# Validation of Lumbar Compressive Force Simulation in Forward Flexion Condition
These studies imply that the spine stability should be considered to estimate the CF. In this study, we apply only the 3.4 kN CF criterion, beyond which damage to lumbar disks is expected to cause LBP. It is worth noting that the CF exerted at a lumbar verteb changes depending on the posture and motions even though they require a positive amount of maximum assistive force. This force is linked with a lumbar burden reduction ratio defined in JJS B 8456-1 which was determined using the machine/dummy method [13; 14]. However, the machine/dummy method cannot represent the soft tissues nor mimic different motion patterns; these characteristics can be represented in human simulators. Simulators are expected to provide useful data to determine whether a caregiving posture that is captured and modeled for the later purpose of biomechanical analyses is sufficiently safe. To avoid using any motion capture system, [PERSON] et al. proposed a static model to estimate the CF that exploits mechanical stability [15]. The focus in this model is the muscular and intersegmental forces concerned with the external load and human upper body weight. [PERSON] et al. developed a quasi-dynamic model in which the muscular and intersegmental forces induce a static CF, and their coefficients are determined by the regression analysis of human flexion motion in a 3D simulator called 3 DSSPP [16]. Finally, the quasi-dynamic CF is obtained by multiplying the static CF by a coefficient related to the flexion speed. The existing estimation methods are limited to static/quasi-dynamic calculations, and their applicability to dynamic scenes has not been statistically verified. It has been reported that the static estimation is less than the dynamic estimation by 11-38%; thus, the static model cannot be applied in dynamic conditions [17]. Therefore, it is important to validate CF simulators with inverse dynamics functionality to estimate the CF. Currently, CF simulators can be easily constructed and combined with the moment arm proposed by [PERSON] et al. [18]. In these simulators, the human movement is first recorded by a motion capture system. Second, the lumbar intersegmental force and moment are estimated using inverse dynamics [19; 20]. Third, the CF is determined as a component of the resultant lumbar intersegmental force and the erect spinae force, which is perpendicular to the sacrum. By modeling the lumbar moment with a single muscle, the erect spinae is assumed to have a constant moment arm in the motion [21; 22]. Although different CF estimation methods have been compared in previous studies, their dynamic performance was not comprehensively evaluated and invasive CF data were not considered [17; 23]. Without the dynamic performance of the model and the invasive CF data, it is difficult to assess the accuracy of the models and their applicability in lumbar support robots. In this study, we validate a CF simulator based on inverse dynamics computation using an ergonomic simulator. We also compare the data estimated by the CF simulator and those obtained by the regression models proposed by [PERSON] et al. and [PERSON] et al. with the results of previous invasive CF experiments [15; 16].