## 5 Study Results and Discussion
### Human Chest Motion Features  
Besides the head, it is the chest, and in particular the spine, that is the important body part to protect. The study of the spine's behaviour during the different situations evaluated in this study can be performed through the analysis of vertical (YY) and horizontal (XX) accelerations. The global axes XX and YY remain fixed throughout the analyses. For the spine, the upwards vertical accelerations (Figure 13a) are significant at the beginning of each analysis, reaching their highest absolute values when the damper tends to lead the velocity in this direction to zero, which happens before the first tenth of a second. Once these maximums are passed, with the impact on the bumpers, a second set of maximums appears, with an opposite sign and with absolute values greater than the first ones, in the case of tilted falls (situations 2 and 3). This behaviour arises from the impacts of the body against the chest cushions, which is not the case for the vertical fall (situation 1). Thereafter, the vertical accelerations are reduced, with some episodes caused by the collisions with the chest and back cushions or with the return of the body to the seat of the chair. Directly before the half-second, a new set of peaks of ascending vertical accelerations appears, now corresponding to the second cycle of vertical movement and in the downwards phase, which presents lower absolute values when compared with the first set of peaks. The horizontal accelerations present high values in the first half (1/2) tenth of a second of the analyses, when the vertical velocities are higher, induced by the collisions of the chest against the cushions that surround it. With the reduction of vertical velocity, these impacts become less intense, and consequently, the values of horizontal accelerations are reduced. For tilted falls (situations 1 and 2), the moment of the impact of the columns of the chair against the bumpers is critical, since the subsequent impact of the chest against its cushions returns the highest absolute values of horizontal acceleration in the spine, which are higher than the peak vertical accelerations. Thereafter, the horizontal accelerations repeat a pattern similar to that of vertical accelerations. The comparison performed between the average accelerations of the human body, thus considered as a whole concentrated mass, and the limits of the \"Eiband curves\" are good indicators for human tolerance to the impacts that the body suffers during the building collapse, although the movement of the human body, particularly the chest, has important components of relative translation and rotation between vertebrae (Figure 7) that are not considered in these curves. Thus, these conditions do not allow for accurate assessment of whether the compressions in the spine exceed their tolerated limit, one of the main concerns of this study. For this reason, the human body model was also used to individually evaluate the axial and shear forces installed in the spine, as well as in the rib cage/sternum and in the abdomen (the latter in absolute value). The tilted fall of the \"upper\" chair (situation 2) is the situation that returns the greatest compression forces on the spine (Figure 14a), and it is also clear that the lower vertebrae are the most affected (all lumbar, L1 to L5 and some of the thoracic, T6 to T12), which happens for the three analysed situations. It is important to stress that the limit established for the compressive strength of the vertebrae (7 kN) is never exceeded. The upper thoracic vertebrae (T1 to T5) turn out to be the least compressed, not only because the chest/sternum supports part of the axial load, but also because the rotation of the body to the fetal position (bracing for impact) compresses the thoracic cage more (in particular the sternum) and relieves the compression of the spine. In this movement, the rib cage is subjected to a force lower than 2 kN (Figure 14c).