The dynamics of electric powered wheelchair sideways tips and falls: experimental and computational analysis of impact forces and injury

Abstract

Background: To reduce the occurrence of wheelchair falls and to develop effective protection systems, we aimed to quantify sideways tip and fall dynamics of electric power wheelchairs (EPWs). We hypothesized that driving speed, curb height and angle of approach would affect impact forces and head injury risk for wheelchair riders. We further expected that fall dynamics and head injury risk would be greater for unrestrained riders compared to restrained riders.

Methods: Sideways wheelchair tip and fall dynamics were reconstructed using a remotely operated rear wheel drive EPW and a Hybrid III test dummy driving at different approach angles (5 to 63°) over an adjustable height curb (0.30 to 0.41 m) at speeds of 0.6-1.5 m/s. Rigid body dynamics models (Madymo, TASS International, Livonia, MI) were developed in parallel with the experiments to systematically study and quantify the impact forces and the sideways tip or fall of an EPW user in different driving conditions.

Results: Shallower approach angles (25°) (p < 0.05) and higher curbs (0.4 m) (p < 0.05) were the most significant predictors of tipping for restrained passengers. Unrestrained passengers were most affected by higher curbs (0.4 m) (p < 0.005) and fell forward from the upright wheelchair when the approach angle was 60°. Head impact forces were greater in unrestrained users (6181 ± 2372 N) than restrained users (1336 ± 827 N) (p = 0.00053). Unrestrained users had significantly greater head impact severities than restrained users (HIC = 610 ± 634 vs HIC = 29 ± 38, p = 0.00013) and several tip events resulted in HICs > 1000 (severe head injury) in unrestrained users.

Conclusions: Sideways tips and forward falls from wheelchairs were most sensitive to curb height and approach angle but were not affected by driving speed. Sideways tips and falls resulted in impact forces that could result in concussions or traumatic brain injury and require injury prevention strategies. Seat belts eliminated the risk of falling from an upright chair and reduced head impact forces in sideways wheelchair tips in this study; however, their use must be considered within the ethical and legal definitions of restraints.

Fig. 1

The experimental set up using the rear-wheel drive power wheelchair driven by a remote joystick showing an example of a tip event. The Hybrid III dummy was positioned in the chair and was either unrestrained or belted with a lap belt. The curb height was adjusted using manual jacks under the platform. Lines on the platform guided the operator to different approach angles. The dynamics of each experiment were captured using eight optical tracking cameras mounted around the test structure using reflective markers attached to the dummy and the wheelchair

Fig. 2

Example MADYMO power wheelchair model showing local coordinate system. The origin of the coordinate system was used as the landmark to describe head impact location relative to the wheelchair after a tip or fall

Fig. 3

Comparison of simulation (Madymo) and experimental (Qualysis) roll angle for occupied EPW model validation. The slope of the roll data between the experiment and simulation showed good agreement, indicating an accurate moment of inertia in the simulation. Peak roll values are missing from two experiments due to reflective markers being occluded by the dummy arm during tipping. Curbs heights were 0.3 m for all tests. Driving speed and angle varied between tests; T003 - speed = 3.3 m/s, angle = 39.5°, T004 – speed = 1.99 m/s, angle = 20.12°, T007 – speed = 2.35 m/s, angle = 23.12°

Fig. 4

Impact force as a function of impact position for wheelchair tips. Colors represent increasing contact force. All restrained passenger simulations showed contact forces below 3000 N. Unrestrained passengers showed a broad range of impact forces with most simulations exceeding 7000 N. There was a significant relationship between impact position and increased impact force. In the restrained simulations (blocks) the head position is tightly clustered, whereas the unrestrained occupants (triangles) show greater variation in head impact locations relative to the wheelchair. Positive x value indicates the rider moving forward relative to the chair and positive z values indicate the rider moving upward normal to the seat plane