## 1 Introduction  
The main cause of lower limb amputations is car accidents, followed by diabetes and related vascular diseases. According to the National Health and Nutrition Examination Survey (NHANES) 2012, about 6.4 million people have diabetes in Mexico [1]. Mexico currently ranks eighth worldwide in terms of the prevalence of diabetes [2], and Projections of International Reports estimate that, by 2025, Mexico will rank sixth or seventh, with 11.9 million Mexicans with diabetes. Prostheses, together with the amputee, present a complex biomechanical system whose behavior is influenced by various design factors. To determine the operating conditions to which the prosthesis will be subjected, it is necessary to analyze the gait cycle [3]. The operating conditions are the reaction times and forces, as well as the angles between each joint during the running cycle. In terms of biomechanical design, the main factors to consider are the mechanical properties [4], the length of the prosthesis [5; 6], and the weight of the prosthetic components [7]. Most studies in the literature have focused on assessmentsof the kinematic and kinetic gait [8] and foot plantar pressure [9]. The kinematic method [10; 11] describes body movements and the relative movements of body parts during different gait phases; for instance, the study of the angular relationships between lower limb segments during the gait cycle is a kinematic method.  
In recent decades, numerical methods have gained popularity, especially when it is necessary to obtain detailed and complex information about the behavior of the prosthesis or the interaction between an amputee and the prosthesis [12; 13; 14]. Through the Finite Element Method (FEM), the stress and strain distribution in a prosthesis can be determined, which is non-viable using experimental or analytical methods. Despite the many articles dealing with modeling by FEM and the interaction of limbs, only a few have explored the behavior of the components of lower limb prostheses [15; 16; 17]. FEM modeling of a SACH(r)-type foot was performed to treat the effect of the viscoelastic heel performance [18]. Additionally, [19] presents a complete analysis of a prosthetic foot, validating it through FEM and an experimental analysis to improve footwear testing.  
Nevertheless, to the best of the knowledge of the authors at the time of the writing of this paper, there are no published works describing the behavior of the entire prosthesis. Some approaches can be found in studies examining the mono limb(r) prosthesis model [20]. In cases where a prosthetic foot is included, the ground reaction force (GRF) is considered as a load for FEM analyses. However, GRF has usually been directly applied to model nodes of the prosthetic foot or surfaces, despite geometric nonlinearity in the prosthesis design [18; 19; 20]. This assumption has been reported as the most likely source of error, so the incorporation of elements of a contact and friction floor-standing model in future works could sensibly reduce such error [18].  
This work has three main goals: to define the operating conditions of lower limbs during a human gait cycle at a medium-high speed; to design a transtibial prosthesis for medium-high impact activities, through an analytical method, supported by a well-defined methodology; and, finally, to validate the obtained results through analytical methods and numerical simulations.