Which sequence correctly describes the primary energy systems as exercise intensity increases from short to longer durations?

• A. The aerobic system dominates for up to ~10 seconds; anaerobic glycolysis provides energy for high-intensity efforts from about 10 seconds up to 2 minutes; the aerobic system becomes the primary energy source for longer-duration activity.
• B. The phosphocreatine (ATP-PC) system dominates for up to ~10 seconds; anaerobic glycolysis provides energy for high-intensity efforts from about 10 seconds up to 2 minutes; the aerobic system becomes the primary energy source for longer-duration activity.
• C. Anaerobic glycolysis provides energy for the entire duration.
• D. ATP-PC dominates throughout all durations.

Answer: (B)

As exercise duration increases, the body relies on a shifting sequence of energy systems to meet the growing and changing demand for ATP. The immediate source for very short, high-intensity efforts is the ATP-PC system, using stored phosphocreatine to quickly regenerate ATP for up to about 10 seconds. Once effort continues beyond that, energy comes mainly from anaerobic glycolysis, which can provide rapid ATP for high-intensity activity roughly from 10 seconds up to about 2 minutes, though it also produces lactate as a byproduct. For longer, sustained activity, the aerobic system becomes the primary energy supplier, generating ATP more slowly but efficiently over time.

This pattern matches the sequence described: ATP-PC up to around 10 seconds, anaerobic glycolysis from about 10 seconds to 2 minutes, then the aerobic system predominating for longer durations. The other descriptions don’t fit the timing and capacity of the systems—for example, claiming aerobic dominance at the very start, or that anaerobic glycolysis covers the entire duration, or that ATP-PC dominates throughout all durations.