# Validation of Lumbar Compressive Force Simulation in Forward Flexion Condition
## 5 Limitation  
Validation was performed with a relatively small group of ten subjects. They did not represent a wide range of ages, body weights, and body heights; hence, these parameters are not investigated in this study. These healthy subjects did not include the elderly. In different age groups, the compressive strength at the lumbar disc of the elderly is less, approximately 1.7 kN, than that of young people under the 10% risk level [53]. Under the same CF, the elderly may have a higher risk of low back pain.  
With the exception of lumbar criterion, arm strength is an important factor of flexion/lifting motion. The criterion of arm strength is not included in this study because the strengths of nonpreferred and preferred arms show a considerable difference. This difference may cause errors in standardization for the safety requirement.  
We did not simulate the restricted hip motion of caregivers with hip pathology, which is another limitation of this study. One reason is that the different severity of hip pathology may lead to different restricted range of hip motion; thus, it is difficult to select an appropriate restriction level. Another reason is that the restriction degree of one joint motion may affect other joint motions. Caregivers with hip pathology have to adopt inappropriate posture in caregiving tasks, thus experiencing more CF at the lumbar. Therefore, in this study, we asked the subject to maintain a stopo posture to present the worst case in forward flexion.  
Considering that the peak CF usually occurs at the flexion/lifting period in transfer tasks, we validated the CF simulator in the dynamic flexion condition. The twisting motion is not presented in this model because the erector spinae muscular force cannot provide most of the required twisting torque; hence, the evaluation of twisting motion requires a muscle model that can accurately present all the activated muscles at the lumbar region. Because the recruitment pattern of the lumbar muscles in twisting cannot be accurately obtained, the way in which twisting with external load in the musculoskeletal model is presented remains to be investigated.  
The lumbar correction factor \(c\) range from 0.55 to 0.77 with the mean value of 0.66 [30; 36]. Even though the correction factor varies for each individual, we cannot clearly determine the variations. Thus, we directly took the mean value 0.66 for all individuals as it was used by [PERSON] et al. and [PERSON] et al. [30; 36].  
The individual differences affect the estimation results; the participants in this study were different from those in the invasive experiments. Because the variance of the invasive measurements affects the reliability of the data, it is necessary to investigate the factors causing the variance. The coefficient of variation of the erector spinae's EMG is reported in resisted back extension with a wide range of 0.16-0.38 [54]. However, only a slight difference was observed in the body parameters of the subjects: ages of 18 \(\pm\) 1.2 yr, heights of 176.5 \(\pm\) 3.2 cm, and body masses of 71 \(\pm\) 4.5 kg. This implies that the error on invasive measurements could not be reduced by only normalizing the body parameters or fixing the motion pattern. Even though this study includes almost all previous invasive data, the difference between the invasive data of our participants and those of previous studies is difficult to estimate and introduces inaccuracy in the quantitative evaluation of the three models.