Measuring Ergonomic Risk Factors: Part I of II
Posted by Jenn Preziosi on Mon, Jul 26, 2010 @ 02:46 PM
Now that you know how to determine ergonomic risk factors, read this post to learn the methods recommended for measuring and quantifying these risk factors.
In this post we will discuss the risk factors of Magnitude, Repetition, Duration and Force.
Risk Factor Properties
Magnitude
Magnitude quantifies the extent to which each physical stress is involved.
Measurements of force, posture, recovery, vibration and cold magnitude is reported in terms of the average, peak, or root means square levels for the duration of the specific exertion, motion, or posture.
Repetition
Repetition measure depends on the level. At the most detailed level, each time a specific exertion is repeated, such as striking a nail with a hammer, one repetition about the elbow occurs. Likewise, striking a key on a keyboard would be considered one repetition. A higher order of quantification for repetition may be the frequency that a cycle for a task repeats. In the case of the hammer, one repetition may be counted every time another nail is hammered. A less detailed study may quantify for many times a hammer is used in the course of a shift. Consequently, different studies quantifying repetition may not be measuring the same thing. This has made it difficult to compare different studies. It is therefore important that repetition measurements specify the specific action and level of detail being measured (micro, element, cycle, task, etc.).
Duration
Duration is quantified as the time elapsed when a specific exertion is performed or a posture is assumed. At the most detailed level, duration is the movement time or exertion time for a specific muscle-tendon group or body region (e.g., right hand and wrist). Less detailed analyses use element times, cycle times, or task time as measures of duration.
Force
Forces may be either external or internal forces. An external force can be defined as a force applied, voluntarily or involuntarily, to the surface of the body. In general, internal forces increase joint's torque, angular velocity or acceleration. Since internal forces are not conveniently measured, the term "force" shall refer to external forces unless specifically indicated.
The following methods are available for quantifying force:
- Rough approximations
- Simple mechanical scales
- Electronic load cells
Simple mechanical devices such as a spring scale or dynamometer can be used to estimate lift/pull/push forces in many instances. (Note: Care must be taken to ensure the alignment of the scale or dynamometer with the actual axis of the exertion.) Electronic versions of these devices can be connected to a recording device or computer to provide more accurate force-time data for dynamic tasks.
Direct force measurements are often difficult to obtain. Forces can often be roughly calculated using the weight of the objects, estimates of the frictional forces, the power settings on tools, and simple physics equations.
Internal muscle forces are difficult to measure, but can be estimated with Electromyography (EMG) by trained personnel using specialized equipment. Under controlled conditions, internal muscle forces can be estimated by simulating the motions and exertions in a laboratory setting. If internal forces are measured, sufficient replicate measurements should be made to account for variability within and between individuals performing the task.
Pressure and force can be measured using ink force sensors and strain gauges, but this is rarely done due to difficulties in using the equipment. The conditions that cause high contact stresses are well recognized and are usually eliminated without measuring the level of stress.
Element exertion and recovery periods should be measured for each relevant muscle- tendon group, body region or joint.
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In Part II of this post series, we will discuss: Posture and Motion, Vibration, Cold Temperatures, Work Organization and Psychosocial & Psychophysical factors.
