T'Pol's back and upper buttocks
A lifeform's back was the part of their body their spine ran down. It started at the end of the neck on the side the lifeform was facing away from and ended at the buttocks.
Sometimes, Humans could feel pain in their backs as a result of a muscle spasm. When Q was turned into a Human as punishment, a back spasm was the first experience he had with pain. (TNG: "Deja Q")
Klingons had ridges going down the middle of their backs. (TNG: "Ethics")
When Kathryn Janeway was experiencing headaches as a result of being experimented on by the Srivani, The Doctor mistook her symptoms as those of stress or sleep deprivation and rubbed her back in hopes of making them go away, however, Janeway did not enjoy it. (VOY: "Scientific Method")
While pregnant, B'Elanna Torres experienced pain in her back, but ignored it. (VOY: "Endgame")
In 3189, Jett Reno's back was injured from time traveling through a wormhole. For her injury, Reno was given neuroblockers which put her into a drugged state. (DIS: "Far From Home")