This paper presents a survey on recent advances of wearable actuated ankle-foot orthoses (AAFOs). First of all, their medical functions are investigated. From the short-term aspect, they lead to rectification of pathological gaits, reduction of metabolic cost, and improvement of gait performance. After AAFO-based walking training with sufficient time, free walking performance can be enhanced. Then, key design factors are studied. First, primary design parameters are investigated. Second, common actuators are analysed. Third, human-robot interaction (HRI), ergonomics, safety, and application places, are considered. In the following section, control technologies are reviewed from the aspects of rehabilitation stages, gait feature quantities, and controller characteristics. Finally, existing problems are discussed; development trends are prospected.
GageJRSchwartzMHKoopSE, et al. The identification and treatment of gait problems in cerebral palsy. New York, NY: John Wiley & Sons, 2009. Vol. 180.
4.
HakakzadehAShariatAMoradiV, et al. Multimodal impact of acupuncture, exercise therapy, and concurrent functional electrical stimulation on osteoarthritis of the knee: a case report. Acupunct Electrother Res2019; 44(2): 135–145.
5.
LusardiMMJorgeMNielsenCC.Orthotics and prosthetics in rehabilitation-e-book. Amsterdam: Elsevier Health Sciences, 2013.
6.
van der WilkDDijkstraPUPostemaK, et al. Effects of ankle foot orthoses on body functions and activities in people with floppy paretic ankle muscles: a systematic review. Clin Biomech2015; 30(10): 1009–1025.
7.
ISO 8549-1:2020. Prosthetics and orthotics-vocabulary-part 1: general terms for external limb prostheses and external orthoses. Pub ISO, Geneva Switzerland, 2020.
8.
ShorterKA.The design and control of active ankle-foot orthoses. Champaign, IL: University of Illinois at Urbana-Champaign, 2011.
9.
DuerinckSSwinnenEBeylP, et al. The added value of an actuated ankle-foot orthosis to restore normal gait function in patients with spinal cord injury: a systematic review. J Rehabil Med2012; 44(4): 299–309.
10.
ZhouCYangZLiK, et al. Research and development of ankle-foot orthoses: a review. Sensors2022; 22(17): 6596.
11.
HussainSJamwalPKGhayeshMH.State-of-the-art robotic devices for ankle rehabilitation: mechanism and control review. Proc IMechE, Part H: J Engineering in Medicine2017; 231(12): 1224–1234.
12.
ShiBChenXYueZ, et al. Wearable ankle robots in post-stroke rehabilitation of gait: a systematic review. Front Neurorobot2019; 13: 63.
13.
KhalidYMGouwandaDParasuramanS.A review on the mechanical design elements of ankle rehabilitation robot. Proc IMechE, Part H: J Engineering in Medicine2015; 229(6): 452–463.
14.
AdiputraDNazmiNBahiuddinI, et al. A review on the control of the mechanical properties of ankle foot orthosis for gait assistance. Actuators2019; 8: 10.
15.
KubasadPRTodetiSRKamatYD. A review on designs of various ankle foot orthosis (AFO) used to treat drop foot disease. In: Sen D, Mohan S and Ananthasuresh GK (eds.) Mechanism and machine science. Springer, Singapore, 2021, pp.789–807.
16.
HussainSJamwalPKVlietPV, et al. Robot assisted ankle neuro-rehabilitation: state of the art and future challenges. Expert Rev Neurother2021; 21(1): 111–121.
17.
AlamMChoudhuryIABin MamatA.Mechanism and design analysis of articulated ankle foot orthoses for drop-foot. Sci World J2014; 2014: 867869.
18.
Arnez-PaniaguaVRifaiHMohammedS, et al. Adaptive control of an actuated ankle foot orthosis for foot-drop correction. IFAC-PapersOnLine2017; 50(1): 1384–1389.
19.
KimSJNaYLeeDY, et al. Pneumatic AFO powered by a miniature custom compressor for drop foot correction. IEEE Trans Neural Syst Rehabil Eng2020; 28(8): 1781–1789.
20.
ChoiHParkYJSeoK, et al. A multifunctional ankle exoskeleton for mobility enhancement of gait-impaired individuals and seniors. IEEE Robot Autom Lett2018; 3(1): 411–418.
21.
KwonJParkJHKuS, et al. A soft wearable robotic ankle-foot-orthosis for post-stroke patients. IEEE Robot Autom Lett2019; 4(3): 2547–2552.
22.
AllenDPLittleRLaubeJ, et al. Towards an ankle-foot orthosis powered by a dielectric elastomer actuator. Mechatronics2021; 76: 102551.
23.
YeungLFLauCCYLaiCWK, et al. Effects of wearable ankle robotics for stair and over-ground training on sub-acute stroke: a randomized controlled trial. J Neuroeng Rehabil2021; 18(1): 1–10.
24.
de Miguel-FernandezJPescatoreCMesa-GarridoA, et al. Immediate biomechanical effects of providing adaptive assistance with an ankle exoskeleton in individuals after stroke. IEEE Robot Autom Lett2022; 7(3): 7574–7580.
25.
ChenJHanJZhangJ.Design and evaluation of a mobile ankle exoskeleton with switchable actuation configurations. IEEE/ASME Trans Mechatron2022; 27: 1846–1853.
26.
SteeleKMJacksonRWShumanBR, et al. Muscle recruitment and coordination with an ankle exoskeleton. J Biomech2017; 59: 50–58.
27.
WangTMPeiXHouTG, et al. An untethered cable-driven ankle exoskeleton with plantarflexion-dorsiflexion bidirectional movement assistance. Front Inf Technol Electron Eng2020; 21: 723–739.
28.
MazumderOKunduALenkaP, et al. Robotic AFO to enhance walking capacity: initial development. Electron Lett2016; 52(22): 1840–1841.
29.
KollerJRRemyCDFerrisDP.Biomechanics and energetics of walking in powered ankle exoskeletons using myoelectric control versus mechanically intrinsic control. J Neuroeng Rehabil2018; 15(1): 1–14.
30.
LernerZFHarveyTALawsonJL.A battery-powered ankle exoskeleton improves gait mechanics in a feasibility study of individuals with cerebral palsy. Ann Biomed Eng2019; 47(6): 1345–1356.
31.
OrekhovGFangYLuqueJ, et al. Ankle exoskeleton assistance can improve over-ground walking economy in individuals with cerebral palsy. IEEE Trans Neural Syst Rehabil Eng2020; 28(2): 461–467.
32.
BayónCKeeminkAQLvan MierloM, et al. Cooperative ankle-exoskeleton control can reduce effort to recover balance after unexpected disturbances during walking. J Neuroeng Rehabil2022; 19(1): 1–16.
33.
Acosta-SojoYStirlingL.Individuals differ in muscle activation patterns during early adaptation to a powered ankle exoskeleton. Appl Ergon2022; 98: 103593.
34.
DurandauGRampeltshammerWFKooijHVD, et al. Neuromechanical model-based adaptive control of bilateral ankle exoskeletons: biological joint torque and electromyogram reduction across walking conditions. IEEE Trans Robot2022; 38: 1380–1394.
35.
JainTKCarterVLernerZF.Effects of lightweight wearable ankle exoskeleton in an individual with Parkinson disease: a case report. Top Geriatr Rehabil2020; 36(3): 146–151.
36.
LernerZFGasparriGMBairMO, et al. An untethered ankle exoskeleton improves walking economy in a pilot study of individuals with cerebral palsy. IEEE Trans Neural Syst Rehabil Eng2018; 26(10): 1985–1993.
37.
GalleSMalcolmPCollinsSH, et al. Reducing the metabolic cost of walking with an ankle exoskeleton: interaction between actuation timing and power. J Neuroeng Rehabil2017; 14(1): 1–16.
38.
QuinlivanBTLeeSMalcolmP, et al. Assistance magnitude versus metabolic cost reductions for a tethered multiarticular soft exosuit. Sci Robot2017; 2(2): eaah4416.
39.
AwadLNBaeJO’DonnellK, et al. A soft robotic exosuit improves walking in patients after stroke. Sci Transl Med2017; 9(400): eng1015050861946.
40.
GasparriGMLuqueJLernerZF.Proportional joint-moment control for instantaneously adaptive ankle exoskeleton assistance. IEEE Trans Neural Syst Rehabil Eng2019; 27(4): 751–759.
41.
FangYOrekhovGLernerZF.Improving the energy cost of incline walking and stair ascent with ankle exoskeleton assistance in cerebral palsy. IEEE Trans Biomed Eng2022; 69(7): 2143–2152.
42.
SawickiGSDomingoAFerrisDP.The effects of powered ankle-foot orthoses on joint kinematics and muscle activation during walking in individuals with incomplete spinal cord injury. J Neuroeng Rehabil2006; 3(1): 1–17.
43.
MoltedoMBačekTVerstratenT, et al. Powered ankle-foot orthoses: the effects of the assistance on healthy and impaired users while walking. J Neuroeng Rehabil2018; 15(1): 1–25.
44.
PourghasemATakamjaniIEKarimiMT, et al. The effect of a powered ankle foot orthosis on walking in a stroke subject: a case study. J Phys Ther Sci2016; 28(11): 3236–3240.
45.
LeeTKimIBaekYS.Design of a 2DoF ankle exoskeleton with a polycentric structure and a bi-directional tendon-driven actuator controlled using a pid neural network. Actuators2021; 10: 9.
46.
ChoiHSBaekYS.Effects of the degree of freedom and assistance characteristics of powered ankle-foot orthoses on gait stability. PLoS One2020; 15(11): e0242000.
47.
Gomez-VargasDBallen-MorenoFBarriaP, et al. The actuation system of the ankle exoskeleton T-FLEX: first use experimental validation in people with stroke. Brain Sci2021; 11(4): 412.
48.
NomuraKYonezawaTTakemuraH, et al. Development of six-DOF human ankle motion control device using Stewart platform structure for fall prevention. J Robot Mechatron2016; 28(5): 654–663.
49.
HarveyTAConnerBCLernerZF.Does ankle exoskeleton assistance impair stability during walking in individuals with cerebral palsy?Ann Biomed Eng2021; 49(9): 2522–2532.
50.
OlsonJ.Design of active ankle foot orthotics for gait assistance and fall prevention. Int Robot Autom J2020; 6(3): 106–113.
51.
PetrucciMNMacKinnonCDHsiao-WeckslerET.Modulation of anticipatory postural adjustments using a powered ankle orthosis in people with Parkinson’s disease and freezing of gait. Gait Posture2019; 72: 188–194.
52.
Russell EspositoESchmidtbauerKAWilkenJM. Experimental comparisons of passive and powered ankle-foot orthoses in individuals with limb reconstruction. J Neuroeng Rehabil2018; 15(1): 1–10.
53.
AwadLNKudziaPReviDA, et al. Walking faster and farther with a soft robotic exosuit: implications for post-stroke gait assistance and rehabilitation. IEEE Open J Eng Med Biol2020; 1: 108–115.
54.
FangYLernerZF.Bilateralvs.Paretic-limb-only ankle exoskeleton assistance for improving hemiparetic gait: a case series. IEEE Robot Autom Lett2022; 7(2): 1246–1253.
55.
EmmensARvan AsseldonkEHFvan der KooijH.Effects of a powered ankle-foot orthosis on perturbed standing balance. J Neuroeng Rehabil2018; 15(1): 1–13.
56.
GonzalezSStegallPCainSM, et al. Assessment of a powered ankle exoskeleton on human stability and balance. Appl Ergon2022; 103: 103768.
57.
JacobsDAKollerJRSteeleKM, et al. Motor modules during adaptation to walking in a powered ankle exoskeleton. J Neuroeng Rehabil2018; 15(1): 1–15.
58.
RinaldiLYeungLFLamPCH, et al. Adapting to the mechanical properties and active force of an exoskeleton by altering muscle synergies in chronic stroke survivors. IEEE Trans Neural Syst Rehabil Eng2020; 28(10): 2203–2213.
59.
FangYOrekhovGLernerZF.Adaptive ankle exoskeleton gait training demonstrates acute neuromuscular and spatiotemporal benefits for individuals with cerebral palsy: a pilot study. Gait Posture2022; 95: 256–263.
60.
ChungCLPaquetteMRDiAngeloDJ.Impact of a dynamic ankle orthosis on acute pain and function in patients with mechanical foot and ankle pain. Clin Biomech2021; 83: 105281.
61.
BoesMKBollaertREKeslerRM, et al. Six-minute walk test performance in persons with multiple sclerosis while using passive or powered ankle-foot orthoses. Arch Phys Med Rehabil2018; 99(3): 484–490.
62.
van der WilkDReintsRPostemaK, et al. Development of an ankle-foot orthosis that provides support for flaccid paretic plantarflexor and dorsiflexor muscles. IEEE Trans Neural Syst Rehabil Eng2018; 26(5): 1036–1045.
63.
McCainEMDickTJMGiestTN, et al. Mechanics and energetics of post-stroke walking aided by a powered ankle exoskeleton with speed-adaptive myoelectric control. J Neuroeng Rehabil2019; 16(1): 1–12.
64.
OrekhovGFangYCuddebackCF, et al. Usability and performance validation of an ultra-lightweight and versatile untethered robotic ankle exoskeleton. J Neuroeng Rehabil2021; 18(1): 1–16.
65.
ConnerBCRemecNMOrumEK, et al. Wearable adaptive resistance training improves ankle strength, walking efficiency and mobility in cerebral palsy: a pilot clinical trial. IEEE Open J Eng Med Biol2020; 1.
66.
TamburellaFTagliamonteNLPisottaI, et al. Neuromuscular controller embedded in a powered ankle exoskeleton: effects on gait, clinical features and subjective perspective of incomplete spinal cord injured subjects. IEEE Trans Neural Syst Rehabil Eng2020; 28(5): 1157–1167.
67.
AwadLNEsquenaziAFranciscoGE, et al. The rewalk restore™ soft robotic exosuit: a multi-site clinical trial of the safety, reliability, and feasibility of exosuit-augmented post-stroke gait rehabilitation. J Neuroeng Rehabil2020; 17(1): 1–11.
68.
TamburellaFTagliamonteNLMasciulloM, et al. Gait training with achilles ankle exoskeleton in chronic incomplete spinal cord injury subjects. J Biol Regul Homeost Agents2020; 34(5 Suppl. 3): 147–164.
69.
MoltedoMBačekTSerrienB, et al. Walking with a powered ankle-foot orthosis: the effects of actuation timing and stiffness level on healthy users. J Neuroeng Rehabil2020; 17(1): 1–15.
70.
KirtasOSavasYBayrakerM, et al. Design, implementation, and evaluation of a backstepping control algorithm for an active ankle–foot orthosis. Control Eng Pract2021; 106: 104667.
71.
ZhangYKleinmannRJNolanKJ, et al. Preliminary validation of a cable-driven powered ankle–foot orthosis with dual actuation mode. IEEE Trans Med Robot Bionics2019; 1(1): 30–37.
72.
ThalmanCDebeurreMPLeeH.Entrainment during human locomotion using a soft wearable ankle robot. IEEE Robot Autom Lett2021; 6(3): 4265–4272.
73.
KimCKimGLeeY, et al. Shape memory alloy actuator-embedded smart clothes for ankle assistance. Smart Mater Struct2020; 29(5): 055003.
74.
ObaTKadoneHHassanM, et al. Robotic ankle–foot orthosis with a variable viscosity link using MR fluid. IEEE/ASME Trans Mechatron2019; 24(2): 495–504.
75.
LeeMKimJHyungS, et al. A compact ankle exoskeleton with a multiaxis parallel linkage mechanism. IEEE/ASME Trans Mechatron2021; 26(1): 191–202.
76.
WatersRLMulroyS.The energy expenditure of normal and pathologic gait. Gait Posture1999; 9(3): 207–231.
77.
YeungLFOckenfeldCPangMK, et al. Randomized controlled trial of robot-assisted gait training with dorsiflexion assistance on chronic stroke patients wearing ankle-foot-orthosis. J Neuroeng Rehabil2018; 15(1): 1–12.
78.
KotakPWilkenJMAndersonKM, et al. Carbon fiber-based twisted and coiled artificial muscles (TCAMs) for powered ankle-foot orthoses. J Biomech Eng2022; 144(1): eng79095840148.
79.
HongJCOhashiHIwataH.High-dorsiflexion assistive system for passive swing phase dorsiflexion training and preventing compensatory movements. J Robot Mechatron2022; 34(1): 121–130.
80.
LevangiePKNorkinCC.Joint structure and function: a comprehensive analysis. Philadelphia, PA: FA Davis, 2011.
81.
ChoiHSLeeCHBaekYS.Design and validation of a two-degree-of-freedom powered ankle-foot orthosis with two pneumatic artificial muscles. Mechatronics2020; 72: 102469.
82.
EleryTReznickEShearinS, et al. Design and initial validation of a multiple degree-offreedom joint for an ankle-foot orthosis. J Med Device2022; 16(2): 021001.
83.
BolusNBTeagueCNInanOT, et al. Instrumented ankle–foot orthosis: toward a clinical assessment tool for patient-specific optimization of orthotic ankle stiffness. IEEE/ASME Trans Mechatron2017; 22(6): 2492–2501.
84.
MoltedoMCavalloGBačekT, et al. Variable stiffness ankle actuator for use in robotic-assisted walking: control strategy and experimental characterization. Mech Mach Theory2019; 134: 604–624.
85.
Gomez-VargasDBallen-MorenoFRodriguez-GuerreroC, et al. Experimental characterization of the T-FLEX ankle exoskeleton for gait assistance. Mechatronics2021; 78: 102608.
86.
KobayashiTGaoFLeCursiN, et al. Effect of shoes on stiffness and energy efficiency of ankle-foot orthosis: bench testing analysis. J Appl Biomech2017; 33(6): 460–463.
87.
LevineDRichardsJWhittleMW.Whittle’s gait analysis-E-book. Amsterdam: Elsevier Health Sciences, 2012.
88.
Arnez-PaniaguaVRifaïHAmiratY, et al. Adaptive control of an actuated ankle foot orthosis for paretic patients. Control Eng Pract2019; 90: 207–220.
89.
LiuYZangXZhangN, et al. Design and evaluation of a wearable powered foot orthosis with metatarsophalangeal joint. Appl Bionics Biomech2018; 2018: 9289505.
90.
ZhaoJYangTSunX, et al. Sliding mode control combined with extended state observer for an ankle exoskeleton driven by electrical motor. Mechatronics2021; 76: 102554.
91.
MoonHMaitiRSharmaKD, et al. Hybrid half-gaussian selectively adaptive fuzzy control of an actuated ankle–foot orthosis. IEEE Robot Autom Lett2022; 7(4): 9635–9642.
92.
WuMIStegallPSiuHC, et al. Impact of haptic cues and an active ankle exoskeleton on gait characteristics. Hum Factors2022; 0(0): 00187208221113625.
93.
ShaoYZhangWSuY, et al. Design and optimisation of load-adaptive actuator with variable stiffness for compact ankle exoskeleton. Mech Mach Theory2021; 161: 104323.
94.
DeBoerBHosseiniARossaC.A discrete non-linear series elastic actuator for active ankle-foot orthoses. IEEE Robot Autom Lett2022; 7(3): 6211–6217.
95.
WangJTangBPangM, et al. Self-adaptive particle swarm optimization with human-in-the loop for ankle exoskeleton control. Sens Mater2021; 33(9): 3125–3151.
96.
ShaferBAPhiliusSANuckolsRW, et al. Neuromechanics and energetics of walking with an ankle exoskeleton using neuromuscular-model based control: a parameter study. Front Bioeng Biotechnol2021; 9: 615358.
97.
SchubertTWollesenBWeidnerR.Development and preliminary evaluation of a lower body exosuit to support ankle dorsiflexion. Appl Sci2021; 11(11): 5007.
98.
ZhangYLiSNolanKJ, et al. Shaping individualized impedance landscapes for gait training via reinforcement learning. IEEE Trans Med Robot Bionics2022; 4(1): 194–205.
99.
ZhangJCollinsSH.The iterative learning gain that optimizes real-time torque tracking for ankle exoskeletons in human walking under gait variations. Front Neurorobot2021; 15: 65.
100.
YinKJinYDuH, et al. Virtual neuromuscular control for robotic ankle exoskeleton standing balance. Machines2022; 10(7): 572.
101.
JategaonkarCSinghYVashistaV.Effect of external damping on ankle motion during the swing phase of walking. IEEE Robot Autom Lett2022; 7(3): 7612–7619.
102.
IngrahamKARemyCDRouseEJ.The role of user preference in the customized control of robotic exoskeletons. Sci Robot2022; 7(64): eabj3487.
103.
TanidaS.3rd prototype of intelligently controllable ankle foot orthesis with compact MR fluid brake and its application for a patient with flaccid paralysis in ankle joints. Trans Jpn Soc Med Biol Eng2010; 48(1): 50–58.
104.
Fournier BelleyABouffardJBrochuK, et al. Development and reliability of a measure evaluating dynamic proprioception during walking with a robotized ankle-foot orthosis, and its relation to dynamic postural control. Gait Posture2016; 49: 213–218.
105.
AdiputraDMazlanSAZamzuriH, et al. Fuzzy logic controller for current induced ankle foot orthoses based on electromyography biosignal and ankle angle. J Adv Veh Syst2016; 1(1): 28–40.
106.
ChenBZiBWangZ, et al. Development of robotic ankle–foot orthosis with series elastic actuator and magneto-rheological brake. J Mech Robot2021; 13(1): 011002.
107.
van DijkWMeijnekeCvan der KooijH. Evaluation of the achilles ankle exoskeleton. IEEE Trans Neural Syst Rehabil Eng2017; 25(2): 151–160.
108.
FatoneSJohnsonWBTuckerK.A three-dimensional model to assess the effect of ankle joint axis misalignments in ankle-foot orthoses. Prosthet Orthot Int2016; 40(2): 240–246.
109.
ChenBZiBZhouB, et al. Implementation of robotic ankle–foot orthosis with an impedance-based assist-as-needed control strategy. J Mech Robot2022; 14(5): 051001.
110.
ConnerBCLuqueJLernerZF.Adaptive ankle resistance from a wearable robotic device to improve muscle recruitment in cerebral palsy. Ann Biomed Eng2020; 48(4): 1309–1321.
111.
MancholaMSerranoDGomezD, et al. T-flex: variable stiffness ankle-foot orthosis for gait assistance. In: International symposium on wearable robotics, 2018, pp.160–164. Springer.
112.
IslamMHsiao-WeckslerET.Developing a classification algorithm for plantarflexor actuation timing of a powered ankle–foot orthosis1. J Med Device2016; 10(3): 030961.
113.
HongJCYasudaKOhashiH, et al. Optimal assistance timing to induce voluntary dorsiflexion movements: a preliminary study in healthy participants. Appl Sci2022; 12(4): 2248.
114.
AdiputraDAbdul RahmanMAMazlanSA, et al. Control reference parameter for stance assistance using a passive controlled ankle foot orthosis—a preliminary study. Appl Sci2019; 9(20): 4416.
115.
SiviyCBaeJBakerL, et al. Offline assistance optimization of a soft exosuit for augmenting ankle power of stroke survivors during walking. IEEE Robot Autom Lett2020; 5(2): 828–835.
116.
BisheSSPANguyenTFangY, et al. Adaptive ankle exoskeleton control: validation across diverse walking conditions. IEEE Trans Med Robot Bionics2021; 3: 801–812.
117.
FangYLernerZF.Feasibility of augmenting ankle exoskeleton walking performance with step length biofeedback in individuals with cerebral palsy. IEEE Trans Neural Syst Rehabil Eng2021; 29: 442–449.
118.
ZhangYNolanKJZanottoD.Oscillator-based transparent control of an active/semiactive ankle-foot orthosis. IEEE Robot Autom Lett2019; 4(2): 247–253.
119.
BarriaPPinoATovarN, et al. Bci-based control for ankle exoskeleton T-FLEX: comparison of visual and haptic stimuli with stroke survivors. Sensors2021; 21(19): 6431.
120.
TuckerMROlivierJPagelA, et al. Control strategies for active lower extremity prosthetics and orthotics: a review. J Neuroeng Rehabil2015; 12(1): 1–30.
121.
HuoWArnez-PaniaguaVDingG, et al. Adaptive proxy-based controller of an active ankle foot orthosis to assist lower limb movements of paretic patients. Robotica2019; 37(12): 2147–2164.
122.
MoLFengPShaoY, et al. Anti-disturbance sliding mode control of a novel variable stiffness actuator for the rehabilitation of neurologically disabled patients. Front Rob AI2022; 9: 864684.
123.
KochharRKanthiMMakkarRK.Neuro-fuzzy controller for active ankle foot orthosis. Perspect Sci2016; 8: 293–297.
