## 2 Materials and Methods
### Research Models--Flat Bar Model and Human Body  
The human body in the model can be simplified into two elements, the support platform (i.e., the foot) and the rest of the body, which tilts at the upper ankle joint. This simplification allows for focusing on the relationship between the centre of gravity, the support platform (which bears the weight of the tilting flat bar), and the ground (which supports the weight of the entire model). It was not concerned with how the weight is distributed within the foot itself. Instead, it was focused solely on the loads at the extreme support points at specific tilt angles of the flat bar, while maintaining the system's balance. By translating this to the human foot, it was aimed to capture the load on the head of the first metatarsal bone and the calcaneal tuberosity. A model system was with a height of \(l_{1}\) = 2.06 m, which consisted of a metal flat bar (\(l_{2}\) = 2 m, width: 0.03 m) with a threaded rod with nuts imitating the weight of the human head placed at one end, and on the other side with a hinged platform imitating the footplate (_lamina pedis_). The height of the foot with the hinge is \(l_{3}\) = 0.06 m. The hinge in this system imitates the talus bone with a trochlea. The weight of the tilting part of the model (flat bar + rod with nuts) was 2.648 kg, and the entire system, together with the platform imitating the foot, was 2.972 kg. The centre of gravity for the tilted part of the model (\(CoG_{2}\)) (without the support platform) is located at a height of 1.1 m (\(h_{2}\)) from the axis of rotation, and 1.16 m from the ground (1.1 + height of the platform with a hinge of 0.06 m) (\(h_{CoG2}\)). On the other hand, the general centre of gravity for the entire system (\(CoG_{1}\)) together with the platform and hinge is 0.01 m lower, i.e., at a height of 1.15 m (\(h_{CoG1}\)) measured from the ground, and measured from the axis of rotation at a height of 1.09 m (\(h_{1}\)). The line indicating the projection of the centre of gravity on the support platform was obtained by mounting a cable with a weight at the centre of gravity point of \(CoG_{2}\). The hinge connecting the tilted part and the support platform imitates the upper ankle joint. Subsequent degrees of flat bar tilt relative to the support platform were stabilised by tightening the screw on the axis of rotation. Two scales (EKS002W, 1-5000 g, Digital scale, Esperanza-Extrim-Titanium, Ozarow Mazowiecki, Poland) were used to measure the ground load of the object (Figure 3A). The readings on the scales were checked based on standard weights of 0.5 kg and 1 kg. Before each weighing, the scales were tared. It was performed twenty-two repetitions of the gravity measurements at points \(A\) and \(B\) at each degree of flat bar inclination. In addition to the tests on the technical model (flat bar model), the ground loads at the level of the extreme support points of the human foot were also exam tied. The calculations were made for an example human with a total mass of 94.2 kG (\(CoG_{1}\)), height of 1.82 m (and a distance from the axis of the upper ankle joint to the top of the head of 1.73 m), foot length of 0.23 m (\(r_{D}\)), the distance from point \(A\) to the projection of the upper ankle joint axis (\(r_{A}\)) equal to 0.13 m, and the distance from point \(B\) to the projection of the upper ankle joint axis (\(r_{B}\)) equal to 0.045 m (Figure 3B). The \(CoG_{1}\) of the human body was located at a height of 1.064 m (\(h_{CoG1}\)) from the plantar side of the feet, and at a height of 0.974 m (\(h_{1}\)) from the axis of rotation. To calculate the \(CoG_{2}\) of the leaning body, the total weight of the human was reduced by the weight of the feet [19], which was finally 91.6 kG. The \(CoG_{2}\) of the leaning part of the human body was located at a height of 1.077 m (\(h_{CoG2}\)) and a height of 0.987 m (\(h_{2}\)) from the axis of rotation. Figure 3: Model for measuring the ground load through the extreme areas within the support platform of the model object.