April Merritt by April Merritt

The Fick EquationVO2 = Q x A-VO2 diff

The Fick equation. You may have read about it in your Essentials of Exercise Science for Fitness Professional’s book (page 228) or heard about it on the street, but do you really understand it?

The Fick equation is used to determine the rate at which oxygen is being used during physical activity. Don’t worry…it’s not an equation you are going to use on a daily basis. You’ll never actually directly measure the stroke volume or oxygen extraction of your clients, so if you get this question on the certification exam we have to supply that information.

So if you don’t use it in real life and won’t be tested on it, why are we talking about the Fick equation? Because it is the basis for how the body responds to the demand of physical activity. The principle behind the Fick equation is the foundation to a lot of what we are learning.

The Fick equation determines the rate at which a person uses oxygen in their body – which is also known as VO2 (the volume of oxygen uptake). Your maximum VO2 is the greatest amount of oxygen you can take in and utilize, and is a value that can be influenced by genetics and training. Maximal VO2 is typically used to measure how aerobically fit an individual is. The greater the VO2 max value, the more efficient the individual is at getting oxygen to their tissues and getting their tissues to utilize that oxygen. As fitness professionals, we usually talk about max VO2 it in relation to maximal exercise or maximal exercise testing.

During every activity we are using some percentage of our maximum VO2. But how do we measure VO2? Well, we look at two things: 1) the amount of oxygen delivered to the cells and 2) how much oxygen is used by the cells.

  1. The oxygen delivery to cells is calculated by determining cardiac output (amount of blood pumped per minute). Cardiac output (aka Q) is made up of two components – how much blood is pumped per beat (aka stroke volume) and how many times the heart beats per minute (aka heart rate). The short version? Cardiac output is equal to stroke volume times heart rate. The really short version? Q = SV x HR
  2. How much oxygen is used by the cells is called oxygen extraction. Oxygen extraction looks at how much oxygen is taken from the blood to be used by the cells to make ATP for energy. Oxygen extraction rates are measured by the difference in oxygen saturation of the blood going to the tissues (in the arteries) and of the blood returning to the heart (in the veins). This is known as the arterio-venous difference or A-VO2 difference.

Do you see how the Fick equation comes together now? You take your Q and your A-VO2 and put them together. Vo2  = Q x A-VO2 difference Do you see the importance of understanding Fick? If you know what components make up aerobic capacity (HR, SV, A-VO2 diff) you may have a better understanding of how you can influence those variables to increase an individual’s Max VO2.

Questions? Contact an Education Consultant at 1-888-825-3636 x782 or via email at support@acefitness.org

Many thanks to Julia Valentour, Program Coordinator, for her insight and words on this subject!