J-03

[Queequeg]

A jumbo is not your couch guitar.
If you're accustomed to placing the waist of the instrument on your thigh like this guy with a dreadnaught, your shoulder will be really out of kilter and your upper arm resting way up on the lower bout of the guitar. See how much higher his right shoulder is than his left?
This will very likely result in pain over time.



And not like this:



You'll have to assume a more classical style position, as B0WIE suggests, in order to play a jumbo while seated, with the lower bout resting between your thighs, which is how I hold all guitars, anyway.
OK, this guy has a dread, but note how his right shoulder has dropped to a normal position, more in line with his left shoulder.

Compare the position of the elbows, left & right in each photo.

[StringPicker6]

All good advice!  I have tried the classical position a few times, and it is indeed more comfortable.  However, it's just not "rock and roll" enough, so I would never do it in front of my groupies or my dog (I have neither). 
One day I will try a jumbo guitar if I see one in the wild.  They must sound magnificent!

[BlueBowman]

All good advice!  I have tried the classical position a few times, and it is indeed more comfortable.  However, it's just not "rock and roll" enough, so I would never do it in front of my groupies or my dog (I have neither). 
One day I will try a jumbo guitar if I see one in the wild.  They must sound magnificent!

Ah, but if you're the only one playing in the classical position (while everyone else is on the right knee), that is rock and roll 

[William2]

Here is my favorite jumbo demo. I love a jumbo in rosewood. I also think his sitting position looks very natural. And I'm a recovering classical guitarist.

https://www.youtube.com/watch?v=cOv9yKaouXM&t=3s

[B0WIE]

FWIW, classical position is actually becoming quite modern. I've seen a lot of younger players, particularly in the electric world, picking it up. And, some of the new headless electric guitar designs are being made to accommodate classical position by cutting away some of the lower right bout.

Not that we're seriously debating this or anything. I just found it interesting.

[unclrob]

Get a nice chair, place feet flat on the ground, sit upright,play guitar on one leg,hold head high and enjoy.

[BlueBowman]

For those with shoulder pain, you could also try the strap and sit method. Just like it sounds, throw a strap over the shoulder while sitting. It allows the guitar to be angled away from the right shoulder a little bit, maybe enough to reduce or remove the pain. Not as much as the classical position, but better than nothing.