Thoracolumbar spine kinematics and injuries in frontal impacts with reclined occupants.

OBJECTIVE: Highly automated vehicles may permit alternative seating postures, which could alter occupant kinematics and challenge current restraint designs. One predicted posture is a reclined seated position. While the spine of upright occupants is subjected to flexion during frontal crashes, the orientation of reclined occupants tends to subject the spine to high compressive loads followed by high flexion loads. This study aims to investigate kinematics and mechanisms of loading in the thoracolumbar spine for a reclined seated posture through the use of postmortem human subjects (PMHS). METHODS: Frontal impact sled tests (50 kph delta-v) were conducted on five adult midsize male PMHS seated with the torso reclined to 50 degrees with respect to the vertical. The PMHS were seated on a semi-rigid seat and restrained by a seat-integrated three-point belt with dual lap-belt pretensioners and a shoulder-belt pretensioner with a 3 kN load-limiter. 3-D kinematic trajectories of five chosen vertebrae, and the pelvis were measured relative to the vehicle buck. Intervertebral pressure transducers were installed at three locations in the lumbar column to detect load timing. RESULTS: Three PMHS suffered fractures at L1. Combined compression and flexion of the thoracolumbar spine occurred in all tests, but the magnitude of peak flexion varied across the PMHS. During the PMHS' forward excursion, the pelvis rotated anteriorly in two tests and posteriorly in two tests (lap-belt submarining occurred in one). In one test, the pelvis mount interacted with the seat, but did not affect kinematics. CONCLUSIONS: Anterior rotation of the pelvis caused increased extension of the lumbar spine, which exacerbated lumbar compression in two of the PMHS; the one subject whose pelvis kinematic tracking was lost exhibited similar compression kinematics. Posterior rotation of the pelvis enabled lumbar flexion, which decreased lumbar compression, but lead to lap-belt submarining in one case. Lumbar kinematics for these reclined frontal impacts were sensitive to changes in initial posture of the spine (magnitude of lordosis or kyphosis) and pelvis (pitch angle). To our knowledge, this study is the first to analyze thoracolumbar kinematics and resulting injuries of a reclined seating posture using PMHS.

Kinematics of inboard-leaning occupants in frontal impacts.

Objective: Up to one-half of drivers swerve before a crash, which may cause vehicle motions that displace an occupant from a normal seated position. How these altered postures affect occupant restraint in a crash is unknown. The goal of this study was to quantify the effect of an initial inboard lean on occupant kinematics in a frontal impact.Methods: 30 km/h frontal impact tests were performed with three postmortem human subjects (PMHS) seated in a neutral, upright posture and in a 20° inboard-leaning posture identified from simulated swerving tests with human volunteers.Results: In comparison to the upright posture, the inboard-leaning posture increased the initial distance from the D-ring to the belted shoulder by 105-156 mm. In the inboard-leaning tests, the occupant's head displaced 45-70 mm farther forward than in the upright tests and was also located 123-147 mm farther inboard at the time of maximum forward excursion. The peak resultant velocity of the occupant's head relative to the vehicle interior increased 1.40-1.54 m/s in the inboard-leaning tests.Conclusions: The posture-induced increase in the distance between the D-ring and the shoulder permitted the increased maximum forward head displacement and increased maximum head resultant velocity relative to the vehicle interior. Thus, an initial inboard lean in a frontal impact may increase the risk and severity of a head strike to the vehicle interior, and alter the location, timing, and nature of airbag engagement.

Kinematic and Injury Response of Reclined PMHS in Frontal Impacts.

Frontal impacts with reclined occupants are rare but severe, and they are anticipated to become more common with the introduction of vehicles with automated driving capabilities. Computational and physical human surrogates are needed to design and evaluate injury countermeasures for reclined occupants, but the validity of such surrogates in a reclined posture is unknown. Experiments with post-mortem human subjects (PMHS) in a recline posture are needed both to define biofidelity targets for other surrogates and to describe the biomechanical response of reclined occupants in restrained frontal impacts. The goal of this study was to evaluate the kinematic and injury response of reclined PMHS in 30 g, 50 km/h frontal sled tests. Five midsize adult male PMHS were tested. A simplified semi-rigid seat with an anti-submarining pan and a non-production threepoint seatbelt (pre-tensioned, force-limited, seat-integrated) were used. Global motions and local accelerations of the head, pelvis, and multiple vertebrae were measured. Seat and seatbelt forces were also measured. Injuries were assessed via post-test dissection. The initial reclined posture aligned body regions (pelvis, lumbar spine, and ribcage) in a way that reduced the likelihood of effective restraint by the seat and seatbelt: the occupant's pelvis was initially rotated posteriorly, priming the occupant for submarining, and the lumbar spine was loaded in combined compression and bending due to the inertia of the upper torso during forward excursion. Coupled with the high restraining forces of the seat and seatbelt, the unfavorable kinematics resulted in injuries of the sacrum/coccyx (four of five PMHS injured), iliac wing (two of five PMHS injured), lumbar spine (three of five PMHS injured), and ribcage (all five PMHS suffered sternal fractures, and three of five PMHS suffered seven or more rib fractures). The kinematic and injury outcomes strongly motivate the development of injury criteria for the lumbar spine and pelvis, the inclusion of intrinsic variability (e.g., abdomen depth and pelvis shape) in computational simulations of frontal impacts with reclined occupants, and the adaptation of comprehensive restraint paradigms to predicted variability of occupant posture.