The human body uses three main energy systems when we exercise for a long time: the phosphagen system, the glycolytic system, and the oxidative system. The way we switch between these systems depends on how hard and how long we exercise, as well as how much oxygen and energy sources are available.

1. Phosphagen System (ATP-PCr System)

• Duration: Works for about 0 to 10 seconds.
• How It Works: This system uses a form of energy called ATP stored in the muscles and another substance called creatine phosphate (CP) to quickly make energy.
• Energy Output: It produces about 1 unit of ATP for every unit of CP used.
• When It’s Used: It’s mainly used for short, intense activities like sprinting or heavy weightlifting.

2. Glycolytic System

• Duration: Active for about 10 seconds to 2 minutes.
• How It Works: When the phosphagen energy runs out, the body moves to another way to create energy called anaerobic glycolysis. This breaks down sugar to make energy without oxygen, which leads to the buildup of lactate.
• Energy Output: This system creates about 2 units of ATP from one unit of sugar.
• Lactate Buildup: High-intensity efforts can cause more lactate. Studies show that lactate can increase when workout intensity is more than about 80% of your maximum oxygen use (VO2 max).
• When It’s Used: Important for efforts like a 400m sprint or several tough sets of weightlifting in a row.

3. Oxidative System

• Duration: Starts to take over after about 2 minutes of exercise and can keep going as long as there is enough oxygen and nutrients.
• How It Works: This system uses oxygen to break down carbohydrates, fats, and sometimes proteins for energy through processes called the Krebs cycle and the electron transport chain.
• Energy Output: It can produce 36 to 38 units of ATP from one unit of sugar, and even more from fats depending on the type.
• Energy Sources: At rest, the body mainly burns fat, but it uses more sugar as exercise intensity increases.
• When It’s Used: It’s used in long activities like distance running, cycling, or swimming where you need endurance.

Switching Between Energy Systems

• As you keep exercising, your body gradually goes from the phosphagen system to the glycolytic system and then to the oxidative system. This change isn’t like flipping a switch; all three systems can work together at the same time.
• For example, during a 10k run:
  • First 30 seconds: Mainly uses the phosphagen system.
  • 30 seconds to 2 minutes: Switches to glycolysis to get more energy for the muscles.
  • 2 minutes onward: The oxidative system becomes the main source of energy, especially as heart rate and breathing increase to bring more oxygen.

Recovery and Replenishing Energy

• After exercising, the body needs to recover and refill its energy stores. Research shows that CP can be back up to 80% within 2 to 5 minutes after intense exercise, but it may take longer to completely restore.
• Lactate levels drop quickly and usually return to normal within 30 minutes after finishing intense exercise.

Conclusion

Knowing how these energy systems work and switch can help with better training plans and nutrition choices for better performance during workouts. By mixing different activities of various intensities and durations, athletes can make the best use of their energy systems and boost their overall endurance.