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.