Lead researcher Jonathan Lifshitz, assistant professor in the UK Spinal Cord and Brain Injury Research Centre and colleagues are collecting data to document a visible, involuntary response to head trauma.
The response, dubbed as "fencing response," has a forearm posture that resembles the en garde position in competitive sword fighting. It also can appear as a defensive boxing pose.
According to Lifshitz, the fencing response - which has also been observed in rats under experimental conditions - indicates damage to blood vessels and neurons in a critical brainstem region that controls balance.
During the study, the research team reviewed some 2,000 "knockout" videos on YouTube, eventually narrowing their sample to three dozen that showed moderate-to-severe impacts to the head, where the person receiving the blow did not immediately get up.
Of those, two-thirds exhibited a clear fencing response. The response was noted particularly in football and mixed martial arts.
"The fencing response frequently takes place before the player even hits the ground," said Lifshitz.
Moderate-to-severe head trauma can cause permanent brain damage or death if ignored by medical staff.
However, sometimes these injuries are not readily apparent. The fencing response provides an immediate visual cue that could help injured players get the attention they need, Lifshitz said.
The findings are published in journal Medicine and Science in Sports and Exercise.