## 4 Discussion
### Trunk Yaw Is Not Valid to Predict Walking Turns as It Tracks the Actual Trajectory of the Subject  
In our study we did not find quantitative data regarding the synchronization between Trunk Yaw and Trajectory Heading. In any case, considering the analyzed literature, we can consider that both evolve synchronously, and Trunk Yaw is thus not valid to predict walking turns. Papers [19; 20] reported no significant difference between Trunk Yaw and Trajectory Heading. This was additionally supported in [29], where the authors reported a short delay (\(-\)26.52 \(\pm\) 73.45 ms) between Trunk Yaw and pelvis rotation trajectory that was not significantly different from zero. Results in [10] were also consistent with this statement, showing that the turn can be detected from the upper trunk with a delay of 69 ms and from the lower trunk with an anticipation of 5 ms.  
An apparently contradictory result was reported in [17; 31], where Trunk Yaw was found to predict the geographical corner pivot from 0.9 m to 1.14 m. This result can lead to think that Trunk Yaw anticipates walking turns. However, they may be explained by different turn negotiation strategies or simply by the experimental conditions. For instance, in some experiments described in [31], subjects had to walk along a trajectory painted on the floor that actually started to bend in the turning direction before the corner pivot, using concentric circles (0.5 m of radius) around it. Therefore, users had to modify the walking trajectory before the corner pivot, which could justify the anticipated detection Trunk Yaw onset. Our analysis also showed that Trunk Yaw onset was not significantly different from Medio-Lateral COM Displacement onset, further confirming the statement, as Medio-Lateral COM Displacement is also an indicator of the change in the trajectory of the subject. Head Yaw Is a Reliable Choice to Consistently Anticipate Heading Direction in Real Time by Around 200 ms  
It is well known in neuroscience that the head anticipates motion direction to provide a stable reference frame that helps to coordinate the motion of the other body segments (see for instance [12; 28]). As expected, results from the addressed studies are consistent with this behavior: changes in Head Yaw precede changes in other upper body segments and in the trajectory orientation. This was even found under different experimental conditions (eyes open/closed or walking forwards/backwards [22]). Discrepancies to this general finding have been occasionally reported. However, they can be attributed to the particularities of the addressed experiments. In [17], Trunk Yaw was delayed 600 ms from Head Yaw. However, in this study, the experiments were designed to study the effect on locomotor coordination when pushing a wheelchair and this may perhaps have biased the results. In [16], Head Yaw was found to be synchronized with Trunk Yaw, however, in this work, subjects were involved in a very particular steering maneuver (obstacle avoiding). Initial studies found an anticipation of about 440 ms from Head Yaw to Trunk Yaw [21]. However, results of [23; 28; 29] advance that this anticipation is expected to be lower. An interesting remark is that results of [29] agree with those of [23; 28], even though they were obtained under a completely different experimental setup, involving a greater diversity of turns. Our aggregated analysis revealed a significant anticipation of Head Yaw to Trunk Yaw of 149 ms, on average. This anticipation increases to 271 ms if we consider only preplanned turns. The results of these studies were also consistent with others obtained from different experimental setups using body-worn inertial sensors [10], where head onset anticipated the upper back yaw onset by 273 ms. A recent work [32], also based on inertial sensors, reported a greater anticipation (528 ms on average from the head yaw to the pelvis yaw).