CrossFit: A Scientific Explanation

By: Clint Russell

Our goal is to help each and every one of our clients lead healthier, more fulfilling lives.  A lot of people join a gym with the goal of losing weight.  We have been told for years that in order to lose weight, we need to adhere to the “Standard American Diet,” and do longer duration aerobic exercise for 20-30 min at least.  The fallacies of the SAD diet are for another post.  For now I will detail how the before mentioned methodology is largely ineffective and why shorter higher intensity exercise is significantly more effective.

First we need to understand what happens to our body during exercise, and then what exercise is mathematically.  Everything needs a number so it can be measured.  Our body is always trying to remain in homeostasis.  In order to function properly, it must maintain MAP, or mean arterial pressure.  MAP is the average pressure in one cardiac cycle which is the total peripheral vascular resistance (TPR) x cardiac output.  Total peripheral vascular resistance is the total resistance of all the systemic circulation (TPR=what the heart must work against).  Cardiac output  (CO) is the heart rate (HR) x stroke volume.

So TPR x CO = MAP. When we sit down our blood pressure slightly decreases as opposed to standing.  When we stand up, the nervous system must tell the heart to increase CO in order to maintain MAP so that we don’t pass out.  Our body is constantly working to maintain this homeostasis.[1]

So, exercise mathematically can be equated by Power = Work/Time.  Work = Force x Distance.

That means Power = Force x Distance / Time.

So lets review an example to get a baseline.  If I weigh 185 pounds and run 1 mile in 10 min, I get a much smaller power output than if I run that mile in 5 minutes.  You can correlate the final figure with how bad your chest burns:).  By moving faster and you work harder and thus Power is directly affected by Intensity.  According to the American Heart study on the safety of high intensity exercise on patients with coronary artery disease, high intensity exercise is found to have a greater increase in aerobic capacity and greater cardioprotective effects than at moderate intensities.  While this is another article, this was performed on cardiac rehab patients, people recovery from thing like heart attacks…  So I don’t want to hear its too hard:)  Okay, so we can see that the higher the load, the farther and faster we move it, the greater the power output and when you apply yourself, the greater the intensity.[2]

Now, back to TPR x CO=MAP.  We used our example of sitting to standing to understand that our body manipulates CO and TPR to maintain MAP.  Now lets go to our running example.  The body only recruits the muscles necessary to do the task at hand.  So our 10 minute mile doesn’t recruit as many muscles as our 5 minute mile.  This is a VERY IMPORTANT concept.  We have to recruit more muscles to run faster, so our body has to work much harder to maintain MAP.  When our body can’t get the job done, we slow down so that it can.  Now running is a single modality event, meaning our body is doing the same thing over and over.  The body will shunt blood to the legs from the abdomen in order to fuel the working muscles. The nervous system is going to reach homeostasis and attempt to maintain it as long as possible. Understand that in this running position, the center of mass stays very close to the same height and orientation for the duration of the activity. In other words, the equilibrium doesn’t drastically change. Now, lets analyze thrusters and burpees.  Both of these movements require a load to move quite a long way, very fast, and we are going back and forth between them.  There is a very large change in center of mass and the bodies equilibrium, especially in a burpee! This is part of the reason burpees suck so much. Your body is going crazy trying to maintain MAP. In something like this, vasoconstriction and dilation are rapidly occurring to fuel the muscles needed to complete the exercise.  By switching back and forth between exercises, the cardiovascular system is having to constantly adjust to the changing environment can never really achieve any sort of steady state.  Think about what a thruster and burpee entail.  This puts tremendous stress on the cardiovascular system as it is never able to hit cruise control.  There is nothing like this stimulus in any other form of training.  The couplets and triplets that we do here allow the cardiovascular system to be taxed far more than any steady state single modality event.

Now, what I’d imagine you have all been waiting for, programming. What type of stimulus does a wod produce. This is a very complex topic, and it will be discussed in our next blog post. Stay tuned for it next week!



Guyton, Hall. Textbook of Medical Physiology 12th edition.  2011 Saunders.  1600 John F. Kennedy Blvd. Set 1800 Philadelphia, PA  19103.  Pp. 163, 230-231.

Moholdt, Bakken, Hole, et el al.  “Circulation”.  American Heart Association.  August 9, 2012.  DOI: 10.1161/CirculationAHA. 112.123117.

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