Metcons and Anaerobic Exercise

Guy recovering from a metcon workout, 8/24/10

The following is a compilation of several posts published on our blog nearly two years ago.  I’ve decided to stitch it together and publish it again primarily because we have so many athletes who have never seen it.  However, it is also a good reminder that our “metcon” workouts are intense for a reason, and that they must be performed with intensity to be most effective.

In this post, I’ll explain the differences between anaerobic and aerobic exercise, and how we use them in CrossFit.

This requires a brief overview of something called metabolic pathways, which are the chemical processes that provide energy in the human body.  There are three such pathways: the phosphagen pathway, the glycolytic pathway, and the oxidative pathway.  The phosphagen pathway is the primary source of energy for high-powered efforts lasting less than 10 seconds.  The glycolytic pathway is the main contributor to moderate-powered efforts lasting up to 2 or 3 minutes.  The oxidative pathway is the lowest-powered and is used for longer efforts.

The goal of the CrossFit program is balanced development of all three metabolic pathways.

Each metabolic pathway can be classified as either aerobic or anaerobic.  Aerobic means the presence of oxygen is required.  Of the metabolic pathways, the oxidative pathway is the only one that is aerobic.  Thus, aerobic exercise is that in which the majority of the energy is generated through the oxidative pathway.  This means lower-powered exercise that typically lasts longer than a few minutes (for instance, a 5km run).

The other two metabolic pathways are anaerobic in nature, meaning they don’t require the presence of oxygen.  The first is the phosphagen pathway, which is the highest-powered of the three metabolic pathways, but also the shortest-lived.  Our muscles only contain about 5-8 seconds worth of the chemicals required for the phosphagen pathway.  The other is the glycolytic pathway, which is moderately-powered and can typically be used for up to a few minutes.  The chemical “waste” from the glycolytic pathway is the primary contributor to muscle fatigue.

The key point is: anaerobic exercise is shorter and more intense; aerobic exercise is longer and less intense.

Aerobic (lower-powered, less intense) exercise has a number of benefits.  It burns body fat and improves cardiovascular function, thereby improving your ability to perform further aerobic exercise.  Unfortunately, aerobic exercises also burns muscle and decreases your capacity to perform anaerobic exercise.  Check out the physique of any competitive endurance athlete to see this — elite marathoners look like cancer patients and usually have a vertical leap of only a few inches.  Also, while aerobic exercise does burn fat, it largely only does so during exercise.

Anaerobic (higher-powered, more intense) exercise has its own benefits.  It builds muscle, improves bone density, and of course, improves your capacity for further anaerobic exercise.  It also burns fat to a greater degree than aerobic exercise, because it does so not just during exercise, but also afterward.  The beauty of anaerobic exercise is that it can also be used to improve your capacity for aerobic exercise.

To summarize, anaerobic exercise has the same benefits as aerobic exercise, without any of the negative effects.

It makes sense then, that anaerobic exercise is the staple of the CrossFit exercise regime.   Anaerobic exercise can be used to condition all three metabolic pathways.  At the heart of our approach is something called “high intensity interval training” (HIIT, for short).  HIIT means doing short bouts of exercise at unsustainable (anaerobic) levels of intensity with periods of rest interspersed.  The work and rest intervals of HIIT can be manipulated to condition different metabolic pathways.  Below is a table from the October 2002 issue of the CrossFit Journal (page 5, figure 3) that shows the details.  A very similar table can be found in exercise science textbooks as well, such as the “Essentials of Personal Training” and “Essentials of Strength and Conditioning” from the National Strength and Conditioning Association.

CrossFit workouts implement these intervals in a number of ways.  In workouts like “Tabata Something Else” or “Fight Gone Bad”, you are asked to exercise and rest for certain periods of time: 20 seconds on, then 10 seconds off for Tabata; for “Fight Gone Bad”, it’s five one-minute rounds, followed by a one-minute rest.  In a workout like “Barbara”, you have to do a certain amount of work (20 pullups, 30 pushups, 40 situps, 50 squats) before you get to rest (three minutes).  Similarly, we occasionally do rowing or running workouts in which you go a certain distance, then rest.  Finally, in many CrossFit workouts, the intervals are created by the rest you have to take to get through it.  An example of this is “Angie” (100 pullups, 100 pushups, 100 situps, 100 squats).  These self-enforced intervals are probably the most common form of intervals in CrossFit workouts.

CrossFit uses anaerobic exercise through a variety of intervals to condition all three metabolic pathways.

So what is this term “metcon” that we throw around so often?  The term “metcon” is short for “metabolic conditioning”.  In reality, even heavy lifting is a form of metabolic conditioning (for the phosphagen pathway), but it is almost never what is meant when the term “metcon” is used.

In common usage, “metcons” refer to the CrossFit workouts that involve functional movements executed at high intensity, resulting in high heart rate for a short to medium lengths of time (usually 3-30 minutes).