Extensor muscles were muscles of the hand located in the back of the forearm with long tendons connecting them to bones in the hand.
In 2373, Kathryn Janeway's extensor muscles showed a gradual build-up of lactic acid during her Nechani endurance ritual. (VOY: "Sacred Ground")
Later that year, Janeway ruptured her dorsal extensor muscle while dodging a macrovirus in the mess hall. (VOY: "Macrocosm")