Which ATP source powers only the first approximately 15 seconds of a muscle contraction before another source must take over?

1. Because stored ATP in muscle lasts only a few seconds, muscle relies on a sequence of regeneration mechanisms. 2. Creatine phosphate transfers its phosphate to ADP very quickly via creatine kinase, powering the first few seconds of contraction. 3. However, creatine phosphate stores can only sustain about 15 seconds of activity. 4. Anaerobic glycolysis and aerobic respiration are the slower sources that take over afterward, and stored ATP alone lasts only a few seconds, all shorter or different from the 15-second creatine phosphate window. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 10 "Muscle Tissue", section 10.3, p. 419_