Thought Bytes...
January 2004

RSI Risk Factors #3: Repetition

Last month I told you that we are not designed to hold static postures – that we need motion to pump blood and keep our muscles and joints working. But how much is too much? Why can one person do a job for 10 years but another gets injured after 2 months?

Unfortunately, there is no ethical way to determine exactly what the human body can tolerate when it comes to repetition. Sure they can test a tendon in a cadaver until it fails, but a cadaver can’t tell you (it had better not anyway) when it feels the symptoms of tendonitis, nor when the irritation to the tissues is so severe that it doesn’t want to do the movement anymore. A cadaver doesn’t have blood circulating to the tissues bringing nutrients and lubrication. And, furthermore, the cadaver was exposed to activity (prior to it’s current state), so there is no way of eliminating previous strains and scarring to the tissue. Given that most anatomy specimens are donated after many years of use / living, it would be difficult to say that there is such as thing as a “perfect” sample.

In research done on the living, determining tolerance is no easy task because we are limited by pain. Every person has different pain thresholds and these fluctuate depending on the person’s circumstances at any given moment. We have all seen the child that falls and scuffs his knee but doesn’t start to cry until he is close to Mommy. Most people will work through a job because it has to be done, only to feel the aches and pains when the job is complete. The environment around us and the exposures we have are factors in the recognition of pain.

The problem comes with repeat exposure. This is where the “cumulative” in Cumulative Trauma Disorders (CTD’s) comes from. The first time you do a job and feel discomfort you likely will try to work through it. If you don’t have to go back and do the same job the next day, the irritation gradually dissipates and you go on with your life. The next time you do the job or use the same muscles in the same way, the irritation comes back. Sometimes the irritation is worse than the first exposure. Each time you use the muscles the same way, the irritation comes back either worse or in a shorter time span, or both.

I have been asked many times if a worker can develop a Repetitive Strain Injury (RSI) in 2 days/ weeks/months. The answer is, given the right circumstances, a person could develop an RSI in a matter of hours. While I may be able to type for hours on end, 2 hours of chopping wood or hammering nails is enough to give me tennis elbow – as experience has proven repeatedly.

Unless a person has never used the muscles and tissues prior to the activity in question, it will be impossible to say how much is too much. There will always be history of previous usage “written” on to the tissues. To this, consideration also must be given to the person’s psychological make-up. Each person feels pain differently and this changes from day to day. To one person a pinch may be nothing and to another the same pressure may be excruciating.

So, how much is too much? As an ergonomist, I can only comment what I think is reasonable and try to find out if there are other factors contributing to the perception of discomfort if I think that the perception of discomfort outweighs the repetition associated with the task. Some of the questions I ask are:

• How often do you do the task?
• How long does the task take?
• How long are the breaks between tasks?
• Is this the first episode of pain?
• When was the first episode?
• How was it treated?
• Did you change how you do the task after the first episode?
• Has the discomfort changed?
• What kinds of activities do you do outside of work that use the same muscles / joints?
• Is your work/life stressful?

I observe the worker to see if there are non-neutral postures and static postures associated with the repetitive task. (I also look for force application, vibration, mechanical stress and cold exposure, as I will discuss over the next couple of months). For example, holding the arm away from the body is a static and non-neutral posture that could contribute to developing a strain from high rates of repetition when keyboarding or using the mouse. A common observation is that people do extra / unnecessary movements that they are not aware of that are highly repetitive. A large part of reducing repetition is to teach the worker correct working postures and show them how to use the body in more efficient manner.

I have never yet seen any one risk factor working alone to create an injury. There have always been multiple factors from the work environment in conjunction with the worker’s physical history and psychological situation. Is repetition a concern? Absolutely. But, it has to be looked at in the context of the other risk factors and the worker’s situation.

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