Shake, Rattle, and Roll: Whole Body Vibration and its Impact on the Back
Wood harvesters, farmers, and excavation contractors along with truck drivers will at some point find themselves inside the cab experiencing the vibration from the vehicle and uneven ground. In the United States an estimated 6 million American workers are regularly exposed to whole body vibration for more than two hours a day, this according to the National Institute of Occupational Safety and Health’s Kristine Krajnak. This bouncing around has an impact on the body. According to Nathan Fethke, a biomechanical engineer and associate professor at the University of Iowa, 30% of farmers will have at least one episode of work-limiting back pain during the year and that figure approached 90% of farmers over the course of their career.
Sitting changes the shape of the back and reduces the lower back inward curve. This in turn changes the dynamics of the discs between each vertebrae. Consequently, these changes increase the pressure inside the discs according to research done by Alf Nachemson as illustrated in the graph below. Note the back-pressure difference between sitting and standing.
So, couple the increased pressure of sitting with whole body vibration and you have the recipe for back pain. The act of sitting will change the shape of the disc and create increased pressure to the back wall of the disc. Over time this pressure breaks down the tough fibers of disc. When a worker gets out of a cab their back is in its most vulnerable position.
The disc needs time to return to the “normal” shape of standing upright and this process can take a couple minutes. Workers should never jump down from equipment or immediately lift any load. Because of the posterior shift of the disc it doesn’t take much to go beyond its tolerance and cause an injury. As workers exit machinery or large vehicles they should face the machinery and lower themselves from the cab making sure they have good footing and hand support. This is the “three points of contact rule” in practical use. Once on the ground they need to take a few minutes to get used to being upright and reshape the disc to neutral position. A couple of gentle slow and controlled back bends will assist the process.
Nathan Fethke also recommends adjusting the seat to fit the worker properly. Check the seat suspension and determine if it can handle the weight of the employee. It shouldn’t bottom out during operation. Pay attention to maintaining the seat with proper lubrication and replacing worn suspension. It should be part of the overall machine maintenance program.
Although the hazards of whole-body vibration are becoming more apparent, OSHA currently has no standard addressing it. Those employers who want to dig deeper into preventing injuries from this hazard will have to look to several other countries including the United Kingdom who does have standards addressing vibrations. The International Organization for Standardization (ISO) also has several standards relating to whole-body vibration. Lastly, the American National Standards Institute has voluntary standards like those of ISO as does the American Conference of Governmental Industrial Hygienists. Facing this hazard with a better understanding of how it results in injury and what preventive measures can be taken is a great start. Our backs have to last a lifetime, so start taking care of yours today!
By Allan Brown
