10.3.5 Separation by joint, biomechanical channel, and functional muscle group
The temporal exertion dose is not accumulated globally for the entire body or indiscriminately for a whole joint. A single joint may generate moments about different movement axes and, within each axis, may alternately require the action of antagonist functional muscle groups. The calculation is therefore organized at three levels: joint, biomechanical channel, and functional muscle group.
The joint identifies the anatomical location of the exposure. Left and right joints are calculated separately where applicable because they may adopt different postures and be subjected to different loads.
A biomechanical channel represents a degree of freedom or functional axis of moment generation. For example, flexion-extension constitutes one channel, abduction-adduction another, and rotations about another axis may constitute a third channel. The two opposite directions of the same movement are therefore not considered different channels, but antagonist actions within the same channel.
This distinction is consistent with the way moments and maximum isometric capacities are calculated. A joint may have different capacities depending on the movement axis and the functional muscle group that must act. The demand corresponding to flexion-extension should therefore not be combined with that corresponding to abduction-adduction or rotation.
Functional muscle groups are distinguished within each channel. A muscle group does not represent an individual anatomical muscle, but the functional group that must generate the resisting moment: for example, flexors or extensors, abductors or adductors, and rotators in either direction.
The direction of the required moment, according to the convention established for each channel (see Section 8), identifies the active muscle group at each instant. Once it has been identified, the magnitude of relative exertion is obtained from the absolute value of the ratio defined above, and the corresponding temporal dose is assigned exclusively to that muscle group.
This separation is necessary because antagonist exertions should not accumulate as though they constituted continuous exposure of the same functional group. For example, a phase in which the flexors act and another in which the extensors act belong to the same flexion-extension channel, but generate doses in different muscle groups. When the exertion changes direction, the new dose is added to the group that becomes active, while the cumulative history of the antagonist group is retained separately and may decrease through recovery.
Conceptually, the cumulative load corresponding to joint a, biomechanical channel c, and functional muscle group m is represented by Da,c,m(t). Each combination of joint, channel, and muscle group maintains its own time course. When a channel contains several antagonist muscle groups, its summary value is not obtained by adding their loads. Instead, the greatest cumulative load among the functional muscle groups in the channel is selected:
\[D_{a,c}(t) = \ _{m \in M_{a,c}}^{\max}{D_{a,c,m}(t)}\]
Equation 18
where Ma,c is the set of functional muscle groups associated with channel c of joint a. Thus, if the flexors have a cumulative load of 0.60 and the extensors a load of 0.40, the summary value for the flexion-extension channel is 0.60. The group that determines the result may change over the course of the activity as the doses and recovery evolve.