# Simulating 3D Human Postural Stabilization in Vibration and Dynamic Driving
## 3 Results
### Vibration Validation  
Frequency domain responses of body segments (head, trunk and pelvis) were validated for both translational and rotational body segment motion in fore-aft (Figure 4), lateral (Figure 5) and vertical (Figure 6) vibrations. Reduced neck and spine postural control gains were found to optimally predict head and trunk rotation. Hip and knee control gains were set at minimal levels such that they hardly affected the frequency domain results but prevented lateral drift of the knees. Arm control gains were set at zero level, in line with the applied instruction to relax the arms with hands resting on the thighs.  
The best fit parameters with low postural gains (green lines) provided a much better fit as compared to the high gains fitted towards the slalom data (red and red dotted lines). Translational responses closely matched the 5 Hz oscillation in vertical vibration in the pelvis, trunk and head and matched the lateral response well for the head, while underestimating trunk translation. In fore-aft vibration, the 4-5 Hz peak was well-reproduced for the trunk but underestimated for the head. Rotational responses for head and trunk matched the measured kinematics, but pelvis rotation was strongly underestimated.  
The high postural control gains fitted towards the slalom provided unrealistic low rotations of head and trunk and increased the peak frequency and amplitude of vertical vibrations. This effect was even stronger with high postural control gains and legs pushing the body into the seat back and the thighs into the seat. This shows that while active leg stabilization is not essential to stabilize body roll and pitch with low motion amplitudes, the legs can strongly affect vertical vibration transmission. Increasing the arm postural control gains from zero to 0.5 notably affected trunk acceleration and rotation in fore-aft  
Figure 3: Slalom validation for body **acceleration**. Blue line: experimental; dotted green line: best model fit with high postural gains and with hips and knees pushing the body into the seat back; solid green line: high gains but no pushing; red line: lower postural gains fitted towards the vibration experiment.  
and vertical vibrations (not shown). This can be explained by the resulting stiffening of the shoulder complex coupling the arm inertia to the trunk.  
Figure 4: Fore-aft vibration validation. Model prediction of head (**upper**), trunk (**middle**) and pelvis (**lower**) in 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.  
Figure 5: Lateral vibration validation. Model prediction of head (**upper**), trunk (**middle**) and pelvis (**lower**) in translation (**left**) and roll and yaw rotation (**middle** and **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.  
These results were obtained using a head orientation-in-space control strategy and were slightly more realistic than when using head-on-trunk control. As the seat and cabin were not rotating in space, the head-in-car control strategy would be equivalent to head-in-space and was therefore not evaluated.