Humans have always longed to be stronger, be faster, lift more, run farther, and to continuously push themselves beyond what is normal. Understanding what fuels our body allows us to train smarter and more efficiently. Mammalian creatures rely on 3 different energy systems to create energy for working muscles. Those three systems are the Phosphagen System, Glycolysis, and the Oxidative System. All three systems create energy for the body, known as ATP and all humans need ATP for fuel inside their muscles. ATP, short for adenosine triphosphate, is what makes us move! ATP is the lifeblood of exercise and different types of exercise will use ATP differently in the body. With that in mind, no one energy system is dominating completely, but is merely the main producer of ATP when working at different intensities.
THE PHOSPHAGEN SYSTEM
The Phosphagen System provides energy to the body for very fast explosive types of exercise. Athletes like sprinters, powerlifters, and individuals who rely on quick explosive bouts of energy rely mostly on their Phosphagen System. While the Phosphagen System can create a large amount of energy and yield intense power, it is limited by the stored amount of ATP in the muscles, therefore after about 15-20 seconds, the body will start to rely on other energy systems to provide fuel to maintain the workload.
Glycolysis provides ATP to the body when the intensity of exercise can no longer be maintained by stored ATP in the muscles themselves, and now the ATP must be created by using muscle glycogen and glucose. Glycolysis can maintain high intensities for longer durations than the Phosphagen System, approximately 1-2 minutes, although it is still limited by the production of the enzyme pyruvate, and ultimately until the intensity of the workload can no longer be maintained and the Oxidative System takes over. There are many enzymatic reactions that occur to create ATP through glycolysis, with both creating pyruvate as the end byproduct. With anaerobic glycolysis pyruvate is converted into lactate and builds up in the blood, and is transported into the liver where it is used and converted back to glucose. Eventually the build up of lactate and other metabolic byproducts cause the muscle to fatigue, causing us to work at lower intensity. At the lower intensity, pyruvate is transported into the mitochondria of the cell, where oxygen is present, and becomes part of the Oxidative System.
THE OXIDATIVE SYSTEM
When exercise intensity is lowered and oxygen present in the cell, pyruvate enters the mitochondria of the cell and undergoes what is known as the “Krebs Cycle” to create ATP from glucose and fatty acids. The mitochondria is like the power plant of the cell where ATP is generated. Marathon runners, rowers and other endurance athletes rely on the Oxidative System and the Krebs Cycle to fuel their muscles with ATP. While it can’t be produced as fast as Glycolysis or from the Phosphagen System, the Oxidative System can work indefinitely as long as the intensity of the exercise is maintained at or below the lactate threshold, and if there is a presence of oxygen within the mitochondria. When glucose and glycogen are depleted, fatty acids are also used as fuel in the Oxidative System to create ATP but require more processes to be broken down for energy, therefore take longer to yield ATP.
No one process is only working at a given time. Different people are adapted to different styles of training and sport. Remember to train for the energy system that your sport or activity demands. If I am a sprinter, I want to train in short explosive bouts, if I am a rower I want to increase my body’s ability to use oxygen and build up endurance. Maximize your performance by training your body not just hard, but train smart!