10.3.3 Determining the exposure time limit using MET curves

Once the instantaneous relative exertion \(q_{c}(t)\) has been determined, the next step is to relate it to the length of time for which it could be sustained continuously. Maximum Endurance Time (MET) models are used for this purpose. These models express the relationship between the relative intensity of a contraction and the maximum time for which it can be sustained.

As explained for static exertions, this relationship is nonlinear. As relative exertion increases, maximum endurance time decreases rapidly. Small differences in the required relative exertion can therefore produce substantial differences in the time for which it could be sustained. This behavior has been described in the classic literature on static contractions and in later endurance-time models, including joint-specific models30F¹.

Ergoniza Intelligent uses the power models obtained by Frey Law and Avin from a meta-analysis of endurance times observed in different body regions. For body region z corresponding to the biomechanical channel being analyzed, maximum endurance time is calculated as follows:

\[MET_{z}(q) = b_{0,z} \cdot q^{b_{1,z}}\]

Equation 15

where \(MET_{z}(q)\) is the estimated maximum endurance time, expressed in seconds, for body region \(z\); \(q\) is the relative exertion expressed as a fraction of maximum isometric capacity; and b_0,z and b_1,z are the coefficients for that region (Table 4). In all the models used, exponent b_1,z is negative, so maximum endurance time decreases as relative exertion intensity increases.

The specific Frey Law and Avin models cover the ankle, trunk, hand or grip, elbow, knee, and shoulder. Because the study does not provide a specific model for the hip, Ergoniza Intelligent uses the general model obtained from the complete data set for that region. The hand or grip model is used for wrist exertions.

The value \(MET_{z}(q)\) represents an estimate of the maximum time for which the exertion can be sustained until fatigue. However, using maximum endurance time as a direct operational limit is not appropriate in ergonomic analysis because that value is associated with a limiting condition of exhaustion. Ergoniza Intelligent therefore applies a preventive criterion and defines the exposure time limit as 50% of the estimated maximum endurance time:

\[T_{lim,z}(q) = 0.5 \cdot MET_{z}(q)\]

Equation 16

The factor 0.5 is an internal preventive criterion in Ergoniza Intelligent and establishes a reference time below the estimated maximum time to fatigue. It should not be interpreted as a regulatory limit or as a universal guarantee that fatigue will not occur. Its use is conceptually consistent with the preventive approach of the ACGIH limit values31F² for localized muscle fatigue, in which exhaustion is not used as the direct reference for acceptability.

The time limit \(T_{lim,z}(q)\) should be interpreted as the reference exposure time for the relative exertion level observed at a given instant. If relative exertion is low, the time limit will be long and the cumulative contribution of a brief time interval will be small. Conversely, if relative exertion is high, the time limit will be short and the same time interval will consume a larger fraction of the allowable time.

Transforming \(q_{z}(t)\) into \(T_{lim,z}(q)\) converts an instantaneous measure of relative muscle intensity into a temporal quantity. This time limit can then be used to calculate, for each interval in the sequence, the fraction of the preventive reference time limit that has been consumed. That fraction constitutes the temporal exertion dose that will subsequently be integrated in the cumulative-load calculation.