## 2 Materials and Methods
### Motion Capture and Data Processing  
During the takeoff phase, 3D motion was captured, while 2D motion during the flight phase was recorded using three Z Cam E2 cameras (Shenzhen, China) positioned on both sides of the jump platform and at the referee stand (Figure 1A). Cameras were set to record at 50 Hz with a resolution of 1920 \(\times\) 1080 and an exposure time of 1/800 s. Synchronization was achieved using sync cables. The jump platform was scanned using a terrestrial laser scanner (FARO Focus S150, Lake Mary, FL, USA), and point cloud data were processed with CloudCompare software (v2.12.4). Calibration was carried out with 28 existing platform markers (calibration volume: 9.4 \(\times\) 7.9 \(\times\) 4.2 m, Figure 1B). The known point on the top platform served as the origin of the coordinate system, with the \(x\)-axis pointing horizontally backward, the \(y\)-axis horizontally to the right, and the \(z\)-axis vertically upward (Figure 1A). A three-dimensional motion analysis software (FastMove 3D Motion, v1.1) was used to track 21 markers [20] placed on the athlete's body during the takeoff phase. The 3D coordinates of these markers were derived using the direct linear transformation (DLT) method. Calibration accuracy showed small errors in reconstructing control point coordinates compared to known values (3.2, 3.7, \(-\)1.8 m) of the calibration frame, with mean square errors of 0.519% for X, 1.48% for Y, and 0.177% for Z coordinates. Identified marker trajectories were further smoothed using a Butterworth low-pass filter with a cutoff frequency of 6 Hz [21]. The takeoff phase was defined as beginning when a change was detected in the horizontal flexion/extension angle of the shoulder joint, signifying the initiation of the upper limb swing, and ending when the tail of the snowboard left the edge of the platform; the flight phase started at the moment the snowboard's tail left the platform and concluded upon contact with the landing slope. Performance metrics were calculated as follows: takeoff time was defined as the interval from the start of the takeoff movement to the moment of platform departure; flight time was measured from takeoff to landing; flight height was identified as the maximum z-coordinate of the center of mass (COM) during the  
Figure 1: Camera deployment and lidar-based SBA platform reconstruction. (**A**) Diagram of camera positions and perspectives of each camera. (**B**) Reconstructed SBA platform based on lidar point cloud technology. flight phase; departure combined velocity was calculated as the total velocity of the COM at the end of takeoff; departure anteroposterior velocity referred to the horizontal component of the resultant velocity (in the forward-backward direction) at the end of takeoff; departure vertical velocity was the vertical component of the resultant velocity at the end of takeoff; and departure angle was defined as the arctangent of the ratio between the vertical and anteroposterior velocities at takeoff completion. The method for constructing body segments and joint angles in this study is consistent with previous research [22; 23; 24]. Anatomical coordinate systems for the upper arms, upper trunk, lower trunk, and thighs were established using unit vectors derived from key anatomical landmarks (shoulder, elbow, wrist, hip, knee, and ankle), with orthogonal axes determined via vector cross products.