During a 2-minute maximal effort, which energy systems are most active and what is the primary substrate for ATP production?

• A. The aerobic system is most active; fatty acids are the primary substrate.
• B. The anaerobic glycolysis is most active, with some contribution from ATP-PCr; glucose/glycogen is the primary substrate.
• C. ATP-PCr is most active; phosphocreatine is the primary substrate.
• D. Oxidative phosphorylation is most active; amino acids are the primary substrate.

Answer: (B)

During a two‑minute maximal effort, the body relies mainly on anaerobic glycolysis to meet the very high ATP demand. This pathway can ramp up quickly and generate ATP rapidly by breaking down glucose or stored muscle glycogen, producing pyruvate that is converted to lactate when oxygen is limited. The ATP‑PCr system provides a fast initial burst at the very start, but its stores are exhausted within seconds, so it can contribute only a small amount by the two‑minute mark. The oxidative (aerobic) system, while always active, cannot supply ATP fast enough to sustain maximal effort for this duration, and fat oxidation is too slow to be a significant energy source at such high intensity. So the primary substrate is glucose or glycogen, with anaerobic glycolysis driving the majority of ATP production, and a minor early contribution from phosphocreatine.