# Preliminary Biomechanical Evaluation of a Novel Exoskeleton Robotic System to Assist Stair Climbing
## 1 Introduction  
An exoskeleton is a wearable device with rigid and/or soft components, that augments, assists, enables, and/or enhances physical activity through mechanical interaction with the body [1]. Furthermore, an exoskeleton robotic system is an exoskeleton with all of the associated components such as a power supply, equipment, software, and communications that are necessary to make it fully functional. In recent years, more and more exoskeletons have been developed and tested for a wide range of applications [2]. Thus, a number of systems were also developed to assist people, especially elderly people, exclusively during stair climbing [3, 4]. For example, an active system has been developed by [5] that consists of five parts and weighs about 13.5 kg. In this system, additional torque is applied to the knee and ankle joints of both legs by tension elements, which are actuated by two DC motors that are located on the back with a power of 140 W each [5]. Another example is a lower limb exoskeleton that is described in [6]. This carbon fibre bilateral system assists the extension and flexion moment of the knee joint by two pneumatic actuators each. The actuators are controlled by five non-invasive surface electromyographs (sEMG) that have to be placed on the user's thigh muscles in advance [6]. All systems of the state of the art devices for the exclusive assistance of elderly people during stair climbing have different actuator configurations. However, biomechanical studies to evaluate the need for assistance have shown that only knee extension should be assisted during ascent and descent [3]. In this way, the natural muscle weakness that occurs with age can be compensated [7]. The current systems do not meet this requirement and assist or actuate more than is actually needed. Furthermore, the effectiveness of these exoskeletons has not been evaluated or has only been evaluated in very simple biomechanical studies. For example, trials were conducted with the active system by [5] and it was described that the system helps the user to climb stairs. However, this statement is only based on the experience and sense of the user and has not been evaluated by measurements. The lower limb exoskeleton was tested with only one subject, but for a period of 10 days [6]. From the recordings of knee angle, sEMG of knee extensors and flexors, and metabolic costs, a reduction in both flexor muscles activity by up to 61% could be concluded [6]. Also, only one subject (age 37 years, height 1.70 m, weight 69 kg) was examined in the preliminary walking experiments for the test of the robotic hip exoskeleton [8]. Although the test was carried out on a standardized staircase (step height 0.18 m, tread 0.30 m) according to [9], the aim of the test was not the biomechanical evaluation of the exoskeleton. A more comprehensive investigation was carried out on the Hiteexosuit with three test persons [10]. Here, the tensile force of the actuator and the knee joint angular displacements were measured during stair climbing [10]. The lower extremity exoskeleton achieved a knee joint moment of 0.8 Nm/kg in stair climbing experiments, which were not further described [11]. It was concluded that the design and hybrid control strategy are functional and improve mobility during stair climbing. Furthermore, trials were conducted with Beihang University's lower limb exo on various indoor and outdoor staircases [12]. However, only the kinematics were measured to evaluate a gait algorithm, but not to investigate the biomechanical effectiveness of the system itself [12]. The passive exoskeleton PKAExo was only mechanically verified in one experiment [13; 14]. Nevertheless, the authors noted that this experiment does not sufficiently prove that the passive system neither reduces muscle activity during weight-bearing ascending locomotion, nor that it improves human metabolic productivity [14]. They already mentioned that sEMG signals, metabolic costs, and also external forces should be measured in order to evaluate the system [14].