## 4 Discussion  
In this study, we demonstrated that, with the use of the BA, the patients who required some assistance to get into or out of the bathtub could perform bathing activities by themselves with monitoring by the caregivers. Moreover, we found that use of BA resulted in a significant reduction in the activity of all measured muscles in the lumbar, trunk, hip, and upper limb regions in caregivers when assisting the patients out of the bathtub. More specifically, for bathing assistance without BA, the caregivers had to lift the patient while maintaining a forward leaning posture beside the bathtub. The quadratus lumborum and erector spinae muscles have roles in trunk extension and maintaining a forward leaning posture. The biceps brachii muscle is also activated when lifting heavy objects with the upper limbs, as in this experiment. However, when providing assistance with BA, the caregiver was able to raise and lower the seat by operating the lever in the BA control box. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Caregiver ID** & **1** & **2** & **3** & **4** & **Median** \\ \hline Physical burden & Control & 5 & 5 & 4 & 3 & 4.5 \\ (min: 1, max: 7) & BA & 2 & 1 & 1 & 1 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Results of the questionnaire on the physical burden on caregivers. Figure 3: Comparison in muscle activity of left and right quadratus lumborum (**A**), erector spinae (**B**), gluteus maximus (**C**), and biceps brachii (**D**) when assisting a patient out of the bathtub without or with BA. The symbols (triangles and circles) filled in blueish colors in each graph represented EMG data obtained from the caregivers caring for patients with moderate (ID1) and mild assistance (ID2-ID4), respectively. The red square represented the averaged EMG data in each muscle. In most cases, strenuous physical assistance was no longer necessary, and the task could be completed by simply monitoring the patient sitting on the seat of BA. Thus, there was a significant change in the method of care when using BA, from assistance to monitoring, at least when assisting the patient to get into and out of the bathtub. The results were also confirmed by the questionnaire that all of the caregivers answered, showing physical burden was decreased with BA. This accords with our results showing a substantial decrease in the activity of the recorded muscles when using BA. On the other hand, it was shown that the activity level of the quadratus limborum in one caregiver exceeded 200% MVC without BA (Figure 3A), suggesting that excessive effort was involved. Excessive stress or impairment of the lumbar muscles, such as the quadratus lumborum and multifidus muscles, is associated with an increased risk of lower back pain in younger adults [27, 28]. Therefore, our findings suggest that the long-term use of BA can encourage more independent bathing activities in care recipients as well as lead to a reduction in the risk of lower back pain in caregivers, resulting in safer and more secure care. Although the results are preliminary, we have also measured the heart rate of caregivers while they are performing various types of care tasks such as transfer, toileting, bathing, and meal assistance at nursing facilities. Notably, we found that heart rate increased significantly during bathing assistance (data not shown). Since there is currently no objective, reliable indicator for estimating effort and exhaustion in caregivers, it will be necessary to accumulate evidence from various perspectives using multiple indicators related to physical burden to assess this issue. Our results also indicate that for the patient who required a greater amount of assistance, there was a clear decrease in caregiver muscle activity during the task (Figure 3). This may suggest that caregivers assisting patients who require a higher level of assistance may benefit more from the use of BA.