Ergo Tip of the Month

January 2006

Sandalwood Enterprises is a consulting firm that specializes in Ergonomics, Process Optimization, and Information Management. The Ergo Tip of the Month is offered to our customers and friends for the benefit of the Ergonomics community. If you choose not to receive future editions, contact us at ergotip@sandalwood.com

An advanced measurement technique for assessing physical task demands: When the standard ergonomic assessment tools just don't 'cut it'

Have you ever tried to assess a task where the physical demands are difficult to quantify and/or the traditional ergonomic assessment tools do not provide a definitive answer? Then electromyography (EMG) could be the answer you are looking for.

Electromyography (EMG) is a measure of the electrical activity within a muscle. Information from EMG can be useful in the design and assessment of workstations; ultimately aiding in reducing the risk of work-related injury. For example, EMG can assist in identifying which muscles are active and to what intensity during a specific task. It has also been used to estimate the force production of a muscle and compare this to a maximum

voluntary contraction (MVC: The largest force that can be voluntarily exerted by a muscle).

How is Surface EMG used in work settings?

Surface electromyography (sEMG) detects muscle activity through the skin's surface. Small rubber electrodes (2 per muscle belly) are fixed to the surface of the skin overtop of the muscle(s) and one additional electrode acts as a ground or reference (located over a bony surface).

Portable EMG systems are best suited for work settings. With the system attached to the worker, he or she can continue to perform their usual activities while the EMG collection device records and stores data that can be later transferred to a computer software program for analysis.

What do you do with the Data?

As mentioned, EMG measures the electrical activity that causes a muscle contraction. As such, the amplitude of the EMG signal is highly correlated with the muscle force. If a maximum effort is obtained, the amplitudes can be normalized to indicate the relative exertion of each muscle that was monitored. The time-history of these amplitudes can be compared to published standards (for example: Jonsson, 1982).

How much is too much?

According to Jonsson (1982), an unacceptable risk of cumulative trauma disorder and/or fatigue exists if muscle activation times deviate from the following guidelines:

  • For 10% of the total activity time, muscle activation must be below 2% of maximum (termed: Static level)
  • For 50% of the total activity time, muscle activation must be below 10% of maximum (termed: Median level)
  • For 90% of the total activity time, muscle activation must be below 50% of maximum (termed: Peak level)
    •  In other words, the guidelines determine if a muscle is getting enough rest (termed the Static limit), if the average levels are too high (termed the Median limit) and if the muscle is spending too much time at high levels of force (termed the Peak limit).

    See the EMG tracing below for an example:

    Practical Examples for using EMG in the Workplace

  • Repetitive typing and mouse work in an office environment.
  • Awkward, static postures assumed by nursing aids and homecare workers during patient care (i.e. Bathing).
  • Continuous cutting, sewing and folding of material required by seamstresses or clothing manufacturers.
  • Continuous tilting and twisting of the neck by crane operators or lift truck drivers.
  • Repetitively dealing and shuffling cards, as done by card dealers in a Casino.
  • Do you have an example in your workplace?
    •  The next time you're having difficulty with an ergonomic analysis; consider using EMG as an alternative assessment technique.

    Reference:
    Jonsson, Bengt. (1982). Measurement and Evaluation of Local Muscular Strain in the Shoulder During Constrained Work. J. Human Ergol. 11: 73-88.

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