## 2 Materials and Methods
### Experimental Protocol  
The participants were instructed to initiate gait at maximal velocity with their preferred limb while clearing an obstacle, and to continue walking at the same velocity to the end of a five-meter track (Figure 1). The task was self-triggered, that is, the participants initiated gait only when they felt ready. As in our previous study [11], the obstacle height was 10% of the subject's height and its distance from the subject was 30% of the subject's height. Two conditions of gait initiation were performed. Under the first condition, the participants were instructed to strike the ground with the heel first after stepping over the obstacle (rearfoot strike (RFS) condition). Under the second condition, they were instructed to strike the ground with the forefoot first (forefoot strike (FFS) condition). We showed in our previous study [11] that participants used both the FFS and the RFS patterns under the condition where the obstacle had the same distance/height features as in the present study. In other words, no clear preferred foot strike pattern could be detected, suggesting that participants of the present experiment were not much constrained by the instruction on the foot strike pattern.  
The order of the conditions was randomised to avoid rank effects. For each condition, the subjects were allowed two familiarisation trials, after which five trials were recorded. In the initial posture, the participants stood upright barefoot, with their feet hip-width apart, their arms hanging loosely alongside their body and their body weight evenly distributed between their legs.  
Gait was initiated on a force plate (600 \(\times\) 1200 mm, AMTI, Watertown, MA, USA) positioned at the beginning of the track. The force plate was embedded in the track and was long enough so that the participant's swing foot always landed on it at the end of gait initiation. The obstacle consisted of  
Figure 1: Schematic illustration of the experimental setup. (**A**) Gait initiation with a forefoot strike (FFS) pattern. (**B**) Gait initiation with a rearfoot strike (RFS) pattern. 1, walkway; 2, force plate; 3, obstacle; 4, reflective marker; 5, Vicon camera; 6, visual target.  
a lightweight wooden rod (length, 65 cm; diameter, 1 cm) resting on two adjustable upright stands (Figure 1). The participant's does served as the reference point for positioning the obstacle. Reflective skin markers (9 mm in diameter) were placed bilaterally at the hallux (toe markers), head of the fifth metatarsal (metatarsal markers) and posterior calcaneus (heel markers). In 10 subjects, a marker was also placed at the top spine level (C7) to investigate the transmission of the collision forces from the swing foot to the upper part of the musculoskeletal system. An additional marker was positioned at the middle of the obstacle to determine the safety distance, that is, the distance between the swing foot and the obstacle at the time of clearance. A V8i VICON eight-camera (Mcam2) motion capture system (Oxford Metrics Ltd., Oxford, UK) with a 64-channel analogue card was used to collect both the position of these markers and the force plate data at a sampling rate of 500 Hz. Kinematic and force plate data were acquired and synchronised using the Vicon acquisition software (Workstation, Version 5.1, Oxford, UK).