## 1 Introduction  
The current global number of people with diabetes mellitus is approximately 536 million and is estimated to increase to 700 million by 2045 [1]. This will also increase the incidence of diabetic foot syndrome [2]. Here, foot ulceration is the most common complication [3]. The main risk factor for the development of foot ulceration is diabetic polyneuropathy. The associated altered plantar pressure decreases pain sensation and altered skin function in combination with abnormal repetitive pressure overload and causes foot ulcers. An additional major problem of diabetic foot syndrome is the high ulcer recurrence rate. Here, a history of foot ulceration is among the greatest risk factors [4]. The probability of recurrence after an healed ulcer within one year is 40%, but it is 65% after three years [3]. As foot ulcers recur, the likelihood of minor and major amputation also increases [5; 6; 7]. Therefore, diabetic foot ulceration is associated with a high rate of lower extremity amputation and is among the major causes of mobility impairment worldwide [2; 8; 9; 10]. Prevention and remission measures are becoming increasingly important to reduce the first occurrence of an ulcer and the recurrence rate [11; 12]. A major aim is to reduce the increased plantar pressure associated with diabetic foot syndrome using specific shoe and insole care [11; 12; 13; 14; 15; 16; 17; 18]. In this context, customized insoles, so-called diabetes-adapted insoles, play an important role [13; 17]. However, the conventional production of these diabetes-adapted insoles is time-consuming [19]. The insoles are adjusted for shape and stiffness over multiple patient trials by removing and filling material until the desired reduction in peak plantar pressure is achieved. The increased effort is ultimately accompanied by increased costs as well [20; 21]. Additionally, the available materials are limited in terms of their mechanical properties [19]. The finite element (FE) method, combined with additive manufacturing processes, represents a promising alternative, as the method enables insole customization via parametric analysis. Additive manufacturing processes can directly produce the insoles that have been adapted in shape and stiffness. Moreover, they offer a more comprehensive range of physical material properties by changing the internal structure and filling degree of the 3D-printed material [19]. Currently, various FE models exist that simulate the plantar pressure distribution between the foot and the insole in order to investigate the influence of different sole designs and modifications, or to adjust the sole stiffness. These models greatly differ in their complexity. From a clinical application, the FE model should be as simple as possible, but as complex as necessary [22; 23]. Both 2D and 3D models have been described [24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35]; the former has a lower computational time to advantage, so dynamic analyses are mostly used [24; 25]. However, a disadvantage of 2D models is the limited analysis area. Therefore, recent studies have used static or quasi-static 3D simulations [27; 28; 29; 30; 31; 32; 33; 34; 35]. Although the International Working Group of Diabetic Foot recommends adjusting the stiffness of diabetes-adapted insoles based on the plantar pressure distribution during gait [13; 17], most studies analysed plantar pressure during a balanced stance, and only some studies have simulated a gait cycle [27; 32; 35]. FE models that analyse gait or time points of gait are usually very complex due to the detailed geometries (joints, cartilages, ligaments, etc.) and the integration of muscle forces [27; 29; 30; 32; 33; 35; 36; 37]. Furthermore, the implementation of muscle forces proves to be challenging and unsuitable for daily clinical practice [22; 38]. In this context, [PERSON] et al. [32] investigated the plantar pressure distribution between the insole and the foot by simulating three different time points of the gait.