12 LUMBAR COMPRESSION AND SHEAR RISK ANALYSIS

External loads and the weight of body segments above the lumbar region generate moments that must be balanced by muscle forces and joint reactions. Consequently, the intervertebral discs withstand compression and shear forces that can increase during load handling, especially when the load is heavy, held away from the body, or the trunk adopts unfavorable postures. Estimating these forces is therefore used as a biomechanical indicator of lumbar demand. Ergoniza Intelligent performs this calculation at the L4/L5 intervertebral level.

In its 1981 manual-handling guide, the U.S. National Institute for Occupational Safety and Health (NIOSH)67F¹ established two reference values for estimated compression force at L5/S1: an Action Limit (AL) of 3400 N and a Maximum Permissible Limit (MPL) of 6400 N. The revised NIOSH equation subsequently retained 3400 N as a biomechanical criterion associated with increased risk of low-back injury.

These values were established for L5/S1 and for the ergonomic assessment of tasks. They must therefore not be interpreted as individual injury limits or as a precise boundary between a safe and an injurious situation. Ergoniza Intelligent uses them as operational references to classify the compression force estimated by its model at L4/L5. Applying them to a different, albeit adjacent, lumbar level is an approximation that must be considered when interpreting the result.

Subsequent research examined how age and sex influence the load-bearing capacity of the spine, particularly at L5/S168F². Aging causes progressive degeneration of the intervertebral discs and loss of mechanical strength in the vertebrae and supporting tissues. Biomechanical studies show that the compressive load capacity of the discs decreases substantially after age 40 or 50. Some authors therefore propose more conservative compression limits for older populations. In general, women also have less bone and muscle mass than men, which may reduce their capacity to withstand high compressive loads. Studies suggest that women's intervertebral discs have smaller diameters and lower absolute resistance to compressive loads.

Although the NIOSH limit of 3400 N remains a general reference for mixed populations, more advanced ergonomic applications such as Ergoniza Intelligent can adjust safety criteria for age and sex. Table 6 shows the recommended L5/S1 intervertebral compression-force limits used by Ergoniza Intelligent according to age and sex. These values are based on the studies cited above and are calculated automatically when the corresponding model data are entered.

Ergoniza Intelligent also uses a conservative operational threshold of 500 N for resultant shear at L4/L5. This resultant combines the anteroposterior and lateral components. The value originates from classic biomechanical studies69F³ that examined the resistance of ligamentous and disc structures to shear stress. Later reviews proposed values of 1000 N for occasional exposures and 700 N for repetitive exposures; the 500 N value used by the program must therefore be understood as a conservative screening criterion. Exceeding it indicates that the task should be reviewed, but does not by itself predict injury.