Training harness

From Memory Alpha, the free Star Trek reference

Training harnesses were used by the thralls of Triskelion in sessions of combat training. They consisted of straps wrapped around the thrall's shoulders and waist, and their exact purpose remains unknown. There were two variants. There was a black variant, which was apparently worn by male thralls. James T. Kirk, Pavel Chekov, Lars, and Kloog wore black training harneses, whereas the male Andorian thrall did not wear a harness. The second variant was silver one, which was apparently worn by females. This variant had laces in front. It was worn by Uhura, Shahna, and Tamoon. (TOS: "The Gamesters of Triskelion")

James T. Kirk and Pavel Chekov wearing black training harnesses

Uhura wearing a silver training harness