Bodybuilding Science: What is Glycolysis?

Aspartic acid molecule. Alpha-amino acid nonessential in mammals. Precursor to several amino acids including methionine, threonine, isoleucine and lysine. Atoms are represented as spheres and are colour-coded: carbon (grey), hydrogen (blue-green), nitrogen (blue) and oxygen (red. Getty Images

Whether you are training in the gym, making breakfast in the kitchen, or doing any sort of movement, your muscles need constant fuel in order to properly function. But where does that fuel come from? Well, several places is the answer. Glycolysis is the most popular of the reactions that take place in your body to produce the energy, but there are also the phosphagen system, along with protein oxidation and oxidative phosphorylation.

Learn about all these reactions below.

Phosphagen System

During short-term resistance training, the phosphagen system is mainly used for the first few seconds of exercise and up to 30 seconds. This system is capable of replenishing ATP very quickly. It basically uses an enzyme called creatine kinase to hydrolyze (break down) creatine phosphate. The released phosphate group then bonds to adenosine-5’-diphosphate (ADP) to form a new ATP molecule.

Protein Oxidation

During long periods of starvation, protein is used to replenish ATP. In this process, called protein oxidation, protein is first broken down to amino acids. These amino acids are converted inside the liver to glucose, pyruvate, or Krebs cycle intermediates such as acetyl-coA en route to replenishing 


After 30 seconds and up to 2 minutes of resistance exercise, the glycolytic system (glycolysis) comes into play. This system breaks down carbohydrates to glucose so it can replenish ATP.

The glucose can come fromeither the bloodstream or from glycogen (stored form of glucose) present in 
muscles. The gist of glycolysis is glucose gets broken down to pyruvate, NADH, and ATP. The generated pyruvate can then be used in one of two processes.

Anaerobic Glycolysis

In the fast (anaerobic) glycolytic process, there is a limited amount of oxygen present.

 Thus, the generated pyruvate is converted to lactate, which is then transported to the liver through the bloodstream. Once inside the liver, lactate is converted to glucose in a process called the Cori cycle. The glucose then travels back to the muscles via the bloodstream. This fast glycolytic process results in a rapid replenishment of ATP, but the ATP supply is short lasting.

In the slow (aerobic) glycolytic process, pyruvate is brought to the mitochondria, as long as an ample amount of oxygen is present. Pyruvate gets converted to acetyl-coenzyme A (acetyl-CoA), and this molecule then undergoes the citric acid (Krebs) cycle to replenish ATP. The Krebs cycle also generates nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2), both of which undergo the electron transport system to produce additional ATP. Overall, the slow glycolytic process produces a slower, but longer lasting, ATP replenishment rate.

Aerobic Glycolysis

During low-intensity exercise, and also at rest, the oxidative (aerobic) system is the main source of ATP. This system can use carbs, fats, and even protein. However, the latter is only used during periods of long starvation. When the intensity of the exercise is very low, fats are mainly used in 
a process is called fat oxidation.

First, triglycerides (blood fats) are broken down to fatty acids by the enzyme lipase. These fatty acids then enter the mitochondria and are further broken down into acetyl-coA, NADH, and FADH2. The acetyl-coA enters the Krebs cycle, while the NADH and 
FADH2 undergo the electron transport system. Both processes lead to the production of new ATP.

Glucose/Glycogen Oxidation

As the intensity of the exercise increases, carbohydrates become the main source of ATP. This process is known as glucose and glycogen oxidation. The glucose, which comes from broken down carbs or broken down muscle glycogen, first undergoes glycolysis. This process results in the production of pyruvate, NADH, and ATP. The pyruvate then goes through the Krebs cycle to produce ATP, NADH, and FADH2. Subsequently, the latter two molecules undergo the electron transport system to generate even more ATP molecules.