8.2.2 Lower-limb joint loads in a supported model

8.2.2.1 Calculating ankle loads with foot support

When support on the analyzed side occurs only at the foot, with no simultaneous knee or hip contact with the floor, the known reactions at the foot supports allow the ankle load to be calculated. Following the notation shown in Figure 33, this load consists of force F_Tob, with components F_Tobx, F_Toby, and F_Tobz, and moment M_Tob, with components M_Tobx, M_Toby, and M_Tobz. Because this is a static system, the sum of the forces and moments acting on the foot must be zero.

To establish moment equilibrium about the ankle, the moments generated by the reaction at the heel (F_T), the reaction at the forefoot (F_P), and the foot's own weight (P_P) are considered. Where present, the reaction moments calculated at the heel and forefoot supports, represented by M_T and M_P, respectively, are also included. These equilibrium conditions are expressed in Equation 4.

\[\overrightarrow{F_{Tob}} + \overrightarrow{F_{T}} + \overrightarrow{F_{P}} + \overrightarrow{P_{P}} = \overrightarrow{0}\]

\[\overrightarrow{M_{Tob}} + {\overrightarrow{r}}_{Tob \rightarrow T} \times \overrightarrow{F_{T}} + {\overrightarrow{r}}_{Tob \rightarrow P} \times \overrightarrow{F_{P}} + {\overrightarrow{r}}_{Tob \rightarrow CdG} \times \overrightarrow{P_{P}} +\]

\[+ \overrightarrow{M_{T}} + \overrightarrow{M_{P}} = \overrightarrow{0}\]

Equation 4

8.2.2.2 Calculating knee and hip loads with foot support

When support occurs exclusively at the feet, once the forces and moments transmitted through the ankles are known, the loads at the knees can be calculated using the same procedure presented at the beginning of Section 8. This calculation includes the load transmitted by the ankle, the lower leg's own weight, and any external loads applied to this segment. The loads calculated at the knees, together with the weight of the thighs and the corresponding external loads, are then used to obtain the loads at the hips. The calculation therefore proceeds from the supports toward the pelvis by successively applying the equilibrium equations.

8.2.2.3 Calculating ankle and knee loads when knee support is present

As explained in Section 7.5, Ergoniza Intelligent detects knee contact with the floor and calculates the corresponding reaction forces. When this type of contact exists, the joint loads no longer depend only on the reactions at the foot supports, but also on the reaction exerted by the floor on the knee. Therefore, the calculation of ankle and knee loads differs from the procedure described in Sections 8.2.2.1 and 8.2.2.2.

As an example, Figure 34 shows a leg supported simultaneously at the knee and forefoot. The same procedure can be applied to the opposite side and to other equivalent configurations. To facilitate interpretation, the figure shows only the reaction forces at the supports and the joint loads; moments are not included.

Two reaction forces appear in the situation shown: R_R, applied at the knee support, and R_P, applied at the forefoot. The load F_T calculated at the ankle must balance reaction R_P and the foot's own weight (PP_PIE). The ankle moment is obtained by considering the moments produced by these forces about the joint and, where applicable, the reaction moments calculated at the supports.

The force F_R calculated at the knee must balance the load transmitted by the ankle, the lower leg's own weight (PP_Pierna), and the reaction (R_R) exerted by the floor on the knee. In the simplification used by Ergoniza Intelligent, R_R is considered to be applied at the joint center and therefore generates no moment about it. The knee moment is obtained from the moment transmitted by the ankle and the moments produced by the ankle load and the lower leg's own weight.

8.2.2.4 Treatment of the knee joint moment under extreme lower-leg flexion

When the feet and/or knees are in contact with the floor and flexion of the lower legs relative to the thighs is extreme, the thighs are supported on the lower legs. Because of this support, the muscles acting at the knee joint do not need to overcome the moment associated with loads located higher in the kinematic chain.

When the foot is supported on the floor and knee flexion is very pronounced, Ergoniza Intelligent assumes that the back of the thigh contacts the lower leg and provides passive support. Under this simplification, the moment generated by loads located above the knee is not attributed to the muscles acting on the joint. Contact between the thigh and lower leg is inferred from the flexion angle and the presence of foot support.

Figure 35 shows two configurations consistent with this simplification. In the main image, the model adopts a squatting posture in which the backs of the thighs can rest on the lower legs. The detail image shows that this situation can also coexist with knee support on the floor, provided that the corresponding foot remains in contact.

When the foot is in contact with the floor and the knee flexion angle is greater than 157°, Ergoniza Intelligent sets the calculated knee joint moment to zero. This condition can coexist with support of the knee itself on the floor. The joint force is not set to zero and continues to be calculated using the equilibrium equations.

Even though Ergoniza Intelligent sets the load moment to zero in this situation, the posture should be analyzed carefully. Setting the moment to zero is a model simplification and does not mean that the knee is unloaded. The joint force continues to act, and neither the contact pressures nor the local distribution of loads over the cartilage, menisci, or ligaments are calculated explicitly.

Epidemiological studies have found an association between cumulative exposure to kneeling or squatting tasks and a higher frequency of symptomatic knee osteoarthritis. This association may be stronger when these postures are maintained or repeated for prolonged periods and combined with handling additional loads¹.

Consequently, a zero joint moment in this case must not be interpreted as an absence of load or ergonomic risk. The calculated knee force and the results of postural assessment methods should be considered together when analyzing this type of posture.

8.2.2.5 Calculating lower-limb joint loads when hip support is present

When the hip on the analyzed side contacts the floor, a reaction force arises at that point and changes the calculation of the corresponding lower-limb joint loads. The procedure differs from that described for support exclusively at the feet in Sections 8.2.2.1 and 8.2.2.2, and is analogous to that used when knee support is present, as described in Section 8.2.2.3.

As an example, Figure 36 shows a leg supported simultaneously at the heel and hip. The same procedure can be applied to the opposite side and to other equivalent configurations. To facilitate interpretation, the figure shows the reaction forces at the supports and the joint loads; moments are not included.

Two reaction forces appear in the situation shown: R_T, applied at the heel, and R_C, applied at the hip. Both are represented by their X, Y, and Z components. The joint forces calculated at the ankle, knee, and hip are also shown.

In this situation, the ankle load F_Tal corresponds to the combination of the foot's own weight (PP_PIE) and the reaction at the heel R_T, accounting for their opposite signs. The reaction moment at the ankle—not shown in the image—is the moment about the ankle produced by R_T and PP_PIE.

The joint force F_Tal calculated at the ankle must balance the reaction R_T exerted by the floor on the heel and the foot's own weight, PP_PIE. The ankle moment is obtained by considering the moments produced by these forces about the joint. Where applicable, the residual moments described in Section 8.3 are incorporated into the result.

The joint force F_R calculated at the knee must balance the load transmitted by the ankle, the lower leg's own weight, PP_Pierna, and any external loads applied to this segment. The knee moment is obtained from the moment transmitted by the ankle and the moments produced by these forces about the joint.

Finally, the joint force F_C calculated at the hip must balance the load transmitted by the knee, the thigh's own weight, PP_Muslo, any external loads applied to this segment, and the reaction R_C exerted by the floor on the hip. Ergoniza Intelligent considers R_C to be applied at the joint center, so it generates no moment about the joint. The hip moment is obtained from the moment transmitted by the knee and the moments produced by the remaining forces.