Ergoniza Intelligent

10.2.2 Recommended %MVC for static exertions

When an exertion is static, meaning that it is sustained over time, continuous muscle contraction can produce fatigue. Muscle fatigue can be defined as a reduction in the ability to generate force or power as a consequence of the activity performed, and it can contribute to the development of musculoskeletal disorders15F1. Various biomechanical models therefore incorporate procedures for estimating how fatigue develops16F2, particularly in virtual human simulation applied to ergonomics17F3. Their purpose is to facilitate the assessment of sustained exertions and support the design of tasks and recovery periods.

Maximum Endurance Time (MET) is the maximum time for which a static contraction of a given intensity can be maintained before the exhaustion criterion is reached or the required exertion can no longer be sustained. MET decreases as contraction intensity increases and differs substantially among joint regions.

Muscle-fatigue modeling in ergonomics frequently uses Maximum Endurance Time models. These models make it possible to estimate the endurance of a muscle group or joint region without individually representing the activation and fatigue of each muscle. They are constructed from experimental data obtained for different contraction intensities and body regions. One of the best-known is Rohmert's model18F4, traditionally used to analyze static contractions. This and other empirical models19F5 provide reference values for limiting the intensity and duration of sustained exertions and for guiding the design of work and recovery periods.

For static exertions, Ergoniza Intelligent calculates the recommended %MVC using the joint-region-specific power models obtained by Frey Law and Avin20F6 from a systematic review and meta-analysis of studies on static contractions.

These models relate relative contraction intensity to Maximum Endurance Time and show that this relationship varies among joint regions. Their general expression is presented in Equation 11, in which the parameters b0 and b1 depend on the joint region being analyzed (z).

\[METz = \ b_{0,z}\ {MVC}_{z}^{b1,z}\]

Equation 11

where:

  • METz is the maximum endurance time, expressed in seconds.

  • MVCz is the contraction intensity expressed as a fraction of maximum capacity, between 0 and 1.

  • b0,z and b1,z are the parameters corresponding to the joint region.

In Ergoniza Intelligent, the user can use the dialog shown in Figure 42 to set the exertion duration after selecting “Does not repeat; it is sporadic but is sustained for a certain duration” in the “Repetitiveness” dialog. Given the entered exertion duration, Ergoniza Intelligent calculates the recommended maximum voluntary contraction by solving Equation 11 for MVC.

As a preventive criterion, Ergoniza Intelligent requires the entered duration not to exceed 50% of the maximum endurance time estimated by the models, following the Threshold Limit Value (TLV) guidelines published by ACGIH21F7 (American Conference of Governmental Industrial Hygienists). This introduces a margin relative to the experimentally predicted time to exhaustion. The criterion is used to obtain a recommended contraction intensity and should not be interpreted as an exact prediction of when fatigue will occur in a particular worker. ACGIH recommends using this threshold as a practical reference for limiting the duration of static muscle contractions.

  1. R. H. Westgaard, “Work-Related Musculoskeletal Complaints: Some Ergonomics Challenges upon the Start of a New Century,” Applied Ergonomics 31, no. 6 (2000): 569–80, doi:10.1016/S0003-6870(00)00036-3.↩︎

  2. L. Ma et al., “A Novel Approach for Determining Fatigue Resistances of Different Muscle Groups in Static Cases,” International Journal of Industrial Ergonomics 41, no. 1 (2011): 10–18, doi:10.1016/j.ergon.2010.11.005; T. Xia and L. A. Frey Law, “A Theoretical Approach for Modeling Peripheral Muscle Fatigue and Recovery,” Journal of Biomechanics 41, no. 14 (2008): 3046–52, doi:10.1016/j.jbiomech.2008.07.013.↩︎

  3. H. Honglun, S. Shouqian, and P. Yunhe, “Research on Virtual Human in Ergonomic Simulation,” Computers and Industrial Engineering 53, no. 2 (2007): 350–56, doi:10.1016/j.cie.2007.06.027.↩︎

  4. W. Rohmert, “Ermittlung von Erholungspausen F¨ur Statische Arbeit Des Menschen,” European Journal of Applied Physiology 18, no. 2 (1960): 123–164.↩︎

  5. K. El Ahrache, Da. Imbeau, and B. Farbos, “Percentile Values for Determining Maximum Endurance Times for Static Muscular Work,” International Journal of Industrial Ergonomics 36, no. 2 (2006): 99–108, doi:10.1016/j.ergon.2005.08.003.↩︎

  6. Laura A.Frey Law and Keith G. Avin, “Endurance Time Is Joint-Specific: A Modelling and Meta-Analysis Investigation,” Ergonomics 53, no. 1 (2010): 109–29, doi:10.1080/00140130903389068.↩︎

  7. ACGIH, “Ergonomic Threshold Limit Value (TLV) for Hand Activity Level (HAL) and Muscle Fatigue,” in 2016 TLVs and BEIs (Cincinnati, OH: ACGIH, 2016), ERG-1–ERG-14.↩︎

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