Head Impact Telemetry (HIT) is the industry standard software system that measures linear and angular head accelerations impacting football helmets. This system is used to help detect concussion types by measuring the magnitude, duration, location, and direction of a hit to the head of a player. The HIT system has been used since 2003 to study the magnitude of concussions and brain injury in youth and collegiate football. The system consists of six battery-powered sensors located in the padding of the helmet.
Study: In 2013, a study was conducted to investigate the accuracy of the HIT system used in football helmets. The study focused on the helmet fit in high school aged football players and the assessment of the appropriate helmet size for the Hybrid III, the gold standard dummy used for impact testing. Twenty-four FlexiForce sensors were designed into a nylon skullcap worn by volunteers to measure the pressure between the player's head and helmet padding to determine how tightly the players wore their helmets and how evenly they fit their heads. The sensors were also used to measure the pressure between the Hybrid III dummy and both a medium and large Riddell helmet. The pressure was transmitted from the head to the sensor via thin plastic shims adhered to the sensors. Each sensor was calibrated three times with dead weights and a custom signal conditioning system was fabricated on a printed circuit board while the data analysis was conducted in Matlab.
Results: Researchers analyzed the fit of the helmet by computing the average and max pressure of all the sensor data from each volunteer and collected a pressure distribution mapping of the helmet pressure. They also conducted linear impact testing on both the medium and large sized Riddell helmets on the Hybrid III dummy. The large helmet on the Hybrid III was more representative having average pressure and max pressures in the 39th and 35th percentile of the volunteer data while the medium helmet tests produced peak pressures and high levels of discomfort. This study verified the rationality of manufacturer's suggestion to use the large helmet for Hybrid III testing, however most studies use a medium helmet.
Conclusion: When the wrong size helmet is used for acceleration testing, for example the medium Riddell helmet on the Hybrid III dummy, the results are less accurate. The measurement of head acceleration in football must be accurate in order to properly report the severity of on-field collisions. The consequences of these inaccuracies result in the risk of concussions being undetected. As this topic becomes increasingly more popular, the football and concussion community need to invest in further research of concussion detection and helmet fit in order to reevaluate existing data and future studies.