The fencing response is not a parry or a repost in fencing or sword fighting, but rather a medical term used often in competitive sport. The fencing response is an unusual and unnatural looking position of the arms adopted by individuals when suffering a concussion. Here, immediately following moderate forces affecting the brain stem of the individual, the forearms are held awkwardly, with one in a flexed position, and the other extended up into the air lasting a few seconds following the impact. This often occurs in sports such as football, hockey, boxing, rugby, martial arts and others, and is used as an indicator of an injury force magnitude and midbrain localization which can aid in injury identification for medical professionals in sports activities and possibly other scenarios.

The reason for the term ‘fencing response’ is of course because the position resembles that of the en-guard position used in fencing. The movements of the arms occur usually once the individual has lost consciousness so it is not a conscious reaction, and they will not be affected by gravity or momentum. Once the individual regains their consciousness they will resume normal movement of the arms.

Cause

This rather puzzling effect seems to resemble reflexes caused by vestibular stimuli (the vestibule being the part of the ear). These activate very primitive motor reflexes – such as when we ‘jump’ when startled by a noise – particularly in infants. One such reflex is that of the asymmetric tonic neck reflex and parachute reflexes (named after how they look).

The reason that this occurs may be to do with the location of the lateral vestibular nucleus which contains many of the vestibular nerves. This has fibers extending into the motor nuclei and it seems as though the close anatomical location of the two mean that the excitation of one can result in the firing of nearby neurons in the motor regions causing a ‘reflex’ action.

Similarly the cerebellum of the brain is highly important for motor control. As it is located on the brain stem near the top-rear of the neck (dorsal-posterior in psychological terms) this means that sudden movement of the neck – such as that which occurs in a concussive blow – would be enough to cause the sudden movement seen in the fencing response. Specifically the mechanical forces to the head can stretch the ‘cerebellar peduncles’ and thereby activate the LVN.

Use

The occurrence of the fencing response is actually useful in many ways, for diagnosing injuries and for scoring in sports. Two thirds of sporting injuries that involve a blow to the head result in a fencing response. The most challenging aspect of treating brain injury tends to be in identifying the damaged area. The fencing response can help to achieve this, and furthermore looking for it makes it possible to tell the severity of an injury – and whether it is mild traumatic brain injury (concussion) or a more severe injury that needs more attention. This can help judges and referees also to discern whether a player can return to the game and they should generally regard the fencing response as a sign that there is more moderate damage and that the player needs to be removed from the game. This may also lead to penalties for the other team/player.