Biomechanics · functional reading

Shear

Shear is a deformation produced by forces parallel or tangential to a surface, which cause neighboring layers or regions of a tissue to slide relative to one another.

Scientific basis

Any force applied to a surface can be broken down into a perpendicular component (pressure) and a parallel component (shear). Shear tends to make the layers of a tissue slide over one another.

In the seated position, the forces at the interface between the body and the seat include both components; their distribution varies with posture and with the inclination of the trunk and pelvis (Hobson 1992). In the deep tissues of the buttock region, imaging and modeling studies show that shear deformations combine with compressive deformations (Linder-Ganz et al. 2007).

Blue Portance / SBNFA™ reading

In the SBNFA™ framework, shear is one component of the stresses that sitting imposes on tissues. It is observed indirectly, through the orientation of the pelvis and the variation of support points.

Contribution to the analysis of sitting

Seating analysis does not measure shear. It observes the parameters that change its distribution: pelvic inclination, forward sliding on the seat, and the distribution of support between the sit bones (ischial tuberosities) and the thighs.

Not to be confused with

Friction, the frictional force between two surfaces in contact, most often the skin and the support surface; Pressure, the component perpendicular to the surface; Compression, a load that tends to push the elements of a tissue closer together; Fascial gliding, the physiological sliding of fascial layers over one another; Nerve gliding, the movement of a nerve relative to the surrounding tissues.

Sources and scope

An established biomechanical concept. Shear is not always harmful, a sensation of rubbing does not prove pathological tissue shear, and no seat eliminates it.