*2.1. Inertial Sensors, Synchronization and Calibration*

XSens MVN motion capture system (Xsens Technologies B.V., Enschede, The Netherlands) consisting of 17 body-wired inertial motion trackers (Figure 1) was used to record the participants' kinematical data at a frequency of 240 Hz. Each motion tracker is 36 × 24.5 × 10 mm in dimension, weighs 10 g, and is mounted at a specific body location with the help of a wearable lycra suit. The wired motion trackers are connected to an on-body data hub (known as bodypack), which is responsible for synchronization and gathering data on its internal memory (Xsens MVN Technical Report, March 2018).

**Figure 1.** (**Left**) Front and back view of the position of 17 inertial motion trackers that are attached to the Xsens bodypack worn by the skiers. (**Right**) One Xsens inertial motion tracker depicting the local *x*, *y*, and *z* axes.

Calibrations were performed by the system software, *Xsens MVN Analyze*, prior to data collection, which requires the subject's height and foot length to estimate the dimensions and proportions of the person being tracked. After subject calibration, he/she is asked to stand still in a T-pose and walk a few meters back and forth for the purpose of the sensor to segment calibration and development of a biomechanical human model. This is followed by a slight forward movement for defining the positive *X*-axis. The *Y*-axis is perpendicular to the *X*-axis in the horizontal plane while the *Z*-axis is perpendicular to the horizontal surface. Each motion tracker has 3 sensors: accelerometer, gyroscope, and magnetometer, which provide raw recorded data of linear acceleration, angular velocity, and magnetic field intensity along the *x*, *y*, and *z* axis local to each sensor, respectively.

The on-body recording function was used for data collection, which allows recording of the subject's motions without the need for a laptop or PC by storing the motion trackers' data on the body pack. After finishing the data recording, the body pack was connected with a laptop to import the recordings for further processing and analysis.
