7.7.4 Simplifications assumed when modeling seated posture

Seated posture in a chair introduces a more complex biomechanical situation than bipedal stance or direct support on the floor. In reality, contact between the body and the chair does not occur at points, but over relatively large surfaces: primarily the ischial tuberosities, a variable portion of the back of the thighs, and the backrest, where applicable. In addition, pressure distribution depends on the type of seat, its deformability, trunk posture, and foot position.

The following simplifications are introduced to make the problem manageable within the Ergoniza Intelligent biomechanical model:

Equivalent point supports.

The actual pressure distribution on the seat is replaced by point supports located at the knees and hips. This assumption maintains a calculation scheme consistent with that used in the rest of the model.

The actual pressure distribution between the body and the seat is replaced by the equivalent point supports defined in Section 7.7.2: a front support at the knee and a rear support at the hip on each side. This assumption maintains a calculation scheme consistent with that used in the rest of the model.

Two supports on each side of the body.

As in the other situations considered by the system, each side of the body is modeled using a maximum of two support areas. When seated in a chair, these areas correspond to the knee and hip on the respective side.

Partial unloading of the thigh.

When the model is seated in a chair and the foot of one leg is supported, Ergoniza Intelligent considers a substantial portion of the thigh's weight to be supported directly by the seat. In the current model formulation, the chair is assumed to support 80% of the thigh's weight, with only the remaining 20% contributing to the reaction force transmitted to the hip. This assumption provides a simplified representation of support under the back of the thigh and avoids overestimating the force transmitted through the hip joint.

Separation between reaction forces and joint moments.

When seated in a chair, Ergoniza Intelligent distinguishes between the reaction force at the hip resulting from the overall equilibrium of the model and the moment that must be supported by the joint itself. This distinction is particularly important in postures with foot support, where the seat relieves some of the joint loading.

When seated in a chair, Ergoniza Intelligent separately calculates the reaction force transmitted through the hip and the moment supported by the joint. When the corresponding foot is supported, the hip may transmit a nonzero force required for model equilibrium, while its joint moment is taken as zero. This assumption is described in greater detail in Section 7.7.5.