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Training Client's Energy System's in Accordance with their Goals copy

Training Client’s Energy System’s in Accordance with their Goals

As a personal trainer, you will encounter clients with a range of fitness goals, some aesthetic and others performance-based. Performance-based goals could be training your client for a long-distance race, enhancing performance in a HIIT-based group fitness class, or adding plyometric training to your client’s routine because your client read about the positive correlation of load-bearing exercises and bone density. Whichever individual goal interests your client, a one-size-fits-all approach won’t work for addressing each client’s unique goals. To tailor one’s approach, a personal trainer must thoroughly understand the body’s three energy systems and how to train each system.  

This article will examine how rate of perceived exertion (RPE) and heart rate zones (HR zones) can be applied to these systems to help our clients achieve their performance-based fitness goals. This article will provide a high-level bioenergetics introduction, however, IFTA’s Personal Trainer Certification and all of their courses and CECs include a more in-depth exercise science block to best equip learners

Phosphagen System

This is our body’s first responder and quickest energy system during short bouts (1-6s) of physical activity. It uses no oxygen, carbohydrates, or fat. This system primarily utilizes skeletal muscle stores of creatine phosphate, which are limited in quantity, to make energy (ATP) and is therefore not sustainable for a long time period. 

A personal trainer may work with a client to train this system if the client is looking to improve sprints or bouts of explosive heavy exercises. To train a client effectively, exercise should achieve an RPE of 7-10 and a HR zone of 88-99% within a few short-second training periods.

Glycolytic System

This second quickest high-intensity energy system (7-180s) consists of the breakdown of blood glucose or muscle glycogen stores to create energy (ATP). This process occurs in the watery cytoplasm of cells. Once ATP is synthesized, pyruvate is formed. Pyruvate has two destinies depending on the presence or absence of oxygen. If the body’s oxygen demand is higher than the availability of oxygen (lactate threshold) pyruvate will convert to lactate and cause a state of acidosis in the muscles due to metabolite accumulation. This causes clients to feel a burning sensation and muscle weakness. 

If enough oxygen supply is present, aerobic glycolysis occurs and pyruvate is shuttled into the cell’s mitochondria via acetyl-CoA. A personal trainer may train this system if a client has a goal of short-distance runs emphasizing speed or is looking to improve HIIT intervals. This system should be trained at an RPE of 5-6 and a HR zone of 82-87% for around two minutes.

Aerobic or Oxidative System

This last system is our slowest energy-producing system, however, it has the highest production of energy (ATP). The Aerobic system works in the presence of oxygen and primarily utilizes free fatty acids. This process occurs in the mitochondria of muscle cells. Acetyl-CoA feeds the Kreb’s Cycle where the yielded electrons enter the Electron Transport Chain. 

This energy system is valuable to train when a client is looking to perform better at endurance activities such as long-distance runs, step aerobic classes, and very high repetition resistance training. A personal trainer should adhere to an RPE of ≤5 and a HR zone of 55-82%.


The body has three very unique energy systems. The Phosphagen System uses muscle creatine stores to create rapid energy (ATP) in the first 1-6 seconds of exercise. The Glycolytic System occurs during minutes 1-3 and utilizes glucose for energy without oxygen. Lastly, the Aerobic System uses free fatty acids in the presence of oxygen to yield the most energy for endurance exercise greater than three minutes. 

Although each energybsystem has its distinctions, personal trainers must remember the body isn’t solely using one energy system at a time. During exercise, the contributions of each energy system will fluctuate. This is why it is important to always curate a dynamic training program for clients.

For more information on how bioenergetics further connects to exercise science principles, we encourage you to try any one of IFTA’s courses to further understand and apply your knowledge!

Morrison, S., Ward, P., & duManoir, G. R. (2017). Energy System Development and Load Management Through the Rehabilitation and Return to Play Process. International Journal of Sports Physical Therapy, 12(4), 697–710.

Jason Karp, P. (2021, May 5). The Three Metabolic Energy Systems. IDEA Health & Fitness Association. 


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