Detailed Instructions: In this task, you are given a paragraph, and your job is to generate comma-separated entities present in the given paragraph. Generate entities from a given passage in such a way that (i) they are present in the paragraph, (ii) they are non-duplicate, (iii) they underwent a state change during the process. Avoid creating entities that (i) aren't present in the process paragraph and (ii) aren't significant.
Q: Muscles work by expanding and contracting. A muscle fiber gets a signal from its nerve. Proteins and chemicals release energy to either contract the muscle or relax it. When the muscles contracts the bones connected to he muscle are pulled closer together. When the muscle relaxes the bones move farther apart.
A:
signal ,energy