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This Page Last Updated: Sat Feb 11 2006:

Vibration Weakens Backs The following information was written by David G. Wilder, Ph.D., P.E. and Malcolm H. Pope,DR. Med. Sc., Ph. D. I do not know the original source, I found this on an other BBS. Back problems among truck drivers and other motor vehicle operators are more common than you might imagine. Do you know why? Compare your spine to a paper clip. Take the paper clip, straighten it out and jiggle the ends back and forth. you can do this only so long before it breaks or snaps. Now sit down. Sitting flattens out your back's natural curve. If you expose your back to vibration in this position, for instance by bouncing around in a back hoe driving over a dirt road, you can jiggle your spine like a paper clip. Low frequency (4 to 6 Hz) cyclic motions like those caused by a vehicle's tires hitting the road can put the body into resonance. Just one hour of seated vibration exposure can cause muscle fatigue, weaken the soft tissues and make a worker more susceptible to back injury. If you put a worker in that position for eight hours a day with no protection, it won't be long before...SNAP! you've got a back injury on your hands. Truck drivers, traveling sales reps, subway operators, tractor drivers, and construction vehicle operators are common victims of back problems. One study shows that truck drivers are four times more likely to have a herniated disk. Another says that commuters traveling more than twenty miles a day have twice as many back problems as those that don't. But the vibration that people are exposed to while sitting and driving is rarely linked to the cause. That's because vibration weakens the spine through "cumulative trauma". You can't see it happening. And usually some event, such as lifting a heavy object, triggers the pending injury. Doctors and workers' comp records may blame injuries on single acts like lifting. But this article will tell you how vibration gradually weakens backs, and offer you tips for minimizing its' effects, particularly when workers are operating vehicles. Cause and effect: First, here's how sitting and vibration combine to trigger back problems: 1) Sitting is an extreme position for the back. It flattens out the back's natural curve. Driving a vehicle will cause the back to flex back and forth, even more when the pelvis rocks. 2) Sitting down stretches out the back of the discs. The back of the disc is made of fewer, thinner fibres and is not as strong as the rest of the disc. 3) When a person sits, the interlocking small bony elements in the back disengage somewhat and allow more motion between the vertebrae. 4) Sitting also increases pressure inside the disc. 5) Vibration softens and weakens the disc's fibers, and has lead to tears in the disc during experiments. 6) The disc and muscles need time to readjust to the standing posture after sitting. Too much to bear: Any structure is more likely to fall apart or break when it is being vibrated at it's natural frequency, because vibration at the natural frequency overloads the structure. Take for instance, the Hyatt Regency sky walk collapse in Kansas City in 1980. A large group of people dancing to one rhythm created a vibration that overloaded the sky walk's supports, causing it to collapse. The natural frequency vibration for a worker while seated is between 4 1/2 and 5 1/2 times per second. At that frequency the most stretching of discs in the spine occurs. If you can make workers aware of how vibrations affect their bodies and take steps to reduce exposure you can minimize their chances of suffering back injuries. One solution: Richard Elliott, assistant engineer at Virginia International Terminals, recently took steps to reduce worker's comp costs and painful injuries caused by vibration. Vibration's effect on yard tractor, or "hustler", drivers is generally a costly burden in the port industry. Virginia Int. Term. had worker's comp claim costs in the millions of dollars last year. According to Elliott, an inordinate number of claims came from low back pain in hustler drivers. Several drivers had surgery for blown out disks. After Dr. David Wilder tested Elliott's yard tractor crew, he made recommendations to minimize the problem. First, studies revealed that Elliott's practice of installing suspension systems and air ride seats to reduce vibration in the tractors actually worsened the effects. He was unknowingly setting up two separate harmonic ranges that worked against each other, compounding the effect of vibration on the driver's backs. Instead, Wilder recommended redesigning tractor cabs to change the way drivers sit, reach for controls, and enter and exit the cab. For one thing, he wanted to stop drivers from twisting their bodies awkwardly to get out of the compact cab. Injury is common when a worker twists or puts stress on the back after significant vibration exposure. Wilder also recommended changes to the tractor that would minimize vibration. Turning those recommendations into bid specs for a prototype tractor, Elliott approached tractor vendors. Now his drivers operate three new hustlers with modifications including larger cabs, isolation systems, and a three-point cab mount. Elliott has yet to study the benefits of the new cabs. But if the injuries are reduced like he thinks they will be, his company will purchase more than 100 new tractors over the next seven years, he says. At fifty thousand dollars a tractor, it will be a significant investment. But in the long run, Elloitt estimates millions of dollars in comp costs will be saved. What you can do: If our costs aren't significant enough to warrant management approval of a multi-million dollar investment, there are still steps you can take to reduce vibration. Here are some tips: Workers should avoid bending over or lifting immediately following driving. Tell them to walk around for a few minutes first. Drivers should frequently adjust their postures, for instance, by stretching. Asymmetric postures or twisting while driving should be avoided. Seating environments should be arranged so that not much bending to the side is required. Make sure the seat back can recline 20 degrees or more from the vertical position. The seat pan should have adjustable inclination. Arm rests should be used. Be sure the lumbar area is supported. Decrease vibration as much as possible. Wilder and Pope believe that reducing occupational vibration is one way to prevent the low back injuries that cost the U.S. economy some $85 billion annually. They have received numerous grants for their research on vibration and low back problems.

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