If your muscles or joints stiffen, or if something stiffens them, they become difficult to bend or move. The blood supply to the skin is reduced when muscles stiffen. Stiffen up means the same as stiffen. These clothes restrict your freedom of movement and stiffen up the whole body
make stiff or stiffer; "Stiffen the cream by adding gelatine" become stiff or stiffer; "He stiffened when he saw his boss enter the room" restrict; "Tighten the rules"; "stiffen the regulations
If something such as cloth is stiffened, it is made firm so that it does not bend easily. This special paper was actually thin, soft Sugiwara paper that had been stiffened with a kind of paste
If you stiffen, you stop moving and stand or sit with muscles that are suddenly tense, for example because you feel afraid or angry. Ada stiffened at the sound of his voice