10.3.2 Instantaneous relative exertion
As explained in the description of customized exertion assessment (Section 10.1.1.1) and the maximum voluntary contraction percentage recommended for an isolated posture (Section 10.2), the demand at a joint can be expressed in relative terms by comparing the required joint moment with the maximum isometric capacity estimated for the biomechanical channel and active functional muscle group. In multiple-posture analysis, this same logic is applied successively throughout the entire time sequence.
For each biomechanical channel \(c\), instantaneous relative exertion is defined as:
\[q_{c}(t) = \mid \frac{M_{req,\ c}(t)}{M_{isom,max,c,m}(t)} \mid \]
Equation 14
where:
\(M_{req,c}(t)\) is the joint moment required in biomechanical channel c at time t,
\(M_{isom,max,c,m}(t)\) is the individualized maximum isometric moment estimated for that same channel, posture, and functional muscle group m, and
m is the functional muscle group that must act to resist the moment required at that time.
The direction of the required moment determines the active functional muscle group. For example, depending on the sign convention established for each channel, one moment direction may require flexor action and the opposite direction may require extensor action. Once the muscle group has been identified, relative exertion is calculated using the absolute value of the ratio, because the quantity relevant to determining the temporal dose is the proportion of its maximum capacity that the group must develop.
The value \(q_{c}(t)\) is dimensionless and represents the fraction of maximum isometric capacity required at that time. Thus, a value of 0.25 indicates that the required moment equals 25% of the maximum capacity estimated for the corresponding channel and muscle group. A value of 1 equals 100% of that capacity, while a value greater than 1 indicates that the required moment exceeds the estimated maximum isometric capacity.
This calculation uses the individualized maximum isometric capacity without applying the reduction defined for the duration or repetitiveness of an isolated posture. The Base Maximum Moment calculated in single-posture analysis is therefore not used as the denominator. In the cumulative analysis, the influence of time is introduced later by transforming instantaneous relative exertion into a temporal exposure dose. This avoids counting the effects of duration and repetitiveness twice.
This normalization means that the procedure does not directly accumulate joint moments expressed in newton-meters (N·m), but rather exertions relative to estimated functional capacity. Consequently, the same required moment may represent different relative intensities depending on the joint, the adopted posture, and the functional muscle group that must generate it.
When the demand in a channel is resolved by a passive joint limit rather than the action of a muscle group, that interval does not add a new ordinary muscular dose. However, the previously accumulated load continues to decrease according to the partial-recovery procedure described in Section 10.3.7.