The phrase "in torsion" implies a force that is twisting or turning something, so its antonyms would be words that describe qualities that are opposite to that. For example, "stable," "straight," or "rigid" could all be antonyms of "in torsion." Other possible antonyms include "non-rotating," "stationary," or "immobile." These words suggest a state of being which is free from any twisting or torque. Additionally, we can use "fixed," "steady," "constant," or "unchanging" to communicate the idea of stability, and "static," "motionless," or "unmoving" to convey the absence of movement.