## 2 Materials and Methods  
### Impactor Test Setup  
An instrumented rigid headform with appropriate geometry was developed to support the multibody modelling portion of this study. This custom-manufactured headform comprises a solid aluminium dome with a radius of 0.0825 m and integrated accelerometer mounting points at its base. This dome is connected to a 0.32 m long aluminium cylinder with a wall thickness of 0.02 m. The cylinder serves two primary functions: it stabilises the headform during the acceleration phase with the assistance of small plastic fins and it adjusts the centre of gravity towards the flat side of the dome, positioning it closer to the accelerometer mounting locations to ensure accurate measurements. The overall mass of the headform is 10 kg, which enables the headform to effectively simulate impacts involving body regions with similar geometries, such as the shoulder. Based on a previous NYC Subway epidemiology study, the most common impact velocity is 12 m/s [7]. To accelerate the headform to the desired velocities, a HYGE Dynatest 500 Component Crash Simulation System was used. This system utilises differential gas pressure acting upon two surfaces of a thrust piston within a sealed cylinder.  
To identify the suitable foam type for the proposed impact tests, three distinct commercial foam samples were subjected to compression testing with an Instron 3366 up to 80% strain. The examined foam types included firm foam (40 kg/m\({}^{3}\)), a mixed firm foam with a memory-foam topper (50 kg/m\({}^{3}\)), and a mixed bound foam (35 kg/m\({}^{3}\)) with a memory-foam topper. The bound foam exhibited the highest stiffness, making it the most suitable candidate for the intended application. For testing without a hard shell, the foam was held within a rigid mounting box. A total of six hard shells were created using 1 mm aluminium and 3 mm ABS (Figure 2). The material was ordered in sheets, then bent into shape using rolling pins and a heat strip. The side panels of the mounting box were removed when using the hard shells to facilitate lateral flexion of the shell.  
Figure 3 outlines the basic instrumentation schematic for the headform impact tests. A Fastec 5G high-speed camera was mounted on the ceiling, facing the top surface of the energy-absorbing countermeasure, and was used to track the position of the headform. The camera's frame rate was 2500 fps, with the foam illuminated by two spotlights. A MEMS piezoresistive PCB 726 ch accelerometer was positioned inside the headform, with its sensitive axis oriented in the direction of motion. The accelerometer's sample rate (51.2 kHz) was regulated by a 24-bit ADC PCB9234 data acquisition device. The accelerometer was powered by a battery pack housed within a Faraday cage, as the PCB9234 does not provide excitation voltage to piezoresistive accelerometers. Both the camera and accelerometer were activated via an RS PRO Retroreflective Photoelectric Sensor positioned as close as possible to the headform within the Dynatest mount. This arrangement ensured a synchronous start time for both the accelerometer and video footage for each test. The software utilised in this study comprised LABVIEW, Matlab 2024a, and MADYMO, with the raw accelerometer data passing through a 150 Hz Butterworth lowpass filter [17].  
Figure 3: Headform impact test schematic.  
Figure 2: Foam (**Left**), 3 mm ABS-shelled foam (**Middle**), and 1 mm aluminium-shelled foam (**Right**).