The phrase "flexes muscles" typically refers to demonstrating strength, power or authority. The antonyms of this phrase could be "concede defeat", "yield", "surrender" or "retract". While flexing muscles emphasizes a physical or symbolic show of strength, the antonyms carry negative connotations of backing down, giving up or losing. Other antonyms could include "apologize", "agree", "compromise", "defer" or "withdraw", which emphasize a more conciliatory, humble approach. Ultimately, the opposite of flexing muscles is to avoid confrontation, seek common ground and work towards diplomacy, collaboration, and peaceful resolution of conflicts.