• Introduction
  • The basics of the phosphagen system
  • Creatine kinase
  • Adenylate kinase
  • Final thoughts
  • Sources

Introduction

Human energy production is based on complex interconnected pathways that break down nutrients derived from food into usable energy, adenosine triphosphate (ATP). It can be divided into three main systems; aerobic respiration, anaerobic respiration, and the phosphagen system.

Each of these has a specific function and relies on varying metabolic processes to generate ATP. This energy is used for exercise and maintaining regular body functions like breathing, heartbeat, cell repair, hormone activity, etc.

The phosphagen system is the fastest of the energy systems. It provides energy rapidly during quick and intense exercises that last ~10 seconds. Therefore, it is especially important in short and explosive sports like weightlifting, javelin and discus throwing, shot putting, as well as various sports that rely on quick sprints.

This article explains the basic mechanisms of the phosphagen system, and what makes it so important for everyday life as well as athletic performance.

The basics of the phosphagen system

The phosphagen system, also called the ATP-PC system, utilizes stored adenosine triphosphate (ATP) and creatine phosphate (CP) during the first few seconds of an exercise. This process relies on the hydrolysis of an ATP molecule, where the bond is split by adding a water molecule, as well as breaking down a high-energy phosphate called creatine phosphate. This process of regenerating ATP via the transfer of phosphate groups occurs through either of two reactions;

  • Creatine kinase
  • Adenylate kinase

Although the phosphagen system is anaerobic and does not require oxygen to work, it provides so little energy that it does not produce lactic acid. Hence, the phosphagen system is also known as the anaerobic alactic system. 

During the first 2-3 seconds of an exercise, ATP is broken down to produce adenosine diphosphate (ADP) and inorganic phosphate (Pi) in a process called ATPase. Since muscle fibers have such a limited amount of free ATP, your body needs to constantly produce more energy to meet the demands of physical activity. This is done by breaking down creatine phosphate into creatine (Cr) and phosphate. This process is catalyzed by an enzyme called creatine kinase. The resulting energy of this process also fuels the resynthesis of ADP and CP into ATP. Thus, generating more energy for muscle contraction.

Regardless of the intensity of the exercise, any activity relies on the phosphagen system and the stored ATP during the beginning of an exercise. Due to the small amount of stored ATP and CP, the phosphagen system is the main source of energy for only the first 10s of an exercise. Thus, the body must quickly break down creatine phosphate to generate more energy. After this, the main energy production method quickly switches to glycolysis. Whether this occurs aerobically or anaerobically depends on the intensity of the exercise.

During a maximal exercise, the energy received from the phosphagen system is fully depleted in 10-15s. However, it also recovers very quickly. In fact, it is nearly 70% regenerated in just 30 seconds and fully recovered in 3-5 minutes. However, this also depends on the acidity inside the muscles (slower if more acidic) and the extent of CP depletion. 

Duration

Classification

Energy Source


1-3s

Anaerobic

Stored ATP


3-10s

Anaerobic

ATP + CP


10-45s

Anaerobic

ATP + CP + Muscle Glycogen


45s-2mins

Anaerobic, Lactic

Muscle Glycogen


2-4mins

Aerobic + Anaerobic

Muscle Glycogen + Lactic Acid


>4mins

Aerobic

Muscle Glycogen + Fatty Acids


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The Phosphagen System


Without oxygenOccurs in the cytoplasmVery fast energy productionVery small energy storages(stored CP & ATP)Does not produce lactic acid

Creatine kinase

Creatine kinase is an enzyme that has several functions in energy metabolism. Most importantly, it catalyzes the reaction of ATP and creatine, forming creatine phosphate and ADP. This also makes it the most immediate means to generate ATP.

The process is also reversible, meaning that ATP can also be generated from ADP and creatine phosphate. Thus, Creatine kinase is used to maintain the energy balance (ATP homeostasis) inside the muscle cells. 

CP + ADP + H+

Creatine kinase

ATP + Cr

Adenylate kinase

Much like creatine kinase, adenylate kinase also produces ATP, albeit with a significantly lower energy yield. More importantly, adenylate kinase also produces adenosine monophosphate (C10H14N5O7P), which is a nucleotide consisting of a phosphate group, the sugar ribose, and a nucleotide base adenine. AMP, also known as 5′-adenylic acid, is one of the components of RNA and present in all forms of life.

Although AMP levels are naturally very low during rest, they quickly begin to rise in long and intense exercises. This is due to the fact that for every depleted ATP molecule, the AMP levels increase six times. The reason why this is important is that AMP is an allosteric regulator that sends a signal when the cell’s energy levels are running low. 

A rise in AMP activates;

  1. Phosphorylase, which increases glycogenolysis and boosts glucose-6-phosphate (C6H13O9P) production. This can further be used for glycolysis. 
  2. Phosphofructokinase, which allows more glucose molecules through glycolysis for increased ATP production.

Thus, a higher concentration of AMP activates metabolism through other pathways such as glycolysis and the citric acid cycle.

ADP + ADP

Adenylate kinase

ATP + AMP

Around 70% of stored ATP and CP are replenished in just 30 seconds of recovery.

Final thoughts

Although the phosphagen system only provides energy for the first ~10s of an exercise, it is still crucial in sports that rely on quick and intense bursts of muscle movement (shot put, javelin, weightlifting, etc.). On top of that, ATP and CP storages also replenish quickly, making them especially useful in sports that require repeated sprint ability (soccer, ice hockey, American football, etc.)

Interestingly, the amount of stored ATP and CP is directly related to the muscle fiber type. Fast-twitch muscle fibers (type IIa & type IIb/IIx) do not contain mitochondria like slow (type I) ones do, making them unable to generate energy via oxygen. However, fast muscle fibers make up for this by being larger in size and containing a higher amount of ATP and CP. Thus, they are able to contract with more force in explosive exercises.

Luckily, the amount of stored ATP and CP can also be improved with consistent exercise. This occurs especially after very strenuous activities like strength and speed training. Simply put, if you want to improve your performance in short (~10s) maximal exercises, you must train in a similar fashion.

Did you learn anything new about the phosphagen system? Let us know in the comments.

Sources

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