A gravity bar was a device which could have its weight altered. Field Major Styre used one in his experiments on humans by having Erak and Krans hold one above Vural. He hoped to discover the limitations of muscular strength and the pressure a human could withstand on its breast cage.
Styre gradually increased the weight from forty pounds to six hundred. At this point, he was interrupted by the Fourth Doctor, who challenged him to single combat. While they fought, Sarah Jane Smith and Harry Sullivan helped Erak, Vural and Krans to break free of their bonds, allowing them to drop the bar. (TV: The Sontaran Experiment)