Measures to Design Chairs or Seats

Measures to Design Chairs or Seats

For the design of all furniture, it is necessary to know the anthropometric habits of the human being, the design of the seat dates back to antiquity. The footstool, for example, acquired in time of the Egyptians, 2050 BC, category of the valuable element of furniture and the same happens with the chair, dating to around 1600 BC the seat, despite its ubiquity and long life, continues to be one of the worst designed elements of the interior space. Creating a chair is the test of fire of every designer; one of the most significant difficulties encountered in this task is that it is often understood to sit as a static position when it is dynamic.

Hence, the exclusive application of two-dimensional static data in the resolution of a three-dimensional problem, which involves biomechanical facets, is a wrong approach. Paradoxically, an anthropometrically correct chair doesn’t have to be comfortable unlike church pews, and a design that is not based on the weight and size of the human body will be unfailingly annoying.

The insufficiency of available data concerning the biomechanics of this design and publications of research on comfort adds more difficulties to this question.

Elemental Dynamics of a Seat

For a better understanding of the dynamics of sitting it is worth studying the mechanics of the support system and the general bone structure that operate in it. The supporting axis of a sitting torso is a line located in a coronal plane that passes through the projection of the lower point of the ischial tuberosities that rest on the seating surface.

First, in a sitting position, about 75% of the total body weight is supported only by 26 cm2 (4 square inches) of said tuberosities. It is a high load that is distributed on a small surface, which results in considerable compressions in the buttocks, which is valued between 6 and 7 kg / cm2 or 85 to 100 pounds/square inches.

Other information estimates the compression experienced by the skin surface in contact with the seat between 2.5 and 4 kg / cm2 (40 and 60 pounds / inch2) when in slightly more distant points it is reduced to 250 g / cm2 (4 pounds) / inch2).

The conjunction of these pressures causes fatigue and discomfort and results in changes in posture to relieve discomfort. Otherwise, a prolonged stay in the same position and under the same state of forces, produces ischemia or interference in irrigation of blood, causing pain and possible numbness. The anthropometric data are irreplaceable to determine the necessary measures and gaps. Though, structurally, tuberosities are a two-point support system that, in itself, is already unstable.

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