10.5 Conceptual differences between single- and multiple-posture analysis

Single-posture analysis, described in Section 10.2, and the temporal procedures used in multiple-posture analysis, described in Sections 10.3 and 10.4, represent the activity in different ways and provide complementary results.

In single-posture analysis, the activity is represented by a fixed postural configuration and specified loads. Ergoniza Intelligent first calculates the maximum isometric capacity of each joint and biomechanical channel, individualized according to the worker's characteristics. The exertion duration and repetitiveness conditions then determine the recommended Maximum Voluntary Contraction Percentage (%MVC). Applying this percentage to the individualized maximum isometric capacity yields the Base Maximum Moment used to assess the posture.

The result can be displayed as % Recommended Joint Capacity or, when Calculate Max. Recommended Exertion (%) is enabled in the Epidemiological Priority Adjustment section of the Biomechanical Analysis Settings dialog, as % Maximum Recommended Exertion. The recommended joint capacity percentage relates the required moment to the Base Maximum Moment. The maximum recommended exertion percentage then weights this result according to the prevalence of musculoskeletal disorders assigned to the body region. In both cases, the indicator assesses the analyzed posture under the temporal conditions declared by the user, but does not reconstruct the time course of exertion throughout an activity.

In multiple-posture analysis, Ergoniza Intelligent obtains, on the one hand, an exertion indicator for the represented time. When Include Cumulative Exertion Load in Recommended Exertion (%) is enabled in the Biomechanical Analysis Settings dialog, the required moment is compared with the 3CC-r Recommended Moment, which may vary according to the exertion and recovery history of the active muscle group. If this option is disabled or the 3CC-r calculation is unavailable, the required moment is compared with the individualized maximum isometric capacity. The operation of the 3CC-r model is explained in Section 10.4.

The indicator name depends on the controls selected in the Biomechanical Analysis Settings dialog. % Maximum Recommended Exertion is displayed when Calculate Max. Recommended Exertion (%) is enabled. If this control is disabled, % Recommended Joint Capacity is displayed when Include Cumulative Exertion Load in Recommended Exertion (%) is enabled and a valid 3CC-r result is available. If the latter option is also disabled or the 3CC-r calculation is unavailable, % Maximum Isometric Capacity Used is displayed. Although the indicator corresponds to the represented time, when 3CC-r is involved it may reflect the previous history of exertion and recovery.

Cumulative exertion load, on the other hand, is calculated using the independent procedure described in Section 10.3. In each interval, the required moment is related to the individualized maximum isometric capacity, transformed into a temporal dose using Maximum Endurance Time curves, and integrated while accounting for partial recovery. The cumulative dose does not modify the state calculated by 3CC-r. When Calculate Max. Recommended Exertion (%) is enabled, the displayed cumulative-load value incorporates the weighting corresponding to the prevalence of musculoskeletal disorders assigned to the body region.

The active instantaneous indicator and cumulative exertion load should therefore not be interpreted as equivalent quantities. The former expresses the relationship between the moment required in the visible posture and the applicable reference moment under the selected settings. When 3CC-r is involved, that reference moment may reflect the previous history of exertion and recovery. Cumulative load, by contrast, represents the state of biomechanical exposure reached up to the time being considered as a result of the complete time course of exertions and partial recovery.

When several cycles of a repetitive activity are considered, the cumulative load for the selected repetition also incorporates residual load from previous repetitions and recovery during rest periods between cycles.

A posture may have a high value for the active instantaneous indicator yet produce a moderate cumulative load if it is held briefly or followed by sufficient recovery. Conversely, moderate instantaneous exertions may produce a high cumulative load when sustained for prolonged periods or frequently repeated using the same muscle group without sufficient recovery.

Single-posture analysis is appropriate when the activity can reasonably be represented by a characteristic posture and simple temporal conditions. Multiple-posture analysis is more appropriate when there are postural changes, load variations, alternation between muscle groups, phases of lower demand, rest periods, or repeated cycles.

The two procedures should be used as complementary tools. Instantaneous indicators identify the postures or joints closest to their reference limits, while cumulative load identifies the biomechanical channels and muscle groups that concentrate the greatest exposure over the course of the activity.