Occupant to seat interaction for the prediction of posture, motion and muscular activity

One of the major performances to be offered by a seat in its environment is the capability for the occupant to reach in a comfortable way all the equipments of the car compartment, such as the pedals, the hand brake or the glove box.

Currently, the seat design process starts with the car compartment definition including the location of all those equipments. Then, for a chosen human anthropometry, ergonomists are able to specify the posture to be adopted in order to ensure a good accessibility to the pedals and the steering wheel. This ideal posture is characterized by the Seat Reference Point (SRP). At this stage, the designer has to imagine the seat which will actually provide a H-Point at this SRP location. Assuming this, the ergonomist computes a motion from the SRP to check the accessibility to other equipments like glove box. This motion can be afterwards imposed to musculoskeletal human models to evaluate the necessary muscle force to perform this motion.

This process presents several issues. First, the actual H-Point of a seat is never exactly at the SRP location. Secondly, when the motion is computed, the current tools do not take into account the interaction with the seat which has an influence on the way the motion is performed. For instance, pushing on a clutch pedal induces a strong interaction between the thigh and the seat bottom cushion which limits the possible thigh angle. Finally, in order to be accurate, the computation of the muscular force has also to take into account the exact force induced by the seat reaction force.

The objective of this study is to demonstrate the accuracy improvement in the evaluation of posture and motion by simulating the whole process chain taking into account the exact interactions between the seat and the occupant.