Okay so Spock was born and raised on Vulcan, a planet that canonically has a higher gravity than Earth. At least partially due to this, Vulcans have much denser bone and muscles than humans (part of why they are so much stronger than us) meaning that per m^3 Vulcans weigh more than the average human. Considering Spock, the beanpole that he is, has been stated to have the strength of 3 average men it is logical to deduce that his muscle mass is up to 3 times as dense as that of the average human meaning it is up to 3 times as heavy (I’m not a physician so don’t quote me on this). All this considered, no matter the math, Spock is heavy af, so
how strong is Kirk????????