Review Article Balance and stability issues in lower extremity exoskeletons: A systematic review Mukhtar Fatihu Hamza Q1 a,b, * , Raja Ariffin Raja Ghazilla a , Bashir Bala Muhammad c , Hwa Jen Yap a a Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia b Department of Mechatronics Engineering, Bayero University, 3011 Kano, Nigeria c School of Mechanical Engineering, Northwestern Polytechnical University, Xi'an, 710072 China 1. Introduction Losing balance is a terrible incident that causes falling. This is a common phenomenon that creates a high risk for many people in domestic and professional scenarios. Information from the US Department of Labour (https://www.osha.gov/ oshstats/commonstats.html) indicated that falling is the major cause of the death of workers in construction industries, which accounted for about 39.2% of total fatalities in 2017. Age b i o c y b e r n e t i c s a n d b i o m e d i c a l e n g i n e e r i n g x x x ( 2 0 1 9 ) x x x – x x x 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 a r t i c l e i n f o Article history: Received 22 September 2019 Received in revised form 31 August 2020 Accepted 6 September 2020 Available online xxx Keywords: Exoskeleton Stability Lyapunov Zero moment Point Centre of mass Extrapolated Centre of mass a b s t r a c t The lower extremity exoskeletons (LEE) are used as an assistive device for disabled people, rehabilitation for paraplegic, and power augmentation for military or industrial workers. In all the applications of LEE, the dynamic and static balance, prevention of falling, ensuring controller stability and smooth human-exoskeleton interaction are of critical importance for the safety of LEE users. Although numerous studies have been conducted on the balance and stability issues in LEEs, there is yet to be a systematic review that provides a holistic viewpoint and highlights the current research challenges. This paper reviews the advances in the inclusion of falling recognition, balance recovery and stability assurance strategies in the design and application of LEEs. The current status of research on LEEs is presented. It has been found that Zero Moment Point (ZMP), Centre of Mass (CoM) and Extrapolated Center of mass (XCoM) ideas are mostly used for balancing and prevention of falling. In addition, Lyapunov stability criteria are the dominant methods for controller stability confirmation and smooth human-exoskeleton interaction. The challenges and future trend of this domain of research are discussed. Researchers can use this review as a basis to further develop methods for ensuring the safety of LEE's users. © 2020 Published by Elsevier B.V. on behalf of Nalecz Institute of Biocybernetics and Biomedical Engineering of the Polish Academy of Sciences. * Corresponding author. E-mail address: mfhamza@um.edu.my (M.F. Hamza). Abbreviations: LEE, lower extremity exoskeletons; ZMP, Zero Moment Point; CoM, Centre of Mass; XCoM, Extrapolated Centre of mass; GRF, Ground Reaction Force; HAA, Powered hip ab/adduction; SWAA, Step-Width Adaptation Algorithm; HFE, hip flexion/extension; SM, Stability margin; GCP, Ground Contact Point; IP, Inverted pendulum; APO, Active Pelvis Orthosis; ANN, Artificial Neural Network; MLPNN, Multi-Layer Perceptron Neural Network; SMC, Sliding Mode Control; PRM, Poincare return map; CaM, Centroidal angular momentum. BBE 492 1–14 Please cite this article in press as: Hamza MF, et al. Balance and stability issues in lower extremity exoskeletons: A systematic review. Biocybern Biomed Eng (2020), https://doi.org/10.1016/j.bbe.2020.09.004 Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/bbe https://doi.org/10.1016/j.bbe.2020.09.004 0208-5216/© 2020 Published by Elsevier B.V. on behalf of Nalecz Institute of Biocybernetics and Biomedical Engineering of the Polish Academy of Sciences.