10.3.7 Partial recovery between exertions

Temporal integration of the dose makes it possible to estimate how cumulative load evolves while a functional muscle group remains under exertion. In a real activity, however, demand is seldom constant. There may be intervals in which it decreases, changes direction, shifts to an antagonist muscle group, or temporarily ceases. During these periods, the previously accumulated load may be partially reduced through recovery.

Ergoniza Intelligent applies recovery separately to each joint, biomechanical channel, and functional muscle group. In each time interval, the loads previously stored by all muscle groups are attenuated. If a new dose is generated during the same interval, it is then added exclusively to the active muscle group. Recovery is therefore applied continuously, including while a new exposure is being generated: net load increases when the new dose exceeds the reduction produced by recovery and decreases when recovery predominates.

Recovery is represented by two exponential components: a fast component, which loses relevance more rapidly, and a slow component, which retains part of the previous exposure for longer. For an interval of duration Δt, the retention factors for the two components are:

\[\rho_{rap}(\Delta t) = 2^{- \Delta t/h_{rap}}\ \]

\[\rho_{len}(\Delta t) = 2^{- \Delta t/h_{len}}\ \]

Equation 22

where h_rap and h_len are the half-lives of the fast and slow components, respectively. Half-life is the time required for a component to retain half the value it had at the beginning of the interval.

In the absence of a new dose, the two load components are updated as follows:

\[D_{a,c,m}^{rap}(t + \Delta t) = D_{a,c,m}^{rap}(t) \bullet 2^{- \Delta t/h_{rap}}\]

\[D_{a,c,m}^{len}(t + \Delta t) = D_{a,c,m}^{len}(t) \bullet 2^{- \Delta t/h_{len}}\]

Equation 23

and the remaining load is:

\[D_{a,c,m}^{}(t + \Delta t) = D_{a,c,m}^{rap}(t + \Delta t) + D_{a,c,m}^{len}(t + \Delta t)\]

Equation 24

When a new temporal dose is generated, it is distributed between the two components according to their relative weights:

\[\omega_{rap} + \omega_{len} = 1\]

\[ \]Equation 25

The complete update, simultaneously including recovery and the new dose, is defined in the preceding section.

For an isolated dose initially distributed between the two components, the proportion remaining after an interval Δt can be expressed as:

\[R(\Delta t) = {\omega_{rap} \bullet 2}^{- \Delta t/h_{rap}}{{\ + \ \omega}_{len} \bullet 2}^{- \Delta t/h_{len}}\]

Equation 26

This expression is useful for representing the recovery of an isolated dose. When the load results from multiple doses generated at different times, the system retains and updates the fast and slow components separately because each may represent a different proportion of the total value at that time.

Using two components with different half-lives is a simplified approximation of a recovery process that does not occur at a single rate. Yates et al. 32F¹ observed that recovery of dynamic muscular endurance progressed rapidly at first, reached approximately 50% after 2 minutes and 15 seconds, was slightly below 90% after 20 minutes, and was best fitted by a three-component exponential curve. Iridiastadi and Nussbaum33F² highlighted the difficulty of representing recovery in intermittent tasks and its dependence on loading conditions. Looft et al.34F³ adapted a compartment model to intermittent tasks, and Looft and Frey Law35F⁴ showed that increasing the recovery rate during rest intervals improves the representation of these tasks. Taken together, these findings support distinguishing a faster initial recovery from a more persistent recovery component, without attempting to reproduce the complete physiology of recovery for an individual.

The following values are used in the default Ergoniza Intelligent configuration:

\[{\omega_{rap} = 0.70,h_{rap} = 120\text{\:\,}s }{\omega_{len} = 0.30,h_{len} = 600\text{\:\,}s}\]

Thus, 70% of each new dose is assigned to the fast component, which has a half-life of 2 minutes, and the remaining 30% is assigned to the slow component, which has a half-life of 10 minutes. After 120 seconds, the fast component retains 50% of the value it had at the beginning of the interval; after another 120 seconds, it retains 25%, and so on. The slow component evolves in the same way, but with a longer half-life.

Considering both components together, an isolated dose retains approximately 61% of its value after 120 seconds and 17% after 600 seconds. A short break therefore partially reduces the influence of previous exposure, while a longer break produces a greater reduction without eliminating it instantaneously.

These values should be interpreted as default model parameters, not as universal constants. They were selected to represent an intermediate, prudent behavior: appreciable recovery during the first few minutes without immediately eliminating the effect of repeated exposures. The user can modify them in the Activity Repetitions dialog (Figure 46), under Temporal Fatigue Decay. The weights of the two components are complementary: when one is changed, the program adjusts the other so that they sum to 1. Half-lives are entered in seconds. In Multiple Postures mode, this dialog can be opened from the Timing button on the left sidebar or from the clock icon at the bottom of the Exertion panel shown in Figure 38. The Temporal Fatigue Decay section is collapsed by default; click its heading to expand it.

This procedure distinguishes between activities that generate similar doses but have different time distributions. A sequence of repeated exertions with short breaks will normally produce a greater cumulative load than the same sequence separated by longer rest periods. Similarly, alternation between antagonist muscle groups allows each group to recover partially during the intervals in which the opposing group acts.

The calculated partial recovery should not be interpreted as complete restoration of muscle capacity or as an individual physiological simulation. Its purpose is to represent, in an ergonomic and operational manner, the progressive loss of relevance of previous exposure as demand on a biomechanical channel decreases. This approach is especially important in intermittent tasks, in which the time history of exertion, rest periods, and distribution of active periods determine the final cumulative load36F⁵.