Which statement accurately describes the three primary energy systems and their typical substrates and time frames?

• A. ATP-PC system (anaerobic, up to ~10–15 seconds; fuel: phosphocreatine); glycolytic system (anaerobic glycolysis, ~30–120 seconds; fuel: glucose/glycogen); oxidative system (aerobic, >2 minutes; fuel: carbohydrates and fats).
• B. ATP-PC system is aerobic and uses fats; glycolytic system is aerobic and uses glucose; oxidative system is anaerobic and uses glycogen.
• C. Oxidative system is anaerobic and uses carbohydrates; ATP-PC is aerobic; glycolytic system is aerobic.
• D. All three systems are correctly described as listed.

Answer: (D)

Understanding how the body fuels ongoing muscle work involves three energy systems, each defined by how it generates ATP, what fuel it uses, and how long it can sustain activity. The ATP‑PC (phosphagen) system is anaerobic and relies on phosphocreatine stored in muscles to rapidly resynthesize ATP; it powers very short, explosive efforts, roughly up to about 10 seconds (often listed as 10–15 seconds) before stores run out. The glycolytic system also operates without oxygen for the initial phase of energy production, breaking down glucose or glycogen to make ATP and producing lactate as a byproduct; this system is most active for moderate durations, typically from about 30 seconds to around 2 minutes. The oxidative, or aerobic, system requires oxygen and can use either carbohydrates or fats as fuel, supporting longer-duration work beyond a couple of minutes as intensity and duration demand are sustained. Because the statement aligns these systems with their primary fuels, oxygen use, and typical time frames, it accurately reflects how the three energy systems function.