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I can imagine I never got into it myself and I am not sure why but it does look like such fun
Not that I would try it now my knees aren’t what they used to be

Yeah, it's probably not a good idea, because especially slides can be rather hard on the knees.

The last time i tried skiing my knee popped out of joint and i understand long boarding is harder on the knees so for sure not for me

Uh, that sounds bad. I hope you recovered completely after that accident!
I think it depends on what your want to do with a longboard.
If you just use it for cruising around and you start to learn it step by step, it's rather soft on the knees.
But all further from that has a higher risk for injury.

It was maybe 20 Years ago and I recovered fine from it has happened a few times over the years I smashed both knees when I was 19 but that didn’t slow me down for long but I feel it now as approach my 60s lol

Thats very good, that you recovered that fast!
I had broken my foot while climbing 6 or 7 years ago and I still have to deal with it today sometimes.
Injuries at the joints can be nasty.

I was stubborn back then was told I would need surgery or I would never run again, and the surgery was no guarantee I would run again so I put it off, and not long after started to run and ended up doing a few marathons and triathlons so it was all good, now I feel the aches and pains but I have done pretty well I think :)

Broken feet can take a while to heal, I have broken both of mine a few times, the worst one was when a truck ran over it, I didnt even know they were broken most of the times I thought it was a strain or whatever and taped it up myself, I only found out for sure they were broken about 5 Years ago when I had feet issues and they did xrays on both feet, the funny thing is the foot I call my good foot, just about every bone in it has been broken at some stage been broken, but they healed fairly well so I must have taped them up well

Lets just say I was crazy when I was younger