training programmes have many variables such as central nervous system (CNS) demand, training intensity, mixed modality, mixed time domains, competitions, sleep, nutrition, stress, etc.… which all affect an athletes heart rate from workout to workout.  “Functional fitness" athletes must be able to visualise a much broader picture of fitness and be coached around sustainable, repeatable, and paced efforts.  Typical aerobic metrics are not a successful predictor of intensity or effort; athletes must rely on their Rate of Perceived Exertion (RPE).

There are three stages of the aerobic energy system pathway, Glycolysis (we mentioned this last week), the Krebs cycle (TCA), and the Electron Transport Chain (ECT).  Each stage can work to produce a more significant amount of energy. However, the time taken to do so is significantly longer than the anaerobic systems.  The aerobic energy system is the MOST adaptable energy system when it comes to room for improvement. Athletes who work on high-intensity training session 3-4 times per week quickly hit plateaus with their levels of fitness because they fail to develop their aerobic fitness capacity maximally. A variety of intensity and volume must be utilised, not just high-intensity. Low-Intensity, Steady-State (LISS) training has become a forgotten principle of building superior levels of fitness and promoting recovery.

When training the aerobic energy system, the primary goal is to increase blood flow and therefore the rate at which oxygen can be delivered, and CO2 extracted.  The aerobic adaptation is achieved by: 

  • Delivering more blood to the muscles and tissues by increasing the functional capacity of the heart (hypertrophy, elasticity, contractibility, etc…).  The heart is a muscle that can be trained to become more efficient and effective

  • Deliver more blood and oxygen throughout the body by increasing the vascular networks (blood vessels and capillaries)

  • Increasing the function and number of mitochondria, the cell power plants that create ATP.  The health and volume of your mitochondria are essential to life and longevity.

  • Developing proper breathing techniques, during exercise, the body’s demand for oxygen increases and our breathing volume must also rise.  Respiratory muscles must contract more forcefully and more rapidly to keep pace with the body’s substantial increase in metabolism.

Aerobic energy production in a multidiscipline environment can be developed when the oxygen-transportations system is working at a higher capacity, and the lactate accumulation in the muscles is under control.   This means both the Krebs cycle and the ETC can sustain the energy demands places on the athlete while also clearing the lactate at a rate higher than or equal to lactate accumulation.   Training the aerobic system and its many levels depends on the athlete’s current fitness level and function of the athlete. A novice lifter would not be programmed to squat 100kg on their first day of training; the same approach applies to Energy System Training, aerobic metabolism plays a vital role in human performance and is the base foundation of fitness.  Can you build a pyramid without a solid base?  This is one common weakness in the group exercise model; workouts fail to elicit aerobic fitness improvements as the intensity and exercise selection is not suited to the individual.

The athlete's education on the characteristics of aerobic training comes down to the skills and knowledge of the coach and the quality of the training plan. Merely programming a workout with a duration of more than 60seconds does not guarantee aerobic conditioning. The intensity of the exercise, the athletes level of fitness, the athlete's ability to stay aerobic under specific movement and loads, and the athletes understanding of aerobic training all have a role to play in term of energy system activation.  Far too often athlete push energy expenditure beyond their aerobic capacity and into lactate accumulation which quickly limits performance and forces intra-workout rest.  Athlete pushes the accelerator to the floor for 30-90seconds and then blow up, taking rest and allow the energy systems to recharge, before repeating the cycle.  This is a very inefficient way to attack a workout, and it's certainly not going to help build aerobic fitness.

Would you go to the gym every single day and put your 1RM load on the bar?  Would you go to the gym every single day and train legs?  Why do so many athletes go to the gym and focus primarily on anaerobic conditioning?  How many “functional fitness” programmes take the time to educate their athletes about dynamic energy control and the importance of build aerobic capacity? There are thousands of WODS, or so-called "conditioning programmes" published all over the internet that fail to deliver fitness periodisation.  Far too many gyms are only focusing on the high-intensity endorphin that offers short-term improvements but feeble long-term health and performance benefits. Our bodies can adapt to acute stress, but a chronic overload of stress is both unhealthy and unproductive in strength and fitness development. 

The “work harder”, “keep moving”, “move faster”, etc…. the mentality of training is not the answer to improving our athlete's health, performance and longevity. The overload of “high-intensity” workouts are failing to deliver the results that many people are seeking. We are starting to see the long-term detrimental impacts associated with this style of “fatigue based training."  How many clients are making ongoing progress with their fitness levels?  We see short-term improvements in the first 6-18months, but then what happens? There is a lack of education and awareness around how to building aerobic and anaerobic fitness alongside the building and maintenance of strength.  The classic fitness programme exposes athletes to chronic levels of high-intensity training with the majority and frequency of sessions being dedicated to high-intensity.

In the coming weeks we are are going to dive deeper into the aerobic pathways to build a better understanding of the biology and an awareness of the “why” and “how” to improve fitness levels.


  1. Does Physical Activity Increase Life Expectancy? A Review of the Literature, C.D Reimer et al, 2012

  2. Leisure Time Physical Activity of Moderate to Vigorous Intensity and Mortality: A Large Pooled Cohort Analysis. Steven C. Moore et al  2012

  3. Exercise and longevity, Vincent Gremeuax, et al, 2012

  4. Increases in physical activity is as important as smoking cessation for reduction in total mortality in elderly men: 12 years of follow-up of the Oslo II study, S. A. Anderson, 2015

  5. Fifteen minutes of moderate daily exercise lengthens life, Taiwanese study finds, 2011