Energy System Training - Is the old model all wrong?
You’ve heard of some different energy system targets before—anaerobic training, lactate threshold or tempo runs, VO2 max workouts, and aerobic development—but there are some problems with these traditional concepts. They’re actually pretty far off target. As a triathlete or endurance athlete, most of the training we do revolves around energy system training. The reasons why we train include other targets like skills training or psychological development, but in endurance sports, we have to spend a lot of our time working on our energy systems. Let’s talk about what the traditional energy systems look like, some of the problems with that model, and a new way to think about our targets for training.
TL;DR
Okay, there’s a lot of science-y stuff ahead. Here’s the elevator version. Traditional energy systems suppose you can use energy anaerobically—without oxygen—and that energy system produces fatiguing byproducts that make you tired. Well, the problem with that is your muscles begin using oxygen immediately when you start doing work (proof below). So if there is no such thing as “anaerobic” training and our bodies use oxygen at all intensities, then what are we actually trying to do? We’re training one or more of three things: our muscles’ ability to use oxygen, our heart’s ability to pump oxygen-carrying blood to the muscles, or our lungs’ ability to exchange CO2 for oxygen.
Traditional Energy Systems Thought
So what is the old school of thought on energy systems? Well, in order for our bodies to move, be that in the water, on the bike, or on the run, our muscles need a molecule called adenosine triphosphate, or ATP. Our bodies have a few ways to make and recycle ATP, and the brief version is that can happen anaerobically (without oxygen) and aerobically (with oxygen).
Anaerobically, our bodies take glycogen (the way our bodies store and use carbohydrates) and go through glycolysis to produce pyruvate. Think of it like refining crude oil into gasoline our bodies can use. Then, our bodies take pyruvate and convert it either to acetyl-CoA or lactate. Our mitochondria (the powerhouse of our cells, you may remember from high school biology) can use acetyl-CoA to produce energy through the Krebs cycle. We can also break down lactate for fuel, but there are more byproducts through this process.
Aerobically, our bodies start with the Krebs cycle in the mitochondria. The Krebs cycle uses acetyl-CoA, which can be produced from glycogen (carbs) as described above, or also from fats and proteins. So the acetyl-CoA produced anaerobically through glycolysis still has to go through the Krebs cycle to produce ATP for the muscles—remember this for later.
The traditional school of thought on how our bodies use energy says that we use mostly anaerobic energy for the first 45 seconds or so of exercise. 45 seconds is the crossover point where our aerobic systems take over the majority of work, but it takes 90 seconds to 2 minutes for the aerobic system to rev up fully. Unfortunately, this means that people tend to think that your body doesn’t really need oxygen for the first 45 seconds of a work. That’s where the disconnect starts.
(Steve Magness in The Science of Running provides a thoroughly detailed description of these energy systems)
Problems with Traditional Concept of Energy Systems
The belief that our bodies work anaerobically exclusively for the first few seconds of exercise is where the problem lies. If you want to test the fallibility of this, go for a 30 second sprint and hold your breath. Tell me how that feels, compared to how it feels when you can breathe. Let me illustrate that with a graph from a recent workout.
First, let me describe what I did in this workout. I did two sets of eight 30 second maximal hill sprints (yes, it hurt a lot). This is a snapshot of one of those intervals, where I started from a standstill. The yellow line is my power output, which you can see increases dramatically and then plateaus. My heart rate, the red line, is similar. Now, let’s look at the pink line, which depicts my muscle oxygen saturation. With the Moxy meter I’ve described before, we can see how my muscles (the vastus lateralis, outside of my quadricep) use oxygen. A decreasing muscle oxygen saturation like that pink line means that the muscle is using oxygen. Well the line starts to drop the second I start working! So that means my muscle is using oxygen immediately when I start exercise, not working anaerobically while the aerobic system starts to rev up.
Well remember when we talked about the Krebs cycle and glycolysis? It seems that when our bodies go through glycolysis to produce pyruvate and acetyl-CoA out of stored carbs, the acetyl-CoA still has to go through the Krebs cycle, which requires oxygen. So even when some of the products our bodies use to make energy come from anaerobic glycolysis, we still need to use oxygen to actually get the ATP our muscles need. So it’s no wonder that our bodies start using oxygen as soon as we start exercise!
New Way of Thinking
Well in that case, what are we actually training when we perform workouts? There are three targets.
1. Our heart and its ability to pump oxygen-carrying blood to the muscles
2. Our lungs’ ability to exchange CO2 produced by the muscles for oxygen
3. Our muscles and their ability to use the oxygen delivered to them
So it’s the combination of these three factors that determine our athletic performance. This is a much more effective way to think about how we structure our training. So for example, if we want to improve an athlete’s sprint triathlon performance, and this athlete has a muscle oxygen utilization limitation, we would target workouts that deplete her muscle oxygen. Conversely, if the athlete has a delivery limitation, like the heart not being able to pump enough blood to the muscles, then we need to work on the heart more than the muscles’ ability to use the available oxygen. We’ll talk through each of these relative limitations and how we train to improve them in a future post.
Conclusion
Congratulations, you made it through the science-y stuff! So to sum, our bodies can produce the ATP we need to make our muscles move in a few different ways. One of those ways happens anaerobically, but only to a point. We still need oxygen to make the actual ATPs we need, even though we can get most of the way there with the anaerobic breakdown of glycogen. So that means that even for your 10 second sprint, you still use oxygen!
So instead of trying to improve our anaerobic metabolism, really, we need to focus on our muscles’ ability to use oxygen through the mitochondria. Or, depending on the athlete’s relative limiter, maybe we need to improve how much oxygen can get to the muscles, or how much CO2 we can exchange for oxygen in the lungs.
This shifts our training paradigm. Instead of saying, “that athlete needs to improve her sprint power,” we should look at what is limiting her sprint power. If it’s her oxygen utilization in the muscles, then we should try some repeated sprint training to help her deoxygenate. But if her muscles are so strong that when she’s sprinting, she’s actually squeezing the muscles so hard that they block some of the blood flow in, that’s not really going to help her! Instead, we would want to train her heart’s ability to push blood into those strong muscles. That’s exactly why we need to individualize training based on what our athletes need!