11.2.1 Measuring postural discomfort: psychophysical basis

Postural discomfort and postural risk are not equivalent quantities. The former estimates a perceptual response associated with joint configuration; risk methods, by contrast, combine different exposure factors to obtain a preventive classification or score.

Ergoniza Intelligent calculates joint forces and moments using the procedure described in Section 8 and assesses biomechanical exertion using the indicators in Section 10. Discomfort analysis supplements those results by quantifying a perceptual component of posture that is not described by joint moments or exertion percentage alone. Interpreting both types of information together provides a more complete characterization of the analyzed situation.

Ergoniza Intelligent expresses postural discomfort on a percentage scale based on regression equations obtained for different joints and movements44F¹. These equations come from psychophysical scaling experiments in which participants maintained static postures and rated their comfort using the free-modulus magnitude-estimation method. The responses were then normalized and fitted using regressions that relate joint angle to perceived comfort. In the original studies, most fits had R² values above 0.90.

The validity of psychophysical measurements for comfort assessment has been supported by previous studies. This approach integrates participants' subjective perceptions with quantitative analysis methods, providing reliable and representative data. Values obtained through psychophysical scaling show a strong correlation with physiological and biomechanical measures. In addition, statistical normalization of the responses minimizes interindividual variability, making the results applicable to broader populations45F². Nevertheless, the equations represent the responses of the populations and experimental conditions studied; their results should therefore be interpreted as ergonomic estimates rather than exact predictions for a particular person.

To interpret the values obtained from the regression equations, comfort intervals were established based on analysis of the normalized comfort values reported during the experiments. These intervals were defined by converting subjective ratings into qualitative categories and assigning them normalized values on a scale from 0 to 100. The main categories are shown in Table 5.

These values relate the estimate obtained from the comfort equations to a verbal category and facilitate its interpretation. The model distinguishes between men and women, showing significant sex-related differences in the perception of postural comfort. These differences are reflected both in the absolute values in Table 5 and in the distribution of comfort levels across the range of motion. Ergoniza Intelligent therefore differentiates its calculation by sex, underscoring the importance of adapting ergonomic analysis to the specific characteristics of each population.

The regression equations and comfort intervals were obtained from postures maintained for 60 seconds. In other words, they measure comfort in relatively prolonged static postures without supporting a load.

To incorporate posture-holding time into discomfort, Ergoniza Intelligent first calculates postural discomfort using a base time of 60 seconds and the regression equations (referred to as absolute discomfort). Once absolute discomfort has been obtained, it is modified to account for posture-holding time using the procedures described in Sections 11.2.3 and 11.2.4. These adaptations extend the operational use of the original equations and should be interpreted subject to the limitations described later.