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Aerobic Capacity – What Is It & Why Is It Important?

Our regular members have noticed that along with our current strength cycle, we have introduced “Aerobic Capacity Wednesdays.”  You may be wondering what it’s for, and why you should attend these or other aerobic capacity workouts, which are lower intensity than our usual metabolic conditioning (“met-con”) work.  Traditionally, CrossFit has used only short, explosive efforts like the Olympic lifts and high-intensity metabolic conditioning (“met-cons”) to develop the body’s three known energy systems.  Workouts designed specifically to develop aerobic capacity is the biggest change in CrossFit programming since the founding of CrossFit, and it’s reasonable to ask why we should include it in our programming.

Which system provides energy as duration of effort increases

Let’s start to answer that with a discussion of the three energy systems and some important terms.  Two energy systems function without oxygen and are therefore known as “anaerobic,” meaning “without air.”  These are the phosphagen system (also known as the ATP-CP system) and the glycolytic system.  The phosphagen system only lasts about 10 seconds before the body has used up the stored creatine phosphate (the “CP” in “ATP-CP”), and can no longer supply enough energy this way.  The glycolytic system uses a process called “anaerobic glycolysis” to produce energy.  It’s fast, but inefficient, and byproducts of the process build up in your muscles.  You know it’s dominating when your muscles feel like they’re on fire, get stiff, and your brain starts screaming at you to slow down.  That’s the result of the buildup of those byproducts.  Aerobic glycolysis, in contrast, generates energy without that buildup, so you can sustain an “aerobic” level of effort.

Related: Intensity– The Key to Improving Your Physical Fitness

Aerobic capacity means the amount of oxygen your body can consume in a given time frame, usually one minute.  It is measured by the formula:

VO2 max = Q(CaO2 – CvO2)

Where: Q = the Quantity of blood your heart can pump in the given time frame, and CaO2 and CvO2 represent the Concentration (C) of Oxygen (O2) in your arteries (a) and veins (v), respectively.

In other words, how good is your body at (1) pumping blood and (2) drawing oxygen from that blood as it passes by your cells?  The better you are at both, the higher your VO2 max will be, and the easier a given work level will feel, because you’ll be in the aerobic system, without the buildup of byproducts that come with using the glycolytic system.

Aerobic capacity workouts push our anaerobic threshold higher, so we can remain aerobically dominant and able to continue without rest at a higher work output.  In CrossFit language, it increases our work capacity.  

While we get the most bang for our exercise buck from high-intensity work, the effect of that high intensity work is concentrated on our phosphagen system for our sprints and heavy lifts and the glycolytic energy system for met-cons.  While met-cons definitely train the aerobic system, the very definition of “high intensity” is that we are pushing ourselves to the upper limit of our ability, which means we are constantly pushing against the line between which of the aerobic system and the glycolytic system predominates.

This line is known as our “anaerobic threshold.”  We can go over the line into glycolytic system domination to finish our thrusters unbroken.  The price we pay is the need to rest in order to clear some of the byproducts and “catch our breath” before we can start our pull-ups.

Related: CrossFit ‘As Rx’ vs. Personal Progress

Aerobic capacity workouts push our anaerobic threshold higher, so we can remain aerobically dominant and able to continue without rest at a higher work output.  In CrossFit language, it increases our work capacity.  In practical terms, it means you can finish your thrusters and go right into your pull-ups, dramatically improving your performance.  While aerobic capacity work does not increase our work capacity across all time domains, it certainly impacts everything over the 10 seconds or so that the phosphagen system lasts.  And that’s a lot of performance.

–John Bryant

Founder & Head Trainer

john@crossfitslipstream.com