11.2.3 Time-relative postural discomfort in single-posture analysis
As explained in Section 11.2.2, Ergoniza Intelligent defines absolute discomfort as the discomfort measured for a posture held statically for 60 seconds. Time-relative postural discomfort is absolute discomfort adjusted to the actual time for which the analyzed posture is held. To calculate relative discomfort, Ergoniza Intelligent uses two different procedures depending on whether a single posture or an activity composed of multiple postures represented in an animation is being analyzed (see Section 11.2.4).
Figure 53: Postural discomfort in the shoulder over time.
When Ergoniza Intelligent is used in Single Posture mode, the analysis focuses on the single posture adopted by the biomechanical model. In this case, the user must specify how long the posture is held. To do so, the dialog shown in Figure 42 can be used to specify the duration and repetitiveness of the posture. This dialog can be opened with the Timing button on the left sidebar or with the clock icon at the bottom of the Postural Risk panel shown in Figure 59. The following sections explain how relative discomfort is calculated according to the duration and type of posture holding.
The Postural Discomfort Options dialog shown in Figure 52 includes an option for excluding posture-holding time from the postural discomfort calculation. When Relative to Time is disabled, the discomfort results shown by Ergoniza Intelligent correspond to absolute discomfort, that is, the discomfort calculated for postures held for 60 seconds.
11.2.3.1 Postural discomfort in a static posture
Static or sustained postures are those in which the body maintains the same posture for a more or less prolonged period, with little or no movement. Unlike dynamic or repetitive postures, which involve continuous movement or alternating body segments, static postures involve sustained muscle contraction that can cause localized fatigue.
In Ergoniza Intelligent, the user can use the dialog shown in Figure 42 to specify how long the posture is held by selecting Does not repeat; it is sporadic but is sustained for a certain duration in the Repetitiveness box. Once absolute discomfort has been calculated as explained in 11.2.2, its variation according to the duration of a static posture is considered to obtain time-relative discomfort. For this purpose, Ergoniza Intelligent uses an adaptation of the Law and Avin47F1 models (Equation 34) to predict Maximum Endurance Time (or “endurance time”) in static muscle contractions according to relative exertion intensity (%MVC) for different joints. Each equation follows a power law whose parameters, obtained by regression from experimental data, characterize how rapidly tolerance decreases when a posture is maintained at each joint.
\[t = \ b_{0}\ {MVC}^{b1}\]
Equation 34
The scale constant (b₀) establishes the temporal reference point, while the exponent (b₁) defines the decay curve: the more negative b₁ is, the more sharply endurance falls as relative load increases. Specific values for the ankle, knee, elbow, shoulder, and torso, together with a general case, allow these models to finely adapt the estimate of static fatigue to the anatomical and functional variability of each joint. This provides a robust tool for assessing risk and designing exposure guidelines that is suitable for sustained postures because it reproduces the nonlinear relationship observed between sustained exposure and exertion tolerance.
Ergoniza Intelligent adapts the models to postural discomfort over time as follows. First, it solves for MVC, yielding Equation 35.
\[D\left( t,{b_{0},b}_{1} \right) = MVC(t,{b_{0},b}_{1}) = \ \left( \frac{t}{b_{0}} \right)^{\frac{1}{b1}}\]
Equation 35
This curve is anchored to the discomfort value measured after the posture has been held for 60 seconds (absolute discomfort), yielding Equation 36.
\[D_{est}\left( t,b_{1} \right) = \ D_{absoluto}\left( \frac{t}{60} \right)^{\frac{1}{b1}}\]
Equation 36
Anchoring the curve to the value measured at 60 seconds ensures that the model reflects the experimentally verified discomfort level (absolute discomfort), while also capturing the change in fatigue as posture-holding time varies. The resulting value is again expressed as a percentage; 100% discomfort corresponds to the level at and above which the posture can be considered uncomfortable.
Figure 53 shows an example of the type of potential curve obtained with this approach. The curve was calculated for the shoulder joint when absolute discomfort in the adopted posture is 88%. The power exponent, fitted using joint-specific muscle endurance data, causes discomfort to increase rapidly during the first few seconds and more slowly as the posture continues. This behavior is consistent with endurance time studies48F2 and with subjective perception curves for static postures, in which the sensation increases more slowly after a rapid initial “jump” in fatigue.
11.2.3.2 Postural discomfort in a repetitive posture
To account for posture duration when assessing discomfort in repetitive or intermittent tasks—those in which a static posture is held for only part of a work cycle and alternates with pauses—while in single-posture analysis mode, Ergoniza Intelligent uses a method based on the ACGIH49F3 (American Conference of Governmental Industrial Hygienists) Threshold Limit Values (TLVs) for muscle loading.
When the Ergoniza Intelligent user selects Occurs periodically in repeating movement cycles in the Repetitiveness box shown in Figure 42, the duration of the work cycle, the duration of one exertion, and the number of exertions performed in each work cycle must be entered. Ergoniza Intelligent uses these data to calculate the effective time for which the posture is held within the cycle (t), that is, the number of seconds per minute (or other units of time) during which the posture—or postures similar to the one analyzed—is held. For example, if a person performing a cyclic task holds a certain posture for 10 seconds in every 60-second cycle, the duty cycle (DC) is 16.7%.
The procedure described in Section 11.2.3.1 is used to calculate the discomfort associated with the effective posture-holding time. This discomfort value is then corrected using the TLV factor corresponding to the observed duty cycle, as shown in Equation 37. In this equation, j represents the joint being analyzed, t is the effective posture-holding time, and DC is the percentage of effective posture-holding time relative to the total cycle time.
The parameter αj is the time exponent associated with joint j. Its function is to modulate the influence of posture-holding time on estimated discomfort. This parameter is obtained from the exponent b1,j of the Frey-Law and Avin Maximum Endurance Time models, expressed in Equation 34 as t = b0,j · MVC^b1,j. In these models, b1,j is negative and specific to joint j. Solving for relative intensity and normalizing the result against the 60-second reference time used by Ergoniza Intelligent to calculate absolute discomfort yields the positive exponent αj = -1 / b1,j. This exponent allows absolute discomfort, measured for 60 seconds of posture holding, to be rescaled to the actual exposure time t.
\[D_{rep,j}\left( t,DC,\alpha_{j} \right) = \ \frac{D_{absoluto,j}\left( \frac{t}{60} \right)^{\alpha_{j}}}{\left\lbrack - 0.143\ \bullet ln\left( \frac{DC}{100} \right) + 0.066 \right\rbrack}\]
Equation 37
The resulting value is again expressed as a percentage; 100% discomfort corresponds to the level at and above which the posture can be considered uncomfortable.
When the assessed joint belongs to the torso or lower extremities, Ergoniza Intelligent applies the same internal adaptation coefficients described for calculating recommended %MVC in repetitive tasks (see Table 3 in Section 10.2.3). These coefficients are not region-specific TLVs, but rather a conservative approximation based on relative differences in fatigue resistance described in the literature.
11.2.3.3 Postural discomfort in a momentary or unsustained posture
The procedures described in the preceding sections make it possible to assess postural discomfort when a posture is held for a certain time or forms part of a repetitive task. In these cases, exposure time is a fundamental variable because holding a posture away from neutral can cause fatigue, reduce exertion tolerance, and progressively increase the perception of discomfort.
However, not all postures last long enough to be interpreted as sustained postures. In some tasks, the posture being analyzed may be a momentary, transient, or nearly instantaneous position. Ergoniza Intelligent considers this to be the case when the user selects Does not repeat; it is sporadic and is sustained only momentarily in the dialog shown in Figure 42.
In these situations, directly applying a temporal fatigue model is inappropriate. A posture adopted for an instant does not consume endurance time in a way comparable to a posture held for several seconds or minutes. However, this does not mean that every momentary posture should be considered acceptable. A joint positioned near the limits of its range of motion can cause discomfort, tissue strain, reduced functional margin, or exposure to an inadvisable postural configuration, even if that position is not held for long.
For this reason, Ergoniza Intelligent uses a specific procedure for momentary or unsustained postures. Instead of estimating the increase in discomfort caused by posture-holding time, it assesses the instantaneous proximity of each joint to the end of its range of motion. The objective is to identify postures that, although brief, place joints in regions far from the comfortable functional zone.
The complete mathematical formulation of the procedure and the rationale for the parameters used are provided in Appendix 15.2.
11.2.3.3.1 Biomechanical and psychophysical basis
The procedure is based on the principle that a joint's range of motion does not constitute a homogeneous comfort zone. A joint may be within its anatomical range yet still adopt an uncomfortable position or one close to a functional limit. The literature therefore distinguishes between the total anatomical range, comfortable movement zones, and resting or maximally comfortable positions.
Kee and Karwowski's50F4 studies of joint angles of isocomfort show that the relationship between joint angle and perceived comfort depends on the joint, movement, and direction being assessed. Comfort does not decrease in the same way in all body regions: some show an appreciable reduction with relatively moderate deviations, while others tolerate wider ranges. Consequently, applying a single angular threshold to every joint is inappropriate.
This distinction is also reflected in the concepts of Comfort Range of Motion (CROM) and Range of Rest Posture (RRP). CROM represents the portion of the range of motion associated with comfortable conditions, while RRP identifies a more restricted zone associated with resting or maximally comfortable postures. Apostolico et al.51F5 place RRP within CROM for the neck, shoulder, elbow, wrist, and ankle, and emphasize its usefulness for characterizing the most favorable joint configurations.
Other studies52F6 have used the body's angular configuration to classify postural discomfort or to develop predictive models using regression and machine learning53F7. These findings support the proposition that joint positions contain information relevant to estimating the discomfort associated with a specific posture, although they do not eliminate individual variability or turn that estimate into a direct physiological measurement.
Finally, discomfort arising from angular configuration must be distinguished from discomfort associated with mechanical demand. Studies of discomfort and joint moment show nonlinear, joint-dependent relationships54F8. Ergoniza Intelligent addresses this second dimension through the exertion analysis described in Section 10. The momentary-posture procedure developed here is therefore limited to instantaneous angular configuration and does not estimate the muscular demand produced by joint moments or external loads.
11.2.3.3.2 General calculation principle
For momentary or unsustained postures, Ergoniza Intelligent calculates an instantaneous discomfort indicator based on the relative position of each joint within its effective range of motion. A central functional zone in which momentary discomfort equals 0 is defined for each joint and degree of freedom. Once the limit of that zone is exceeded, the indicator increases continuously as the joint approaches the end of its range of motion.
This calculation does not estimate fatigue caused by sustaining a posture over time. Its purpose is to detect brief or transient joint configurations that are close to functional limits, even when they are not held for a prolonged period.
For each joint or body region j and each degree of freedom or movement d, the normalized angular position is first calculated:
\[r_{j,d} = min\left( \frac{\left| {ang}_{j,d} \right|}{{ROMef}_{,j,d}},1 \right)\]
Equation 38
where:
angj,d is the angle adopted by joint or region j in movement and direction d, measured relative to the neutral or reference position;
ROMefj,d is the effective range of motion used as a reference in that direction;
rj,d is the relative position within the effective range: r=0 represents the neutral posture and r=1 represents the effective ROM limit. If the angle exceeds that limit, the normalized value remains at 1.
The effective ROM does not physically modify the movement permitted by the biomechanical model. It is used only as a reference for measuring proximity to the joint limit and may differ from the base ROM because of age-related adjustment factors. The procedure used to obtain it is detailed in Appendix 15.2.
Next, a threshold r0j,d is defined to represent the upper limit of the central functional zone. While the joint remains below this value, momentary discomfort is considered zero or negligible. When the joint exceeds this threshold, discomfort increases progressively.
\[r_{j,d} \leq {r0}_{j,d\ } \rightarrow \ D_{puntual,j,d} = 0\]
\[r_{j,d} > {r0}_{j,d\ } \rightarrow \ D_{puntual,j,d} > 0\]
Equation 39
The value r0j,d is not the same for every joint. Ergoniza Intelligent calculates it separately from the relative sensitivity assigned to each joint and degree of freedom. For the upper extremities, neck, and torso, this sensitivity is obtained from the maximum scores and angular intervals in the LUBA method55F9. LUBA is based on assessments of static postures and must not be interpreted as a model specifically developed for instantaneous postures. Ergoniza Intelligent therefore does not use LUBA to calculate momentary discomfort directly, but only as a source of relative information about the sensitivity of each joint and degree of freedom. For the lower extremities, an alternative procedure based on the Kee and Karwowski isocomfort equations56F10 is used. The method for determining r0j,d, including the use of LUBA for the upper extremities, neck, and torso and the alternative isocomfort-equation procedure for the lower extremities, is described in Appendix 15.2.
The value r0j,d is determined separately for each joint, movement, direction, and postural condition. For the upper extremities, neck, and torso, Ergoniza Intelligent combines the maximum score and the entry angle for the highest-scoring interval in the LUBA method57F11. LUBA is not applied as a method for assessing the momentary posture or as a temporal model; it is used only to establish relative differences in sensitivity among joints and degrees of freedom. For the lower extremities, an alternative procedure based on the Kee and Karwowski isocomfort equations is used. The detailed calculation of \(r_{0,j,d}\) is provided in Appendix 15.2.
The 100% momentary-discomfort value does not coincide with the absolute ROM limit, but with a zone close to that limit. In the current implementation, 100% is reached when the joint reaches 95% of its effective ROM (r100=0.95).
This criterion introduces a preventive margin: a momentary posture does not need to reach the exact end of the range of motion to be classified as extreme. If the movement continues beyond that threshold and approaches the effective ROM limit even more closely, the indicator can exceed 100%, up to a bounded maximum. In the current implementation, the maximum value considered is 125%. The transition function used to produce this progressive increase in the indicator is detailed in Appendix 15.2.
11.2.3.3.3 Interpreting the result
The momentary discomfort calculated using this procedure is expressed as a percentage. A value close to 0% indicates that the joint is within a central functional zone or sufficiently far from its joint limit. A value close to 100% indicates that the joint is approaching an extreme zone of its effective ROM. Values above 100% indicate that the operational threshold for an extreme posture has been exceeded and that the joint is moving even closer to the effective ROM limit.
The result must be interpreted as an indicator of instantaneous postural deviation, not as an indicator of cumulative fatigue. It therefore does not replace the temporal analysis of sustained postures or the analysis of repetitive tasks. Nor does it replace the biomechanical analysis of joint exertion: a posture may have low angular discomfort but high muscular demand due to an external load, or conversely, it may have a low joint moment but high postural deviation.
The purpose of the procedure is to provide a continuous indicator for identifying and comparing brief or transient postures close to functional limits. Its interpretation must be combined with the analysis of joint exertion, observation of the actual task, and, when appropriate, temporal assessment of sustained or repetitive postures.
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ACGIH, “Ergonomic Threshold Limit Value (TLV) for Hand Activity Level (HAL) and Muscle Fatigue.”↩︎
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Kee and Karwowski, “LUBA: An Assessment Technique for Postural Loading on the Upper Body Based on Joint Motion Discomfort and Maximum Holding Time.”↩︎