Disclosed is an adjustable support frame for a breathing apparatus having an adjustable longitudinal dimension and a user-facing side configured to substantially overlay a user's back in use, the adjustable support frame comprising: a first frame portion and a second frame portion, the first and second frame portions being moveable relative to each other so as to adjust the longitudinal dimension; an adjustment mechanism having a locked configuration which inhibits relative movement of the first and second frame portions and an unlocked configuration which permits relative movement of the first and second frame portions; wherein the adjustment mechanism comprises one or more actuating elements which are operable to actuate the adjustment mechanism from the locked configuration to the unlocked configuration, the one or more actuating elements being provided on a surface which is substantially perpendicular to the user-facing side of the support frame.