# Effect of Parameters on Lumbar Compressive Force during Patient Transfer
## 4 Discussion
### Limitations  
There are some limitations to our study. 1. This experiment did not include a considerable weight and age group or a large number of subjects because of the massive number of tasks and large analysis, which is a reason the R-square value was not high in the regression analysis. In addition, it is unclear to what extent the small sample can explain the peak CF timing when considering the individual differences requires validation in the real transfer tasks. 2. The subjects were college students who could not perform the tasks as professional caregivers or patients with physical constraints. Usually, well-trained caregivers will start at a lower speed than those without experience in transfer. It will take away the peak timing of all the factors, thereby causing a reduction of the peak CF. If we use well-trained caregivers, we can observe the difference among the peak CF timing, peak angular velocity timing, and peak H timing. We can probably use peak angular velocity or peak H to estimate the timing of peak CF instead of using peak trunk angle or angular acceleration. The peak trunk flexion angle in the PT task was 48.5\({}^{\circ}\), which is greater than 31\({}^{\circ}\) as reported by [PERSON] et al. [32], but smaller than 84.1\({}^{\circ}\) as demonstrated by [PERSON] et al. [33]. Different trunk angles result in errors in the CF estimation. There are three possible reasons for the difference between trunk angles in our study and other studies. The first reason is the knee angle. In lifting, an increase of knee angle leads to a decrease of trunk angle and a decrease of peak CF. A lifting sample is provided in ISO 11228-1, which states that the caregivers need to perform a larger trunk flexion when they keep their legs straight compared to when they bend their knee, and the peak CF in the latter is much smaller than the former. The second reason is the location where the caregiver holds the patient. As the caregivers support the patients at their hip, waist, and underarm, the peak trunk angles of the caregivers vary. Third, the experimental setup also affects the peak trunk angle. For example, using a taller chair leads to a smaller peak trunk angle. 4. For the safety of the lumbar, this study referred to the description of the acceptable external load in the international standard for manual lifting ISO 11228-1, which states, \"In order to lower the risk for people at work, particularly those with less physical capability, the recommended limit for mass should not exceed 15 kg\" [34]. Therefore, the maximum change in the \(GRF\) on the caregivers was limited to 150 N in the experiment. When the external force exceeds 15 kgf, assisting devices should be used to facilitate the transfer. 5. Not controlling the knee angle is a limitation of this study because the knee angle could affect the upper trunk angle, resulting in a variation in the estimation of the CF. In our study, the Kruskal-Wallis test showed that both the left and right knee angles varied among subjects (\(p<0.001\)), while the trunk angle did not (\(p=0.1754\)). One possible explanation is that the posture of each subject was well coordinated to maintain the trunk angle. This posture may have been similar among all subjects, with each subject adjusting their hip and knee angles at the same time. Therefore, a change in knee angle did not lead to a large variance of trunk angle in this study. Since this relationship between the peak CF and the simple factors has not been reported in the literature, we needed to investigate this relationship under known safety conditions. Thus, we conducted the experiment with healthy students with a limited lumbar burden instead of actual tasks conducted by real caregivers and care-receivers. Although this study has several limitations, we provide a quantitative estimation method for the peak CF in PT tasks, which is expected to contribute to the standardization of PT tasks.