This article will be the first in a 4 part series diving into how our muscles function at the cellular level and how training intensity and volume affect cellular function and overall health. In this article, we will learn what our muscles use to fuel activity and what systems our cells use to replenish the energy needed for movement and exercise. This article will lay the scientific foundation for a deeper discussion of training intensity and how to maximize our time training to get the biggest bang for our buck. In the second article, we will define exercise intensity, heart rate zones, and discuss how training intensity affects which energy system our muscles rely on to replenish our energy supply. In the third article, we will explore what VO2Max is, how we improve it, and the health benefits of a better VO2Max. In the last article, we will take a look at a popular trend regarding Zone 2 training; we will explore what all the hype is about and see what the evidence supports. Now on to learn how our body is able to keep working hard during workouts.
Adenosine triphosphate (ATP) is the primary energy molecule our muscles use to produce muscle contractions. We won’t get into the nitty gritty details of how muscle contractions occur but we need ATP to get the work done.
When our muscles use ATP for energy, ATP breaks down into adenosine diphosphate (ADP). Because there is a limited supply of ATP ready to go in our muscles, our cells have to replenish our ATP supply to keep lifting that barbell. Our body does this by taking adenosine diphosphate and transforming it back into ATP. Our cells have three systems to recycle ADP back into ATP: [1]
These systems can be classified as aerobic or anaerobic.
This system provides ATP for quick, short, high intensity activities such as heavy lifting or sprinting. It is often the first system to be utilized regardless of exercise intensity. This is a simple one step chemical reaction that occurs in the sarcoplasm of the cell. The sarcoplasm is basically the fluid inside of the cell that other structures called organelles hang out in. The sarcoplasm is the pool water and the organelles are the people in the pool, make perfect sense? The phosphagen system takes ADP + creatine phosphate to create ATP (yay!) and creatine. [1]
Sounds like a great, efficient, fast acting system, which it is. But, our muscles and body can only store a limited supply of creatine phosphate (CP), which limits how long this system can function, thus, the phosphagen system is not the primary energy supplier for longer, more continuous physical activity. [1]
This reaction is regulated by the overall abundance or concentration of ADP, CP, creatine, and ATP. As your muscles use more ATP to exercise, ADP concentrations will increase. With more ADP present in the cell, it drives the reaction pictured above to occur more easily, as long as there is sufficient creatine phosphate present.
At submaximal exercise intensities, glycolysis and the oxidative system are able to help replenish ATP, thus keeping ADP concentrations lower, which decreases how much our cells rely on the phosphagen system. If there is a sufficient supply of ATP from the other systems, it allows for the phosphagen system to work in reverse to restock our creatine phosphate reserves. This preps our muscles to be ready for that next burst of maximal effort. [1]
Glycolysis uses carbohydrates stored in the muscle as glycogen or delivered from the blood via glucose to replenish ATP. Glycolysis involves multiple reactions which makes it a slower process than the phosphagen system, but glycolysis is able to provide more ATP for a longer duration effort because there is more glycogen or glucose available in our bodies than creatine phosphate (which the phosphagen system relies on). Glycolysis is a more complicated process but here is basic summary of how our body gets ATP from glycogen or glucose: [1]
There are two phases of glycolysis, an initial investment phase and a payoff phase [2]. In the investment phase, blood glucose or glycogen are converted into fructose-6-phosphate. [1] This process requires energy from ATP to occur, hence being termed the investment phase. [2] Fructose-6-phosphate is then broken down into 2 pyruvate molecules and ATP. [1] For each molecule of glucose, glycolysis creates a net gain of 2 ATP and for each molecule of glycogen, we get a net gain of 3 ATP. [1]
Pyruvate (the other product of glycolysis) can then undergo 2 different reactions, which path it takes depends largely on exercise intensity and oxygen availability:
The mitochondria is an organelle hanging out in the sarcoplasm and can be described as the power plant of the cell. Mitochondria can take carbohydrates, fats, or even proteins and break them down to create ATP. The process is longer, more complicated, and requires adequate oxygen supply to function. Therefore, the oxidative system is better suited for longer duration, less intense activities or during recovery periods. We will talk more about exercise intensities in the next article.
The oxidative system involves the Krebs Cycle and electron transport chain. Glycolysis discussed above, can occur separate from the oxidative system, but is the first step in the oxidative system for breaking down carbohydrates. Glycolysis + the oxidative system, creates a net 38 to 39 ATP (39 if started with glycogen versus blood glucose). [1]
Our mitochondria can also use fats to create ATP. This process is called fatty acid oxidation [3] and will be important when we discuss zone 2 training in the 4th article of this series. The amount of ATP produced by fatty acid oxidation will depend on the type of fat molecule used but is capable of producing significantly more ATP than from glucose or protein. For example, breaking down palmitic acid, a triglyceride molecule, creates more than 300 ATP molecules. Wow!!
Every time you move—whether you’re lifting weights, sprinting, or going for a long run—your muscles need energy. That energy comes from a molecule called ATP. Since your muscles only store a small amount of ATP, your body has three different ‘backup systems’ to keep producing more: the phosphagen system (for quick bursts like sprints or heavy lifts), glycolysis (for medium-length, harder efforts), and the oxidative system (for longer, steady activities like running or cycling). These systems work together all the time to replenish ATP. At different exercise intensities, our body will use certain systems more than others but “at no time, during either exercise or rest, does any single energy system provide the complete supply of energy". [1]
Ok, we now have a basic understanding of the 3 energy systems our cells can use to replenish ATP so that we can keep moving that barbell or keep pedaling on that echo bike. In the next article, we will define training intensities and heart zones and relate how different intensities affect how our body uses our 3 energy systems to keep us moving.
[1] Haff, G. G., & Triplett, N. T. (Eds.). (2016). Essentials of strength training and conditioning (4th ed.). Human Kinetics.
[2] Chaudhry R, Varacallo MA. Biochemistry, Glycolysis. [Updated 2023 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482303/
[3] Talley JT, Mohiuddin SS. Biochemistry, Fatty Acid Oxidation. [Updated 2023 Jan 16]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK556002/
