# Quasi-Passive Resistive Exosuit for Space Activities: Proof of Concept
## 1 Introduction  
Bone density loss and muscular atrophy are an inevitable outcome of exposure to micro-gravity [1, 2, 3, 4]. The average rate of bone loss is about 2.5% per month in at least one skeletal site [5]. This is very significantly higher than the bone density loss due to osteoporosis. In addition to bone density loss, muscle atrophy is also of great concern during spaceflights. [PERSON] et al. [6] have observed muscle strength reductions of up to 30% during extended space activities, with an exponential decay related to the duration of the flight [7]. Many countermeasures have been evaluated during flights, and in recent years, promising results have been shown [8]. Pharmaceutical countermeasures, either alone or in combination with exercise, show potential benefits [9]. Physical exercise has also been tested in many forms over the past four decades of spaceflight. Typical exercise equipment available on long duration space missions (Mir and Skylab) is mainly designed to promote aerobic and muscular endurance and is based on Combined Operational Load Bearing External Resistance Treadmills (COLBERT), Stationary Bikes, and elastic expanders such as the Exer-Genie [10]. With the start of operations on the International Space Station (ISS),the interim Resistive Exercise Device (iRED) [11], and later in 2008, the Advanced Resistive Exercise Device (ARED) [12] have been used as countermeasures to promote resistance exercise and are used together with applied loads to simulate earth's gravity. A recent study that examined the effects of space flight coupled with exercise, showed promising results on mitigation of micro-gravity side-effects without the use of pharmaceutical agents [8]. Although the use of exercise devices does provide promising results. The average time spent doing the aerobic exercise while in space, with cycle egometer (CEVIS) and treadmill (TVIS) is 120 min/wk and 160 min/wk respectively [13]. The cycle exercise generally ranged between 100 and 150W (light to moderate effort activity), and the walking/running activities on the treadmill were performed at an average speed of 3.0 to 9.0 km/h. Cycling and walking/running activities generate a metabolic expenditure ranging from 2 METS (3 km/h walking speed) to 8 METS (9 km/h walking speed) with 5.5 METS and 7 METS for the 100 and 150 watts cycling activities respectively [14]. In spite of the positive potential of these systems, undesirable side-effects are that the form-factors of these training systems restrict the crew to a specific location within the station and require a large vibration isolation system to minimize disturbances imparted into the spacecraft during workouts [15]. Therefore, these concentrated exercises regimes require the crew to train in isolation or semi-isolation for about an hour and half every two days in a dedicated training room. Moreover, the lack of monitoring systems does not allow for real-time activity feedback on specific physical parameters or muscle strength assessments, although this would be very beneficial. Wearable robotic device used as countermeasures for micro-gravity activities could represent a solution to address several of these drawbacks in one single device as presented in [15]. In the past decade, the use of exoskeletons has grown in several fields. The most common such domain is rehabilitation, spanning from stationary exoskeletons, such as the Lokomat [16] and LOPES [17], to mobile exoskeletons to provide assistance to paraplegics when walking [18; 19]. Recently, exosuits (soft exoskeletons), which form a new generation of exoskeletons have been presented, [20; 21; 22], with ankles, knees or hips that are supported by a soft or hybrid structure driven by active tendon connected actuation systems. These exoskeletons use a soft wearable structure, which helps to reduce the burden created by the assistive device. They address not only the degree of assistance but also the usability/acceptance by end users.