Energy (in the form of ATP) is the lifeblood of human physiology, fueling our daily activities and enabling us to perform various tasks, from walking and running to cognitive processes. The human body employs a sophisticated network of energy systems to generate and utilize energy efficiently. We want to discuss the three primary energy systems: the ATP-PCr system, the glycolytic system, and the oxidative system. 

First is the phosphagen (ATP-PC) system. If you remember biology class in high school, you recall that ATP, short for adenosine triphosphate, is a molecule that provides energy for all movement. Your body breaks down ATP to fuel your every move, from standing up, to doing a box jump. The phosphagen energy system harnesses ATP for highly intense activities that last 10 to 30 seconds. So for explosive activities like sprints and plyometric exercises in your training sessions, your body is predominantly engaging the phosphagen system for energy. Since your body stores a limited amount of ATP, activities lasting more than 30 seconds must also tap into energy generated by the glycolytic system.

The glycolytic system uses carbohydrates to produce ATP. Activities lasting 30 seconds to 3 minutes are primarily fueled by energy produced by this system. Think of boxing rounds, which last 1 to 3 minutes. Sometimes we use interval training in the training center in which you might see similar brief circuits, intervals, and drills. After a few minutes of sustained activity, your body starts to rely on the oxidative system to help meet your energy needs.

Unlike the phosphagen and glycolytic system, the oxidative system is aerobic and uses oxygen to help with energy production. While the glycolytic system uses carbohydrates to generate energy, the oxidative system dips into other macronutrients as well: fat and protein. The oxidative system is heavily engaged in low- to moderate-intensity activities. You'll harness a lot of energy via the oxidative system for your longer training sessions, including distance running. Not everybody needs to train this system. For example, a football lineman does not need to train in the oxidative system as much as a distance runner because of sport specificity. To prepare for the quick bursts of blocking and tackling demanded by the sport's position, a lineman may spend most of his training time engaging the phosphagen and glycolytic energy systems. However, this athlete may still engage the oxidative system for overall health and conditioning.

Keep in mind that all energy systems are active at all times during activity. Your body stores a limited amount of ATP, so the three energy systems work together to provide the energy you need in order to keep moving. The extent to which each one is engaged varies depending on the duration and type of activity, whether it's an explosive move or sustained exertion over time. This all matters because each person's goals in training are different, and because we are going to talk more about some specific ways to train each system in later blogs.