Understanding the energy systems of the human body is essential to understand how we should train and work on cycling performance optimization. How efficient your energy systems and metabolism is, is the foundation of your level as a cyclist and athlete. Therefore, it is the primary decider in racing outcomes along with tactics and fueling. Knowing where our energy comes from is essential to plan our fueling strategy for races.
What are the energy systems used in cycling?
The energy used by our muscles comes from a molecule called adenosine triphosphate (ATP). The human body uses three primary energy systems to create ATP and provide the necessary energy for various activities. The intensity of the exercise dictates what energy system is used:
- Immediate Energy (Phosphagen System)
For short bursts of intense effort, such as a sprint or a quick acceleration, your body uses the phosphagen system. Creatine phosphate is broken down to rapidly regenerate ATP. This system provides immediate but short-lived energy. These energy stores are also replenished quite fast. - Short-Term Energy (Glycolysis)
As the duration of your cycling activity increases, your body increasingly relies on glycolysis. Glycolysis involves the breakdown of glucose (from carbohydrates) to produce ATP. This process doesn’t require oxygen (anaerobic). It’s efficient for quick energy, but it can lead to the build-up of lactate, or what we know as the burning sensation in our muscles causing fatigue. - Long-Term Energy (Aerobic Metabolism)
- For longer and sustained efforts, especially during activities like endurance cycling, your body shifts to aerobic metabolism. This involves the breakdown of both glucose and fatty acids (from fats) in the presence of oxygen. The main energy source is fat, but we need glucose continuously to keep the process running. The primary pathway for this is cellular respiration, which includes the citric acid cycle (Krebs cycle) and the electron transport chain.
Why does it matter for cycling performance optimization?
During physical activities, the body often utilizes a combination of these energy systems, with one system dominating depending on the intensity and duration of the exercise. For instance, a sprinter might rely heavily on the phosphagen system, while a marathon runner primarily uses the oxidative system.
If you ride as hard as you can, you will rely the Phosphagen System for about 10 seconds. Once those energy stores are depleted, you will switch to Glycolysis. Studies show that after 60-90 seconds of all-out exercise, we rely primarily on the aerobic metabolism (using fat as fuel), while glycolysis becomes a smaller and smaller part of the energy contribution. It’s important to say, that there is an overlap between durations and energy source. It’s not just one or the other.
Any low to moderate intesity level will use aerobic metabolism as the primary energy system. So for pretty much any ride, you will rely on aerobic metabolism for the majority of your energy contribution. Hence why cycling is categorised as an endurance sport. Therefore, aerobic metabolism is the primary factor in determining your skill level as a cyclist. Then there’s also the tactical element of course. Often enough it’s not the strongest rider who wins the race.
Cycling races, depending on discipline, typically last a minimum of 2 hours for road races and up to 12 hours for the epic gravel events. You might think the aerobic metabolism is the only thing that matters then. But cycling is unique in the way that races are typically decided by high intensity efforts, due to the energy that can be saved by drafting on other riders. Unless you can drop other riders on technical sections or a hard climb, chances are you will ride in a group, where some riders will save more energy than others. See our racing strategy guide, to improve your tactical skills, save energy, and improve your chances of winning races.
How much energy is stored in the body?
As stated above, high intensity efforts are powered by glycogen (carbohydrates). Low intensity efforts are powered primarily by fat. But why don’t we just ride fast all the time? Because our glycogen stores are quite limited. Not in daily life, but in terms of a 5 hour bike race, we don’t have enough stores, nor can we take enough in, to ride at high intensity the whole time. On average, an adult can store approximately 350-500 grams of glycogen:
- Liver Glycogen: The liver stores glycogen as a form of glucose reserve. It typically holds about 70-100 grams of glycogen.
- Muscle Glycogen: Muscles store a larger amount of glycogen compared to the liver. The storage capacity can vary significantly but is estimated to be around 300-400 grams, and this can increase with regular physical training.
1 gram of glycogen/carbohydrate equals 4 kcal. That means we have up to 2000 kcal in reserve, but our body will not let us empty the tank completely to protect core functions. So let’s say we have 1500. Taking an easy example of riding at 200 watts on average, that equates to roughly 720 kcal burned per hour. That means we will empty all our stored energy in 2 hours if we relied solely on carbohydrate at that intensity. Emptying the tank will lead to bonking or hitting the wall, at which point we only have the lowest tempo available.
Refueling while riding
Taking in energy while riding is therefore essential for pretty much any ride longer than an hour. The science is still developing. In the 90’s it was said we could take in 40 grams of carb per hour. Nowadays, there’s guides for aiming at 90 grams an hour in intense exercise. Pro Tour riders are reporting much higher intakes, like Matteo Jorgenson reporting eating 170 grams of carbs an hour.
But even if we can eat 100 grams of carb per hour, we are only taking in 400 kcal. That’s nowhere near the 720 kcal we are burning at, let’s be honest, a quite modest 200 watts. Of course we are also relying on fat. But if you are a well trained athlete you could perhaps ride at 350 watts for an hour, relying mostly on carbs. In that case, you could possibly empty your glycogen stores in that hour, if you are not developing your aerobic metabolism and taking in carbs while riding.
Check our guide on developing your fueling strategy for racing.
Obviously, we want to be able to ride at the highest power output possible, while relying as much on fat as possible, because fat is essentially an unlimited fuel source. We all have kilos of fat, which is more energy dense than carbs, sitting at 9 kcal per gram of fat. That’s why developing your aerobic system and reliance on fat as a fuel source is the most important thing in cycling performance optimization.
How to distribute your training for improving your energy systems and cycling performance
Knowing how the energy systems contribute to our performance, we can now decide on a training philosophy. In short, a training philosophy is how we decide to distribute duration and intensity. Normally, we also want to pair that with periodization, so we can reach our peak form for our most important races.
On the Cycling Bible we are big advocates of the Polarized training approach, if an athlete has enough time. Developing the aerobic metabolism and ability to utilize fat requires training at that intensity. However, 3 one-hour rides per week at low intensities is not enough to develop that energy system. Training at low intensities requires a lot of volume to stimulate adaptations.
Developing the aerobic metabolism is something that takes time. In order to reach your genetic potential, we are talking about up to 10 years of good training stimuli and progression. However, you should be able to see great results in a timeframe of 1-2 years, by adapting the polarized training approach, rather than an “all out every ride” type of approach.
Conclusion on energy systems and cycling performance optimization
Training your aerobic metabolism is the most important aspect of cycling performance optimization. It is the energy system you will rely on for 98% of your time in the saddle. Improving your aerobic metabolism not only makes the easy to moderate intensity parts easier, it also allows you to be able to ride at a higher power output, while still relying mostly on fat. Thereby saving your precious glycogen stores to when the race is decided and you need to deploy the turbo.
