What you need to know: How your body generates energy

There are two key types of metabolic/energetic pathways used for generation of energy in all our cells:  Aerobic and Anaerobic.   Aerobic means with oxygen and requires the presence of oxygen in the cell producing energy.   Anaerobic means without oxygen and cells can produce energy without the presence of oxygen using these pathways. 

A single cell can either produce energy aerobically or anaerobically at any given moment in time, but never both at the same time!   However during any activity there are some cells producing energy aerobically and other cells producing energy anaerobically.    

Muscles obtain energy from three important sources*: 

Glucose which is sugar can be burned aerobically or anaerobically

Fat can ONLY be burned aerobically!

Protein (not a primary fuel  but protein can be broken down to be used including break down and creation of glucose in certain circumstances)

* all energy comes from ATP but ATP comes from fats, sugars, and proteins

Overview of Aerobic Generation of Energy

When a muscle has sufficient oxygen present it will burn glucose aerobically which provides much more energy compared to burning glucose anaerobically when the cell does not have enough oxygen.     

Muscles can also generate energy from fat aerobically.  Fat provides MUCH higher amounts of energy than glucose, but burning fat requires even more oxygen availability relative to the amount of energy needed per unit of time!   

Overview of Anaerobic Generation of Energy

When a cell needs to produce a lot of energy in a hurry it is forced to use anaerobic energy pathways including burning glucose which results in a byproduct of lactic acid (the burn).  When it is forced to burn glucose anaerobically the muscle will go through lots of glucose really quickly..   This rapid depletion of glucose along with the build-up of lactic acid limits how long a muscle cell can keep generating energy anaerobically.

So the higher the intensity of exercise the more that muscle cells will be forced to use anaerobic metabolism to burn glucose rather than burning glucose aerobically or using fat for a fuel.   

That being said intensity is different for different people depending on their fitness level so intensity is a relative term.  For example walking on flat ground at 3 MPH is low intensity exercise for a fit runner so their muscles can generate most of the energy required aerobically because they have lots of oxygen delivery capacity to the working muscles relative to the amount of energy required per unit of time so the cells can use aerobic pathways.   However for a severely de-conditioned person walking at the same pace this is an anaerobic exercise because they cannot deliver sufficient oxygen quickly enough to generate the required energy aerobically so their muscles are forced to go anaerobic.

Exercise physiologists use the intensity and duration of an exercise to define it as being aerobic or anaerobic.     Exercises requiring short intense efforts that cannot be maintained for long periods are classified as anaerobic and exercises that are less intense and can be maintained for a long period of time are classified as aerobic.  Generally speaking exercises that are so intense they cannot be continued for more than 2 minutes are classified as “anaerobic” but this is a bit of a generalization as we will see below.    

In fact there are several different anaerobic energy pathways used depending on how much energy is required and how fast it is required (see chart below).   For example an 8 second all out sprint will use a different anaerobic energy pathway than a 30 second sprint or 2 minute all out run – yet they all can be classified as “anaerobic”.   

So if an exercise can be done for more than two – four minutes – it means that most of the energy is being supplied aerobically so a physiologist would say this is an aerobic or cardiovascular exercise.   On the other hand if someone cannot keep doing an exercise continuously for 2 minutes it is an Anaerobic Exercise

The key point to understand from these energy pathways is that the best conditioning response will occur if the work interval time, work/rest ratio, and work interval intensity of the conditioning program is matched to the specific energy requirements of a sport or a position within a sport.


Written by: Greg Maurer 


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