11.2.2 Absolute discomfort assessment
Absolute discomfort is the value estimated for a posture maintained for 60 seconds without handling external loads. Ergoniza Intelligent calculates it for each joint, movement, and direction using the regression equations described in Section 11.2.1.
The regression equations provide a continuous assessment of the effect of angle on comfort, avoiding artificial changes when discrete thresholds are crossed. They also associate the verbal categories (e.g., very good, good, moderate, marginal, somewhat poor, …) with a numerical value on the same normalized scale. This correspondence makes it possible to use these categories as interpretable comfort/discomfort references and, above all, to anchor the indicator at a selected level according to interpretation needs and convergence with other frameworks (such as ISO 11226, EN 1005-4, RULA, or REBA).
Ergoniza Intelligent generates an absolute-discomfort indicator as a percentage, with 0% representing the optimal condition and 100% the value that should not be exceeded. This is performed as follows. A regression equation is available for each joint/movement j46F1:
\[y(ang) = f_{j}(ang)\]
Equation 31
where:
ang is the movement angle (in degrees, within the allowable range or ROM),
y is the continuous normalized comfort value predicted by the model.
A comfortable reference value is established so that the indicator starts at 0 in the neutral movement posture (ang=0):
\[y_{ref,j} = y(0) = \ f_{j}(0)\]
Equation 32
Finally, a value Ck,j is selected for each joint or body region j as the operational limit, that is, the comfort value associated with 100% discomfort. This value is not the same for all joints; it is established specifically for each body region, movement, movement direction, and analyzed postural condition. Absolute discomfort (Dj) is thus defined as:
\[D_{j}(ang) = 100\ \frac{y_{ref,j} - y_{j}(ang)}{y_{ref,j} - C_{k,j}}\]
Equation 33
where yref,j is the comfort value predicted by the equation for joint j in the neutral posture, yj(ang) is the comfort value predicted for the analyzed angle, and Ck,j is the comfort value used as the reference for the maximum recommended limit at that joint and movement.
As defined, discomfort Dj satisfies the following:
Dj(0) = 0; that is, discomfort at joint j in the neutral posture is 0.
Dj(ang) = 100 when yj(ang) = Ck,j; that is, discomfort reaches 100% when the comfort predicted by the regression reaches the value established as the recommended limit.
Ergoniza Intelligent selects Ck,j using two criteria. First, it uses the recommendations of established standards and methods such as ISO 11226, EN 1005-4, RULA, or REBA when they provide relevant recommendations. When the standards do not specify a clear limit or do not consider a particular movement at a joint, the value corresponding to the Marginal verbal category in Table 5 is used.
This approach simultaneously satisfies two requirements: it preserves the progression (slope/curvature) of the regression equations, and therefore of the psychophysical data, while allowing an operational threshold (100%) to be established in alignment with an external criterion.
This adjustment does not alter the shape of the regression curve; it only rescales the axis so that the 100 threshold has a meaning aligned with the literature and preventive practice while preserving continuity, angular sensitivity, and differentiation by sex.
As an example, Figure 51 shows the calculation of absolute discomfort (purple line) in the forearm from the elbow-flexion angle for women. Discomfort is assessed as a percentage and increases as the angle increases. The figure also shows the regression equation (y(x)) and the comfortable reference value (97.77) corresponding to y(0).
Figure 51: Calculation of postural discomfort in the forearm.
The vertical red line marks the angle at which regression equation y(x) reaches the “Marginal” value (58.0 in Table 5), beyond which comfort begins to deteriorate. In Ergoniza Intelligent, this angle corresponds to 100% discomfort, that is, the point beyond which the posture can be considered uncomfortable. Postures that reach a discomfort value of 85% are shown in orange.
As shown in Figure 51, the discomfort graph extends beyond the joint's maximum ROM. If the user does not limit joint ranges while manipulating the model (by disabling Biological Constraints in the interface), unnatural joint angles adopted by the model may produce inconsistent displayed discomfort values.
11.2.2.1 Considering upper-limb orientation relative to gravity
When assessing upper-limb postural discomfort, Ergoniza Intelligent can consider segment orientation relative to gravity. This option affects the calculation of upper-arm and forearm discomfort and prevents a posture from being assessed solely from the relative angles between segments when trunk inclination substantially changes the actual demand of supporting the limb.
The basic postural-discomfort calculation uses the position of each joint relative to the preceding segment. For the upper arm, this means assessing its position relative to the trunk; for the forearm, its position relative to the upper arm. This approach is suitable in many situations, especially when the trunk is upright. In that posture, an upper arm aligned with the trunk approximately corresponds to an arm hanging vertically toward the floor and is therefore favorably oriented relative to gravity.
However, when the trunk tilts, flexes, or bends laterally, the anatomical and gravitational references no longer coincide. For example, if the trunk is flexed forward, an upper arm parallel to the trunk may be close to the neutral position relative to the shoulder but oriented horizontally in space. In that situation, even though the relative upper-arm-to-trunk angle is small, maintaining the arm requires greater muscle activity to support its weight against gravity. Conversely, an arm forming a large angle relative to the trunk may hang vertically toward the floor and have a more favorable orientation.
Figure 52: Postural-discomfort calculation options
For this reason, when Gravity Adjustment (Figure 52) is enabled, Ergoniza Intelligent introduces a correction to upper-limb postural discomfort. This correction does not replace the joint calculation based on relative angles; it partially reduces the result when the orientation of the upper arm or forearm is clearly favored by gravity.
For the upper arm, the system calculates trunk orientation relative to vertical and upper-arm orientation relative to downward vertical. The influence of the correction increases as the trunk moves away from vertical. When the trunk is nearly upright, the correction is zero or very small because the calculation based on the relative upper-arm-to-trunk angle is sufficiently representative. As the trunk inclines, the actual orientation of the upper arm relative to gravity becomes more important. Its influence is greatest when the arm is aligned with downward vertical and disappears as the arm moves away from that direction.
For the forearm, the system calculates the orientation of the segment between the elbow and wrist relative to downward vertical. The correction is applied only when the forearm is close to a natural hanging orientation toward the floor. As with the upper arm, maximum gravity assistance is considered to occur when the forearm is aligned with downward vertical and progressively disappears as it moves away from that direction. This correction is applied less strongly than for the upper arm because elbow discomfort depends primarily on joint flexion, and the mechanical demand associated with forearm weight is also reflected in the biomechanical exertion analysis.
The selector shown in Figure 52 defines the maximum intensity of this correction (to access this dialog, click the gear-shaped button at the bottom of the Postural Load panel shown in Figure 59). This value does not directly indicate the reduction in discomfort, but the maximum weight that favorable orientation relative to gravity can have in the final calculation. For example, Medium (70%) indicates that, when the posture meets the conditions for the correction to act at maximum intensity, orientation relative to gravity can appreciably reduce the final assessment, although the relative-joint criterion still contributes to the result.
The correction is applied progressively. For the upper arm, if the trunk is only slightly inclined, the effect of gravity will be small even if the selector has a high value. As trunk inclination increases, the correction approaches the selected maximum weight. For the forearm, the correction is more moderate and is applied according to its orientation relative to downward vertical.
This option does not turn the postural-discomfort calculation into a biomechanical analysis of moments or muscle exertion. Its purpose is to adjust continuous upper-limb postural assessment to account for the fact that the same relative position between segments may involve different demands depending on its orientation relative to gravity. Detailed analysis of moments, joint forces, and maximum recommended exertion percentage is performed in the Biomechanical Exertion module (Section 10).
Kee and Karwowski, “The Boundaries for Joint Angles of Isocomfort for Sitting and Standing Males Based on Perceived Comfort of Static Joint Postures”; Kee and Karwowski, “Joint Angles of Isocomfort for Female Subjects Based on the Psychophysical Scaling of Static Standing Postures.”↩︎