A muscle fibre relax when?

A muscle fibre relaxes when the nerve signals that were causing it to contract stop, allowing the muscle to return to its resting state. This can happen when the brain signals the muscle to relax, or when the nerve signals that were causing the muscle to contract are interrupted. Additionally, muscle fibres can also relax when they are no longer being stimulated by electrical impulses.

When acetylcholine (ACh) is broken down by acetylcholinesterase (AChE)