## 2 Materials and Methods
### Procedures
#### 2.4.1 Data Processing  
Marker trajectories were labelled using Qualisys Track Manager (version 2021.2, Qualisys AB, Sweden). Trajectory deviations or interruptions were interpolated using the software's built-in polynomial algorithms. All marker trajectories were then low-pass-filtered with a 6 Hz bidirectional filter. Similarly, the force platform's CoP data were filtered using a 10 Hz low-pass bidirectional filter [67; 71]. The filtered marker trajectories were imported into Visual3D Professional (version 6.01.36, C-Motion, Germantown, MD, USA), where a six degrees-of-freedom biomechanical model was constructed based on the marker set and manufacturer recommendations (Figure 1) [73]. To identify GI onset (T0) and the surrounding time window (A: \(-\)600 ms to B: +50 ms), the CoP velocity was calculated as the first derivative of the CoP position [15]. A threshold was defined as the mean plus three standard deviations of the first 10 s of the recording. The T0 event was identified when this threshold was crossed in the AP (\(\gamma\)-axis) or ML direction (\(X\)-axis), provided the crossing persisted for at least 250 ms [74]. Visual inspection was performed to validate and correct the automatic detection of T0. Events A and B were defined relative to T0, with event A occurring 600 ms before T0 and event B being marked 50 ms after T0. To account for the signal shift introduced by the derivative calculation and threshold placement, the event was adjusted backward by 3 frames [15; 74]. Figure 2 illustrates the CoP velocity in both the AP and ML directions, highlighting the selected events and their temporal alignment with the signals. Figure 1: Three-dimensional representation of the full-body biomechanical model constructed in Visual3D software (Version version 6.01.36). The global coordinate system is depicted, along with the tandem force platforms and the ground reaction force vector during stance position. Figure 2: CoP velocity in the ML and AP direction. The red dashed lines represent the selected events: event A (\(-\)600 ms), T0 (black solid line), and event B (+50 ms). These time points define the analysis window used for extracting joint angles and CoP variables. The linear kinetic variables were characterized by measures of magnitude and variation in displacements, including total displacement (TDISP), mean displacement (MDISP), position (POS), amplitude (AMP), and mean velocity of the CoP (MVEL) in the mediolateral (ML-CoP) and anteroposterior (AP-CoP) directions. The non-linear kinetic variables were characterized by position (POS) and displacement (DISP). For the kinematic analysis, joint angles of the LL, UL, and axial skeleton were calculated based on the angles formed between the distal and proximal segments. The extracted linear variables included joint position (POS), total displacement (TDISP), mean displacement (MDISP), and amplitude (AMPL). The non-linear variables were position (POS) and displacement (DISP). Joint moments were normalized to the subject's body mass [17]. The kinematic variables analyzed encompassed the angles of the head, shoulder, elbow, wrist, upper trunk, lower trunk, pelvis, hip, knee, and ankle across the sagittal (X), frontal (Y), and transverse (Z) planes. Joint angle values were interpreted according to the following conventions: positive values (+) correspond to specific movements such as extension, adduction, or internal rotation, while negative values (-) indicate opposing movements like flexion, abduction, or external rotation. Detailed descriptions for each joint and axis are provided in the Supplementary Materials (Table S2). The outcomes were used to describe the feedforward phase of GI for the swing limb (lower and upper limbs on the same side as the first step) and the stance limb (lower and upper limbs on the contralateral side of the first step). In addition to linear analyses, two non-linear measures were calculated for the CoP POS, DISP, and VEL and for the joints POS and MDISP to evaluate the behavioural characteristics of the feedforward phase of GI.