A program for estimating risk of work related musculoskeletal disorders
1. Introduction
2. Data Collection
A. Efforts per Minute
B. Percent Duration
C. Intensity of Effort
D. Postures
E. Speed
F. Hours per Day
3. Calculated Items
C. Multipliers
a. Efforts per Minute Multiplier
b. Percent Duration of Effort Multiplier
c. Intensity of Effort Multiplier
d. Posture Multiplier
e. Speed Multiplier
f. Hours per Day Multiplier
D. Strain Index
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The StrainAnalysis program is intended to aid in the identification of jobs and job tasks that may present an increased risk for work related musculoskeletal disorders in the distal upper extremities (from the elbow down to the fingers). The program is based upon the Strain Index equation, which was published by Moore and Garg in the American Industrial Hygiene Association Journal in May 1995. This equation takes into account the interplay among a number of weighted ergonomic stressors and calculates a numerical figure which is associated with the risk of workers developing musculoskeletal disorders of the upper extremities. The work of Moore and Garg suggests that job tasks with a strain index of 5.0 or greater present such a risk, while tasks with a strain index less than 5.0 are unlikely to be associated with such risk. Interested readers and users of this system are encouraged to obtain a copy of this article.
In addition to the strain index itself, the program provides the numerical multipliers for each of the ergonomic stressors. This is helpful in job analyses in that it aids in the identification of the specific components of a task which are most in need of ergonomic remediation.
In its present form, the strain index does not address neck problems, nor does it address proximal upper extremity problems (such as shoulder strain). Vibration related problems are also not included in this job analysis system.
To use this system for job task analysis, the job analyst or ergonomics team collects data for task variables. This analysis takes place during observation of the job being performed by a worker or workers. Thus the system could be used for task analysis and immediate task remediation.
The specific data to be collected will be discussed below.
A. Efforts Per Minute. Over the course of observation of the task, the actual number of efforts observed (or job cycles) is tracked using a counter, and the figure for Efforts/Min is calculated.
B. Percent Duration of Effort. This refers to the amount of time during a job cycle that the worker is actually performing the effort, divided by the total time of the job cycle (effort time plus rest time). For example, if a job cycle takes one minute and the worker is applying force for 30 seconds, then Percent Duration of Effort is 50%.
C. Intensity of Effort. There are five categories which are somewhat subjective and are based on the use of verbal descriptors: Light, Somewhat Hard, Hard, Very Hard, Near Maximal. If in doubt, a consensus should be sought among ergonomic team members and workers performing the job task.
D. Posture. There are five categories which are somewhat subjective and are based on the use of verbal descriptors: Very Good, Good, Fair, Bad, Very Bad. These refer to hand/wrist postures and correlate somewhat with measured ranges of motion. If in doubt, a consensus should be sought among ergonomic team members and workers performing the job task. It is important to realize that these are estimates of overall upper extremity postural demands. For example, even if the actual hand/wrist posture qualified for a "Good" rating, a job that required awkward positioning of the thumb joints would push the posture rating into a less favorable category.
E. Speed. There are five categories which categories are somewhat subjective and are based on the use of verbal descriptors: Very Slow, Slow, Fair, Fast, and Very Fast. If in doubt, a consensus should be sought among ergonomic team members and workers performing the job task.
F. Hours per Day. This refers to the number of hours per day (assuming a 5-day workweek) the worker performs the job task being analyzed.
After all data are entered, click the Calculate button to perform the calculations.
The StrainAnalysis program calculates efforts per minute, percent duration of effort, the strain index for the task, as well as multipliers for each of the task variables. The multipliers provide a weighted assessment of the ergonomic stress on the distal upper extremity for the associated task variable. For each multiplier, the lower the value the more favorable the task variable.
Specific calculated items are as follows.
A. Efforts per Minute. This is simply based on the number of efforts observed and the total observation time.
B. Percent Duration of Effort. This is based on the total effort time and total observation time, and is simply the percentage of time during the task in which work is being done (as opposed to rest time).
C. Multipliers:
a. Efforts per Minute Multiplier. This is a function of efforts per minute, and varies from 0.5 to 3.0, depending on how repetitive the task is. However, in the case in which percent duration of effort is 100%, this multiplier is automatically set to 3.0 regardless of the efforts per minute.
b. Percent Duration of Effort Multiplier. This is a function of percent duration of effort, and varies from 0.5 to 3.0.
c. Intensity of Effort Multiplier. This is a function of intensity of effort. It varies from 1.0 to 13.0. As you can see, this is heavily weighted and thus intensity of effort is an important task variable.
d. Posture Multiplier. This is a function of the hand/wrist posture estimate, and varies from 1.0 to 3.0.
e. Speed Multiplier. This is based on the speed of the job and varies from 1.0 to 2.0. f. Hours per Day Multiplier. This is a function of the hours worked (at the job being evaluated) per day, and varies from 0.25 to 1.5.
D. Strain Index. This is an overall estimate of the relative risk of an association between a job task and distal upper extremity work related musculoskeletal disorders. A figure lower than 5.0 is suggestive of a lower such risk, while the incidence of these types of problems may climb when the Strain Index is 5.0 or greater.
4. Worksheet
This worksheet can be printed and used at the job site for data collection.