A stretchable adjustable-stiffness assembly includes a casing and first and second friction layers. The casing can be fluidly sealed and includes an interior compartment containing a fluid. The fluid can shift between a first interior pressure and a second interior pressure less than the first interior pressure and an exterior pressure. The first and second friction layers are disposed within the interior compartment. The assembly can shift between relaxed and fixed configurations. In the relaxed configuration, the fluid has the first interior pressure, the casing is stretchable between a first dimension and a second dimension greater than the first dimension, and the assembly has a first stiffness. In the fixed configuration, the first fluid has the second interior pressure, the casing length is fixed, the first and second friction layers are engaged, and the assembly has a second stiffness greater than the first stiffness.