## 2 Materials and Methods
### Dependant Variables  
Initial posture, APAs and swing foot lift. Gait initiation was divided into three phases, namely APAs (from t0 to swing heel-off), swing foot lift (from heel-off to toe-off) and swing phase (from swing toe-off to swing foot contact) (Figure 2). The ML and AP centre of mass position in the initial upright posture was estimated by averaging the centre of pressure position during a 250 ms period, lasting from t0 minus 1500 ms to t0 minus 1250 ms. No dynamic phenomena were detected during this period. APAs amplitude was characterised by the peaks of the backward and ML centre of pressure shift obtained during the APAs time window. The duration of APAs along the ML and AP axes was computed separately, as the t\({}_{0}\) times for these two axes did not necessarily occur simultaneously as stated above. The centre of mass velocity and displacement along the ML and AP axes were determined at swing toe-off and foot contact. Figure 2: Typical biomechanical traces and representation of the main experimental variables (horizontal plane). Traces are from one representative subject initiating gait in the (**left**) rearfoot strike condition and the (**right**) forefoot strike (FFS) condition. Anteriorposterior (AP) direction. xM\({}_{\text{TO}}\), xM\({}_{\text{FC}}\): centre of mass (COM) velocity at toe-off and foot contact; xPmax: peak of centre of pressure (COP) displacement during anticipatory postural adjustments (APAs); F: forward; B: backward. Mediolateral (ML) direction. yM\({}_{\text{TO}}\). y\({}_{\text{TO}}\). y\({}_{\text{TO}}\): COM velocity at toe-off and foot contact; yM\({}_{\text{TO}}\), y\({}_{\text{TO}}\): COM displacement at toe-off and foot contact; yPmax: peak of COP displacement during APAs; ST: stance limb; SW: swing limb. Vertical dashed lines: t\({}_{0}\) onset variation of biomechanical traces; HO: swing heel-off; TO: swing toe-off; FC: swing foot contact. Horizontal arrows. APAs: anticipatory postural adjustments; FL: swing foot lift; SWING: swing phase of gait initiation. Motor performance. Motor performance was quantified with the AP centre of mass velocity at foot contact, step length and swing phase duration. Step length corresponded to the distance covered by the heel marker of the swing leg from the initial posture to foot contact. Safety distance. The vertical distance between the obstacle and the marker of the swing/rear heel, and between the obstacle and the marker of the swing/rear toe was measured at the instant these markers passed over the obstacle. The smaller of these two vertical distances corresponded to the safety distance. ML dynamic stability. ML dynamic stability at foot contact (hereafter called ML stability) was quantified by the ML boundary of the base of support (\(\text{BOS}_{\text{ymax}}\)) and an adaptation of the margin of stability (MOS) [25]. The \(\text{BOS}_{y}\) was measured as the distance at foot contact between the marker on the fifth metatarsal of the stance foot and the heel marker of the swing foot in the case of a rearfoot strike, or the marker on the fifth metatarsal of the swing foot in the case of a forefoot strike. The MOS corresponded to the difference between \(\text{BOS}_{\text{ymax}}\) and the ML position of the extrapolated centre of mass at swing foot contact (\(YcoM_{FC}\)), that is, \(\text{MOS}=\text{BOS}_{\text{ymax}}-YcoM_{FC}\). Based on the study of [PERSON] et al. [25], the ML position of the extrapolated centre of mass at foot contact (\(YcoM_{FC}\)) was calculated as follows:  
\[Y_{CO}M_{FC}=\ yM_{FC}+\ \frac{y^{\prime}M_{FC}}{\omega_{0}}\, \tag{3}\]  
where \(yM_{FC}\) and \(y^{\prime}M_{FC}\) are, respectively, the ML centre of mass position and velocity at foot contact, and \(\omega_{0}\) is the eigenfrequency of the body modelled as an inverted pendulum calculated as \(\omega_{0}\ =\ \sqrt{\frac{g}{l}}\), where \(g=9.81\) m/s\({}^{2}\) is gravitational acceleration and \(l\) is the length of the inverted pendulum, which in this study was estimated to \(57.5\%\) of body height [26].