I'd disagree with that generally, but in Mythic Europe it might fit. While one anecdote doesn't prove a point, it has some relevance, so here's an anecdote.
I've been in a sedentary job for most of my life. I now weight 25 kgs more than when I started my job about 25 years ago. I decided I didn't want this weight gain to continue and move on to be 100 kgs when I retire and die of a heart attack, so I joined a gym. I've been going at least 5 days a week, 30 mins - 1 hour a day.
I've mostly done weights and cardio. I helped move some furniture recently. It was definitely easier. I can't see furniture moving being associated with athletics. Not to mention I can't do a long duration handstand, let alone a back flip or stuff associated with athletics. I'd say my strength moved for a +0, to a +1. or -1 to +0 depending on perspective, more than me getting 1 rank of athletics.
Saying all that, "the blood will out", good breeding, etc is more the model of Mythic Europe, so there is an argument stats being immutable fits the setting.
Well, I wasn't looking for a philosophical universal truth, just trying to explain the way I feel what characteristics are in the game, which happens to be placed in Mythic Europe, so I really don't understand that phrase
As for your personal experiences working out, what can I say? Good for you, I guess, I couldn't do that without thinking the hours per year I'd be spending on a gym with the huge amount of things I have to do and that I'd rather be doing instead of being sweating. Anyway real life experiences can't be translated finely into any game mechanics system: on some place or another you are going to get incoherences and things that don't fit well. Still, I'll make a try, because it's fun. What happened to you, obviously, is that by spending that many hours per year you just made an ordeal, and got initiated into the Healthy People Who Does Cardio Mystery Cult, and you got the Minor Virtue Improved Characteristics, which you spent doing something like raising your Str from -1 to 1 and Sta from 0 to 1. Doesn't it make sense?
I love the idea of the gym being a mystery cult. The gym is a chore I do, so I can still go up a staircase without wheasing a few years from now.
I do find these kind of discussion fun, however, I agree stats and game mechanics will never fit perfectly. Someone with superhuman strength, the +5, will lose an arm wrestle with a standard person 1 time in 10. That makes no sense. It's important to put the game and fun ahead of any overthinking of the numbers.
Quite fun indeed.
What I liked the most of the Ars Magica mechanics about characteristics back in the day was the idea that the average was 0. That lead me to the feeling that they were just broad modifiers, etc etc, and also made me look over the shoulders to any other game where a characteristic is a number from somewhere to somewhere else.
Regarding how a guy with Str +5 still may loose against the average guy from time to time due to the nature of stress rolls, yeah, that happens, but that's not because of chars, but due to the very high (mostly) linear range of rolls. Any attempt to make rolls curved like using 2D6 is flawed to me anyway (mostly because I like D10 dice and find D6 kind of childish and boring). Anyway think that characteristics are just part of the character. If you have a character designed to be a beast in brawling (I had a concept for one of these, kind of a kung-fu rogue which came from the other side of the world, but I still hadn't found a not silly enough reason to use it), he is not just going to get high physical characteristics. There is also going to be a whole battery of chained virtues granting him his status and making the fight against the average guy quite less dependent of a lucky roll.
ArM3 suggested using multipliers for things such as this one: it's not stress die + Str vs. stress die + Str. It's stress die + 5xStr vs. stress die + 5xStr! It is a bit inelegant mechanically, though. A better option would be to say:
a) if the SG rules it's relatively unimportant to the story, someone in the troupe gets to narrate the outcome. This could be the SG or even a player.
b) if the SG rules it's important, then run the context as a sequence of rounds, where each round both characters compare stress die + Str and the first to accumulate an advantage of, say, 30 points over the other wins. Interesting twist if time is of the matter: the winner of each comparison, if any, decides if the next comparison takes place in the current or following round. This generally allows a definitely stronger character to win in 1 or 2 rounds!
We're on the same page. My next line was. "It's important to put the game and fun ahead of any overthinking of the numbers."
I'd narrate the arm wrestle not die roll it as well.