7.7.6 Consideration of the chair backrest

Ergoniza Intelligent can also account for the presence of a chair backrest. Posterior trunk support changes the transmission of loads between the body and the chair. Experimental studies using adjustable lumbar supports have observed changes in load distribution, reduced lumbar muscle activity, and better maintenance of lumbar lordosis while seated¹.

Ergoniza Intelligent allows the user to indicate that the trunk is supported by a backrest using the Backrest control under Model settings on the right side of the interface. This option should be enabled only when the model is seated in a chair and the trunk is actually supported by the backrest.

When the backrest is enabled and the posture has been identified as seated in a chair, Ergoniza Intelligent analyzes the local components of the lumbar moment separately. If the flexion-extension component corresponds to a flexion demand that can be relieved by posterior support, the system considers this component to be supported by the backrest and sets it to zero. The lateral-flexion and rotation components are not changed.

The same criterion is applied independently to the flexion-extension component of the moment calculated at the L4/L5 joint. From the corrected L4/L5 moment, Ergoniza Intelligent calculates muscle activity, compression force, and shear forces in the intervertebral disc. Compression is not set to zero because the weights of the body segments, external loads, and the muscle forces required to balance the lateral-flexion and rotation components continue to act. The general procedure used to calculate these results is described in Section 12.

When the flexion-extension component corresponds to extension—a situation in which the trunk does not tend to rest against the backrest—the system applies no unloading. It also does not change the results if the backrest is disabled or if the posture is seated on the floor.

This approach provides a simplified representation of the unloading effect of a backrest in seated postures with posterior trunk support, avoiding the attribution to the trunk muscles of demands that are actually supported by the chair structure. The model does not explicitly calculate the force exerted by the backrest or its point of application. Therefore, enabling the backrest does not alter the reaction forces or their distribution among the other supports. Its effect is represented exclusively by unloading the local flexion component of the lumbar moments.