Why does a sprinter only run as fast as they can for short distance before they begin to slow down? Why don’t they run that fast for longer distances? Well it’s a simple concept really, their body can’t keep up with the energy demands it is asking for. Our bodies use energy called ATP or Adenosine triphosphate to do all things and without ATP our bodies don’t survive. To replenish this ATP our bodies have two energy systems that we use at all times depending on the energy demand. The energy demand is based on the intensity and also the duration of activity. These two energy systems are called aerobic and anaerobic. Aerobic means that the body is using oxygen and anaerobic means that the body is not using or in the presence of oxygen. These systems have different abilities when it comes to replenishing ATP for energy. The anaerobic energy system breaks up into two different systems.
The phosphagen system– This system provides ATP primarily for short-term, high-intensity activities such as sprinting and resistance training. This energy system relies on hydrolysis of ATP (when the energy is released) and the breakdown of another high-energy phosphate called Creatine phosphate or phosphocreatine. ADP combined with Creatine phosphate creates ATP for energy and for this to occur Creatine kinase has to synthesize this combination. This system is used during high explosive activities that last for about 6 seconds. This system has the ability to replenish a small number of ATP very quickly. This becomes a problem when the energy demands are too great for this system, so the body begins to rely on a new system more.
The glycolysis system– This system is also anaerobic and is the breakdown of carbohydrates (the only micronutrient that can be broken-down without oxygen) into glycogen or glucose to resynthesize ATP. This system actually breaks down into fast glycolysis and also slow glycolysis. Fast glycolysis is when Pyruvate (the end result of glycolysis) is converted to lactate, which is faster but is limited in duration. Slow glycolysis is when Pyruvate is shuttled into the Mitochondria to undergo the Krebs cycle (cycle of reactions to produce energy) and this can be kept up for ATP production for longer duration if the exercise is low enough. This system will start up at around 6 seconds and 30 seconds for fast glycolysis or all the way to 2 minutes if the exercise is lower in intensity. Slow glycolysis system kicks in after that.
Once you are beyond these systems the presence of oxygen is the primary source for the aerobic energy system. This system is called the oxidative system.
The Oxidative system– This system is the primary source of ATP at rest and during low-intensity activities. The body uses mainly carbohydrates and fats during this system. At rest you are mainly using fats compared to the carbohydrates until the intensity increases. Just like in glycolysis the glucose is transported into the mitochondria where it is taken up into the Krebs cycle for ATP. Glucose can also be broken down and cycled into the electron transport chain for ATP production. This system is the slowest for production of ATP but has the greatest capacity for it, meaning you get more ATP during this system but it can’t keep up with demands of higher intensity exercise. This system is the source of ATP at activities lasting longer than 3 minutes.
These are the systems of the body that produce the energy for us to move fast, powerful, or maybe just for a long time. Each system has its own strength and weakness but they all work together to keep you as efficient as possible. Now that you know how you replenish ATP through different systems you have an understanding on how your body reacts to different activity levels. Just eat the right things and keep training each system properly and they will keep you moving.