QCE Physical Education Practice Questions

During Player K's 6-second maximal sprint effort in the second quarter, the ATP–PC (phosphagen) system is the dominant energy system. This system operates anaerobically, using stored intramuscular phosphocreatine (PCr) as its fuel source to rapidly resynthesize ATP via the reaction: PCr + ADP → ATP + Cr, catalysed by creatine kinase. At maximal sprint intensities of this duration (~0–8 seconds), the ATP–PC system contributes approximately 85–90% of total energy, as it provides the highest power output of any system with an extremely rapid rate of ATP resynthesis.

However, as the sprint progresses toward 6–8 seconds, PCr stores begin to deplete significantly (reaching approximately 50% depletion by 5–6 seconds). This depletion triggers interplay: the anaerobic glycolysis system begins increasing its relative contribution, mobilising muscle glycogen and breaking it down via glycolysis to produce pyruvate. At this high intensity, pyruvate cannot be fully oxidised, and is reduced to lactate with the concurrent release of hydrogen ions (H⁺). The end-products of anaerobic glycolysis — lactate and H⁺ — begin accumulating, which contributes to peripheral fatigue mechanisms.

During the recovery period immediately following the sprint, the aerobic system assumes dominance. Using oxygen delivered via the cardiovascular system, the aerobic system oxidises glucose and free fatty acids via the Krebs cycle and electron transport chain (end-products: CO₂ + H₂O), and critically, it drives the resynthesis of PCr stores — approximately 50% are restored within 30 seconds and ~95% within 3 minutes of rest. This aerobic-driven PCr replenishment is essential for Player K's repeated sprint capacity across the 120-minute match, given the data shows 38 sprint efforts are required.