10.4 The 3CC-r model and Recommended Moment in multiple-posture analysis

The three-compartment model, known as 3CC (Three-Compartment Controller), was proposed as a simplified representation of the development of peripheral muscle fatigue and recovery during an activity38F¹. Specific parameters for different joint regions were subsequently obtained39F². The 3CC-r model is a modification designed to represent intermittent tasks more accurately by using a multiplier that increases recovery during rest periods40F³. This modification has also been studied specifically for the shoulder joint41F⁴.

The model assumes that, at each instant, muscle capacity is distributed among three states: a resting fraction available for activation, an active fraction used to generate the required moment, and a fatigued fraction that is temporarily unavailable. A controller regulates transitions between the resting and active states to meet demand. During exertion, part of the active capacity moves to the fatigued state at a fatigue rate specific to the body region. When exertion decreases or ceases, fatigued capacity progressively recovers and returns to the resting state. In 3CC-r, this recovery is increased during intervals without demand by recovery multiplier r.

Ergoniza Intelligent applies the model separately to each joint, biomechanical channel, and functional muscle group. At each instant, it calculates relative demand by comparing the magnitude of the required moment with the individualized maximum isometric capacity corresponding to the posture, worker characteristics, and active muscle group. Antagonist muscle groups, such as flexors and extensors, maintain independent states because the activation or recovery of one should not be confused with that of the opposing group.

To use 3CC-r as a recommended criterion rather than a direct prediction of exhaustion, Ergoniza Intelligent incorporates a preventive margin. The time course used to obtain the recommended moment sets the preventive limit at 50% of the maximum endurance time estimated by the model itself for the body region. The result is therefore not intended to predict the exact time at which a particular worker will reach fatigue, but to establish a preventive reference that accounts for the time history of exertion and recovery.

\[M_{rec,3CC - r,z}(t) = M_{iso,z}(t) \bullet f_{3CC - r,z}(t)\]

Equation 30

where:

• M_rec,3CC-r,z (t) is the 3CC-r Recommended Moment for joint or channel z at time t;

• M_iso,z(t) is the moment corresponding to the individualized maximum isometric capacity for joint or channel z at time t; and

• f_3CC-r,z(t) is the recommended capacity fraction obtained from the model, with values between 0 and 1.

As the fatigued fraction increases, this factor decreases and the recommended moment is reduced. During periods of lower demand or rest, recovery may cause the factor to increase again. If Include Cumulative Exertion Load in Recommended Exertion (%) is disabled or the 3CC-r calculation is unavailable, the factor takes a value of 1 and the reference moment equals the individualized maximum isometric capacity.

When the activity contains several repetitions, the 3CC-r states are propagated from one cycle to the next and recovery during the rest periods between them is considered. This procedure is independent of the cumulative exertion load calculation described in Section 10.3: the 3CC-r state is not obtained from the cumulative dose calculated using the MET curves, although both procedures use the same biomechanical sequence.

Where applicable, the resulting reference moment is combined with the epidemiological-priority adjustment described in Section 10.1.1.1.1. The displayed indicator depends on the controls selected in the Biomechanical Analysis Settings dialog. When Calculate Max. Recommended Exertion (%) is enabled, the program displays % Maximum Recommended Exertion. When this control is disabled but Include Cumulative Exertion Load in Recommended Exertion (%) is enabled and a valid 3CC-r result is available, it displays % Recommended Joint Capacity. If both controls are disabled, or a 3CC-r calculation is unavailable, it displays % Maximum Isometric Capacity Used.