## 1 Introduction  
The gait of subjects with primary hereditary cerebellar ataxia (swCA) is characterized by abnormalities in all global and segmental gait parameters, leading to an associated high falls risk [1, 2] and particularly, increased step width, reduced ankle joint kinematics, increased gait variability, lack of intra-limb and inter-segmental coordination, and reduced trunk control and stability [3, 4, 5, 6, 7, 8]. The primary characteristic of the ataxic gait, which has been described regardless of the subtype of CA diagnosis, is the lack of joint coordination, resulting in abnormal coupling of intra-limb joints and upper and lower body segments during walking [1].  
SwCA adopt motor strategies that involve the widening of muscle activation timing and increasing the antagonistic muscle coactivation at a single-joint level [9; 10; 11] in order to stiffen the body segments, as a strategy to compensate for muscle hypononia and irregular trajectories [3; 12; 13]. The trunk plays an important role in these altered movement patterns. Several studies have found increased trunk oscillations which can have a significant impact on stance and gait performance and stability [14; 15; 16; 17; 18; 19; 20; 21].  
Furthermore, wide upper body oscillations that shift the center of gravity to the borders of the base of support may increase body sway while walking, resulting in the characteristic chaotic gait behavior [22] that exacerbates gait instability and raises the risk of falling [23]. To cope with their walking instability, subjects with cerebellar ataxia adopt motor strategies that involve widening and expanding the muscle activation timing to stiffen body segments [3; 14]. However, this compensatory system has some drawbacks, including the possibility of cartilage degradation and increased metabolic costs [24; 25].  
These findings indicate that it may be appropriate to arrange specific rehabilitation therapy focused on trunk control and/or specific equipment for trunk stabilization during walking [10; 14; 26; 27]. In particular, swCA may benefit from elastic or semirigid orthoses (e.g., elastic suits and/or exoskeletons) that may decrease axis fluctuations in the sagittal and frontal planes without affecting lower limb mobility while walking [7]. Back-support passive exoskeletons have been shown to reduce falls risk and improve trunk stability during gait in healthy subjects [28], as well as constraining trunk movements, which may reduce center of mass (CoM) oscillations and step width and optimize the energetic cost of walking [29; 30; 31; 32]. However, the effects of specific assistive devices, particularly elastic or semi-rigid orthoses, on the gait behavior of swCA have not been investigated yet. Reducing trunk oscillations using a passive trunk exoskeleton may help to improve dynamic stability in swCA, resulting in useful tools for supporting the rehabilitation process, reducing falls risk, and assisting swCA in daily activities and work reintegration [33].  
Therefore, this pilot study aimed to assess the feasibility of an elastic textile suit equipped with a passive exospine on trunk kinematics, in terms of effectiveness on center of mass behavior and mechanical energy expenditure and recovery, and on lower limb and trunk muscles coactivation during walking in swCA. We considered two passive exospine versions based on energy accumulation and return, one more flexible, MFE, and one less flexible, LFE. This passive exoskeleton presents a mechanical component, the exospine, which was absent in the previous version [28]. We hypothesized that this orthotic device, equipped with a passive exospine, may be effective in supporting the trunks of swCA, helping to reduce oscillations of the trunk and center of mass, and reducing energy expenditure during walking.