Final answer:
ATP is essential for muscle contraction by enabling cross-bridge cycling; without it, muscles cannot contract and eventually become rigid. ATP is generated in muscle cells through multiple pathways, but if these pathways are blocked, muscle function is severely compromised.
Step-by-step explanation:
ATP (adenosine triphosphate) is crucial for muscle contractions because it provides the necessary energy for cross-bridge cycling between actin and myosin filaments within muscle fibers. Without ATP, the myosin heads cannot detach from the binding sites on actin, resulting in the cessation of muscle contraction and eventual muscle rigidity, a state known as rigor mortis.
Muscle cells have various methods for generating ATP, such as creatine phosphate metabolism, anaerobic glycolysis, and aerobic respiration. If ATP regeneration is prevented, as with drug X that blocks ATP regeneration from ADP and phosphate, muscle cells would eventually become unable to contract due to the lack of energy supply.