10.1.1 Exertion assessment

10.1.1.1.1 Percentage of Maximum Recommended Exertion

The biomechanical indicator obtained without applying the epidemiological priority adjustment shows what proportion of the reference moment is demanded by the task. Depending on the analysis mode and the enabled settings, this indicator is called the Percentage of Recommended Joint Capacity (%RJC) or the Percentage of Maximum Isometric Capacity Used, as explained in the previous section.

The Percentage of Maximum Recommended Exertion (%MRE) supplements this assessment by incorporating the observed prevalence of musculoskeletal disorders in the different body regions. It thereby increases the priority assigned to regions whose prevalence is higher than the reference prevalence and reduces it in regions where prevalence is lower. The ergonomist thus has an indicator that combines the task's biomechanical demands with the observed epidemiological distribution of musculoskeletal disorders, helping prioritize analyses and possible intervention measures.

To enable this adjustment, select Calculate max. recommended exertion (%) under Epidemiological priority adjustment in the Biomechanical Analysis Settings dialog (Figure 41). This dialog is opened with the gear-shaped button at the bottom of the Exertion window (Figure 38).

In the instantaneous assessment, the epidemiological adjustment is applied after the reference moment for the analysis mode has been determined. It does not modify the required moment, the individualized maximum isometric capacity, the Base Maximum Moment, or the internal evolution of the 3CC-r model. Its function is to weight the applicable reference moment and the percentage obtained from it.

The prevalences are taken from the systematic review and meta-analysis by Govaerts et al. (2021)² of workers in European secondary industries. The study's mean 12-month prevalences are used and assigned to the model regions as described below.

Ergoniza Intelligent internally defines a reference prevalence (P_ref) to normalize the adjustment. This reference is the arithmetic mean of the seven prevalences assigned to the model regions and has a value of 31.57%. For each body region z, the weighting factor is calculated as follows:

\[W_{z} = \left( \frac{P_{z}}{P_{ref}} \right)^{\lambda}\]

Equation 8

Here, P_z is the prevalence for the region and λ is the Influence parameter. The user can change this parameter using the Influence control under Epidemiological priority adjustment in the Biomechanical Analysis Settings dialog (Figure 41). It can take values from 0.1 to 0.9, and its initial value is 0.5. λ does not represent a percentage contribution; it controls the strength of the weighting. The higher its value, the greater the difference established between regions with high and low prevalences. If a region's prevalence equals the reference prevalence, its weighting factor is 1.

To express the calculation consistently in both analysis modes, M_ref,z denotes the biomechanical reference moment obtained before applying the epidemiological adjustment. This moment corresponds to:

• The Base Maximum Moment in Single Posture mode.

• The 3CC-r Recommended Moment for the instant being analyzed in Multiple Postures mode when the 3CC-r adjustment is enabled.

• The individualized maximum isometric capacity in Multiple Postures mode when the 3CC-r adjustment is disabled or unavailable.

The reference limit resulting from the epidemiological adjustment is obtained by dividing the biomechanical reference moment by the weighting factor for the region:

\[M_{epi,z} = \frac{M_{ref,z}}{W_{z}}\]

Equation 9

The Percentage of Maximum Recommended Exertion is therefore calculated as follows:

\[{\% MRE}_{z} = 100 \bullet \frac{\left| M_{req,z} \right|}{\left| M_{epi,z} \right|}\]

Equation 10

This result is equivalent to multiplying the percentage that would be displayed without the epidemiological adjustment by W_z. A prevalence above the reference prevalence therefore increases the displayed percentage, while a prevalence below it reduces the percentage. This reduction does not mean that the joint has greater physiological capacity, that the required moment is lower, or that the task is biomechanically safer; it indicates only a lower relative epidemiological priority.

The %MRE should be interpreted as a comparative preventive-priority index, not as a measure of physiological capacity, an individual's probability of injury, or a causal estimate. The adjustment does not modify the mean maximum capacity, its standard deviation, or the Percentage of Protected or Unprotected Population. The same epidemiological weighting is applied to the Cumulative exertion load result, as explained in Section 10.3, which is presented as an indicator independent of the instantaneous %MRE and the temporal adjustment by 3CC-r.

When Calculate max. recommended exertion (%) is disabled, the displayed result depends on the mode and the reference moment used:

• In Single Posture mode, the %RJC calculated relative to the Base Maximum Moment is displayed.

• In Multiple Postures mode with the 3CC-r adjustment enabled, the %RJC calculated relative to the 3CC-r Recommended Moment is displayed.

• In Multiple Postures mode with the 3CC-r adjustment disabled or unavailable, the Percentage of Maximum Isometric Capacity Used, calculated relative to the individualized maximum isometric capacity, is displayed.

10.1.1.1.2 Representation in the interface

When the customized exertion-assessment procedure is used, the Exertions table in the Exertion window (Figure 38) displays the required moment, the active muscle group, the population distribution of joint capacity, and the reference moment used to assess the task demands.

In the results table, the Recomm. column displays the percentage for the active assessment criterion. When the epidemiological priority adjustment is enabled, the Percentage of Maximum Recommended Exertion (%MRE) is displayed. When it is disabled, the Percentage of Recommended Joint Capacity (%RJC) is displayed if the reference is the Base Maximum Moment (in Single Posture mode) or the 3CC-r Recommended Moment (in Multiple Postures mode with this adjustment enabled). If the 3CC-r adjustment is not used in Multiple Postures mode, the percentage of maximum isometric capacity used is displayed.

The name of the indicator shown above the table and on the chart is updated automatically according to the analysis mode and enabled settings:

• % maximum recommended exertion, when the epidemiological priority adjustment is enabled.

• % recommended joint capacity, when the epidemiological adjustment is disabled and the reference moment incorporates the temporal adjustment for the analysis mode.

• % maximum isometric capacity used, in Multiple Postures mode when both the epidemiological adjustment and the 3CC-r adjustment are disabled.

The last column of the table shows the level reached by the calculated percentage. If the value is below the Warning threshold, a green check mark appears. When it reaches the Warning threshold, an orange exclamation mark appears; when it reaches the Risk threshold, the exclamation mark is red. The initial values of these thresholds are 85% and 100%, respectively.

The user can change these values with the Orange threshold (%) and Red threshold (%) controls in the Visualization panel of the settings side menu. The thresholds apply to the actual calculated percentage regardless of the name of the displayed indicator.

A red symbol with a horizontal white line indicates that the joint is in an extreme posture in which exertion is not recommended for the movement and direction being analyzed; the corresponding reference moment is therefore zero. This is the case for left-wrist lateral deviation in Figure 38. Situations involving passive joint structures are described in greater detail in the following section.

The chart at the top of the Exertion window represents the active indicator for the different joints and movements. Bars are green when the value is below the Warning threshold, orange when it lies between the Warning and Risk thresholds, and red when it reaches or exceeds the Risk threshold.

When the 3CC-r temporal adjustment is used, the available recommended capacity may approach zero and produce very high percentages. To preserve readability, values above 500% are identified with the label >500. Bar height is visually limited to the chart's maximum scale; this limitation does not alter the actual value used to determine the risk level.

Figure 38 corresponds to an analysis performed in Single Posture mode. For this reason, the duration and repetitiveness conditions used to calculate the Base Maximum Moment are shown below the chart. In the example, the exertion is not repeated, is sporadic, and is sustained for 11 seconds. Left-shoulder abduction exceeds the Warning threshold, and right-shoulder lateral rotation greatly exceeds the Risk threshold. The worker's selected body build, corresponding to the 50th percentile in the example, is also shown above the chart.

In Multiple Postures mode, the percentage and the 3CC-r Recommended Moment may vary at each instant because of the previous history of exertions and recoveries. The displayed values therefore always correspond to the currently selected instant in the sequence.

10.1.1.1.3 Joint-limit situations at the elbow and knee

In certain postures, some joints—particularly those analyzed by the biomechanical model only through flexion and extension, such as the elbow and knee—may be at one end of their range of motion while the calculated external moment tends to move the joint beyond that limit, as in the case shown in Figure 39.

Under these circumstances, the joint moment still exists and must be resisted, but it should not be interpreted exclusively as a conventional active muscular demand. Part of the resistance may be borne by passive structures such as ligaments, the joint capsule, bone geometry, and periarticular tissues. Passive joint resistance is defined precisely as the torque exerted by viscoelastic tissues whose primary function is to restrict the range of motion³.

When the elbow is extended, the anterior capsule plays an important role in resisting certain loads, and the collateral ligaments contribute to lateral stability when the elbow is extended⁴. At terminal knee extension or hyperextension, stability depends heavily on passive structures and joint locking, so a moment tending to extend the knee further should not automatically be interpreted as an extensor muscle demand.

For example, if the knee is fully extended and the external moment tends to produce further extension, the response is not necessarily equivalent to an extensor muscle contraction comparable with a percentage of maximum voluntary contraction (%MVC). In that case, the result should be interpreted as a joint-limit condition involving passive structures, not as an ordinary active muscular exertion.

In these situations, Ergoniza Intelligent continues to calculate and display the joint moment because the mechanical load on the joint remains relevant. However, when it detects this condition, it identifies it visually in the interface. As shown in Figure 39, the bars in the exertion chart retain the color corresponding to the calculated demand level but include a purple border to indicate that the result occurs in a joint-limit situation. In the exertion table, the same condition is indicated by a padlock icon next to the assessment indicator, warning the user that the value must be interpreted cautiously because the moment may include a substantial contribution from passive structures and should not be interpreted directly as an ordinary active muscular demand.