Background For the past ~20 years, the scientific literature, based on data from inaccurate helmet/head-mounted sensors, has reported that there is little link between head impact magnitude and location and visible signs (VS). The current study found that VS were seen only in impacts in the top 1% as measured by head impact acceleration, velocity and/or energy.

Objective Head impacts were measured with an accurate head impact monitoring mouthguard and confirmed through video and data trace analysis. The athlete/military service member behavior was observed for a visible sign post-impact.

Design Retrospective meta-analysis from military parachute, boxing, mixed martial arts, weapons firing, as well as American football, Boxing, Ice Hockey, Karate, Lacrosse, Mixed Martial Arts, Rugby, Tae Kwon Do, Football.

Setting Sporting field or Military training activity.

Participants 50,000 verified impacts over 10,000 person-days.

Assessment of Risk Factors Visible signs post-impact.

Main Outcome Measurements Kinetic energy transfer (KE), peak scalar linear acceleration (PLA), peak scalar linear velocity (PLV), impact location, impact direction, type of VS.

Results There were fifty (50) single head impacts that caused visible signs. The most common visible signs were loss of consciousness, posturing, dazed, imbalance and ataxia. All visible signs impacts were in the top 1% by magnitude (40–100g, 3–8 m/s and 30–150 J). A total of 90% of the VS impacts were to the side or rear of the head.

Conclusions Visible signs do not occur often, but always occurred in the top 1% of head impacts by magnitude, and 90% of these were to the rear and the sides of the head. By monitoring head impacts with an accurate device, clinical staff can have access to data that identifies head impacts most likely to cause visible signs.