For the third time that night, Spock shifted in his chair, touching the bottom of his back while trying to find a comfortable position. Kirk gave him a worried look, it was unusual for his first officer to be so restless. They were playing chess as it was their custom, and nothing in Spock's behavior seemed to indicate that something was wrong.
Kirk moved one of his pieces. "Is there something bothering you, Mr. Spock?"
Spock made his move. "Nothing, Captain." But his words were belied by his need to hold his spine as to approach the board.
"Well, Spock, there is no reason to lie."
"Vulcans do not lie, Captain."
"But are known to modify the truth when it suits them. Are you still not recovered from the accident in the last mission?"
"Dr. McCoy said it is just a muscle strain and recommended rest."
They continued to play, and Kirk was disappointed with the speed with which he managed to defeat Spock. Usually their games were challenging, but the pain was obviously affecting Spock.
"Spock, you can't go on like this."
"Vulcans aren't affected by pain, captain."
"Right, and you're half human. How about a massage?"
Spock raised an eyebrow. "I would hardly say that's appropriate."
Kirk got up and went to the bed. "Spock, you have given me various massages, at least let me return the favor."
Spock pondered briefly, but he knew the captain well enough to know he wouldn't let him escape easily, so he sat on the edge of the bed. "The Vulcan pressure application techniques can hardly be compared to human massage."
Kirk smiled. "We'll see what's your verdict after the massage. Now take off your shirt."
"Captain, the ship's temperature is too low for Vulcans."
Kirk rolled his eyes, still with a smile. "All right, Mr. Spock. I'll increase the heat while you prepare."
Being unable to find another logical excuse, he obeyed. The room was still very cold, the climate control would need a few minutes to affect the whole environment. Spock laid face down, trying to convince himself that there wasn't any harm in receiving a massage from his superior officer.
Kirk opened a drawer, choosing between lotions available. Finally, he asked. "Lemon, peach or strawberry?"
"The choice is irrelevant, there is no logical difference in product performance based..."
"Lemon, it is." Kirk concluded before returning to bed.
His fingers worked skillfully against the muscles, releasing tension visibly. He began to hear the rare sound of contentment Vulcans made, similar to a purr, which they produced only in situations of extreme relaxation. He didn't say anything, fearing to destroy this rare moment of Spock's vulnerability. His hands went down Spock's back, working all the muscles along the way. When he was nearing the end of the spine, he felt two bumps, and pressed with a gentle force.
"Jim!" Spock said, in a mix of scream and moan, as he jumped out of bed. His cheeks were flushed in a shade of dark green, and in his hurry to get dressed, he put his black undershirt over his blue uniform shirt.
Spock ran before Kirk could understand what was happening. He muttered some excuse as he left, leaving his confused captain behind.
Kirk sat on his bed, perplexed, trying to understand Spock's behavior. When he realized the futility of the exercise, he decided to call Bones.
"Dr. McCoy here."
"Bones, you won't believe what just happened. I was giving Spock a back massage and he suddenly ran away."
The line went dead for a moment, and when Bones spoke again, his tone was annoyed. "Jim, please tell me you didn't touch his lower back."
"Actually, that was exactly what I was doing when he left. I had just touched in a nodule near the spine."
"Those weren't nodules, Jim. What do you remember the Academy's xenobiology lessons? Specifically, what do you remember of the location Vulcan testicles?"
A translation of "Xenobiologia & Massagens" also by me, posted originally on AO3, in honor of K/S Day.