From The Ergoweb® Learning Center

When Does a Task Become Repetitive?

Q:  Can you tell me how many repetitions per minute or hour constitute “repetitive work?”  I need to do a job analysis and the employer does not think the job is “repetitive.”

A:  Many people, including professional researchers and ergonomists, face frustration and questions when trying to quantify risk factors such as force, posture, repetition and duration. The answer is that no one really knows how many repetitions it takes to develop an injury like carpal tunnel syndrome. What scientists do know is that the injury depends upon several factors.

Scientific studies state that the more a person is exposed to a risk factor like force or repetition, the greater the risk of injury. Unfortunately, no one can provide a concrete number like 300 or 10,000. The reason scientists cannot come up with a specific number for how many repetitions it takes to cause an injury is because of the difference in people: human variability.

An individual person’s reaction to MSD risk factors, and how likely it is that exposure to those risk factors will cause an injury is very similar to the way individual people respond to other health risk factors such as smoking and the development of lung cancer.  For example, smoking increases a person’s risk of developing health problems and lung cancer, but there is no clear dose-response curve. Scientists are unable to tell any one person how many cigarettes that person must smoke before an illness occurs. They do, however, state that by reducing the exposure to the risk factor, chances of getting an illness are also reduced.

When quantifying the risks of different risk factors, ergonomists turn to analysis methods like, for example, the Strain Index, ACGIH Hand Activity Level, or the NIOSH lifting equation to help in the job analysis. Such methods help quantify the contribution from risk factors like force, posture, repetition, duration, and how these factors might act together to increase risk of injury. In many cases, repetition may not be the primary risk factor.

This article originally appeared in The Ergonomics Report™ on 2002-12-01.