How to Utilize a VO2 Max & Metabolic Test to Improve Performance
Written by Jim Levesque, Head Coach at Grit Endurance. Learn more about Jim’s background here!
Two weeks ago, I wrote a blog about VO2 max, what it means, and why you should train to improve it. With today’s article, I’m going to dive a bit deeper into how a VO2 max and metabolic test in a lab can help you understand your training zones and substrate utilization to help you improve performance. I’m not going to go back over any information from the last blog, so if you haven’t read that one, I’d recommend reading that blog first to give you better context.
With a traditional VO2 max and metabolic test in a lab, you’ll typically do a ramp test, where you start very easy and gradually increase intensity each minute, while wearing a mask that tracks your respiration. As the intensity increases, the lab is able to track your heart rate, oxygen consumption and CO2 creationb which provide valuable insights that you can use beyond just knowing your VO2 max number.
Understand and/or Validate Your Heart Rate Training Zones
By paying attention to your respiratory exchange rate (RER), which is the ratio between oxygen consumed and CO2 released, you can identify your aerobic threshold (LT1), which occurs at the first point that the trajectory of your RER trendline increases. This is typically near the top of your zone 2, or aerobic endurance training zone. The next point at which your RER trajectory increases is your lactate or anaerobic threshold (LT2), which is typically the top of zone 4, bottom of zone 5, in a traditional 5 zone system. This is the point at which lactate begins to accumulate in your bloodstream at an unsustainable level. Understanding the heart rate at which you hit these levels is very helpful for understanding where you need to be when trying to train in zone 2, as well as validating what your lactate or anaerobic threshold truly is.
Understand Your Substrate Utilization
The second valuable information is your substrate (fuel source) utilization, or how many grams of fat and carbohydrates you are using to create energy at different effort levels. Your body wants to burn fat for energy first, but it also takes longer to convert fat into energy than carbohydrates. When training easy, more athletes stay mainly in the fat burning zone, of which there is near limitless supply, meaning additional fueling is not that critical. However, upon moving into moderate and more intense training, your body begins to look for fuel sources that are quicker and easier to break down, so you begin to utilize carbohydrates. We cannot store much carbohydrate in our bodies, so it becomes increasingly critical to replace the carbs that we are burning. By understanding the amount of oxygen consumed and CO2 created, you can get a very good understanding of how many grams of fats and carbohydrates are being burned. This can help you be more strategic with your pacing, so that you don’t move into the carbohydrate burning zone too quickly, and it can help you create a fueling plan that replaces as much of what you’ve burned as possible.
Below is an example of calories burned and substrate utilization at different effort levels for an example athlete:
100 HR - 535 calories per hour – 84/16 fat / carb ratio – 21g carbs per hour
110 HR - 695 calories per hour – 67/33 fat / carb ratio – 57g carbs per hour
120 HR - 809 calories per hour – 33/67 fat / carb ratio – 135g carbs per hour
130 HR - 895 calories per hour – 10/90 fat / carb ratio – 201g carbs per hour
140 HR - 981 calories per hour – 0/100 fat / carb ratio – 245g carbs per hour
150 HR - 1067 calories per hour – 0/100 fat / carb ratio - 267g carbs per hour
Note how significant the increase in carbohydrate utilization is at higher heart rates in thix example. This information was very helpful for undersanding how this athlete should be training and fueling. His aerobic threhsold is around 110 bpm, meaning his zone 2 training needs to be at a lower heart rate than we had previously thought. Also, he burns far more carbohydrates at heart rates that are lower than we thought, meaning he needs to pace more slowly in the early stages of races but also needs to fuel more proactively to stay on top of his carbohydrate needs.
While this type of testing isn’t needed for most athletes, it can be of great value, and can help you take your training and racing to the next level. If you are interested in talking to a coach in more detail about how we can use this type of testing to assist your training, please reach out!