The antonyms for the word "jerked up down" include smooth, gradual, steady, even, constant, and continuous. Whereas, jerked up and down implies sudden and abrupt movement, the antonyms imply a gradual and steady movement. Smooth indicates a motion without any sudden movement, while gradual shows movement taking place in small increments. Steady and even imply a motion that's constant without any variation, and constant suggests steady movement that doesn't stop. Finally, continuous indicates uninterrupted motion which takes place without any breaks or pauses. The antonyms of "jerked up down" portray a sense of smoothness, steadiness, and consistency in motion.