Many health professionals advocate driving with the hips several inches higher than the knees, usually accomplished by the addition of a forward-sloping seat wedge to the car seat. However, this postural alteration can result in additional risk factors when driving.
Oborne (1982) referred to the tendency of a forward-sloping seat to "destabilize the body and increase its tendency to slip forward. In addition the supporting advantages of the backrest will be less apparent."
These problems with a forward-sloping seat described by Oborne will be even greater when driving compared to sitting in an office chair, due to the movement of the vehicle: vibration, road shock, stops, accelerations. As a result, the forward-sloping seat will greatly increase pelvic instability when driving, as the pelvis slides forward on the seat, away from the backrest.
The resulting rocking motion of the unstable pelvis, intensified with the vibration and road shock of driving, increases the bending stresses to the lower lumbar spine. This is the major risk factor for developing a herniated lumbar disc (L5-S1 or L4-L5) from driving (Sandover and Dupuis, 1987; Branton, 1966, 1969).
The forward migration of the pelvis when driving will be even greater if the forward-sloping seat wedge has a slippery seat cover, such as leather or vinyl.
In their evaluation of office chairs, Bendix et al. (1985) found the pressure against the backrest to be twice as high with a slight backward slope to the seat of 5 degrees compared to a 10 degree forward-sloping seat. When driving, a slight backward slope to the seat of approximately 5 degrees will help prevent the driver's pelvis from sliding forward on the seat.
In addition, a slight backward slope to the seat of 5 degrees will help retain the driver in the seat during a rapid deceleration or emergency stop. In contrast, a forward-sloping seat wedge, bringing the hips several inches higher than the knees, will actually increase the forward sliding force on the driver during a rapid deceleration, emergency stop, or rear-end collision.
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