## 4 Discussion
### Respiratory Response  
Quiet breathing yields nearly constant frequency and pattern, or when metabolic demands increase, a rise in these parameters is expected [62]. The movements of both RC and ABD walls are mediated by the respiratory muscles: the diaphragm (inspiration), the intercostal muscles (inspiration and expiration), abdominal muscles (mainly expiration), and accessory muscles of respiration [63]. When the movements of these muscles are not matched in time, it may indicate increased work of breathing [64], respiratory muscle fatigue [19; 65], or even neuromuscular disease [38; 66].  
The assembly line processes depend on various arm movements, as can be seen in Figure 1. Considering that solely through arm elevation, metabolic and ventilatory demands have been demonstrated to rise [67], it was expected that these tasks would increase workers' respiratory strain. Moreover, these movements rely on muscles of the rib cage that stabilize arm position and posture [68; 69], diminishing their contribution to respiration, leading the diaphragm and ABD muscles to compensate to keep up with physiological requirements. The previous research on diaphragm activity demonstrated that it contracts not just during respiration but also when fast repetitive changes in posture and limb position occur, to raise ventilatory pressures [70].  
As can be seen in Figure 7, at H1, decreased phase synchrony between thoracic and abdominal movements suggest respiratory distress, likely associated with the movements performed near to the upper body demanded by its tasks. H2 shows similar distress, with a marked increase in abdominal motion variation and reduced respiratory wall coordination, indicating a mismatch in muscle effort during respiration, likely due to the bending and weight application required by these tasks. The drop in coordination metrics of the respiratory walls of tasks involving arm movements was also found in [PERSON] et al. for some of the studied subjects. For these, exercise tolerance decreased drastically [69]. In [PERSON] et al., the subjects were put through a fatigue-inducing protocol and also exhibited respiratory wall asynchrony [19].  
Unlike the other workstations, the subjects at the H3 maintained the coordination between their RC and ABD respiratory movements. Thus, it indicates that these tasks do not cause as much breathing distress as the ones in the other workstations.  
The respiratory response by ER rank showed that the moderate risk results point to a rise in the ABD exertion with a decline in RC contribution. Furthermore, the decrease in both measures of thoraco-abdominal coordination indicates a growth in breathing effort from the first to the last 10 min of the recordings.  
The results also point to the low-risk activities having a lower effort of breathing when compared to the other ranks. It appears that the tasks of the moderate-high-risk workstations have more controlled respiration, even though they take a larger toll on the cardiac system. Here it is seen that the breathing load is not compatible with the attributed risk rank. It also gives us another relevant characteristic to discriminate these two ranks.