Q: What is the Strain Index?
A: The Strain Index is a tool used to evaluate a job’s level of risk for developing a disorder of the hand, wrist, forearm, or elbow. The analyst evaluates six task variables (intensity of exertion, duration of exertion, exertions per minute, hand/wrist posture, speed of work, and duration of task per day). The task variable is given a value called a multiplier. The product of the six task variable multipliers produces a number called the Strain Index score. This score is compared to a gradient that identifies level of task risk.
The original reference for the tool is: J. Steven Moore and Arun Garg, “The Strain Index: A Proposed Method To Analyze Jobs For Risk Of Distal Upper Extremity Disorders”, American Industrial Hygiene Association Journal, 56:443-458 (1995).
Q: When is this tool applicable?
A: The Strain Index is an appropriate tool to use when you need to evaluate the risk of developing a musculoskeletal disorder (MSD) in a hand intensive task. Many small assembly tasks will qualify for this.
Q: What are some limitations of the Strain Index?
A: The authors have outlined the following limitations of this tool:
- The Strain Index does not evaluate segmental vibration (such as from hand tools); hence, this tool will not predict the risk of hand-arm vibration syndrome.
- The Strain Index does not evaluate contact trauma; hence, this tool will not predict the risk of hypothenar hammer syndrome.
- The Strain Index is limited to predicting risk for distal upper extremity neuromusculoskeletal disorders.
- Three of the six task variables are subjectively evaluated by the analyst.
- The multiplier values are primarily based on the authors’ professional opinions with support from physiological, biomechanical, and epidemiological principals as opposed to a mathematical relationship between task variables.
- At the time the Strain Index was originally published, it had been tested on only 25 jobs in one industry.
Q: Has there been any additional research on the Strain Index since the original publication?
- Yes. Ergoweb spoke with author J. Steven Moore and learned that additional research and further clarification regarding the reliability and application of this job analysis tool has been completed, and additional research is underway to further enhance the tool and it’s application. Ergoweb will report on these advancements as we learn more.
Q: What is the output of this tool?
A: Once the six task variables have been defined for the task being evaluated and the appropriate multiplier derived, the multipliers are multiplied to give the Strain Index score (SI score).
SI Score = (Intensity of Exertion Multiplier) * (Duration of Exertion Multiplier) * (Exertions per Minute Multiplier) * (Posture Multiplier) * (Speed of Work Multiplier) * (Duration per Day Multiplier).
It was found that jobs associated with distal upper extremity disorders had scores greater than 5.
For more information and assistance with the Strain Index, consider these Ergoweb resources, from which portions of this article were taken: