Final answer:
The function of ATP in muscle contractions is to provide the necessary energy for muscle fibers to contract. Creatine phosphate contributes to ATP regeneration by quickly donating a phosphate group to ADP, enabling rapid muscle contraction in the initial seconds, before other mechanisms like glycolysis and aerobic respiration take over.
Step-by-step explanation:
The function of ATP in muscle contractions is to supply energy for muscle fiber contractions. ATP is vital not only for the cross-bridge cycle between actin and myosin in muscle fibers but also for powering the calcium pumps in the sarcoplasmic reticulum (SR). Muscles store a very limited amount of ATP, which is quickly consumed during contractions, and thus it needs to be regenerated rapidly for sustained muscle work.
When muscle contraction begins, creatine phosphate transfers its energy to ADP, forming ATP and enabling continued contraction. After about 15 seconds, other sources, such as anaerobic glycolysis and aerobic respiration, must be used to generate ATP.