Mocap bodies are [[Robot Kinematics - Frames and Twists|kinematics]] only bodies in [[- MuJoCo -|MuJoCos]] soft dynamics. They are ignored by physics computations and can only be moved manually. It is good practice to attach a mocap body to an actual geometric body and introduce a [[MuJoCo Contacts Cheat Sheet|constraint]] to (softly) track a reference that is forced on the system from the outside. Otherwise, jumps in joint states can easily destabilize the simulation or yield unwanted effects such as jittering.