In this thread already some people mention weight lifting as being helpful

So, I'll look into it

But I don't really want to build up mass.
I always liked being skinny, but defined and with strength/endurance.

I want the body to be able to do what I like.
So, going skiing or surfing and everything else.

I always find those mountains of muscles some bodybuilders move around strange, because they don't seem to be really functional for sport and reduce the ability to move, when they get extreme.

I'm thinking of getting back into fighting sports.
Did Hapkido for some time in my 20s and really liked it. But my joints are aching by just holding the phone to long.
So doing some painful levering on them during training doesn't sound so wise...
I'm currently trying to get them better with some elastic band exercises and try to build up on that, by extending the movement to also include the shoulders.

But obviously, this is more of a health exercise, than a real sport thing