During the first seconds of intense sprinting, before oxidative metabolism ramps up, which reaction provides most of the ATP that sustains cross-bridge cycling?

• A Myokinase converting two ADP to ATP and AMP
• B Substrate-level phosphorylation in glycolysis
• C Creatine kinase transferring a phosphate from phosphocreatine to ADP ✓
• D Adenylate cyclase generating cyclic AMP from ATP

Explanation

Phosphocreatine is the immediate energy reservoir of skeletal muscle. Creatine kinase reversibly transfers its high-energy phosphate to ADP, regenerating ATP within milliseconds, enough to support several seconds of maximal contraction until glycolysis and oxidative phosphorylation take over. The myokinase reaction contributes only briefly, and glycolytic ATP arrives seconds later, making option B correct for sustained effort but not for the very first moments.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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