Thought Bytes...
May 2004

RSI Risk Factors #6B: Whole Body Vibration

I first became interested in whole body vibration in 1988 while I was working on my Master’s thesis. My research explored whether shaking the whole body had an impact on the control of fine motor movements – the answer was, no, it didn’t, as long as the arm was supported. Before I could shake up a bunch of undergraduate guinea pigs / volunteers I had to do some exploration of the risks associated with whole body vibration. And I had to learn a lot about the world of vibration and its terminology.

When it comes to vibration of the body, or of any structure for that mater, the critical feature is resonance. When an object resonates the amplitude or height of the input movement is amplified from the input source through to the other end of the object. So, when the body resonates, an input movement of 2 cm at the buttocks is much larger by the time it reaches the top of the trunk.

The human body resonates at approximately 5 – 7 Hz (or cycles per second). Interestingly, large vehicles such as earth movers vibrate in the same range. Vibration in this range can occur in any vehicle though, depending on road conditions and numerous other factors. Other vibration frequencies also occur, but are not of the same degree of concern because they don’t contribute to resonance of the human trunk.

The problem with resonance is the dramatic increase in amplitude of movement. With each upward movement of the trunk there must be an equal or greater downward movement. As the trunk moves up, the intervertebral disks are stretched slightly. Then as the trunk moves down the disks are compressed. It is believed that the compression of the disks in the downward movement causes progressive damage to the connective tissues within the disk.

Early research found that people exposed to whole body vibration actually shrank during the exposure. When the vibration stopped they regained their height, plus some. Researchers proposed that fluids were lost from the disks due to the repeated compression. Then when the loading stopped the disks essentially sucked back in the fluids they had lost, but because of the trauma got inflamed, drawing in excess fluid. After a good night’s sleep most subjects returned to their normal height. The high incidence of degenerated disks in long distance drivers and heavy equipment operators is thought to relate to this phenomenon.

Much of the emphasis now is on how to reduce exposure to whole body vibration and / or decrease the effect of the impacts. So, here are a few things you can do:

• Firm up the seat. A soft seat decreases the frequency of vehicular vibration, making it more likely to fall into the resonant range. A firm seat increases the frequency.
• Support the back. Having the backrest tilted back slightly – and leaning back into it – allows the impacts to be transferred to the backrest instead of transmitted down the spine, through the disks.
• Get recovery time. Make sure you get adequate prone rest to allow the disks to recover. But no more than 7.5-8 hours because then the disks will swell, increasing the risk of injury.
• Don’t do heavy lifting after exposure. The disks are weak when they are shrunken as well as when they are swollen after the vibration. Flexing the spine or adding compression from a heavy load may cause further injury.

We can’t avoid whole body vibration, but we can learn to live with it.

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