As Guran suggests, it's not just about concertinas, but any instrument: they all carry their own ergonomic challenges.
There is always a physiotherapist or osteopath on hand at the Edinburgh International Harp Festival: is this about carrying big bulky instruments, or the positions adopted while playing?
One of my fiddler friends has to keep stop playing and shake her arms about as the left arm and hand positions cause seize-up.
One of our local instrumental tutors asked me (privately) to look at one of his class who was complaining of upper back pain while playing guitar: just one look was enough: hunched tightly around his guitar - a recipe for discomfort! And you see the same hunched up position with some mandolin and whistle players.
And I, as a button accordion player, favour the two straps, highish on chest approach. Having watched, and tried, the technique of those who balance the right side of their box on their left knee and extend out even further leftwards, it's just inviting left shoulder tendinitis IMHO.