When does the anaerobic system start working?

• A. When oxygen demand exceeds supply
• B. After several minutes of activity
• C. Only when oxygen supply exceeds demand
• D. During rest periods only

Answer: (A)

The key concept is how the body sources energy during exercise, particularly when the anaerobic pathways kick in. At the start of activity, especially at high intensity, ATP demand rises quickly while oxygen delivery and use slow to catch up, creating an oxygen deficit. To bridge that gap, anaerobic systems provide energy right away. The anaerobic contribution begins when oxygen demand exceeds supply and continues until aerobic metabolism can meet the energy needs. That’s why the correct idea is that the anaerobic system starts when demand outpaces supply. It wouldn’t wait for several minutes, it isn’t triggered only when oxygen supply exceeds demand, and it isn’t limited to rest periods.