7.7.5 Lower-limb joint loads when seated in a chair

In addition to support modeling, sitting in a chair requires specific treatment of the loads at the ankles, knees, and hips. Ergoniza Intelligent adopts the following calculation assumptions in this case:

Ankle and knee when the foot is supported.

When the model is seated in a chair and the foot of one leg is supported on the floor, Ergoniza Intelligent takes both the joint reaction force and the moment at the ankle and knee of that leg as zero. This simplification does not mean that the external reaction at the plantar support is zero: that reaction is calculated for the overall equilibrium of the model and displayed under Support reaction loads. However, in this seated-model condition, it is not transmitted as a joint load through the ankle and knee. The foot support and the support provided by the seat are considered sufficient to unload both joints.

Ankle and knee when the foot is unsupported.

When the model is seated in a chair and the foot of one leg is not supported on the floor, that limb is treated biomechanically as a leg suspended in the air. In this situation, ankle force and moment are calculated from the weight of the foot and the loads applied to it. The knee calculation also includes the weight of and loads on the lower leg. The calculation follows the procedure used for unsupported lower limbs, but without transmitting loads from the upper body to that limb.

Hip when the foot is supported.

When the model is seated in a chair and the foot of one leg is supported, Ergoniza Intelligent retains at the hip the reaction force required for system equilibrium, but takes the joint moment at that hip as zero. This simplification is based on the assumption that the simultaneous presence of the seat and plantar support relieves the hip joint of the need to generate a significant moment to maintain the posture. In this situation, therefore, the hip transmits force but no moment.

Hip when the foot is unsupported.

When the foot of one leg is unsupported, the corresponding hip is calculated using the unsupported-lower-limb branch of the model, accounting for the contribution of the lower limb itself and, where applicable, the loads applied to it, but excluding the direct influence of the upper body. This avoids assigning to that hip loads that are supported by the seat in a seated posture.