Traumatic Brain Injury (TBI) contributes to a major number of deaths and cases of permanent disability each year. Falls are the leading cause of TBI with the highest rates for children 0–4 years old and for adults age 75 and older. Accordingly, there is a significant interest in fall-related injury mechanism and head impact. Since the dynamics of human fall and head injury mechanisms are highly variable due to the inherent and complex nature of human falling, the aim of the present study is to describe the dynamics of backward falls and risk of injury due to head impact. In order to have a better understanding of head impact, A HYBRID III 5th Percentile Female test (Denton ATD, Inc.) instrumented with a tri-axial accelerometer with measuring range of ±500g at the center of gravity of the head was dropped from standing posture by using a controlled release mechanism. The dynamic model of fall was captured using a T-series Vicon motion capture system synchronized with a force plate to measure the impact force and a tri-axial accelerometer to measure the impact acceleration of the head. The acceleration impact data measured at 20 KHz and the motion capture system was capable to retrieve 500 samples per second. The primary objective of this study was to determine the equivalent mass involved during head impact due to a backward fall. This effective mass is a key quantity to design the head impact experimental setups, protection devices and computer simulations of head impact. Based on the force and acceleration measurements in several tests, the head impact effective mass is approximately found to be the mass of head itself plus 48% the neck mass. Two scenarios of backward fall were studied and discussed. First, falling while the hip joints are involved and the trunk moves forward and second, falling while the hip joints act like a fixed joint. For the first scenario the impact forces and accelerations peak measured using the HYBRID III were found to be 10±1.8KN and 255±42g, respectively, and for the second scenario the larger impact forces, 14.5±0.9KN, and acceleration peaks, 364±27g, were measured in all tests.

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