## 2 Methods
### Testing Procedure
All the helmets were tested with a 575-size magnesium alloy full headform (Model: 100_04_FMH, Cadex Inc., Saint-Jean-sur-Richelieu, QC, Canada). To vary the value of the friction between the interior of the helmet and the surface of the headform, half of the helmets per test condition were tested with the original metallic surface of the headform (bare headform) and half of them were tested with the headform covered with a uniform thin layer of high-performance silicone rubber (Model: Dragon Skin 10, Smooth-On, Inc., Macungie, PA, USA). A total of 40 g of silicone rubber was uniformly spread in order to meet the thickness requirement of the FRHPhe-01 standard [27]. A spring balance method was used to provide an indication of the friction associated to the coating treatment. The average coefficient of friction (COF) measured between a woven cotton fabric and the bare headform and the covered headform were 0.20 and 0.78, respectively. The headform was positioned inside the helmets using a helmet positioning index (HPI) of 40 mm. The retention system was adjusted under the chin of the headform and tightened to a tension of 75 N [27]. Before each impact, the helmet was re-positioned and the retention system re-tensioned. Three new helmet samples were used per each tangential velocity and headform surface condition. Each helmet was tested on front (Y-rot), left side (X-rot) and right side (Z-rot) (see Figure 1 for the position and orientation of the helmet coordinate system). The first impact was a frontal impact, leading to rotation in the sagittal plane around the \(Y\)-axis. The second impact was a parietal impact on the left side, leading to rotation in the frontal plane around the \(X\)-axis. In these two impacts, the central vertical axis (\(Z\)-axis) of the headform was aligned to the vertical. The third impact was a temporal impact on the right side, leading to rotation in the transverse plane around the \(Z\)-axis. For this oblique impact, the sagittal plane of the headform was positioned parallel to the impact surface of the anvil and the transverse plane of the headform was coincident with the vertical plane of symmetry of the anvil. Figure 1 shows the three oblique impact directions. A wireless system (Model: iCONO, +D, Pozuelo de Alarcon, Spain) was used to measure linear acceleration and angular velocity at the centre of gravity of the headform. The wireless system incorporates three linear accelerometers (Model: 64C-2000, MEAS, Nanshan District Shenzhen, China), three angular rate sensors (Model: ARS PRO-8k, DTS, Seal Beach, CA, USA) and an acquisition system (Model: SLICE NANO, DTS, Seal Beach, CA, USA). Data were recorded at 10 kHz. Head linear acceleration signals were filtered using a low-pass filter CFC-1000 and angular velocities signals were filtered using a CFC-180. High-speed video was captured at 1000 Hz (Model: Eosens mini, Mikrotron, Unterschleissheim, Germany). The end of the impact was estimated when the measured acceleration was lower than 5 g, which correlated well with the instant in which the helmeted headform separated from the anvil. Data post-processing was performed using an in-house developed and validated script of Matlab (Matlab R2013b, MathWorks, Natick, MA, USA).