Final answer:
ATP, or adenosine triphosphate, is the definitive source of energy for muscular contractions, providing the required energy for cross-bridge formation and filament sliding. Option 4.
Step-by-step explanation:
The compound that is considered the definitive source of energy for a muscular contraction is adenosine triphosphate (ATP). ATP is essential for the process of muscle contraction as it provides energy for cross-bridge formation and filament sliding. Muscle contraction requires a continuous supply of ATP because it is used up quickly and needs to be regenerated through mechanisms such as creatine phosphate metabolism, anaerobic glycolysis, and aerobic respiration. Option 4.