The fencing response is an unnatural position of the arms following a concussion. Immediately after moderate forces have been applied to the brainstem, the forearms are held flexed or extended (typically into the air) for a period lasting up to several seconds after the impact. The Fencing Response is often observed during athletic competition involving contact, such as football, hockey, rugby, boxing and martial arts. It is used as an overt indicator of injury force magnitude and midbrain localization to aid in injury identification and classification for events including, but not limited to, on-field and/or bystander observations of sports-related head injuries.
Source: Hosseini, A. H., and J. Lifshitz. Brain Injury Forces of Moderate Magnitude Elicit the Fencing Response. Med. Sci. Sports Exerc., Vol. 41, No. 9, pp. 1687–1697, 2009.