Very stiff, because these are connected to a swivel the wire is constantly finding itself in an uncomfortable position, instead of sitting in a fixed position or just laying softly on my back like most retainers. I bought these because the regular croakies push against my head behind my ears and i thought this would be a decent solution because they clip on to the glasses and do not expand the profile of the frame to push against your skin. But they are constantly uncomfortable and in the way. Wore them once and was frustrated all day.