Thought Bytes...
April 2004

Repetitive Strain Injuries #6a: Segmental Vibration – shocks, impacts and vibration can damage tissues.

One of my favourite experiments from university was vibrating muscles. (Dr. Frank was almost blushing when he described where the little vibrating box was from.) The vibration box was strapped onto the biceps and the subject was blindfolded. When the vibrator was turned on, the biceps would involuntarily contract, flexing the elbow. Then, if the hand was strapped down so that the elbow could not flex, the subject would report that he felt that the elbow was flexing, even though it wasn’t.

This simple experiment shows that vibration not only tricks the motor nerves into firing, but also fools the sensory nerves into thinking that the muscles are doing activities that are not really happening. In our working lives vibration to muscles has the same impact. The problem comes when you get outside the laboratory and have to actually use your muscles while they are being vibrated. The presence of vibration causes the signals to get mixed up. To compensate, we automatically increase the muscle tension so that more of the correct signal goes back to the brain. Increased muscle tension leads to muscular fatigue and more load on the tendons, increasing the risk of injury.

Eventually the nerves that are being vibrated get numbed from the overwhelming amount of signal that they must process and stop sending information. When this is happening, we continue to grip harder in an effort to send proprioceptive information back to the brain. Much like our hearing when exposed to loud noises, the constant overload can cause damage. The longer the exposure, the longer it takes to recover. Eventually, the nerves simply cannot recover and either the sensation is lost or the motor control, or both.

The blood supply to the extremity can also be compromised by segmental vibration. In this situation the vibration causes the smooth muscles around the blood vessels to constrict, restricting the supply to and from the tissues. With repeated and sustained exposures, the blood supply can become permanently compromised. This is what happens (in general terms) in Vibration White Finger Syndrome.

The goal of ergonomic interventions for segmental vibration exposure is to reduce or eliminate the exposure to vibration either at the source or at the contact point. Within the tool that vibrates there may be maintenance issues, calibrations to be done or dampening agents that can be installed. At the contact point, the handles can be wrapped with dampeners or the worker can use vibration dampening gloves. These latter products are usual described as some type of “visco-elastic polymer” and may go under the name Sorbothane or Viscolas.

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