Which energy system primarily provides fuel for short, intense bursts of activity, such as weightlifting or sprinting?

The energy system that primarily provides fuel for short, intense bursts of activity, such as weightlifting or sprinting, is the ATP-PC system, also known as the phosphagen system.

Here's the step-by-step explanation:

1. The ATP-PC system uses a molecule called creatine phosphate (CP) that is stored in the muscles.

2. When intense activity begins, the ATP (adenosine triphosphate) molecules that are immediately available in the muscle are used up in a few seconds.

3. The body then uses CP to quickly replenish ATP, with the help of an enzyme called creatine kinase.

4. This ATP is then available to provide energy for more muscle contraction.

5. The ATP-PC system can provide energy for high-intensity activity for about 10-15 seconds before it is exhausted and the body needs to switch to other energy systems.

6. This is why this system is the primary source of energy for short, intense activities like weightlifting or sprinting.