# Simulating 3D Human Postural Stabilization in Vibration and Dynamic Driving
## 4 Discussion  
To our knowledge, this paper presents the first full-body model validation for 3D head, trunk and pelvis motion combining dynamic driving and vibrations in fore-aft, lateral and vertical directions. The results showed that the slalom simulation (4 m/s\({}^{2}\) cornering) matched the measured data fairly well. The model also correctly predicted frequency domain responses with 0.3 m/s\({}^{2}\) vibrations.  
Salam simulations showed a good prediction of body segment rotations (Figure 2) and accelerations (Figure 3). Trunk and pelvis yaw were well predicted, but the model's predicted head yaw was delayed compared to the measured head yaw. We attribute this delay to the fact that the subjects looked into the corner during the slalom [36].  
In addition to the dynamic driving condition (slalom), the model responses to vibrations were tested in the frequency domain. Gain responses of body segments (head, trunk and pelvis) well matched the 3D measured motion. However, pelvis rotational responses were underestimated by the model.  
Modelling the slalom without active leg control resulted in excessive trunk and head roll (Figure 2), and the model eventually fell from the seat after two cycles of slalom (Figure 1--right). These results show the relevance of active leg control in lateral body stabilization in dynamic driving. However, with low amplitude vibrations, active leg control hardly affected responses to fore-aft and lateral motion. This indicates the trunk to be mainly stabilized by the seat and the seat back in low-amplitude loading. However, vertical vibrations revealed a strong effect of leg control with 4-6 Hz oscillations when leg control gains were enlarged and the effect of legs pushing into the seat was simulated. Leg control stiffens the hips and thereby enlarges the contribution of seat-to-thigh contact to  
Figure 6: Vertical vibration validation. Model prediction of head (**upper**), trunk (**middle**) and pelvis (**lower**) in vertical translation (**left**) and pitch rotation (**right**). Blue line: experimental; solid green line: best model with low postural gains; red solid line: high gains fitted towards the slalom but no pushing; red dotted line: high gains and with hips and knees pushing the body into the seat back.  
vertical vibration transmission. Leg pushing also enlarges the contribution of the seat back in vertical vibration transmission.  
The required neck and spine postural control gains for a good fit with experimental data were much higher in the slalom as compared to the vibration scenario. It seems that postural stabilization is more active in intense dynamic maneuvers. With advanced postural control models [18], we will further quantify the contribution of visual, vestibular and muscle spindle feedback in postural stabilization, including adaptation to motion amplitude.  
There is room for improving the model responses, in particular, for the hip, pelvis and lower back region. As a next step, we aim to improve the model fit by measuring and implementing seat characteristics. Further experiments with advanced seats, varying posture and perturbation type, will refine seat modelling techniques and improve our understanding of postural stabilization in seated vehicle users.  
Conceptualization, R.H.; Data curation, N.K., M.C. and T.I.; Formal analysis, M.M., R.D. and R.H.; Funding acquisition, R.H.; Investigation, M.M., N.K., T.I., B.S. and R.H.; Methodology, M.M. and R.H.; Software, M.M., R.D. and M.C.; Supervision, R.H.; Writing--original draft, M.M., N.K., B.S. and R.H.; Writing--review & editing, M.M. and R.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Toyota Motor Corporation.  
See the underlying experimental papers [1; 31].  
See [31] for experimental data and www.madymo.com (18 May 2022) for the active human model.  
The authors declare no conflict of interest.