In the early phase of high-intensity work, which substrate is primarily used to provide the phosphate for ATP resynthesis?

• A. Phosphocreatine.
• B. Glucose.
• C. Free fatty acids.
• D. Amino acids.

Answer: (A)

In the earliest moments of high-intensity work, energy comes from the phosphagen system. Phosphocreatine serves as the immediate source of a phosphate to regenerate ATP from ADP, via the enzyme creatine kinase. This rapid ATP resynthesis happens without the need for oxygen, letting muscles power through short bursts of effort (roughly up to 10–15 seconds). Glucose would have to go through glycolysis to make ATP, which takes more time to ramp up; free fatty acids and amino acids contribute more to fat- or protein-derived energy in longer, lower-intensity activity and involve slower processes. So the phosphate donor for ATP resynthesis in the very early phase is phosphocreatine.