Visualization of driver's body motion by instantaneous screw axis and line of action

It is well known that the three dimensional motion of a rigid body can be described by a rotation about a fixed axis. Also, the force and moment applied to a rigid body can be regarded as a force along a line. The motions of the pelvis and thorax, and the steering forces and torques of a driver on two different seats are visualized by the instantaneous screw axis and line of action, respectively. The screw axis of the thorax passes close to the center of mass of the head. The relative screw axis between the thorax and pelvis goes through the center of mass of the upper body in both cases. The line of action of the steering force reaches the driver's shoulder girdle when driving on the seat with a larger back support. The proposed method can be helpful when considering the driver's motion from a biomechanical viewpoint.