Kirk finds himself at a loss during beta shift, and heads to the gym.
Spock’s there, running, and the atmospheric controls in his booth are different to the rest of the ship - hotter, with less oxygen. Jim thinks it’s a pretty good way to increase exercise intensity.
About halfway through his workout, just after Spock leaves, Jim is left with a realisation so heavy he all but falls off his treadmill.
It’s nothing to do with the exercise.
Spock was running on Vulcan.