Physical Work Factors of Upper Limb Musculoskeletal isorders

Thomas J. Armstrong

The University of Michigan
Ann Arbor, MI 48109-1522

Copyright © 2002, Thomas J. Armstrong


--- Contents ---

I. Introduction
2. Job Documentation
3. Repetition
4.Force
5. Contact Stress
6. Posture

1. Introduction

Previous studies demonstrate associations between certain physical work stresses, localized fatigue and chronic musculoskeletal disorders of the upper limb (see Figure 6.1) (see: Kuorinka et al. 1994; Bernard et al. 1997). These work stresses are referred to here as ergonomic stresses. This section is intended to show the reader how to look for these stresses and some of the things that can be done to control them. The procedures for identifying these factors range from qualitative observations to quantitative measurements using electromyography and goniometry. Even if quantitative measurements are desired, it is recommended that an observational approach be used first to determine those aspects of the job that are to be quantified. While some instrumental methods are described, this section focuses on observational methods.

Figure 1: Proposed dose-response relationship for work related upper limb disorders (vertical axis) and Repetion (horizontal axis) for high and low force.

Many examples are used to help the reader begin to recognize them when they see similar situations in the work place. In many cases the examples also show how the stresses are controlled. This is to help the reader understand the relationship between work design and ergonomic stresses to help them not only recognize, but to anticipate situations that are more stressful than necessary. This also lays the ground work for the discussion of work design in Section IV. In many cases, interventions for control of stresses will be obvious, but thorough analysis is necessary before control measures or interventions are attempted. It may be found that the same intervention addresses several stresses. The following section offers a more systematic approach to job design and presents additional examples.

1.1 Reasons for Analyzing Ergonomic Stresses

  1. Identifying causes of upper limb disorders
  2. Designing interventions
  3. Evaluating interventions
  4. Placing restricted workers

1.2 Ergonomic Stresses

Ergonomic stresses or Generic Risk Factors are common attributes of different industries and occupations and are associated with discomfort, impaired work performance and chronic muscle, tendon and nerve disorders. Examples include repetitive exertions, static exertions, forceful exertions, localized mechanical stresses, posture stresses, low temperatures, and vibration --- see Table 6.1. These factors may be found alone or in combination with one another. They are distinguished from job specific factors, such as a particular tool that exposes workers to vibration and posture stress or an incentive work standard that results in excessive work rates in given situations.

Table 6.1: Physical ergonomic stresses or generic risk factors.

Repeated exertions: Exertions or movements that are repeated in the same manner
Static exertions: Forces or positions of the body that are maintained throughout each work cycle or for prolonged periods
Forceful exertions: An exertion performed to overcome weight, resistance, or inertia of the body or a work object
Contact stresses: Mechanical tissue stresses in the area of contact with external objects
Posture stresses: Positions of the body that require more effort than others or result in compression or stretching of tissues in or around the joints, e.g. nerves or tendons
Low temperature: Contact of the hand with air or work objects below 20°C or exposure of the worker to low ambient temperatures that result in reduced peripheral circulation
Vibration: Contact of the hands with vibrating objects

1.3 Procedures

2. Job Documentation

2.1 Required Information

Job Title/Objective The name of the job. The job title should be descriptive of what the operator does or the process. For example, the job title "clerical worker" suggests that this job involves clerical work, but does not indicate whether this position involves use of the telephone, copy machine, computer or all of them. The titles "transcriptionist," "receptionist" or "word processor" are more descriptive of what the worker does.
Production Standard The amount and quality of work that is to be completed in a given period. The production standard may be explicitly stated as a standard time, production quota or line speed. It may also be implicitly stated as the completion of certain tasks or goals.
Location The department, line position and/or building location.
Tasks Groups of work elements or steps performed to accomplish a common objective or sub-objective. Tasks are usually somewhat independent of each other and may be performed in varying order, while steps usually must happen in a prescribed order. Examples of tasks are:
  • monitor operation of mold machine,
  • remove parts from machine,
  • trim parts,
  • assemble sub-assemblies,
  • pack parts,
  • service machine.
Methods The sequence of steps or elements required to complete a task. For example the steps required to load and activate a apress might be:
  1. reach for part in bin - LH (left hand)
  2. grasp part - LH
  3. move part - LH
  4. position part in machine - LH
  5. release part - LH
  6. reach for palm button - LH & RH
  7. apply pressure to palm button LH. & RH
A simplified description the press task might be:
  1. get part
  2. put part in machine
  3. activate machine.
Tools and Equipment A list of tools, machines, and personal protective devices used to perform the job. The list should include:
  • make and model
  • weight, torque, or other information affecting force requirements
  • critical dimensions affecting posture
Materials Materials, sub-assemblies, assemblies, or scrap that is handled or used by the worker.
Environment Temperature, vibration, noise, lighting, inside/outside

2.3 Sources of Information

Observations

Video taping

Work sampling

See: Work sampling example.

Interviews

See: McCormick (1982) for guidelines on conducting interviews

2.4 Recording of Information

Detailed written documentation of each job is not necessary for the inspection; however, a cursory attention to each factor will help to assure that the inspection is systematic and that major factors have not been overlooked. The documentation information is used to identify and quantify the ergonomic stresses identified in the next step of the job assessment and to assist with the design of possible interventions. Additional information can be added as necessary to support the ergonomic assessment.