10.1 Maximum joint capacities

In a simplified biomechanical analysis, reaction forces represent the resultant of the actions transmitted through the articular surfaces and the structures that stabilize the joint. Joint moments must be balanced primarily by the muscle groups acting on the joint, although passive structures (such as ligaments, joint capsules, and periarticular tissues) may also contribute, particularly when the joint is near the limits of its range of motion. Moment-generating capacity depends on numerous factors, which makes estimating its maximum value a complex task.

The muscles acting on a joint perform different functions and do not have the same spatial arrangement. During elbow flexion, for example, the flexor muscles act as agonists and generate the primary moment, while the extensors act as antagonists. The latter may reduce their activation but do not necessarily relax completely, because coactivation of agonist and antagonist groups helps stabilize the joint.

A muscle's force-generating capacity also varies with its degree of lengthening. In addition, as movement occurs, the position of its line of action changes relative to the center of the joint and therefore so does the moment arm through which it acts. Muscle and tendon attachments are also distributed over a given surface and cannot always be represented as simple geometric points. Finally, muscular capacity can vary considerably even among people with similar body builds.

To assess joint exertion, Ergoniza Intelligent does not model each muscle individually or explicitly calculate coactivation between agonists and antagonists. The program groups muscular actions into functional muscle groups and, based on the direction of the required joint moment, identifies the group that must generate it.

Maximum Voluntary Contraction (MVC) is the maximum exertion that a person can voluntarily produce with a muscle or muscle group under specified conditions. In the Ergoniza Intelligent biomechanical model, this capacity is represented by the maximum joint moment that the corresponding functional muscle group can generate. This value represents an estimated maximum capacity and does not, by itself, constitute a recommended limit for performing a task.

Essentially, in a static or quasi-static posture, the magnitude of the joint moment required to balance the weight of the body segments and the external loads must be lower than the magnitude of the moment-generating capacity of the active muscle group. When the required moment approaches that maximum capacity, the relative exertion demand is high, even if the action can be maintained only for a very short time.

The Ergoniza Intelligent calculation of maximum moment-generating capacities is based on several studies¹, from which regression equations were developed to estimate the capacity of the different functional muscle groups. In total, Ergoniza Intelligent uses 21 mathematical models: six for the wrist (flexion, extension, radial deviation, ulnar deviation, pronation, and supination); two for the elbow (flexion and extension); six for the shoulder (abduction, adduction, anterior rotation, posterior rotation, medial humeral rotation, and lateral humeral rotation); four for the L5/S1 intervertebral joint (flexion, extension, lateral flexion, and rotation); and three for flexion/extension of the hip, knee, and ankle. For modeling purposes, radioulnar rotation that produces forearm pronation and supination is included in the wrist functional block. Similarly, the ankle model incorporates the corresponding functional movements of the foot.

The variables considered are not identical in all equations. Depending on the joint and movement being analyzed, the models may incorporate the posture of the assessed joint and adjacent joints, sex, age, and laterality or dominance. The applicable variables in each case are used to estimate the mean maximum capacity and the standard deviation of the population distribution. The body build selected under Model Settings is then used to place the worker at a particular percentile of that distribution and obtain an individualized maximum capacity.

An example of this type of model is Equation 5, which shows how to calculate the mean maximum moment-generating capacity at the elbow joint when the active muscle group is the forearm flexor group. As the equation shows, the mean maximum capacity depends on the elbow flexion angle (α_elbow) and the shoulder flexion angle (α_shoulder). It also depends on sex: G is the sex-related factor and takes different values for men and women. Factor Z accounts for whether the limb being assessed is on the person's dominant or nondominant side. Finally, E adjusts the mean maximum capacity based on age and sex.

\[M_{elbow}^{flexion} = \left\lbrack 336.29\ + \ 1.544\ *\ \alpha_{elbow}\ - \ 0.0085\ *\ \alpha_{elbow}^{2}\ - \ 0.5\ *\ \alpha_{shoulder} \right\rbrack \bullet G \bullet Z \bullet E\]

Equation 5

The value obtained with this type of model is the mean maximum capacity of the reference population. Because this quantity is assumed to follow an approximately normal distribution, the models also estimate its standard deviation. An individualized maximum capacity can be estimated from these two values and the percentile associated with the selected body build. This result is a statistical estimate, not a direct measurement of the assessed person's physical capacity.

When biomechanical analysis is enabled, Ergoniza Intelligent displays information on maximum capacity by joint, as shown in Figure 38 and Figure 40. The first column of the tables identifies the joint and type of exertion. The second column shows the component of the required joint moment associated with the movement being analyzed, expressed in a local reference system linked to the corresponding body segment. Therefore, its value does not necessarily match any of the X, Y, or Z components displayed in the Joint loads panel in Figure 37.

The third column of the tables identifies the functional muscle group that must act according to the direction of the required moment. Columns 4 through 6, grouped under the heading Population capacity, show the estimated maximum capacities for the current joint and posture. As noted above, these capacities vary among individuals and are assumed to be distributed approximately normally around the mean. The first of these columns shows the mean maximum capacity, and the second shows the standard deviation of the distribution. The last, % protected population, shows the estimated proportion of the population whose maximum capacity would be equal to or greater than the required moment. This percentage represents the proportion of the population with sufficient moment-generating capacity according to the model; it is not an individual's probability of injury.

For example, the first row of the table in Figure 38 indicates that the required flexion/extension moment at the left wrist is −3.7 N·m. Under the convention used in Ergoniza Intelligent, the negative sign identifies the extensor as the active muscle group. The estimated mean maximum capacity for that group is −6.7 N·m (a magnitude of 6.7 N·m), and the standard deviation is 2.0 N·m. Based on this distribution, the model estimates that 93.7% of the population would have sufficient capacity to generate the required moment.