IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 18, NO. 1, FEBRUARY2013 337 Technologies for Powered Ankle-Foot Orthotic Systems: Possibilities and Challenges Kenneth Alex Shorter, Jicheng Xia, Elizabeth T. Hsiao-Wecksler, Member, IEEE, William K. Durfee, and G´ eza F. Kogler Abstract—Ankle-foot orthoses (AFOs) can be used to ameliorate the impact of impairments to the lower limb neuromuscular motor system that affect gait. Existing AFO technologies include passive devices with fixed and articulated joints, semiactive devices that modulate damping at the joint, and active devices that make use of a variety of technologies to produce power to move the foot. Emerging technologies provide a vision for fully powered, unteth- ered AFOs. However, the stringent design requirements of light weight, small size, high efficiency, and low noise present significant engineering challenges before such devices will be realized. Once such devices appear, they will present new opportunities for clinical treatment of gait abnormalities. Index Terms—Active assist, ankle-foot orthosis (AFO), fluid power, gait. I. MOTIVATION F OR MOST, walking is a fundamental part of one’s daily routine and is a key component in overall quality of life. The efficiency and effectiveness of gait depends on joint mo- bility and muscle activity that is selective in timing and inten- sity [1]. The forces and motion generated during gait are related to three main functional tasks: weight acceptance, single limb support, and limb advancement. Weight acceptance and single limb support occur during stance when the foot is in contact with the ground, whereas limb advancement takes place during swing when the foot is off the ground. The ability to walk is impaired by numerous neurological and muscular pathologies or because of injuries. These include trauma, incomplete spinal cord injuries, stroke, multiple sclerosis, muscular dystrophies, and cerebral palsy [1]. Manuscript received July 20, 2010; revised May 23, 2011; accepted Au- gust 13, 2011. Date of publication December 14, 2011; date of current version September 12, 2012. Recommended by Technical Editor G. Morel. This work was supported by the National Science Foundation (NSF) under Grant 0540834 and by the Center for Compact and Efficient Fluid Power, an NSF Engineering Research Center. K. A. Shorter was with the Department of Mechanical Science and Engineer- ing, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA. He is now with the Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109 USA (e-mail: shorter2@illinois.edu). J. Xia and W. K. Durfee are with the Department of Mechanical En- gineering, University of Minnesota, Minneapolis, MN 55455 USA (e-mail: xiaxx028@umn.edu; wkdurfee@umn.edu). E. T. Hsiao-Wecksler is with the Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana-Champaign, IL 61801 USA (e-mail: ethw@illinois.edu). G. F. Kogler is with the School of Applied Physiology, Georgia Institute of Technology, Atlanta, GA 30332 USA (e-mail: geza@gatech.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMECH.2011.2174799 Biomechanical deficits of the lower extremities and their re- lated pathologies affect joint mobility and muscle activity. This paper focuses on the treatment of lower limb deficiencies with ankle-foot orthoses (AFOs). More specifically, the purpose of this paper is to present the background and rationale for de- veloping new technologies for use in fully powered, untethered AFOs. AFOs can be divided into three groups: passive, semiactive, and active. Passive devices contain no control or electronics, but can have mechanical elements such as springs or dampers to control the motion of the ankle joint during gait. Semiactive devices use computer control to vary the compliance or damping of the joint in real time. Fully active devices have an onboard or tethered source of power, one or more actuators to move the joint, and sensors and a computer to control the application of torque during gait. Passive AFOs make up the bulk of the devices prescribed by clinicians to treat weakness at the ankle joint complex; however, the passive nature of these AFOs lim- its the functional benefit they are capable of providing. These limitations could be addressed with an active AFO, but despite recent advances in computing, sensing, and other enabling tech- nologies, there are currently no practical portable powered AFO systems in existence. The development of light, compact, efficient, powered, un- tethered AFO systems has the potential to yield significant ad- vancements in orthotic control mechanisms and new clinical treatment strategies for rehabilitation and daily assistance. A recent review describes work in lower extremity exoskeletons and active orthoses [2]. The purpose of this paper is to pro- vide a comprehensive review of the state of the art specifically in ankle-foot orthotic technology and to describe the significant technical challenges that remain for AFOs. This paper overviews the biomechanics of normal and pathological gait, reviews ex- isting passive and active AFO devices, and discusses the key enabling technologies required to meet this challenging human scale application. II. NORMAL AND P ATHOLOGICAL GAIT Limb motion during steady-state constant speed locomotion involves intersegment and interlimb interactions during normal and abnormal walking [3]. Each limb segment and joint under- goes a cyclic pattern of flexion, extension, rotation, abduction, and adduction during a stride. An acute injury or pathology that affects a lower limb segment disrupts the cyclic gait pattern and can result in asymmetric deviations during gait [1]. An abnormal gait cycle affects the normal energy conserving characteristics of walking, resulting in increased energy expenditure [4]. 1083-4435/$26.00 © 2011 IEEE