A backpack has a pocket and at least one shoulder strap for supporting the backpack on a shoulder of a user. A generally rigid plate and a flexible inflatable bladder are in the pocket with a load bearing bag connected to the pocket for holding a load. A tubular extension extends from the bladder and into the shoulder strap for inflating and deflating the bladder and a mouth-inflatable check valve is connected to the tubular extension at a location spaced from the bladder and positioned so as to be accessible to the user for inflating the bladder by blowing into the check valve. A curved semirigid liner inside the material of the shoulder strap near the bladder forms a semirigid channel for receiving the tubular extension.