## 4 Discussion
### Correlation between the Indices of Estimated Muscle Torque and the Intensity of LBP in STS  
In addition to Table 4, scatter diagrams of the NRS score with the Max-MT and DMT-MP-stand are shown in Figure 10. The significant positive correlation between Max-MT and LBP intensity implies that higher values of maximum ES muscle torque during STS motion could indicate a higher correlation with LBP. Moreover, a significant positive correlation between DMT-MP-stand and LBP intensity was also observed. Thus, a larger value of maximum ES muscle torque in relation to the standing up motion might be related to more severe LBP symptoms.  
Relationships among the Motion Indices from an IMU, Estimated Muscle Torque, and the Intensity of LBP in STS  
Our previous research revealed that the STS motion particular to pregnant women with LBP can be observed using an IMU attached to the lower torso. For example, a higher maximum angular velocity peak in the sagittal plane was observed in the LBP group. However, other risk factors, such as muscle fatigue, were not considered in that research. In the current study, however, the relationships  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Evaluation Indices of Muscle Torque** & \(\mathbf{r}\)1  & \(\mathbf{p}\)-Value \\ \hline
**Max-MT 2** & **0.743** & **0.022** \\
**DMT-Max-stand 3** & **0.726** & **0.027** \\ MT-MP 4 & 0.143 & 0.382 \\ DMT-MP-stand 5 & 0.367 & 0.331 \\ RMS-MT 6 & 0.376 & 0.319 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Correlation analysis results for muscle torque and low back pain (\(\mathbf{n}\) = 9).  
among the motion index, muscle torque, and LBP during STS motion were investigated for pregnant women using motion, force, and EMG data, respectively. The results in Section 3.2 reveal a statistically significant positive correlation between PP and ES muscle torque (out of muscle torque in the standing position) during STS motion. Simultaneously, the results in Section 3.3 reveal a statistically significant positive correlation between the maximum muscle torque and the intensity of LBP. These results suggest that the PP of angular velocity in the sagittal plane, obtained from an IMU attached to the lower trunk, may have expressed not only the trunk motion characteristics, but also the ES muscle torque exhibited during STS motion for pregnant women. Mechanical information, such as muscle torque estimated by the proposed model, is more useful for quantifying the physical burden related to LBP. In addition, the results reveal the relationship between higher ES muscle torque and LBP severity during STS motion in pregnant women. Thus, this study indicates the potential for conducting pregnancy motion analysis and muscle torque evaluation, which may be associated with LBP, using the proposed IMU-based method, without the need for EMG measurement.  
This is the first study that has focused on the biomechanics of pregnant women and investigated the relationships between ES muscle torque during motion and LBP by motion analysis. However, there are various factors that should be considered when investigating the relationships between biomechanics and LBP during pregnancy. For example, the liiopsoas muscle has a relevant role in STS movement and is related to LBP. Moreover, this muscle might be stretched as well as the rectus abdominis during pregnancy. Similarly, other muscles such as the diaphragm and other abdominal wall muscles might be related to LBP [33; 34]. Moreover, other studies have indicated that there is a relationship between myofascal trigger points in the paraspinal muscles and LBP [35]. Therefore, further study would be required to investigate the existence of an extensive and detailed biomechanical factor related to LBP during pregnancy.