Biomechanics of Household Short Falls in Infants and Children

John  D. Lloyd, Ph.D., M.Erg.S., CPE, CBIS
Board Certified Ergonomist & Certified Brain Injury Specialist

This study involved systematic assessments of falls in a supine (face up) position from heights ranging from 2 to 6 ft in 2ft increments onto varying flooring surfaces including steel/concrete, linoleum, apartment grade carpeting with underlay, berber carpet with underlay, commercial carpeting without pad, and wood laminate.

A CRABI-12 biofidelic mannequin (29.5 in / 22lb), calibrated and certified by Denton ATD, Inc. and a Hybrid III-3 year old (37.2in / 35.65lb) biofidelic mannequin, calibrated and certified by Key Safety Systems, Inc. were used during this systematic evaluation of short falls.

A tri-axial piezo-electric accelerometer (model # PCB 356A02, serial # 37400), calibrated and certified by PCB Piezotronics, was installed at the center of mass of the headforms, in accordance with convention described in SAE J211. A force plate was utilized, measuring 40” x 30”, the top plate of which is 3/8” flat steel and is relatively indeformable. Inset equidistant from the corners of the force plate are installed calibrated and certified four piezo-electric uniaxial force sensors (model # PCB 208C05, serial numbers – 30982, 31041, 31042 & 31043).

Still photography and high-speed video (240Hz) was used to record the fall sequences.

A height adjustable platform was used to represent the fall surface (Figure 1) The platform has trap-doors which are held in place by electromagnets. Interruption of power to the electromagnets causes the sprung trapdoors to open instantaneously, thereby initiating the fall sequence.

Figure 1 –Biofidelic mannequin prepared for supine fall onto carpet-covered forceplate

Data from the 3 channels of the tri-axial piezo-electric accelerometer, mounted in the head of the biofidelic mannequin and the four channels of the uni-axial piezo-electric force cells in the force plate were each acquired at a rate of 10,000 samples per second using LabView software (National Instruments: Austin, TX). Raw data were displayed on-screen for visual verification and recorded to a unique file.

Data were analyzed using MatLab, version 2008a (The Mathworks, Natick, MA). Fast Fourier Transform analysis was performed on the analog data to visualize the frequency spectrum of the data, followed by phase-less filtering using a forward/backward 4th order low-pass Butterworth filter with a cut off frequency of 1000Hz (per SAE J211).

The RMS (Root-Mean-Square) value of head linear acceleration components was calculated and presented in G's (where 1G equals force experienced due to gravity). This value was then used to compute Head Injury Criterion values, according to equation 1 below. Impact force was also summed and presented in Newtons (N).

One-hundred-and-seventy-five trials were completed to investigate biomechanical mechanisms of injury associated with short falls in children. These supine falls range in height from 2 to 6 ft in 2ft increments onto varying flooring surfaces including steel/concrete, linoleum, apartment grade carpeting with underlay, berber carpet with underlay, commercial carpeting without pad, and wood laminate. A summary of results is presented in Tables 1 and 2, below.

Table 1: Head Kinematics and Kinetics Associated with Infant Short Falls

Table 2: Head Kinematics and Kinetics Associated with Toddler Short Falls

As anticipated, the larger Hybrid III 3-year old biofidelic mannequin generated higher linear accelerations, HIC values and forces upon impact associated with short falls. Interestingly, both the CRABI12 infant-representative and Hybrid III toddler representative exceeded injury threshold values from a fall height of only 2 feet (61 cm), based on RMS linear acceleration and Head Injury Criterion, which indicates that such short falls can cause substantial head / brain injuries in young children.