He became violent and had to be physically restrained.

restrained 是动词 restrain 的过去分词形式，在句中表示被动。restrain 由前缀 re-（表示"加强"）和词根 strain（"拉紧"）组成，意思是"制止、约束"。restrained 表示"被制止、被约束"的状态。