A Beginner’s Guide to Energy Systems in the Human Body

Energy is needed by every cell in your body to operate, whether that be muscle contractions for movement and exercise, the regulation of body temperature, sleep, breathing, or any other bodily function. Knowing more about the basics of human energy processes can be helpful for making healthier training and dietary decisions. 

Let’s take a look a closer look at where our “fuel” comes from and how our bodies use it:

It’s common knowledge that the energy that the human body uses each day comes from the food we eat. But what’s less well known is what happens after we’ve eaten our meal to transform that food into something that the countless individual cells that make up our bodies can use.

The simplest useful way we can think about what happens is this: After we consume our meals, the solids and liquids in the foods are digested and broken down into macronutrients, the building blocks of nutrition carbohydrates, protein, and fats.

These are processed even further by the body into the simple compounds that store the energy to be used. Very broadly, these compounds are glucose (from carbohydrates), amino acids (from protein), and fatty acids (from fats). All of these are absorbed into the bloodstream post-digestion and transported to various cells throughout the body (or stored for later use).

Now here’s where it gets a little bit more complicated.

Those broken-down compounds your body got from the foods you ate are formed into molecules of adenosine triphosphate (or “ATP” as it’s commonly known), inside your cells.

Think of the ATP molecules as your fuel. It’s immediately usable in this state, the most readily available source of energy the human body makes.

The body does store a minimal amount of ATP within the muscles, but the majority is produced from the foods we eat, which is why a healthy, well-balanced diet is so important.

During physical activity, three different processes work together to split ATP molecules, which release energy for muscles to use in contraction, force production, and ultimately sporting or fitness performance.

These processes, or energy systems, as they are more commonly known act as pathways to produce energy, and the intensity and duration of the physical activity we are doing determines which pathway acts as the principal fuel source.

As soon as you start exercising, the small amount of ATP stored in our body’s gets used up and needs to be replenished to continue with the exercise, which is where the body’s three different systems come into play, ensuring a constant supply of energy. (We’ll summarize each of these below!)

Different sporting situations have different energy demands. In some situations, like HIIT, energy must be supplied very quickly, whereas in other instances energy does not have to be provided at such a high rate, but must be supplied steadily over a longer period of time.

All three energy systems are always active during exercise, however, the method you rely on most to produce that energy depends on the activity you are doing, or more specifically, its intensity and duration. Understanding exactly what those energy systems are and applying that knowledge to your fitness regime can help you to further your training and elevate your results.

The most immediate energy system available to your body is the Phosphagen system, also known as the ATP-PC system. This energy system is the one the body uses to generate instant energy and can be delivered at a high rate.

The energy source, phosphocreatine (PC), is stored within the tissues of the body and doesn’t require oxygen, making it an anaerobic system which works fast. But, because your cells do not store a lot of phosphocreatine, the total energy it can produce is limited, and as a result, maxes out after around 10 seconds of all-out exertion.

If you are exercising in repeated, brief, maximal, high-intensity bursts (i.e., weightlifting, short sprints, or throwing a ball), it remains the dominant energy system for the duration of your workout, but only if you allow for sufficient rest between bouts to allow for the stores to be replenished.

The Glycolytic system, better known as the anaerobic lactic energy system, can produce ATP quite rapidly for use during activities which require bigger bursts of energy, from around 10-90 seconds max.

The glycolytic system uses carbohydrates in the form of blood glucose and stored glycogen to produce ATP. Like the phosphagen system, it starts out producing energy anaerobically, but as you approach the two-to-three-minute mark, oxygen becomes an increasingly important and eventually necessary part of the process, which is where the next system then kicks in.

The third and final energy system which comes into play is the Oxidative system, commonly known as the aerobic energy system. This pathway requires oxygen to produce ATP, because carbohydrates and fats are only burned in the presence of oxygen. While it is not the main source of ATP at the start of exercise, it can produce a lot of it, making this system the preferred one for long-duration, relatively low intensity cardiovascular activities.

This aerobic energy system must have oxygen to work, or the entire process will slow down and potentially stop completely.

In this energy system, although you may be burning mostly fat, a steady supply of carbohydrate is still necessary for the breakdown of fat into an energy source. The ratio of how much fat vs. carbohydrates are being used depends on the intensity and duration of the exercise, along with the individual’s aerobic training experience.

For example, shorter and more intense workouts tend to burn more carbohydrates for fuel, while longer, less intense workouts will burn a higher ratio of fats. The more an individual is trained aerobically, the more their body will be able to utilize fats for fuel at a given intensity.

Well, for one, it is important to remember that all three of these systems contribute to the energy needs of the body during any physical activity. They do not work independently of each other, but instead dominate at different times, depending on the duration and the intensity of the activity.

No matter what your primary training method or goal, working all three of these metabolic pathways offers benefits. For example, if you typically strength/resistance train, adding cardio to your weekly routine can help enhance your endurance, allowing you to increase your training volume.

If cardio is your go-to, resistance training once or twice a week can help boost your power and lessen the risk of injury. Either way, the payoff is becoming a more well-rounded, better informed athlete, and that really is win-win.