I dunno, ask my wife and she'll tell you the songs I'm usually practicing are beyond the edge of my capabilities.
In seriousness, I don't think that statement is meant for lifting/running or anything that's bounded by your physical fitness. Because you're right, the human body doesn't work like that.
Music is probably a better example. I push myself by learning songs that are beyond what I can do now. I practice them and make a bunch of mistakes. And as I keep practicing, the mistakes go away. And at some point, the song I couldn't play becomes a song I can play. Then I find new even harder songs to focus my practice on.
There is a subtlety of language at play here. When a musician practices a song they "cannot play", usually what they mean is they cannot play it fluidly, at full speed.
There are many degrees of being able or not able to play any given song.
This is what I hear when someone says they are practicing a skill slightly beyond their current capacity.
I mean, do you know what the biggest record jump in 100m running was?
...Usain Bolt beating his own record.
I used to think that genetics must surely play a bigger and bigger role the more a skill is built around pure body function (management < speed typing < wrestling < running), but looking at stuff like this, it doesn't seem so simple.
In seriousness, I don't think that statement is meant for lifting/running or anything that's bounded by your physical fitness. Because you're right, the human body doesn't work like that.
Music is probably a better example. I push myself by learning songs that are beyond what I can do now. I practice them and make a bunch of mistakes. And as I keep practicing, the mistakes go away. And at some point, the song I couldn't play becomes a song I can play. Then I find new even harder songs to focus my practice on.