Effects of Back Support Exoskeleton Use on Postural Stability

Abstract

While occupational back support exoskeletons (BSEs) are being considered as a potential intervention to reduce physical demands in tasks such as repetitive lifting (e.g., Frost, Abdoli-E, & Stevenson, 2009; Koopman, Kingma, Faber, de Looze, & van Dieën, 2019), BSE use in practice may introduce some unexpected or unintended safety challenges (Baltrusch, van Dieën, van Bennekom, & Houdijk, 2018; de Looze, Bosch, Krause, Stadler, & O’Sullivan, 2016). One potential adverse effect is a decrease in postural balance and stability due to the extra weight and the rigid structure of a BSE. However, there is limited empirical evidence on how the use of a BSE affects postural balance and stability. In this study, we investigated the effects of using different BSEs on postural balance and stability during quiet upright stance. A total of 20, gender-balanced, healthy participants were included [males = 25.2 (3.8) years, 176.4 (7.4) cm, and 76.6 (8.8) kg; females = 27.5 (2.7) years, 166.5 (5.4) cm, and 61.2 (8.6) kg]. Each completed multiple trials of quiet upright stance at different levels of difficulty (i.e., bipedal and unipedal stance; each with eyes open and closed), while wearing two different BSEs (SuitX™ AC version, Laevo™ V2) and in a control (no-exoskeleton) condition. Respective masses of the SuitX™ and Laevo™ were 4.5 and 2.8 kg, and both devices were designed to provide external torque parallel to that created by the torso extensor muscles via three body contact points (i.e., thighs, pelvis, and chest). Center-of-pressure (COP) time series were measured using a force-plate during each trial, and traditional COP-based postural sway parameters were used as outcome measures: median frequency (MF), mean velocity (MV), root-mean-square distance (RMSD), and sway area. MF, MV, and RMSD values were obtained in both the anteroposterior (AP) and mediolateral (ML) directions. As MV, RMSD, and SwayAREA were significantly correlated with participant height, these measures were normalized using respective linear regression models to prevent undesired bias. Results from repeated measures analyses of variance indicated that wearing BSEs may adversely affect postural stability during bipedal stance. Compared to the control condition, wearing the Laevo™ increased MFAP by 50% ( p=0.015) with eyes open, and wearing the SuitX™ increased MVAP by 7% ( p=0.029) with eyes closed. However, specific to the unipedal standing with eyes closed, wearing the Laevo™ appeared to positively affect postural stability among males, whereas wearing the SuitX™ appeared to negatively affect postural stability among females. Specifically, males had a 24% decrease in RMSDAP ( p=0.003) and a 31% decrease in SwayAREA ( p=0.012) when wearing Laevo™, while females had a 15% increase in RMSDAP ( p=0.038) when wearing the SuitX™. In conclusion, we found evidence that wearing a BSE can affect postural balance and stability during quiet upright stance. More importantly, such effects appeared dependent on task conditions (i.e., bipedal and unipedal, and with eyes open/closed), and to be BSE-design and gender-dependent. Our findings may suggest complex interactive dynamics between a user and a BSE, potentially due to a change in total center of mass (i.e., body + BSE), restricted motion, and external supportive torques of the BSE. Furthermore, males and females may be differentially affected by such interactive dynamics, depending on the BSE, particularly in more challenging tasks. Future studies need to investigate the specificity of BSE effects on postural control while considering different demographic and individual factors, and also the effects of wearing BSEs on postural stability in dynamic conditions (e.g., walking, negotiating an obstacle, or recovering from an external perturbation).