Final answer:
Acetylcholine receptor sites must be occupied for muscle contraction to occur.
Step-by-step explanation:
When the acetylcholine receptor sites are not occupied, muscle contraction does not occur. Acetylcholine is a neurotransmitter that binds to receptors on the sarcolemma of the muscle fiber, allowing an action potential to move to the endoplasmic reticulum and initiate muscle contraction. If the receptor sites are not occupied, there is no signal for muscle contraction to take place.