Final answer:
During sustained contractions, muscle fibers must manufacture more ATP as needed. ATP is the primary energy source used by the muscles for contraction.
Step-by-step explanation:
The statement is true. During sustained contractions, muscle fibers must manufacture more ATP as needed. ATP is the primary energy source used by the muscles for contraction. The speed of contraction is dependent on how quickly ATP is hydrolyzed by myosin's ATPase to produce cross-bridge action. Fast fibers, which hydrolyze ATP approximately twice as quickly as slow fibers, result in quicker cross-bridge cycling and faster contraction.