Which system is engaged during high-energy activities such as sprinting or heavy lifting, but burns out quickly, leading to a significant decrease in output?

The ATP/PC system, also known as the phosphagen system, is the primary energy source for short bursts of high-energy activities, such as sprinting or heavy lifting. This system utilizes adenosine triphosphate (ATP) and phosphocreatine (PC) stored in the muscles to rapidly regenerate ATP. It allows for explosive movements due to its immediate energy availability.

However, the ATP/PC system has a significant limitation: the available phosphocreatine and ATP stores are depleted very quickly, usually within 10 seconds. After this short duration, the body's capacity to continue supporting high-intensity activity declines sharply. This swift energy expenditure and the subsequent burnout is what leads to a notable decrease in output, as the body must transition to other energy pathways, like anaerobic glycolysis or aerobic metabolism, which are less effective for such intense activity.

The options related to the cardiovascular system and cognitive memory system do not directly relate to the specific high-energy demands of activities like sprinting or heavy lifting, or the rapid depletion of energy resources associated with them. Therefore, the ATP/PC system is the correct choice, as it precisely describes the mechanism at play in these high-energy activities.