## 4 Discussion  
The present study investigated the effect of changing the swing-foot strike pattern (FFS vs. RFS) on ML APAs and active braking of the centre of mass fall, and on the related ML stability and force  
Figure 6: Effects of swing-foot strike pattern on collision force parameters, vertical force braking and swing duration. Reported are mean values (all participants together) \(\pm\) 1 SE. GRF: ground reaction forces; COM: centre of mass; C7: 7 th cervical vertebrae; FC: foot contact; RFS: rearfoot strike condition; FFS: forefoot strike condition. * Indicates a significant difference between bars. collisions. Specifically, it was hypothesised that the active braking of the centre of mass fall and the ML APAs associated with gait initiation over an obstacle to be cleared are both attenuated in an FFS compared to an RFS pattern, but with no alteration of ML stability or increase in collision forces. The results discussed below are in line with these expectations and suggest the existence of an interdependent relationship between balance control mechanisms for optimal stability control. [PERSON] and colleagues first reported that, in young healthy adults, the centre of mass fall during the swing phase of gait initiation was actively braked in anticipation of the swing foot's collision with the ground [2; 14; 15; 27; 28]. This centre of mass brake, which was ascribed to an activation of the soleus of the stance leg (not recorded in the present study), aims to attenuate the vertical ground reaction forces at the time of foot contact (i.e., the collision forces). Therefore, this braking facilitates balance control following foot contact and attenuates the transmission of the collision forces throughout the entire musculoskeletal system by allowing a softer swing foot landing [6; 14; 15]. The results of the present study showed that the indicator of this centre of mass brake (the braking index) reached a significantly lower value under the FFS than under the RFS condition (the difference between the two conditions was \(\Delta=19.4\%\)), thus revealing that the fall of the centre of mass was much less braked under the FFS condition. The much larger downward-oriented centre of mass velocity at foot contact under the FFS condition (\(\Delta=74.5\%\)) can be considered a direct consequence of this reduced braking. Therefore, it could be surprising that the two indicators of the collision forces' perturbing effect (namely, peak collision forces and the slope of this peak) were both smaller under the FFS than under the RFS condition (\(\Delta=27.3\%\) and \(\Delta=26.2\%\), respectively), revealing that the foot landing was softer under the first condition. In addition, the jerk value recorded at the C7 spine level was lower under the FFS than under the RFS condition. This finding suggests that the collision forces acting at the foot were absorbed to a greater extent by the swing leg under the FFS condition, reducing its transmission through the upper part of the musculoskeletal system. In turn, these results are in line with classical studies in the literature comparing collision forces during running [16; 19; 20; 21; 22; 29; 30], or during stepping down during ongoing gait [17]. According to [PERSON] et al. [17], the FFS pattern allows the ankle plantar flexors of the swing leg to produce negative work that can absorb part of the collision forces. This negative work is absent in the RFS pattern which might explain the greater collision forces found under the RFS condition. As stressed in the introduction, it is noteworthy that centre of mass braking has not been investigated in any of these studies. Consequently, a possible modulation effect of this force braking on the collision forces cannot be discounted in these studies. In fact, the result of the present study showing that the collision forces were smaller under the FFS than under the RFS condition, despite the centre of mass braking being attenuated, further stresses the efficiency of the FFS pattern in dampening collision forces.