The Three Energy Systems & Cardiovascular Exercise

Cardiovascular activity is straight forward. It is the main method to elevating your heart rate and testing its workload. It’s walking, running, jumping, swimming, dancing and virtually everything in between.

You’re placing a workload on the body for an extended period of time.

Cardio is synonymous with the term ‘exercise’ more than any other word, but what is actually going on in the body when we do it? And is all cardio the same?

ATP: The Body’s Energy Currency 

To produce energy, our body utilizes a molecule called Adenosine Triphosphate or ‘ATP’. ATP comprises two molecules - an Adenine and a Ribose group - with three attached Phosphates. In order to utilize ATP, the body splits one of the phosphate molecules using the process of hydrolysis (breaking chemical bonds using water). This split creates utilizable energy in the process. 

Once a phosphate is lost, ATP turns into Adenosine Diphosphate (meaning it contains only two phosphates), and must attach another phosphate to its chain to be utilized again for energy production.

Our bodies have three separate energy systems tuned to sustain our physical efforts.  Each system specifically functions to fuel either low, medium or maximal intensity levels. Relatively speaking, though, all 3 are always at work in some capacity regardless of intensity. 

The body is not a manual transmission whereby it specifically gears up or down. The 3 systems are intertwined and working together throughout all physical activity, but the system best suited for the given workload does the majority share. 

The Aerobic energy system

The aerobic system is responsible for fueling longer duration, endurance activity with intensity levels ranging from walking/hiking to jogging/running. It relies on oxygen to convert fatty acids, stored glycogen and circulating glucose for fuel in a process known as Glycolysis. 

Glycolysis provides an abundance of oxygen (due to the more controlled breathing low intensity exercise enables) to split Glucose’s six-carbon ring into two, three carbon Pyruvate molecules. 

A good indicator that you’re in the low intensity range is if you can maintain a conversation while you exercise. If so, this means you are efficiently bringing in enough oxygen to fuel aerobic activity. 

The Pyruvate is then converted into another molecule called Acetyl-CoA; the existence of which triggers two other chemical processes that occur within the mitochondria of the cell named the Krebs cycle and the electron transport chain. These two mechanisms are where glycolysis generates the vast majority of its ATP.

You don’t need to know the in-depth processes of the above to understand how the system works. Essentially, aerobic exercise is low enough in intensity to enable the body to take in large amounts of oxygen, which enables Glycolysis to use carbohydrates to fuel the body effectively. 

This doesn’t work quite as well if the intensity ramps up, though.

The Anaerobic or ‘Lactic Acid’ system

The anaerobic system (sometimes referred to as the Lactic Acid or Oxidative system) does not require the use of oxygen to function. This system takes over when intensity of exercise increases to the point where we cannot bring in enough oxygen to sustain the aerobic system and the glycolytic processes it requires. 

The anaerobic system kicks in during intense exercise generally over ten seconds, such as 200m to 400m running or 100m swimming, for example.

The body converts stored glycogen (mostly from the muscle) into glucose, and then glucose into ATP to sustain the workload initially the same manner as with the aerobic system. 

In both systems, the six-carbon glucose molecule is split into two, three-carbon pyruvate molecules through the initial process of glycolysis.

Without oxygen, aerobic glycolysis cannot progress beyond the initial glucose conversion to pyruvate. This initial process of Glycolysis and the breaking down of glucose into pyruvate creates hydrogen ions that generates an acidic environment. These hydrogen ions are quickly converted and dispelled when there’s enough oxygen in the system, but are not when oxygen deprived. 

Luckily, the body has a few tricks up its sleeve to combat this.

The body joins the newly created pyruvate molecules to the free-floating Hydrogen to create Lactate. 

Contrary to popular belief, it is actually the build-up of Pyruvate and Hydrogen that cause the burning sensation you feel in your muscles, with Lactate acting as an acid buffer to reduce the fatigue and increase performance duration! 

Thanks to Lactate, irrespective of the lack of oxygen, you can continue to generate enough energy to sustain exercise intensity for an extended period. This does not last forever, which is why you’ll only be able to sustain moderate intensity bouts of exercise such as high-octane team sports or middle distance running (400-800m) for a relatively short period.

Once the hydrogen ions do eventually overwhelm the anaerobic system, the acidic environment brings you to fatigue. This acidic environment is the burning sensation in your legs and core you’ll feel after sprinting all out for 45 seconds. 

The ATP-Pc System

The third energy system is the most powerful yet probably the least complex. The ATP-Pc system (Pc standing for phosphocreatine) is utilized for maximal intensity efforts and uses readily available ATP in the working muscle as opposed to creating it through Glycolysis. 

The system’s core mechanism is simple ATP regeneration; splitting a phosphate from the ATP molecule to provide energy. The ATP-Pc system uses the muscles' own stores of phosphocreatine to recycle ADP back into ATP. When these stores run out, you will fatigue and must allow for the muscle to replenish its phosphate stores before it can perform maximally again. 

This is actually how creatine supplementation can benefit your performance for maximal effort exercise bouts. Muscle phosphocreatine stores are not naturally at their full capacity, so you’re able to saturate the muscle with more phosphocreatine than it naturally has and continue to convert ADP into ATP for longer! 

While you can consume creatine naturally through meats, it’d be very difficult to consume the amount you’d need to fully saturate your muscle stores; meaning supplementation is by far the best way to go.

The ATP-Pc system doesn’t last very long at all, making it the perfect tool for high intensity short duration sports such as 100m sprinting or weight lifting. 

The Energy Systems & Different Types of Cardio

There are endless variations of cardio, so let’s have a look at some of the most familiar types people engage in and how we can relate it to the three different energy systems.

Exercise such as the 100m sprint requires immediate power and speed of delivery, and therefore will always predominantly use the ATP-Pc system. 

Cardio activity like HIIT (High Intensity Interval Training) or team sports are perfect candidates for the anaerobic system. These forms of cardio are too intense for the aerobic system, but also not intense enough to require the immediacy of ATP Pc system. This places them nicely within the range that lactic acid can help sustain activity

Distance running and even walking are great candidates for predominant usage of the aerobic system. Steady and controlled, this sort of methodical exercise allows for the rhythmic breathing needed to sustain the oxygen requirements of Glycolysis, and for this reason, you can do it for a long time without succumbing to immediate fatigue. 

It’s important to note that these energy systems are running concurrently regardless of exercise type, but the relative contribution of each is regulated by the intensity of the exercise.

Practical recommendations 

Dietary carbohydrates are the body’s main energy source, and supply the glucose and glycogen used to fuel the energy systems. The main consideration with carbohydrate intake is keeping glucose and muscle glycogen topped up; meaning you want to have enough before exercising and ensure to replenish what you’ve used afterwards. 

A 2014 review concluded that a meal consumed 2-3 hours prior to exercise is adequate for replenishing carbohydrate stores, with studies measuring ingestion times of less than 60 minutes appearing to have less consistent effects. 

Research into post-exercise carbohydrate replenishment appears to be conclusive that providing 24 hours of rest occurs between bouts of exercise, 10g per kg of body weight should maximally replenish glycogen stores. 

Linden Garcia Pepworth is a Sports Nutritionist (BSc Sports Nutrition) and YMCA Accredited Instructor. He is currently working on a review comparing the anabolic differences between plant and animal proteins.