Wheelchair safety: effect of locking or grasping the rear wheels during a rear tip

To test the hypothesis that, with the wheels locked (by the mechanical locks or by the user grasping the wheels), the rear wheels and the chair rotate slowly backwards during a fall, whereas with the wheels unlocked, the rear wheels rotate quickly forwards.

Before-after trial.

Kinesiologic laboratory.

Ten nondisabled adults, a sample of convenience.

In a single representative wheelchair, subjects were dropped from beyond their balance points onto a mat with the rear wheels locked (L), with two hands grasping the rear wheels (TH), and with the wheels unlocked (UL).

From videorecordings, the horizontal displacement of the rear wheels, the rotation of the rear wheels, and the fall times were derived. Using an anthropomorphic test dummy (ATD) and a Kistler force platform, the impacts of the head with the floor were also recorded.

In both the L and TH conditions, the rear wheels moved and rotated backwards. The UL condition was significantly different than the L and TH conditions, with the rear wheels moving and rotating forwards and the fall occurring more quickly, with mean differences of 442 and 455mm, 84.0 degrees and 87.1 degrees, and .52 and .45 sec (p < or = .0001). The forces on the ATD's occiput were 12,280 and 21,118N in the L and UL conditions, respectively, and lasted approximately 20msec.

Locking or grasping the rear wheels has a profound effect on the nature of rear-tipping incidents, a finding with important implications for the training of users in how to fall safely.