Anaerobic and Aerobic Exercises

I often wondered why anaerobic and aerobic exercises are named as they are. Surely, while running, you should not be out of breath to avoid “switching to anaerobic metabolism”. But why is it called anaerobic metabolism when doing short bursts of weight training even though I am barely out of breath? Recently, I refreshed my biology class 11 knowledge with the help of wikipedia and want to share my findings.

Adenosine Triphosphate (ATP) – The Body’s Energy Currency

To make a muscle work, it needs energy. That energy is primarily generated when ATP (Adenosine Triphosphate) splits off one phosphate group, turning into ADP (Adenosine Diphosphate) and releasing energy in the process.

ATP + H₂O → ADP + P + Energy

However, we only have a tiny reserve of ATP in our muscle cells, enough for maybe 2–3 seconds of maximal contraction. To provide a constant stream of energy, ATP needs to resynthesized from other stored energy sources like glycogen and fats (which aren’t always stored right in the muscle cells). An important part of this resynthesis relies on energy carriers such as NADH and FADH₂, which are produced during earlier breakdown steps of these fuels. Inside the mitochondria, these carriers are then used to generate most of our ATP.

Aerobic Metabolism

Aerob means, that this kind of metabolismn is done under use of oxygen. Although this proceses produces a lot of ATP, it takes several sub steps and is therefore slower to deliver energy.

aerob-glykolytic

In a first step, glycogen is broken down to pyruvate in the cell’s cytoplasm (glycolysis). If oxygen is available, the pyruvate enters the powerhouse of the cell – the mitochondria – where oxidative decarboxylation is performed, converting pyruvate into Acetyl-CoA and releasing one CO₂ molecule. In the citric acid cycle, Acetyl-CoA is completely processed, generating the majority of NADH (and some FADH₂). These electron carriers are then used in oxidative phosphorylation to produce the majority of ATP.

We can simplify it to:

Glycogen + Oxygen → Water + Carbon Dioxide + Energy

aerob-lipolytic

Fats consist of long hydrocarbon chains that, given enough time, can be broken down in the mitochondria via β-oxidation, step by step, into Acetyl-CoA. From there, they follow the same path as glucose: into the citric acid cycle and then into oxidative phosphorylation to generate Energy.

Fats + Oxygen → Water + Carbon Dioxide + Energy

Anaerobic Metabolism

Anaerobic metabolism kicks in when oxygen supply can’t keep up with demand. It’s super fast and creates ATP in few steps but only in limited amounts. There are two main types: anaerobic alactic (no lactate produced) and anaerobic lactic (lactate produced).

Anaerob-Alactazid – Phosphat metabolism

After using up the short burst from the limited ATP stored in the muscle cells, creatine phosphate (also stored directly in the muscles) can donate its phosphate group to quickly regenerate ATP.

ADP + Creatine Phosphate → ATP + Creatine

Anaerob Lactacid – Glycolysis Without Oxygen

When the demand for energy is high and oxygen delivery can’t keep up, pyruvate (from the breakdown of glycogen in the glycolysis) is converted into lactate in the muscle’s cytoplasm, producing a small amount of ATP in the process.

Glycogen → ATP + Lactate

Lactate is released into the bloodstream and can later be converted back into pyruvate for use in the citric acid cycle or into glucose via gluconeogenesis. It’s not harmful by itself, but when it accumulates, the associated hydrogen ions cause an increase in acidity (also called acidosis). This slows enzyme activity and muscle contraction, which is a protective mechanism to prevent damage to proteins, but also contributes to the feeling of fatigue.

So what did we learn?

Our muscles store only a tiny amount of ATP, which is enough for a couple of seconds of work. We rely on different systems to replenish our ATP supply.

Aerobic metabolism uses oxygen. It’s slower to start but produces a lot of energy and can keep us going for a long time. Especially

Anaerobic metabolism doesn’t use oxygen. It’s fast but can’t be sustained for long, either because the fuel runs out quickly (creatine phosphate) or because acidity builds up (lactate), making the muscles slow down.

Training Implications

Weight Training: Primarily involves anaerobic metabolism due to the need for quick energy bursts during short, intense efforts. The body relies on ATP and creatine phosphate for immediate energy. Supplementing with creatine can enhance the storage of creatine phosphate in muscle cells allowing for a greater reserve of quick energy. A side effect is that the high concentration of creatine in the muscle cells draws water into the cells through osmosis.

Cardio Training: Relies heavily on aerobic metabolism, which can sustain energy production for longer periods. Fat metabolism needs a short warm-up period and typically becomes a significant fuel source only after about 20 minutes of exercise.

When it comes to long distance running, there are three key areas we can train to boost performance:

• the “aerobic base”, the ability for efficient energy production;
• VO₂max, an indicator of how much oxygen our body can use during intense exercise;
• and the lactate threshold, the highest intensity we can maintain until we reach the tipping point where lactate is produced faster than it can be cleared, and the resulting acidity forces us to slow down (Li et al., 2022)

Improving Aerobic Base (Zone 2 Running)

To improve efficient, sustained energy production over long periods, mainstream media suggest to run at a low heart-rate also called Zone 2. Training in this intensity improves your aerobic foundation (Mølmen et al., 2025) by:

• Stimulating mitochondrial growth in the muscle cells, which boosts energy production efficiency.
• Increasing the capillary network around muscle fibers, improving oxygen delivery.
• while offering a low-fatigue training zone, so you can do it savely, frequently and build endurance steadily.

However, depending on your fitness level and training routine, running in Zone 2 isn’t always the most effective way to improve aerobic fitness. While it’s good for building endurance and supporting recover for trained athletes with higher weekly volumes, its benefits may not fully apply to the “general population”. For people exercising less than 150 minutes per week, Zone 2 might fail to meet the minimum intensity required (Storoschuk et al., 2025, p.1618). That being said, it’s not that Zone 2 has no effect at all. However, contrary to popular belief, Zone 2 training isn’t the only thing that matters and the average person can still benefit by gradually increasing their overall training volume and intensity. As author (Mølmen et al., 2025, p.132) noted, greater training volume and intensity lead to larger improvements in mitochondrial content and VO₂max .

Improving VO₂ max (Interval Training)

VO₂ max is determined by three key factors (Fick equation):

• Stroke volume — the amount of blood the heart pumps per beat.
• Oxygen extraction — the muscles’ ability to pull oxygen from the blood.
• Maximal cardiac output — the total amount of blood delivered to working muscles per minute.

To effectively improve the maximum amount of oxygen our body can use during intense exercises, we need to push our heart, lungs, and muscles close to their limits, forcing them to adapt. We can do this most effectively by doing High-intensity aerobic interval training (Helgerud et al., 2007; Inglis et al., 2024; Mølmen et al., 2025).

Improving Lactate Threshold (Tempo Runs)

As we now know, when lactate builds up faster than the body can clear it, fatigue sets in and performance drops. We can improve handling the lactate threshold by doing tempo runs and progressive overloads as suggested by Vijay et al. (2024, p.8). These methods lead to ” increased lactate clearance, improved mitochondrial function and increased oxidative capacity” (Vijay et al., 2024, p.7)

Now, meassuring the exact threshold point is a topic on its own and beyond the scope of this post, so Id like to refer to other external resources =).

Disclaimer

This is by no means a comprehensive guide to the effects of different training methods. It’s meant to give a general overview of how they work and interact with each othe. For a more in-depth understanding, it’s best to check the references below or consult a certified trainer or professional. What we can take away, though, is that while different training methods focus on specific areas, their effects often overlap and are also nuanced based on the current fittness level. Improving one aspect of fitness usually helps the others too and finding the right balance is what makes training both effective and sustainable.

In this post, we took a quick look at how human metabolism works and how the body produces energy through different systems. We then connected that knowledge to real-life training examples, showing how understanding these processes helps explain why different workouts are designed the way they are.

References

Helgerud, J., Høydal, K., Wang, E., Karlsen, T., Berg, P., Bjerkaas, M., Simonsen, T., Helgesen, C., Hjorth, N., Bach, R., & Hoff, J. (2007). Aerobic high-intensity intervals improve VO₂max more than moderate training. Medicine & Science in Sports & Exercise, 39(4), 665–671. https://doi.org/10.1249/MSS.0b013e3180304570

Inglis, E. C., Iannetta, D., Rasica, L., Mackie, M. Z., Keir, D. A., MacInnis, M. J., & Murias, J. M. (2024). Heavy-, severe-, and extreme-, but not moderate-intensity exercise increase V̇O₂max and thresholds after 6 weeks of training. Medicine & Science in Sports & Exercise, 56(7), 1307–1316. https://doi.org/10.1249/MSS.0000000000003406

Mølmen, K. S., Winfield Almquist, N., & Skattebo, Ø. (2025). Effects of exercise training on mitochondrial and capillary growth in human skeletal muscle: A systematic review and meta-regression. Sports Medicine, 55, 115–144. https://doi.org/10.1007/s40279-024-02120-2

Li, X., Yang, Y., Zhang, B., Lin, X., Fu, X., An, Y., Zou, Y., Wang, J.-X., Wang, Z., & Yu, T. (2022). Lactate metabolism in human health and disease. Signal Transduction and Targeted Therapy, 7, 305. https://doi.org/10.1038/s41392-022-01151-3

Vijay, S. A., Sivakumar, C., Kumar, P. V., Muralidharan, C. K., Rajkumar, K. V., Kannan, K. R., Pradeepa, M., Sivasankar, P., Mariam, A. A., & Anand, U. K. A. (2024). Lactate threshold training to improve long-distance running performance: A narrative review. Montenegrin Journal of Sports Science and Medicine, 13(1), 19–29. https://doi.org/10.26773/mjssm.240303

Storoschuk, K. L., Moran-MacDonald, A., Gibala, M. J., & Gurd, B. J. (2025). Much ado about Zone 2: A narrative review assessing the efficacy of Zone 2 training for improving mitochondrial capacity and cardiorespiratory fitness in the general population. Sports Medicine, 55, 1611–1624. https://doi.org/10.1007/s40279-025-02261-y

Other discussions

A little bit more background information on the lactate threshold:
Lactate Threshold Training: The Definitive Guide — https://training4endurance.co.uk/improve-endurance/lactate-threshold-training/

Discussing the importance of Zone 2 training:
Do We Really Need Zone 2 Exercise for Mitochondrial and Metabolic Health? — https://brokenscience.org/do-we-really-need-zone-2-exercise-for-mitochondrial-and-metabolic-health/

A thread discussing the complex relationship between the focus points mentioned above:
https://www.reddit.com/r/AdvancedRunning/comments/rm6g0y/how_to_improve_your_vo2_max_and_anaerobic/