Playing an anglo three nights per week for dances, plus practising a fourth night, plus general mucking around never gave me tennis elbow. I got minor callouses at the top and bottom of each palm (from constant contact with the straps) and sometimes (after a very long night) a bit of an ache from an abductor muscle in the upper arm but nothing else. Mostly I played standing up but, if I played when seated, I'd have the right hand end resting gently against the inner part of the right knee and the left hand did most of the bellows work.
Being (some lifetimes before) an IV judoka (and something of a tiger in Victorian rockclimbing) I was well practised in the arts of relaxation of all the muscle groups that weren't actually required to move while playing, until you needed to move them. It helps, even for a person known as 'upfront' in his playing.
And I didn't even pick a concertina up until I was 32. Just get stuck into it and learn the relaxation bit as well as keeping tabs on your pre-existing condition and you'll be alright.