## 2 Materials and Methods
### Data Processing  
Data processing was applied in both previous software and in Visual3D ProfessionalTM (v.6.01.36, C-Motion, Inc., Germantown, MD, USA). Anthropometric measurements from each participant were entered into the model [[PERSON], [PERSON]]. In line with previous studies such as [PERSON] et al., a static calibration trial was conducted prior to the movement trials to define segment lengths and identify lower-limb joint centers [11].  
Data from the three stair descent and gait transition trials per participant were collected for the purpose of this study. Therefore, for data collection facilitation, the topmost step of the staircase was labeled \"Step 4\", and the subsequent steps were labeled \"Step 3\", \"Step 2\", and \"Step 1\" (see Figure 1) [11]. The stair descent data selected for further analysis was extracted from the stair cycle, and the gait transition data was extracted from the gait cycle on level ground [11]. The stair cycle was initiated at the moment of the ipsi limb heel-strike (HS) on \"Step 2\" and finished with the ipsi limb HS on level ground (see Figure 1) [30]. The gait cycle was initiated when the ipsi limb HS occurred on level ground and finished with  
Figure 2: Schematic demonstrating the ipsi limb (I—in black) and contralateral limb (CL—in gray) trajectories during the stair descent. The red arrow represents the stair cycle, starting with the ipsi limb on step 2 and terminating with the ipsi limb on level ground. The triangle on the step represents the stance phase, and the dashed line represents the swing phase. Stair descent data selected for further analysis were extracted from the stair-cycle and the gait-transition data from the gait cycle on level ground.  
the subsequent ipsi limb HS [31]. According to [PERSON] et al., the moment of toe-off (TO) was defined by the maximum heel marker's vertical velocity, and the moment of HS was defined by the minimum downwards CoM velocity [32].  
The extracted spatiotemporal parameters included task velocity (m/s) and time (s), single- and double-support time (s) [11], and peak downward CoM velocity (m/s) [33]. In the stair cycle, lower-limb kinematics were characterized by FC and FP; and hip, knee, and ankle angles at the moment of FC and FP [14]. In the gait cycle, the lower-limb kinematics were characterized by the ipsi and contralateral hip, knee, and ankle angles at HS and TO [34]. A low-pass filter from the Butterworth class with a cut-off frequency of 6.0 Hz was used on the joint angles' variables.  
The reference frame was set as a positive \(z\)-axis directed vertically upward, a positive \(y\)-axis directed in the direction of motion, and a positive \(x\)-axis directed from the left to the right (x: flexion/extension; y: abduction/adduction; z: longitudinal rotation) [11]. The segmental CoM location was obtained from the body-segment-parameter data, and the position of the whole-body CoM was estimated as the weighted sum of various body segments [35]. According to [PERSON] et al., FC was established as the minimum resultant horizontal distance between the step-edge and the heel at the instant the heel passed the vertical height of the step-edge [16]. For the ipsi limb, FC data were extracted from the instant the heel passed the vertical height of Step 1. For the contralateral limb, FC data were extracted from the instant the heel passed the vertical height of Step 2. FP was measured as the horizontal distance between the step-edge and the calcaneus at the instant of the initial foot contact [16]. For the ipsi limb, FP data were extracted from the instant of the initial foot contact on Step 2. For the contralateral limb, FP data were extracted from the initial foot contact on Step 1.