When a person experiences an impact that’s strong enough to cause traumatic brain injury (TBI), such as a concussion, their arms often go into an unnatural position. This position — forearms extended or flexed, usually in the air — follows the impact and is known as the fencing response position. It lasts up to several seconds after the collision.

The fencing response is often seen when a player is knocked down or knocked out during full-contact athletic competitions such as football, martial arts, boxing, rugby, and hockey.

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The name comes from the similarity to asymmetrical tonic neck reflex (ATNR), also referred to as fencing reflex, which occurs in newborns.

This is when newborn babies position themselves with one arm flexed and the other extended with their head turned toward the extended arm like a trained fencing athlete. This reflex usually stops after the baby reaches about 4 months old.

This reaction occurs after injury because it’s thought that if a blow impacts the brainstem, it will momentarily reactivate the ATNR.

Doctors use a number of indicators — such as the 15-point Glasgow coma scale — when assessing the severity of TBI. For a variety of reasons, including the fact that concussions can’t be seen on MRI or CT scans, medical professionals are looking for more indicators to make diagnosis more accurate.

Whether or not fencing response was seen by witnesses may become part of that assessment process. If the fencing response is seen after an injury, it could be that it was likely worse than one where no response occurred, since the fencing response is thought to involve the brainstem.

A 2009 study reviewed over 2,000 YouTube videos of high-impact knockout videos and, based on a small portion of these, concluded that that two-thirds of head impacts demonstrated a fencing response.

The researchers also concluded that, based on animal models, fencing response occurs in response to moderate TBI and not to mild TBI even if that mild TBI resulted in knockout or concussion.

A concussion is a mild TBI that results from a blow to the head or body that causes your brain to twist or bounce inside the skull. If you think you might have experienced a concussion, you should see your doctor or receive emergency medical assistance immediately.

Signs of a concussion include:

The fencing response might become an effective tool in helping determine the level of severity of a TBI.

If you feel that you’ve experienced an impact that could have resulted in a TBI, see your doctor. Your doctor might refer you to an expert such as a neurosurgeon, neurologist, or a neuropsychologist.