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Rising ADP signals that the energy supply is falling short of demand. When ATP is used by the cross-bridges, it splits into ADP and Pi, so ADP accumulates. That increase acts as the primary trigger for ATP-producing pathways: it stimulates the rapid phosphocreatine system to donate a phosphate to ADP and quickly reform ATP, and it also accelerates glycolysis and oxidative phosphorylation to generate more ATP as needed. In other words, ADP elevation directly indicates energy shortfall and drives the machinery to restore ATP levels to keep cross-bridge cycling going.

The other possibilities aren’t the immediate triggers. A higher ATP concentration would imply plenty of energy and would actually reduce the drive to produce more ATP. An increase in inorganic phosphate is a byproduct and can influence metabolism, but it’s the ADP rise that most directly signals the need to generate ATP. Oxygen tension rising would support oxidative metabolism but isn’t the immediate signal that more ATP is required.