Final answer:
To inhibit muscle contraction in vitro, you would use a Ca²⁺-ATPase inhibitor to prevent the release of calcium ions from the sarcoplasmic reticulum.
Step-by-step explanation:
To inhibit the contraction of muscle in vitro as a control for an experiment, you would use a Ca²⁺-ATPase inhibitor. This inhibitor would prevent the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum, thereby blocking the contraction process. A Ca²⁺-ATPase inhibitor would specifically interfere with the step where calcium ions are released and initiate filament sliding in the muscle fiber.