10.3.4 Calculating the temporal exertion dose

Once the time limit T_lim,z(c)(q_c(t)) corresponding to the relative exertion observed in a biomechanical channel has been obtained, each time interval in the activity can be transformed into a fraction of the reference time limit. This fraction is called the temporal exertion dose and represents the portion of the time limit corresponding to the exposure produced during an interval of the activity.

For a biomechanical channel \(c\), the elementary temporal dose is calculated as follows:

\[\Delta D_{c}(t) = \frac{\Delta t}{T_{lim,z(c)}(q_{c}(t))}\]

\[ \]Equation 17

where:

• \(\Delta t\) is the duration of the time interval being considered, expressed in seconds,

• \(q_{c}(t)\) is the relative exertion in channel \(c\),

• z(c) is the body region to which the biomechanical channel belongs, and

• \(T_{lim,z}(q_{c}(t))\) is the exposure time limit corresponding to that region and the relative exertion being considered.

This formulation makes it possible to interpret exposure in proportion to the time limit corresponding to the exertion intensity. For example, if the preventive time limit for a given relative exertion were 100 seconds, sustaining that exertion for 10 seconds would represent a temporal dose of 0.10. If, by contrast, relative exertion were higher and the time limit were reduced to 20 seconds, those same 10 seconds would represent a dose of 0.50. The same chronological duration can therefore make a very different cumulative contribution depending on the relative intensity of the exertion.

To prevent very low exertions from generating numerically insignificant doses, Ergoniza Intelligent applies a minimum threshold of 5% of the individualized maximum isometric capacity. When q_c(t) does not exceed 0.05, the interval adds no new dose, although the existing cumulative load continues to decrease through recovery. When calculating dose with MET curves, relative exertions above 1.20 are evaluated as 1.20, that is, as 120% of maximum isometric capacity. This cap prevents the MET curves from being extrapolated beyond the interval adopted for the calculation; it does not modify the required moment or the instantaneous indicator displayed by the program.

The dose is not calculated directly from the joint moment expressed in newton-meters (N·m), but from the moment normalized to the individualized maximum isometric capacity. Consequently, a moderate moment at a joint with lower relative capacity may generate a larger dose than a mechanically greater moment at another joint with higher capacity.

A sequence may contain brief intervals of high exertion, prolonged periods of moderate exertion, and periods of low or zero demand. Each interval whose exertion exceeds the threshold contributes a dose proportional to its duration and inversely proportional to its corresponding time limit. Thus, the result depends not only on the instantaneous maximum, but on the entire time history of the exertion.

The elementary temporal dose is dimensionless. A value of ΔD_c = 0.20 indicates that the interval being considered has consumed 20% of the preventive time limit associated with the observed relative exertion. Doses generated in successive intervals are integrated while accounting for partial recovery to estimate the cumulative load of each biomechanical channel and functional muscle group during the activity.

This approach (Figure 44) makes it possible to compare exposures with different time profiles. Two tasks with the same maximum instantaneous exertion may generate different cumulative doses if one sustains the exertion longer, repeats it more frequently, or provides less recovery between exertions. Similarly, a task without very high peaks may reach a relevant cumulative load if it sustains a moderate demand for prolonged periods.