If you read last week’s post about “Rx”ing a workout, this post will complement that information. If you haven’t yet, click here. The goal of “Rx”ing a workout is to hit the intended stimulus. That means staying in the zone of one of the 3 energy systems.

The reason CrossFit has longer workouts, heavier workouts, shorter workouts, weird workouts and so forth, is because they each tap into our energy systems differently. Varying these types of workouts everyday will keep us from training just one specific metabolic pathway.

When we do a physical activity, our bodies draw energy from these 3 systems:

Anaerobic a-lactic, the anaerobic lactic, and the aerobic.


If these sound familiar it is probably due to the popularity of aerobic exercises, like long distance runs. 

Let’s start with the basics. These metabolic pathways are avenues by which our bodies come up with the energy necessary to do a certain movement. The anaerobic a-lactic (ALA) and the anaerobic lactic (Glycolytic) are both anaerobic systems, which means they do not need oxygen to utilize that stored energy. The aerobic does. The ALA system gets its energy from Phosphagen, the Glycolytic from Glucose and the aerobic system uses Oxygen along with carbs and fats for fuel.

Great, each energy system has a different fuel, so what?

Now, it gets interesting! Each energy system is used for a specific power output and sustainability.



The ALA system is used on max efforts that last up to 10 seconds. Movements like 1 RM squats, full-on sprints, high box jumps are some good examplesIn CrossFit, it can be hard to train this system because it takes a long time to recover and it can fry your Central Nervous System. We don’t like standing around for too long. So whenever we do max efforts, or EMOMs that makes us work for around 10 seconds every minute, or clusters of some movement (not squat cleans thrusters, but clusters of movements, like instead of doing 5×6 pull ups, we do 5×2.2.2) we get the chance to improve our ALA system.

The Glycolytic system is active when we do activities from 10 seconds to 2-3 minutes. To use this system you must be working around 85% of your max effort. After training specifically this system you should be laying on the floor, feeling like a train hit you straight on. Hmm, 2-3 min workout at 85% of your max that leaves your breathless on the ground… did I hear anybody say “Fran”?! Yep, workouts like “Fran”, 200m sprints, 10 fast burpess are all movements that will draw energy from this system. Doing these workouts will produce lactic acid, therefore making you feel sore.

No wonder we dread these WODs the most…


Lastly, the aerobic system is active after 3 minutes and it can work for hours. Also, you cannot be pushing harder than your 75% max effort. These are long activities that you can have a conversation while moving. Jogging, bike riding and chippers usually are what gets this system going. Depending on your ability, even “Cindy” can use energy from this system. If it is easy for you to do 5 pull-ups, 10 push-ups and 15 air squats and you do them at a moderate pace for 20 min you will use mostly the aerobic system.

Obviously, we almost never use only one system when we do activities, hence why we shouldn’t just train one when we workout! We train every energy system so we can improve our efficiency in each of them. By doing that, we become better adapted to deliver energy through any of the 3 pathways.

As long as we are doing the 10 sec workouts with our max effort, the 2-3 min workouts with 85% of our max and keeping a good pace on our longer workouts we will improve not only our physical attributes (muscles, tendons, ligaments), but we will also improve the delivery of energy our bodies are capable of.

How does this tie into last week’s post?


If, for example, we try to do “Fran” and we end up finishing it around 8 minutes, even though we finished a workout, we failed to achieve the goal of the WOD: stay in an anaerobic lactic state. We either spent too much time taking breaks from doing max efforts, if the thrusters were too heavy (too much time in the anaerobic a-lactic zone), or we didn’t push hard enough (too much time in an aerobic zone).

This is the beauty of CrossFit: training for everything and anything. We become good runners without being “runners”, strong without becoming “weightlifters”, we learn to control our bodies in space without becoming “gymnasts”.

We learn new skills everyday, we become fitter and we have a blast doing it.