## 1 Introduction  
Standing upright is a demanding motor task, which has been in the scientific focus for more than four decades [1; 2; 3]. Considering that the upright position is more dynamic than static, scientific attention is focused, among other things, on the acting degrees of freedom, the relevant biomechanical parameters and their physical description. Simplifying the interplay of the joint moments in a follow-up reaction as a function of the horizontal center of pressure (CoP) displacement is the common approach in the modelling of postural stability. The resulting model corresponds to a mathematical inverted pendulum, which is still considered as the scientific consensus today [4; 5; 6; 7; 8]. The natural frequency of the body when oscillating around the ankle axis is 0.2-0.3 Hz. What speaks against the pendulum model is that the amplitude of the oscillation and the occurring frequency are independent of size and weight [9; 10]. Furthermore, it can be assumed that the movements of the head are about the same as those of the hip. There are indications that a reduction of the biomechanical modelling of the immobile stance on the ankle joint is not sufficient enough to fully describe the movement sequence [11; 12]. Studies have shown that not only does the ankle joint form a degree of freedom when standing still,but also the knee and hip joints fluctuate depending on the plane of observation (frontal or sagittal plane) [13; 14; 15]. Furthermore, it could be shown that the hip joint and ankle joint strategy primarily contributes to a calm stance [16; 17]. The knee joint especially seems to play an important role in postural counter-reactions [18]. Based on this thesis, a two-pendulum model was proposed [3] (see Figure 1). In fact, under certain pathological conditions, the mechanics of the body fluctuations can be described remarkably well with this [19]. In recent years, various studies have shown that standing upright is a multiple joint process. When this is integrated into a better model concept, a multiple joint process occurs [14; 15; 20; 21; 22]. For this purpose, double and triple inverted pendulum models are assumed (see Figure 1). In this, a mechanical system that is inherently unstable (multiple inverted pendulum) is represented as a self-stabilizing system with a very high degree of stiffness in the joints [23; 24; 25]. However, it is very unlikely that the human muscle apparatus behaves in the same way [15; 26; 27]. This means that to be able to resist gravity in an upright position, it is necessary to actively influence all mechanical degrees of freedom of the joints. By comparing the spectra of the multi-joint models (Figure 1) with the human spectra, characteristic frequency patterns can be identified. The various models reliably reflect real human balance behavior only within certain limits. However, models based on the multiple joint approach in particular can be used to investigate and explain various phenomena. Various parameters can be determined from the CoP, such as the distance as the sum of the CoP distance over time [29; 30], the speed of the pressure center point [31] and the area enclosed by it. More rarely, the sagittal and lateral center of gravity movements [32] are examined. An effective means of analyzing and displaying these is the frequency analysis or power spectral density (PSD) [33]. Figure 1: Representation of different models for approximating the upright position based on the inverted pendulum with mechanically inert joints to represent the physiological properties of the muscles. Left: Inverted Pendulum with one free joint; center: double inverted pendulum (DIP) with two free joints (ankle and hip); right: triple inverted pendulum (TIP) with three free joints (ankle, knee and hip) [15; 28]. The resulting frequency spectra can be altered by changes in the frequency ranges that occur in various cases as ascribed [34].