# Postural Strategies Used While Donning a Simulated xEMU Spacesuit
## 1 Introduction  
Although critical for human space travel, it is not fully understood how spacesuits affect an astronaut's ability to move. In a general sense, an astronaut's range of motion is restricted, and their center of gravity (COG) is superiorly shifted and moved away from the body when donning a spacesuit and portable life support system (PLSS) [1]. This is evident in the difficulties astronauts experienced once on the moon; as observed from video footage, traversing the moon appeared to be very difficult [2]. The types of postural strategies used by astronauts while on lunar and Martian surfaces have not been previously investigated. Astronauts in future missions will have to perform various activities on the surfaces of the Moon or Mars [3] (e.g., manually drilling/digging to acquire soil samples or unloading supplies and equipment) that will require maintaining stable postures. NASA has an interest in examining gaps that relate to investigating postural control strategies [4]. Developing an understanding of how body segments move during gait, and relative to each other, when donning a spacesuit is beneficial for future generations of spacesuits and astronaut mobility. In particular, here we were interested in how the COG shift up and away from the body, due to the spacesuit, affects postural control. Human locomotion, or gait, can be described as an activity that induces corresponding rhythmic oscillations of the trunk and head [5, 6]. These movements are interpreted as equilibrium strategies, or postural strategies, aimed to provide stabilization while standing or moving (e.g., walking, running, or performing tasks). The various balance strategies adopted by children and adults involve two main functional principles: (1) the choice of the reference frame on which the equilibrium control is based and (2) the degrees offreedom of the various body joints. For the first principle, the reference frame can be either the supporting surface on which the subject is standing or the direction of gravity (e.g., earth-vertical) [7]. When the reference frame is the supporting surface, balance control is temporally organized from the feet to the head, according to an ascending organization. However, when the reference frame is the gravitational vector, balance control is temporally organized from the head to the feet, according to a descending organization [8]. These are known as bottom-up and top-bottom recruitment [7; 8], which are two types of postural strategies. The second functional principle concerns the choice of the degrees of freedom of the various body joints, which must be controlled simultaneously in dynamic equilibrium according to the task's constraints and/or the subject's motor ability. Postural control during stance consists of superimposed modules, which can be controlled independently [8]. Trunk stabilization can be defined as maintaining control over trunk posture and movement, despite the disturbing effects of gravity and external and internal perturbations [9]. Trunk stabilization is dependent on the passive (osteoligamentous), active (muscular), and neural sub-systems that contribute mechanically and in terms of acquiring and processing information to guide mechanical responses [10]. Head and trunk stabilization in space have been shown to be critical in determining stability [5; 6; 7]. The head in normal walking conditions maintains stability during gait, and there is an established relationship between the head and trunk during gait [6; 7]. Using head and trunk stabilization, one study [5] was able to characterize human gait using the absolute angular dispersion and anchoring index (AI) for forward and backward walking. However, in this proposed study, we used the absolute angle with standard deviation. The purpose of our study was to generate new knowledge of how spacecuits may affect how postural control during gait.