# Quasi-Passive Resistive Exosuit for Space Activities: Proof of Concept
## 1 Introduction
Within this paper, we explore the development of the XoSoft exosuit, which represents a soft, modular, bio-mimetic and quasi-passive exoskeleton to assist users with low to moderate mobility impairments such as the elderly, post-stroke and partial spinal cord injury subjects, [23]. Quasi-passive actuators (QPA) that rely on clutches, are used to modulate the forces generated by the passive elements employed to store the mechanical energy. The authors have demonstrated in the latest version of their soft wearable device; \"XoSoft Gamma prototype\" [24], the effectiveness of the technology, where soft pneumatic QPAs [25] have been employed to modulate the level of assistance throughout the gait cycle. This assistive device features a modular and reconfigurable design of hips, knees and ankles actuators. The effectiveness of the assistance provided by this exosuit in terms of improvements to rehabilitation parameters has previously been proven [26; 27; 23], as has the comfort and acceptability [28; 29]. The use of dedicated, comfortable exoskeletons that provide continuous physical training while performing normal activities, without the need to dedicate time and space to physical training, would be extremely beneficial for the crew and their wellness. The introduction of such wearable devices will change the current physical training paradigm for all astronauts. The concept developed in this paper is a functional exosuit with a comprehensive suite of sensors, actuators and soft structures that takes into account both the exercise requirements and long terms user comfort. This will permit continuous all-day training of all body segments (e.g., forearm, shoulders, abdominals, upper and lower legs). The astronauts will train their limbs, as they would under normal gravity on earth, without the need to dedicate long portions of their day to maintaining health/fitness. Previous studies have shown that a low-intensity resistance training with relatively tonic movement and a short rest period causes increases in muscular size and strength [30], and slow-speed resistance training has also shown to effectively cause an increase in muscular strength [31]. In conclusion, prolonged continuous low- to moderate-intensity endurance type exercise training is equally effective as continuous moderate- to high-intensity training in increasing whole body and skeletal muscle oxidative capacity (main responsible for the muscle regeneration process) [32]. The exosuit will be used during normal activity and it will be able to identify and understand the user's motion, generating resistance to the user's limbs and simulating the effects of gravity. To achieve these goals, the exosuit will employ novel, dedicated actuators, sensor strategies and intelligence. The exosuit will also be equipped with wearable sensors to monitor, locally and remotely, bio-signals needed to guide the user during daily activities. Moreover, the transmission of this data to the ground support team will permit remote monitoring and continuous checking of the crew's health. This work describes the conversion of the XoSoft Gamma exosuit, previously developed for terrestrial rehabilitation, to a resistive exosuit to provide countermeasures for micro-gravity activities during medium/long space flights. These modifications and enhancements introduce a number of application specific improvements: (i) bilateral knee flexion and extension actuation units, (ii) a dedicated control strategy designed to generate resistance while carrying out normal walking activity, (iii) assessment of the performance of this control strategy, and (iv) a dedicated soft sensor strategy which is fully integrated and validated. The system design and control strategy are illustrated in Section 2. Experiments to assess the effect on muscle activation, of the sensing subsystems, and the overall resistive device are conducted and presented in Section 3.