How Fartlek Training Can Affect Your Body’s Glycolytic Pathways

Fartlek training is a type of aerobic interval training that can allow you to greatly increase your endurance, stamina, improve your cholesterol, and heart health in general. This is due to many different variables, such as being able to easily diversify your workouts, increasing your VO2 Max, integrating proper progressions with ease (via speed play), enabling your body to generate more mitochondria overtime (to create more ATP), and so on and so forth.

However, to truly understand what fartlek training can do for you, you should first understand how it affects your body. And even more specifically, how it affects your body’s metabolic pathways. 

Obtaining such knowledge will allow you to fuel up and replenish your body with the proper nutrients it craves! Let me first explain what your body’s metabolic pathways are and why they’re important. For the sake of this article, I’ll be discussing glycolytic pathways, which are involved in the breakdown of carbohydrates (glucose molecules).

Glycolytic pathways are essentially long and meticulous processes that your body undergoes in order to yield a very important and vital substrate. The substrate I’m referring to is none other than adenosine triphosphate or ATP.

fartlek training

This high energy phosphate bond allows your muscles to contract and perform work, an absolute necessary function if you desire to perform a bout of exercise of any sort. So, it goes without question as to the importance of this vital phosphate bond, as well as how it allows us to better understand the physiology of exercise.

In this article I’ll briefly discuss the main glycolytic pathways involved in aerobic fartlek training and how we convert carbohydrates into energy that we can actually use and experience for ourselves.

I’ll take you on the journey that a single glucose molecule embarks on as it travels through several glycolytic pathways, yielding several ATP along the way. It is this ATP that gets created during these pathways that allows us to contract muscle tissue and perform work. Thus, allowing you to have an effective and fun fartlek training session!

Let’s get started, shall we?

From the foodstuffs our body consumes, glucose (C6H12O6) can be converted into ATP. This allows our bodies to meet the demands put upon it, whether that demand be anaerobic or aerobic. However, for this article, the discussion will be on the aerobic effects of a fartlek training session.

So, once aerobic exercise begins, the body will use glycogen (stored carbohydrates) found in muscle cells, as well as free flowing glucose molecules found in blood to help fuel your efforts. This is where a glucose molecule’s journey begins as it gets converted into other substrates via enzymes and other chemical reactions to yield several ATP to allow work to be done.

Glucose being used from muscle glycogen will undergo a very short process called glycogenolysis. It is here where a stored glycogen molecule gets converted into two different substrates as it makes its way into the metabolic pathway called glycolysis. Glycolysis starts by either utilizing the foregoing glycogen molecule and/or by using a glucose molecule from blood. Either way, both sugar molecules go in the same direction. Here is what the entire process of glycolysis and glycogenolysis looks like:

Glycolysis and Glycogenolysis

fartlek training

Photo credit: Dr. Kraemer “Exercise Physiology” lecture slides

As intimidating as this may look, all you really need to know is that a total of 2 ATP are yielded from one glucose molecule and 3 ATP are yielded from one glycogen molecule. As enzymes act upon specific substrates, the glucose molecule (or glycogen molecule) that we first started off with is eventually converted into pyruvate or pyruvic acid. This is the result of aerobic exercise.

It’s important to note here that lactate build up only occurs minimally here, as this is the primary result of anaerobic exercise. This has to do with the fact that the Electron Transport Chain (ETC), which is another vital metabolic pathway, gets “backed up” with hydrogen pairs (via the co-enzyme NADH) from the glyceraldehyde-3-phosphate step in glycolysis. So, just know that when enduring a fartlek training session, minimal proportions of lactate are produced and large proportions of pyruvate are produced.

Once two molecules of pyruvate are produced from (slow) glycolysis it is then brought to the Krebs cycle via the intermediate step. The Krebs cycle is located in the mitochondria (the powerhouse) of the cell. The Krebs cycle turns twice per glucose molecule due to the fact that one glucose molecule yields a total of two pyruvate molecules.

See, once pyruvate exits the intermediate step (between glycolysis and the Krebs cycle) it gets converted into Acetyl-CoA with the help of the 4 carbon molecule Oxaloacetate. Two turns of the Krebs cycle yields two ATP. This may not sound like much, but the process of breaking down a glucose molecule happens very rapidly. Also, a total of 4 hydrogen pairs get dropped off into the ETC via the co-enzyzmes NADH (3) and FADH (1). This is very important for generating ATP as the next paragraph will explain.

The Krebs Cycle and The Electron Transport Chain

fartlek training

Photo credit: Dr. Kraemer “Exercise Physiology” lecture slides

The next and final metabolic pathway involved in the breakdown of glucose for energy (ATP) is called the electron transport chain (ETC). This pathway is extremely lucrative for the body during exercise as it is able to generate 3 ATP per NADH molecule and 2 ATP per FADH molecule.

Throughout glycolysis and the Krebs cycle, there are several points where NAD is converted into NADH and where FAD is converted into FADH. Both of these co-enzymes transport their hydrogen bonds (H+H+) to the ETC so ATP can be made. ATP is then made by the help of several enzymes and chemical reactions.

So, why does all of this matter to you and why should you care?

Ok, so the main thing to grasp from all of these reactions is that one glucose molecule yields a total of 32 ATP from glycolysis and 33 ATP from glycogenolysis. Thus, the power of carbohydrates and how important it is for us to consume them for the sake of not only an effective fartlek training workout, but also for the sake of having the energy to perform day to day tasks is imperative.

Depending on how your fartlek training program is designed: Anaerobic (sprinting) to aerobic (jogging) and the duration into consideration, the use of carbohydrates and the availability of them is crucial for having a successful fartlek run without any unwanted complications.

One of the problems that can arise is what’s called “hitting the wall”. This occurs when you’ve completely depleted your glycogen stores in your muscle tissue and have virtually ran out of blood glucose. As you can imagine, this is extremely dangerous and can cause serious problems.

There are in fact other metabolic pathways that come into play when glucose levels decrease, such as Beta Oxidation for the mobilization and utilization of triglycerides (fat cells). However, fat burns in a carbohydrate flame. So, you still need carbs in order to actually burn fat. Be that as it may, that’s for another article.

fartlek training

Okay, how can you ensure that you have enough stored glycogen so you won’t “hit the wall“?

One of the most efficient ways to ensure that your body will be able to handle a very long bout of exercise is by “carb loading”. Carb loading is a staple for endurance athletes such as marathon runners and soccer players alike. It can also be a very good idea if you tend to have intense cardio workouts and like to train on a treadmill or a even on a stationary bike.

Carb loading is essentially the process of increasing the amount of carbohydrates you intake several days before a long fartlek training session or an endurance sporting event. Slightly increasing the amount of carbs you consume each day will allow you to have much greater glycogen reserves for when you’ll need them the most (during exercise).

When you don’t have enough blood glucose or muscle glycogen to complete your fartlek training workout, then you may “hit the wall”. Don’t let this happen to you! Be sure to consume healthy complex carbohydrates that have a low glycemic index.

This will help you have a healthier pancreas as low glycemic carbohydrates have a very small effects on your blood sugar levels. Thus, only relatively small amounts of insulin are needed to bring your glucose levels back to homeostasis. So, give carb loading a try if you have a big track meet coming up, a half marathon, or an intense fartlek training session. Trust me, your body will thank you!


Kraemer, Robert. Physiology of Exercise. Lecture. January 2017.

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