Those of us who are involved in sports – athletes, coaches and those who work with athletes – understand the importance of fueling the body to maximize energy and performance. It also helps to understand how the body converts energy so that healthy strategies can be used to improve athletic performance.

The Fundamental Law of Energy

The first law of thermodynamics states that energy cannot be created, but must be transferred or converted from one form to another. Like an automobile only runs on gasoline, the human body runs on only one kind of energy: chemical energy. More specifically, the body can use only one specific form of chemical energy, or fuel, to do biological work – adenosine triphosphate (ATP).

ATP – The Gas in the Tank

So, how does the human body make ATP, the only fuel it can convert to energy? Our bodies have three different chemical systems that convert energy. Most everyone knows that we use proteins, carbohydrates and fats for energy. Calories are measurement of a unit of heat or food energy. For example, we can achieve four calories per gram of proteins and carbohydrates, and nine calories per gram from fats.

But how do we convert these potential energy substances into ATP? This is where three energy systems come into play.

Energy System 1: Ready Fuel for Immediate Energy

The Immediate Energy system, or ATP-PC, is the system the body uses to generate immediate energy. The energy source, phosphocreatine (PC), is stored within the tissues of the body. When exercise is done and energy is expended, PC is used to replenish ATP. Basically, the PC functions like a reserve to help rebuild ATP in an almost instantaneous manner.

So, in the quadriceps and hamstring muscle groups of an average athlete, a specific quantity of ATP and PC stored within the muscle. These stored substrates are ready and waiting to be chemically transformed to fuel biological work process – such as contracting a muscle. This system gives athletes a readily available store of energy which can be accessed without delay.

What’s The Downside?

The average athlete will have approximately 285 grams of stored ATP in his or her entire body. That amount of ATP will be consumed in a just few seconds of work. At any time, athletes have only about 10 seconds worth of ATP-PC.

A supplement called creatine monohydrate that can increase the amount of PC stored in the muscles. It is one of the most researched ergogenic aids available and it does work. However, it can cause muscle cramps and it is not recommended for use during hot weather.

Energy System 2: Glucose-fueled Quick Energy

The glycolytic system, sometimes called anaerobic glycolysis, is a series of ten enzyme-controlled reactions that utilize carbohydrates to produce ATP and pyruvate as end products.

Glycolysis is the breakdown of glucose. Technically, glycolysis can use glucose or glycogen in its chemical reactions. The glucose must enter the cell membrane to begin the process. Upon entering the cell, the glucose will begin a transformation that will produce a net of two ATP and two pyruvate molecules. These 10 reactions occur very rapidly. Glycolysis is the preferred energy system by the human body when any sort of exercise work is required. The process is fast, there is generally plenty of glucose available and the reactions can occur anywhere within the cell’s sarcoplasm.

What’s the Downside?

Two problems exist with glycolysis. First, only two ATP molecules are produced for each molecule of glucose used in the process. Glucose starts out with six carbons in its structure. In chemical energy, carbons are potential energy – in other words, potential ATP. In chemical terms, that is a waste of potential energy.

Second, the two pyruvate molecules created in the very last reaction have two possible pathways. They can be converted into lactate (lactic acid), or they can be carried into the third energy system and continue to produce ATP.

Aerobic Fitness Reduces Lactate Production in Glycolysis

What actually happens to the pyruvate depends upon several factors – primarily how “aerobically” fit is the athlete, and the degree of work intensity. The lower the relative work intensity and the higher the athlete’s aerobic fitness, the less lactate that will be produced.

Conversely, the more the body uses glycolysis to produce ATP, the more lactate will be produced with it. As most athletes know, a high level of blood lactate does not help sports performance.

How to Use Glycolysis

Generally, glycolysis takes a few seconds to start running and can be utilized for up to approximately two minutes. A classic example is one lap around a 400-meter track. The average athlete will start out super fast, cruise the middle 200, and then crawl across the finish line.

From an energy system perspective, Energy System 1 fuels the athlete’s first three or four steps, and then glycolysis takes control to produce ATP. By the time the 400 meters is finished, so is glycolysis.

Energy System 3: Long-lasting Aerobic Energy

The Aerobic System resides within a specific organelle of the body’s cells. This specific organelle is the mitochondria – the “power house of the cell.” That is precisely true. The bulk of the ATP produced by the human body comes from the mitochondria. Therefore, the bulk of the ATP produced is via “aerobic” processes.

The first two energy systems are anaerobic, meaning they do not require oxygen. The aerobic energy system must have oxygen or the entire process will slow down and potentially stop completely. The oxygen needed by this system is provided by the cardiovascular and respiratory systems via blood flow to the tissues.

Where the Rubber Meets the Road

The aerobic energy system is where we utilize all three of our fuel sources. It is within this system that carbohydrates, fats and proteins may be processed in order to produce ATP. Carbohydrates come through the glycolytic system, producing pyruvate that proceeds into the aerobic system.

The use of proteins and fats is a little more complicated. Proteins must go through a process whereby the nitrogen components are removed. Basically, the protein is changed into its separate amino acids, and the “amino” part is stripped or changed. What is left is simply a carbon molecule that can be processed in either the glycolytic or aerobic systems.

Fats travel around the body in the form of a triglyceride in the blood. Before a fat can be used in the aerobic system, the triglyceride must be broken into its respective pieces, glycerol and fatty acids. Both of these contain carbon molecules that can be used to produce ATP. Glycerol enters through the glycolysis pathways. Fatty acids enter the mitochondria and go through a process called Beta-oxidation. This process requires many chemical reactions, time, and oxygen. Yes, oxygen is needed at two different stages of Beta-oxidation.

Optimal Cardiovascular Condition

So, an athlete needs a very well developed cardiovascular system to provide the oxygen for all of this to occur. The aerobic system takes anywhere from one to three minutes to get up and fully running when we begin to exercise. The speed and efficiency of the aerobic system is directly related to the athlete’s aerobic conditioning. This system is capable of providing ATP for extended periods of time. If the intensity is not too high, an athlete may use this system for hours and hours of work, as in a marathon or IRONMAN triathlon.

Energy Replenishment and Recovery

The aerobic system helps to replenish and recover the first two energy systems. It is this system that helps to clear out the lactate produced from glycolysis and to rebuild the stored ATP and PC needed for the Immediate Energy system. Most team sports are anaerobic in nature. However, all team sport athletes need at least a moderate amount of aerobic conditioning so their aerobic system can provide recovery for the anaerobic systems. It is usually fairly easy to see which team sport athletes do not have the best aerobic conditioning! They will be the ones bent over with their hands on their knees between each play on the football field!

Training the Body’s Energy Systems for Optimal Performance

The three energy systems can be improved by training. What that means is vastly different for each system, but each energy system is just as trainable as your quadriceps and hamstrings by doing squats.

When athletes train, we do basically three things:

  1. Develop muscles to provide more force and/or more efficient use of force
  2. Train motor and muscle skill patterns to more effectively execute a sports skill
  3. Train energy systems to be more effective and efficient at producing ATP

Each of our energy systems provides ATP in a very specific time and intensity range. In order to train these systems, you need to work within these time and intensity ranges.

Immediate Energy System

For example, a training session with the goal of improving the Immediate Energy System would utilize short explosive movements or exercises. A series of repeated maximal vertical jumps or short sprints would be an excellent way to “stress” the first energy system.

Short-term Energy System

Likewise, a training session with a goal to train the Glycolytic System would require a longer session of work but still at a very high intensity level. The 400 meter sprint is a good example. Running intervals on the track or football field is a great way to tax the Glycolytic System.

Long-term Energy System

To train the Aerobic System, an athlete needs to do steady-state work for a minimum of 20 to 30 minutes. Generally, aerobic work occurs in the range of 65 – 85 percent of VO2max. Perform this aerobic work at least four days per week for optimal benefits.

This has been a look at the body’s energy systems, through the eyes of an exercise physiologist. I would argue that when sports performance is at stake, training your energy systems is just as important as how much weight you can bench press or how high you can vertical jump.

Ready to Take Your Performance to the Next Level?

To schedule an appointment for one of our Human Performance tests, please call (713) 897-7912 or click the link below to learn more about our Human Performance Services.

Learn More

Contact Us

For more information on IRONMAN orthopedic doctors or surgeons, treatment for injuries, scheduling Human Performance services, or getting more information about physical therapy, please fill out the form below or call us at (713) 222-2273


Thank you for contacting the IRONMAN Sports Medicine Institute. We have received your inquiry, and a team member will contact you soon.

If you need more immediate assistance, please call us at (713) 222-2273.

If you are experiencing a medical emergency, call 911 or go to the nearest emergency room.

Sports Nutrition
IRONMAN Sports Medicine Institute Orthopedics and Sports Medicine Physical Therapy Human Performance Sports Nutrition Services