Stiff person syndrome (SPS) is an extremely rare progressive neurological disorder characterized by persistent rigidity and spasms of certain voluntary muscles, especially those of legs and feet. In some cases, muscles of the neck, trunk, and shoulders may also be involved. SPS may begin as recurring (intermittent) episodes of stiffness and spasms. A particular type of trigger usually brings on these episodes. Triggers that could set off an episode are mainly sound, touch, bright light, motion or movement. Triggers cause what is referred to as a startle effect. Once startled the body reacts by causing certain muscle groups to fire. Both the agonist and antagonist muscles fire at the same time causing stiffness or rigidity.

An article in laymens terns to explain the illness: click here