Some people say if they use their cell phone while driving, they prop their knee on their steering wheel and use their leg to keep the car steady.

Kathy Simpson, a biomechanics professor at the University of Georgia says there are three big reasons why the steering wheel is designed for our hands and not our knees.

She says the first explanation is leverage. "If we have two hands on the wheel," explains Simpson, "we only need half as much force by each hand, and we need twice as much here because we only have one knee, one source of force to make this steering wheel turn."

She says because our fingers can actually grasp the wheel, that creates more friction, keeping the wheel from sliding out of our hands. Simpson says a driver will have to push up twice as hard on the wheel to have the same effect.

"And a lot of times," she says, "we might be wearing pants or a dress or something with slippery material. So it's very hard to create enough force upwards, and if I don't, I have to really push a lot harder."

Which slides into Simpson's third point. She says, even though the muscles in our legs are much stronger than those in our hands and wrists, driving takes more precision than strength.

"We have a lot more sensation in our finger tips. We have the ability to make fine, precise movements because these muscles are smaller, and because your leg is so much more massive than your wrists and your arms, it takes a lot more effort for this knee to be able to start pushing quickly enough too. It's going to take a while to get the knee to push against the steering wheel, and that may be too late."

Simpson says she can boil it all down to one simple equation; she says driving without your hands on the wheel can result in an increased risk for a car accident, especially if the driver is using a cell phone or other distracting device.