There are several synonyms that can be used in place of the phrase "not move a muscle". These include "remain motionless", "stay still", "freeze", "standstill", "immobility", "rigidity", "inaction", and "lifelessness". Each of these words describes the state of not moving or showing any signs of activity, whether in a physical or emotional sense. While "not move a muscle" is a commonly used expression, using synonyms can add variety and color to your writing or conversation. Experiment with different words to convey your message and create a more interesting and engaging dialogue.