Force and repetition are two well-known risk factors for musculoskeletal disorders (MSDs). Many current ergonomic tools and guidelines consider the factor of repetition to have the same impact on MSD risk no matter the level of force involved. However, a recent article my colleague John Heberger and I published in the journal Human Factors indicates that it may be the way these two factors are combined that is most important in the development of MSDs.
We performed a systematic review of 501 epidemiology studies from which relevant studies were selected for the review. Twelve studies were ultimately identified that evaluated combinations of force and repetition that allowed the authors to evaluate whether these variables had an interactive effect.
Of the twelve studies that met the inclusion criteria, ten provided evidence of interaction between force and repetition. Force x repetition interactions were observed for MSDs for a wide range of joint disorders and symptoms, including low back disorders, carpal tunnel syndrome, hand/wrist tendinitis, wrist discomfort, lateral epicondylitis, shoulder tendinitis, shoulder discomfort, and knee discomfort. The pattern of interaction was quite consistent in most studies: increased repetition for low force tasks resulted in anywhere for a slight decrease to a slight increase in MSD risk, while increased repetition for high force tasks consistently led to a rapid escalation in MSD risk.
Based on our review, it appears that when force demands are very low, repetition is generally well tolerated by the body. With every increase in force, however, fewer and fewer repetitions can be tolerated. When forces get very high, it appears that MSDs can occur with a fairly small number of repetitions.
We go on to show that the interaction of force and repetition may be the result of fatigue failure in musculoskeletal tissues. Fatigue failure begins when a physical exertion leads to a microscopic injury in a tendon, ligament, or disc. Repeated loading on the compromised tissue will cause the injury to expand, and will eventually result in pain and dysfunction.
We also suggest that a third MSD risk factor may be relevant to the force-repetition interaction – awkward postures. Awkward postures usually increase forces experienced by musculoskeletal tissues, so it’s therefore conceivable that a large reason that awkward postures have been identified as an MSD risk factor is simply that these postures increase forces experienced by musculoskeletal tissues.
If MSDs are the result of a fatigue failure process, it may lead to changes in recommendations on how to prevent MSDs. Fatigue failure theory indicates that there is a force level, known as the endurance limit, below which tissues can sustain almost unlimited repetition. This limit is usually about 30% of the ultimate strength of the tissue. People can do highly repetitive activities, such as walking, without injury as long as the forces on tissues are below the endurance limit. Many occupational tasks clearly expose workers to forces that well exceed the endurance limit. Tests on human musculoskeletal tissues indicate that loads up to 40% ultimate strength can be fairly well tolerated. Repeated loading much higher than 40% of the tissue’s ultimate strength can lead to fairly rapid tissue failure.
A limited number of high exertion tasks can be tolerated by the body and may even be beneficial to the musculoskeletal system. Weightlifters gain strength by performing a limited number of high force exertions, for example. However, the data presented in this article suggest that the number of high force exertions that can be tolerated is limited. It appears that any training benefit that might be gained from a limited number of high force exertions can be quickly reversed if too many high force exertions are performed. We can’t provide exact numbers, but clearly high force exertions need to be kept to a minimum.
Sean Gallagher and John Heberger (2012), Examining the Interaction of Force and Repetition on Musculoskeletal Disorder Risk: A Systematic Literature Review, Human Factors, first published on June 28, 2012 as doi:10.1177/0018720812449648, and at the time of this writing available at no charge at http://hfs.sagepub.com/content/55/1/108.abstract.
Sean Gallagher, PhD, CPE has performed research to reduce musculoskeletal disorders since 1984, initially with the U.S. Bureau of Mines and later with the National Institute for Occupational Safety and Health (NIOSH). In January 2012, Dr. Gallagher accepted a position as Associate Professor in Auburn University’s Dept of Industrial and Systems Engineering. He has published over 40 journal articles, several book chapters, and numerous governmental publications detailing his research.
[Editor’s Note: Please join Ergoweb® in congratulating Sean Gallagher and John Heberger for being awarded the 2013 International Ergonomics Association/Liberty Mutual Medal in Occupational Safety and Ergonomics for the research paper summarized by Dr. Gallagher in this article.]
This article originally appeared in The Ergonomics Report™ on 2012-07-11.