Biomechanics of volunteers subject to loading by a motorized shoulder belt tensioner

A biomechanical study using human volunteers.

Motorized shoulder belt tensioning is a new seatbelt technology that is likely to be incorporated into future vehicles. The objective of this study was to characterize the upper torso biomechanics of 3 sizes of adult volunteers (5th percentile female, 50th percentile male, and 95th percentile male) subjected to motorized shoulder belt tensioning in the static environment.

There is a lack of volunteer data concerning the biomechanics of occupants subject to motorized precrash shoulder belt tensioning. Studies of torso repositioning by the air force for ejection seats are much too aggressive to be relevant to motorized systems. Low-level motorized shoulder belt tensioning is well tolerated by vehicle occupants but optimized performance by occupant size is unknown.

Nineteen male and 6 female subjects were instrumented in a fixture designed to support the occupant leaning forward and apply seatbelt tension. The subjects were 5th percentile females, 50th percentile males, and 95th percentile males. Reflective markers were placed on the subjects to monitor torso kinematics during tensioning.

Changes in spinal curvature were small during shoulder belt tensioning and the angular motion of the torso originated within 4.2 cm of the pelvis-femur junction or H-point. Torso repositioning and retraction timing was found to be: 54.3 degrees in 0.78 seconds for the 5th percentile female, 57.6 degrees in 0.95 seconds for the 50th percentile male, and 42.2 degrees in 0.92 seconds for the 95th percentile male.

Occupant size has a significant effect on retraction time to reposition the torso during shoulder belt tensioning. Larger vehicle occupants require more time because of a slower retraction velocity. The results are sufficiently simple that a lumped-mass model can predict tensioning kinetics.