10.2.3 Recommended %MVC for repetitive tasks

Repetitive tasks involve performing similar movements or exertions cyclically. These tasks are organized into work cycles that repeat continuously. Repetitiveness means that the same muscle groups are activated repeatedly within each cycle, which can lead to fatigue accumulation and increase the risk of developing musculoskeletal disorders if exertion and recovery times are not managed appropriately.

When an Ergoniza Intelligent user selects “Occurs periodically in repeating movement cycles” in the “Repetitiveness” dialog shown in Figure 42, the user must enter the work-cycle duration, the duration of one exertion, and the number of exertions performed in each work cycle. Ergoniza Intelligent uses these data to calculate the percentage of the total work-cycle duration occupied by exertion.

Once this value has been obtained, the procedure established by ACGIH (2016) is used to calculate the recommended maximum voluntary contraction percentage (%MVC) for repetitive work tasks, that is, tasks performed in activity cycles. The ACGIH TLV curve is based primarily on psychophysical data concerning maximum acceptable exertions, supplemented by physiological and biomechanical evidence. Its purpose is to establish recommended combinations of intensity and duty cycle to limit the development of persistent localized fatigue in the upper limbs.

The Threshold Limit Value (TLV) curve defines the recommended %MVC as a function of the duty cycle (DC), meaning the percentage of the total cycle time during which muscular exertion occurs. In the ACGIH procedure, activity time is the time during which the exertion exceeds 5% of maximum capacity. In Ergoniza Intelligent, the user identifies this time by entering the duration of each exertion and the number of exertions performed during the cycle; the program uses these data to calculate DC with Equation 12.

\[DC = 100 \bullet \frac{n \bullet t_{e}}{T_{c}}\]

Equation 12

where:

• n is the number of exertions per cycle,

• t_e is the duration of each exertion, and

• T_c is the total cycle duration.

If the actual %MVC exceeds the value recommended by the TLV curve for a given DC, this indicates an unfavorable condition according to the localized-fatigue criterion and suggests that the exertion intensity, duration, or frequency should be reduced.

Equation 13 is used to determine this limit quantitatively.

\[{\% MVC}_{TLV} = 100\ \bullet \ \left\lbrack - 0.143\ \bullet ln\left( \frac{DC}{100} \right) + 0.066 \right\rbrack\]

Equation 13

where %MVC_TLV is the maximum percentage recommended by the curve for the work cycle being analyzed, and DC (duty cycle) is the portion of the cycle time during which the exertion is performed, expressed as a percentage.

To obtain the final recommended %MVC, Ergoniza Intelligent multiplies the value provided by the TLV curve by the adaptation coefficient for the anatomical region, shown in Table 3. In all cases, the result is limited to 100% of the individualized maximum isometric capacity.

The ACGIH TLV curve is defined specifically for localized upper-limb fatigue, considering exertions involving the hand/wrist, forearm, elbow, and shoulder. Its direct application to the trunk and lower limbs therefore cannot be considered a normative equivalence.

For the trunk and lower limbs, there is no general-purpose TLV curve directly comparable to the one proposed by ACGIH for the upper limb. Nevertheless, the literature on maximum endurance times shows that fatigue does not develop in the same way in every joint and that endurance time depends on the body region involved. In particular, published endurance-time models show systematic differences among joints, with greater relative endurance at the ankle and lower endurance in regions such as the shoulder and knee.

Table 3: Internal adaptation coefficients applied to the ACGIH TLV curve to estimate the recommended %MVC for repetitive tasks.

For this reason, Ergoniza Intelligent uses the ACGIH TLV curve as a general reference for representing the effect of the duty cycle on the maximum recommended voluntary contraction percentage, but introduces the internal adaptation coefficients shown in Table 3 to account for relative endurance differences among anatomical regions.

For this reason, Ergoniza Intelligent uses the ACGIH TLV curve as a general reference for representing the effect of the duty cycle on the maximum recommended voluntary contraction percentage, but introduces an internal adaptation coefficient based on the anatomical region (Table 3). This coefficient should not be interpreted as a specific TLV for the trunk or lower limbs, but as an ergonomic approximation intended to prevent a curve defined for the upper limb from being applied indiscriminately to the entire body, consistent with the greater relative fatigue resistance described for some joints in maximum endurance-time models 22F¹.