## 4 Discussion
### Discussion of the Motion Analysis  
As can be seen in Figure 3, the differences at earlier repetitions (i.e., between the first and second dance sequence) remained around zero, meaning that the dancers could repeat the movement with great accuracy. On the other hand, the differences between the first and tenth dance sessions were somewhat above zero (Figure 3). This suggests that the joint ROMs increased in time, indicating that the dancers' movements were less bound during the repetition of the ten dancing sessions. Moreover, the increment of the range of motion of the joint angles increasedthe risk of injury. According to [PERSON] et al., the increased values of whole-body angular momentum accompanied by unfreezing of the upper limb kinematic DOF (increased ROMs) are energetically more expensive [27]. Therefore, it may happen as dancers have to dance for a longer time and cannot pay enough attention to controlling the movements entirely, which may let their limbs move more freely, leading to an energetically more expensive increment in the joints' ROM. Furthermore, [PERSON] et al. showed that in the case of Irish dancers, after a certain amount of fatiguing session, the dancers landed with a reduced ankle plantar flexion and external hip rotation and increased hip-adduction alignment [5]. Figure 2 shows the ROM values, broken down by the dancing sessions. Taking into consideration the red horizontal line, which is the median value calculated from the ten dancing sessions, after around the fifth or sixth session, the medians of the given session's joint angle differed from the absolute mean value inconsistently, whereas in the beginning, they were closer to the absolute median value. This effect was greater when the statistical analysis showed a significant change in a higher significance level (\(\alpha=0.05\)). This inconsistent fluctuation might be caused by less concentration and tiredness as the dancers have to repeat the dancing sessions. Moreover, Figure 2 shows that the median values for the nondominant hip (NH) flexion ROM became lower than the overall median even after the fourth session. However, on the dominant side (DH), the medians became greater, although the changes were not significant. The run-down in the nondominant side of the hip flexion angle suggests that the dancers were less able to concentrate on their nondominant side as they had to repeat the dancing session, which also could be an effect of their tiredness. Note that the \(\eta^{2}\) values of the averages of the ROMs (Table 3) indicated that the effect size was small. Although the eta values were small, this less significant change may already be a sufficient indication of the tiredness of the dancers. In the case of a larger deviation in the motion parameters, the patterns of the dance could already be visibly changed. Based on the post-hoc analysis, the difference between the sixth and ninth dance sequences was significant since the differences between the sixth and ninth dance sequences were below zero (Figure 3), which means that the range of motion of the sixth dance sequence was greater than the ninth, suggesting that after the sixth dance sequence, the ROMs of most parameters had decreased. Figure 3 also indicates that this decreasing trendstagnated towards the end, since the first and tenth dance sequences' comparison showed a positive deficit. Figure 5 shows that the maximum normalized ground reaction force's impulses during cross landing were very similar for each session. The values of the maximum GRF slightly increased when dancers had to repeat the dancing, although the increase was not significant (Table 3). The increased GRF resulted in higher loads on the dancers' joints and means that as the dancers repeated the dance sequence, they could not pay attention to protecting their joints. [PERSON] and [PERSON]'s research showed that the increased ROM of ballet dancers in certain tasks could reduce the reaction forces during landing [11].