I doubt he lost a lot of muscle. Maybe a little, but he's way too young to lose that much without starving himself or sitting still much more than before.
Also, opening a jar isn't much of a test of strength. Sometimes it's just hard to get a good grip, and jars are far from consistent.
Obvious bait is obvious.
Using distractions to win at arm wrestling is weak.
And then she obviously doesn't want to do it again because she knows she's weaker.
Working out together is much more fun than doing it alone.
"I don't even have the strength to work out."
That means you're pushing yourself and getting a work out out of it. It may or may not be too much. But probably not, unless you start hurting in any way other than general exercise pains.
I feel like some “childproof caps” are a struggle as an adult too lol
Temperature also plays a huge role. If you put a jar in the fridge that's been stored on a room-temp shelf, you're going to struggle to open it when you take it out.