Understanding your body – Energy Systems


Have you ever wondered where your energy comes from and how it works?

There are three metabolic pathways that provide the energy for all human action. These “metabolic engines” are known as the phosphagen, the glycolytic, and the oxidative pathway.

Total fitness requires competency and training in each of these three systems. Balancing the effects of these three pathways largely determines the how and why of the metabolic conditioning or “cardio” you usually do. Concentrating on only one or two systems to the exclusion of the others and not recognising the impact of excessive training in the oxidative pathway are arguably the two most common faults in fitness training. Time domain matching of task or sport to training is the first step to effective, legitimate strength and conditioning.





Ok, so yes, we all understand that energy is something we need, similarly to the way a car needs fuel. The only difference is a car will use the same energy system to go from 0 to 100km/h in 10sec, and sit on 100km/h for 5 hours, because it has the fuel ready to be supplied. Our body differs in the way that our fuel, Adenosine Triphosphate (ATP), can only be stored in muscle cells in small amounts.

To replenish this low quantity fuel source we have the so called Phosphagen System – an anaerobic energy
system, which uses the stored ATP for bursts of energy, usually lasting less than 10 seconds. This system replenishes the ATP by converting Adenosine diphosphate (ADP) into ATP, using phosphocreatine. In short: the ADP needs one more Phosphate molecule to work, so it adds one.

An effective workout for this system is short, very fast sprints on the treadmill or bike lasting 5–15 seconds with 3–5 minutes of rest between each set. The long rest periods allow for complete replenishment of creatine phosphate in the muscles so it can be reused for the next interval. A workout could look like this:

  • 2 sets of 8 x 5 seconds at close to top speed with 3:00 passive rest and 5:00 rest between sets
  • 5 x 10 seconds at close to top speed with 3:00–4:00 passive rest



The glycolytic energy system (another anaerobic system) draws on carbohydrates to create ATP for energy. This is a two-phase energy system where glucose (sugar) is broken down to form ATP and pyruvic acid molecules (lactic acid). It is the system used for relatively short periods of high-intensity work, lasting only a few minutes. After a few minutes of intense workout the accumulation of lactic acid will reach a point where pain and fatigue will begin to hinder performance. This is referred to as the lactate threshold.

This system can be trained using fast intervals lasting 30 seconds to 2 minutes with an active-recovery period twice as long as the work period (1:2 work-to-rest ratio). A workout could look like this:

  • 8–10 x 30 seconds fast with 1:00 active recovery
  • 4 x 1:30 fast with 3:00 active recovery



The oxidative system is the most complex energy system and the only aerobic energy system we have. The aerobic oxidative energy system utilizes carbohydrates, fats and proteins to generate ATP for energy. This is a three-stage system comprised of many steps, which are very complex in its chemistry. In order to keep it simple, let’s just say that the end result of the oxidative system is the production of ATP and water molecules. The complexity of this system, along with the fact that it relies on the circulatory system to supply oxygen, causes it to take longer to replenish than the anaerobic systems. It is the system used for long-term, low-to-moderate-intensity work lasting more than just a few minutes. It can be relied upon for long periods of work, making it the primary system used for endurance activities.

While the phosphagen system and glycolysis are best trained with intervals, because those metabolic systems are emphasized only during high-intensity activities, the aerobic system can be trained with both continuous exercise and intervals. A workout could look like this:

  • 60 minutes at 70%–75% maximum heart rate
  • 15- to 20-minute tempo workout at lactate threshold intensity (about 80%–85% maximum heart rate)
  • 5 x 3:00 at 95%–100% maximum heart rate with 3:00 active recovery