STIFFNESS By N., Sam M.S. - 6 Inflexibility of either muscles or joints due to an injury or dystonia. STIFFNESS: "Stiffness may cause muscles to become unable to move in various directions." No related posts. Cite this page: N., Sam M.S., "STIFFNESS," in PsychologyDictionary.org, April 13, 2013, https://psychologydictionary.org/stiffness/ (accessed August 7, 2022).