As Str can be improved on, I can see it as a skill. If you look at people, you will find most are within the same range of STR unless they train or are in a profession that adds to it.So, perhaps Str should be a skill, like in exerting Str with the help of voice ...
Does this means something to you pals ?
( lol )
The thing to look for is "common" attributes across biology. Man vs Ant vs Cat vs Dog vs Man vs Elf vs Giant vs Demon, and so on. You want something that works as a framework.