10.3.8 Consideration of repetitive tasks and rest between repetitions

Many work activities are not performed as a single isolated sequence, but as work cycles that repeat several times. In these cases, cumulative exertion load depends not only on what occurs within a cycle, but also on the repetitions performed previously and the rest between them. Two activities with the same base cycle may produce different results if one is repeated several times without sufficient breaks while the other includes longer recovery periods.

When analyzing repetitive activities, Ergoniza Intelligent interprets the entered posture sequence as a representative cycle of the activity. Within each repetition, the time course of the temporal dose and cumulative load is calculated for each joint, biomechanical channel, and functional muscle group using the procedure described in the preceding sections.

The user selects the activity repetition to be represented and analyzed in the dialog shown in Figure 46. If repetition N is selected, the program assumes that N-1 identical cycles have been completed before the displayed sequence, with the configured rest between them. The selected repetition therefore begins with the residual load from previous repetitions.

Conceptually, let \(D_{a,c,m}^{rap,inicio(i)}\) represent the fast component of cumulative load at the beginning of repetition \(i\), \(D_{a,c,m}^{rap,fin(i)}\) the fast component of cumulative load at the end of repetition \(i\), \(D_{a,c,m}^{len,inicio(i)}\) the slow component of cumulative load at the beginning of repetition \(i\), and \(D_{a,c,m}^{len,fin(i)}\) the slow component of cumulative load at the end of repetition \(i\). The first repetition begins with a cumulative load of zero:

\[D_{a,c,m}^{rap,\ inicio(1)} = D_{a,c,m}^{len,inicio(1)} = 0\]

Subsequent repetitions, however, may begin with a residual load. If \(\mathbf{T}_{\mathbf{d}}\) is the duration of the rest between cycles, the state at the beginning of repetition i+1 is obtained by applying recovery to the state at the end of repetition i:

\[D_{a,c,m}^{rap,\ inicio(i + 1)} = D_{a,c,m}^{rap,\ fin(i)} \cdot 2^{- T_{d}/h_{rap}}\]

\[ {D_{a,c,m}^{len,\ inicio(i + 1)} = D_{a,c,m}^{len,\ fin(i)} \cdot 2^{- T_{d}/h_{len}}}\]

Equation 27

The total load at the beginning of the new repetition is:

\[D_{a,c,m}^{inicio(i + 1)} = D_{a,c,m}^{rap,\ inicio(i + 1)} + D_{a,c,m}^{len,\ inicio(i + 1)}\]

Equation 28

If the rest is short, a substantial portion of the load accumulated at the end of the cycle is therefore retained at the beginning of the next cycle. If the rest is longer, the residual load is smaller. Recovery is applied separately to each functional muscle group, so some groups may begin the new repetition with a greater residual load than others.

During each repetition, load evolves as recovery is applied continuously and the temporal doses generated by the exertions in the sequence are added. Consequently, a later repetition may show higher values than the first even though the biomechanical cycle is identical, because part of the exposure produced in previous cycles remains at the beginning of the repetition.

This approach distinguishes three common situations. An activity repeated with short rest periods will tend to produce progressive accumulation, especially in muscle groups engaged repeatedly. With intermediate breaks, load will increase during exertion periods and partially decrease during rest periods. With longer breaks, residual load may decrease considerably before the next cycle, reducing the cumulative effect of previous repetitions.

10.3.8.1 Duration of the represented activity

If each cycle has a duration \(\mathbf{T}_{\mathbf{c}}\), the rest between cycles is \(\mathbf{T}_{\mathbf{d}}\), and repetition N is selected in the dialog shown in Figure 46, the time elapsed from the beginning of the first repetition to the end of the displayed repetition is:

\[T_{actividad} = N \bullet T_{c} + (N - 1)T_{d}\]

Equation 29

N-1 rest periods are considered because no rest is added after the represented repetition. The configuration dialog displays this total duration so the user can verify the period of activity being represented.

Considering repetitions and rest periods is especially relevant in intermittent tasks. In these tasks, instantaneous exertion intensity alone does not describe the actual demand of the activity. The time distribution of exertions, duration of active periods, repetition frequency, and duration of breaks determine the final cumulative load. This approach is consistent with fatigue-recovery models applied to tasks with complex force-time histories and with studies showing that recovery during rest periods modifies the development of fatigue in submaximal and intermittent exertions37F¹.

The result for a given repetition should be interpreted in the context of the complete work-rest cycle. The same posture may have a low cumulative load during the first repetition and a higher value when it occurs after several cycles with insufficient recovery (Figure 47). The analysis therefore assesses both the demand produced within the base cycle and the effect of repeating it over time.

The Include Repetitions and Rest Periods option shown in Figure 46 activates the cumulative exertion load calculation. When the option is selected and repetition 1 is chosen, the program analyzes the first execution of the sequence, without previous repetitions or rest between cycles, but accounting for accumulation and recovery within the sequence itself. When a later repetition is selected, the previous cycles and rest periods are also included.

If the option is cleared, Ergoniza Intelligent does not calculate or display cumulative exertion load. Instantaneous joint exertion and the other biomechanical results continue to be calculated normally, but the time history of the activity is not projected through this indicator.