Biomechanics · functional reading

Mechanical load

Mechanical load refers to all the forces applied to the body or to a tissue. It is described by several variables: magnitude, direction, area of application, duration, frequency, loading rate and recovery between exposures.

Scientific basis

Mechanical load is not a single value: it is a combination of variables. Two loads of the same magnitude do not have the same effects if their direction, duration, frequency or the surface to which they are applied differ.

A tissue's response depends on the ratio between the load and the tissue's capacity to bear and redistribute it. The same load may be tolerated or excessive depending on the context; certain exposure patterns can contribute to tissue adaptation processes (Khan and Scott 2009). In ergonomics, causation models of musculoskeletal disorders involve magnitude, repetition, duration and recovery, with no single variable sufficient to explain an injury (Kumar 2001).

Blue Portance / SBNFA™ reading

In the SBNFA™ framework, mechanical load is an umbrella concept: seating analysis reads how the body's load is distributed, sustained and varied over time. It does not attribute to load, on its own, an effect on pain.

Contribution to the analysis of sitting

In a seated position, the body's load is transmitted to the seat mainly through the sit bones (ischial tuberosities) and the thighs. Seating analysis is less concerned with the magnitude of this load than with its distribution, its duration and its variations: a constant load, applied for a long time to the same areas, does not have the same profile as a load that shifts with micro-adjustments.

Not to be confused with

Stress, force per unit area or cross-section of the tissue that receives it; Pressure, load perpendicular to a surface, per unit area of that surface; Compression, load that tends to push together and crush the elements of a tissue; Stress concentration, uneven distribution of these forces, with some areas loaded more than others; Pain, a sensory and emotional experience; it cannot be deduced from a load.

Sources and scope

An established biomechanical concept. A high load is not necessarily harmful, a low load is not necessarily harmless, and load alone does not explain pain. Tolerance depends on context, repetition and recovery.