What disorder leads a muscle to stiffen due to agitation or anger?

'Somatization': Most people experience increased muscle tension when frightened or angry so it's very common. In general, whenever your emotions are translated into physical symptoms, it's called 'somatization' from the greek root word for body, 'soma'. As in 'psychosomatic', emotions are powerful in energy and channeled to the wrong place in the body, can create unpleasant symptoms. The symptoms are real!