Biomechanics · functional reading

Stress concentration

Stress concentration refers to an uneven distribution of forces or deformations in the body: some areas are loaded more than others.

Scientific basis

Stresses are not distributed uniformly through tissues. Their concentration depends on the support surface, posture, the support itself and the duration of exposure.

When sitting, the highest pressures are generally found under the ischial tuberosities. Studies combining MRI and finite element modeling show that deep-tissue deformations around the ischial tuberosities can be greater than the pressure measured at the skin surface would suggest (Linder-Ganz et al. 2007; 2008).

Blue Portance / SBNFA™ reading

In the SBNFA™ framework, stress concentration is a key mechanical notion. A compensation is called negative when it leads to an accumulation or concentration of stress, up to a bottleneck; it is called positive when it comes with distribution and redistribution. In static sitting, the possibilities for redistribution are limited.

Contribution to the analysis of sitting

Seating analysis looks at the conditions that favor stress concentration or stress redistribution: support area and load distribution, pelvic orientation, the possibility of micro-adjustments. Stress redistribution can reduce local stress concentration.

Not to be confused with

Mechanical load, the umbrella notion: the set of applied forces; isolated high pressure: a surface measurement does not capture the state of deep tissues; postural compensation, a functional adjustment that can concentrate or redistribute stress; pain: an area of high loading does not, on its own, explain pain.

Sources and scope

Established biomechanical concept. An area of high pressure does not necessarily explain pain, offloading an area does not treat the cause of pain, and pressure measured at the surface does not exactly reflect stress in deep tissues.