actuators Review A Review on the Control of the Mechanical Properties of Ankle Foot Orthosis for Gait Assistance Dimas Adiputra 1,2,† , Nurhazimah Nazmi 1,†, Irfan Bahiuddin 3,† , Ubaidillah Ubaidillah 4,†, Fitrian Imaduddin 4,†, Mohd Azizi Abdul Rahman 1,*,†, Saiful Amri Mazlan 1,† and Hairi Zamzuri 1 1 Advanced Vehicle System Laboratory, Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, Kuala Lumpur 54100, Malaysia; [email protected] (D.A.); [email protected] (N.N.); [email protected] (S.A.M.); [email protected] (H.Z.) 2 Electrical Engineering Department, Institut Teknologi Telkom Surabaya, Surabaya 60234, Indonesia 3 Vocational School, Universitas Gadjah Mada, Jogjakarta 55281, Indonesia; [email protected] 4 Mechanical Engineering Department, Universitas Sebelas Maret, Surakarta 57126, Indonesia; [email protected] (U.U.); fi[email protected] (F.I.) * Correspondence: [email protected] † These authors contributed equally to this work. Received: 20 December 2018; Accepted: 24 January 2019; Published: 28 January 2019 Abstract: In the past decade, advanced technologies in robotics have been explored to enhance the rehabilitation of post-stroke patients. Previous works have shown that gait assistance for post-stroke patients can be provided through the use of robotics technology in ancillary equipment, such as Ankle Foot Orthosis (AFO). An AFO is usually used to assist patients with spasticity or foot drop problems. There are several types of AFOs, depending on the flexibility of the joint, such as rigid, flexible rigid, and articulated AFOs. A rigid AFO has a fixed joint, and a flexible rigid AFO has a more flexible joint, while the articulated AFO has a freely rotating ankle joint, where the mechanical properties of the AFO are more controllable compared to the other two types of AFOs. This paper reviews the control of the mechanical properties of existing AFOs for gait assistance in post-stroke patients. Several aspects that affect the control of the mechanical properties of an AFO, such as the controller input, number of gait phases, controller output reference, and controller performance evaluation are discussed and compared. Thus, this paper will be of interest to AFO researchers or developers who would like to design their own AFOs with the most suitable mechanical properties based on their application. The controller input and the number of gait phases are discussed first. Then, the discussion moves forward to the methods of estimating the controller output reference, which is the main focus of this study. Based on the estimation method, the gait control strategies can be classified into subject-oriented estimations and phase-oriented estimations. Finally, suggestions for future studies are addressed, one of which is the application of the adaptive controller output reference to maximize the benefits of the AFO to users. Keywords: Ankle-Foot Orthosis; gait control; gait assistance; bending stiffness; damping stiffness; assistive torque; motion path; non-linear; walking; mechanical properties 1. Introduction Stroke (brain attack) is caused by the interruption of blood flow to the brain, leading to damage to the nerves in the brain and disrupting the exchange of information through the normal path between the limbs and the brain. Fortunately, the exchange of information can be redirected to another path through learning, which requires the limb to repeat a certain task [1]. This series of repetitions is known Actuators 2019, 8, 10; doi:10.3390/act8010010 www.mdpi.com/journal/actuators Actuators 2019, 8, 10 2 of 28 another path through learning, which requires the limb to repeat a certain task [1]. This series of asrepetitions training oris known rehabilitation, as training where or arehabilitation, therapist assists where patients a therapist in performing assists patients the task. in The performing movements the oftask. the The limb movements during the of training the limb must during be uniformly the training maintained. must be unifor In othermly words, maintained. different In other movements, words, fordifferent instance, movements, caused by for muscle instance, pain, caused should by be muscle avoided pain, [2]. should It is expected be avoided that [2]. with It ais more expected uniform that performancewith a more uniform of repetitive performance tasks, the of recovery repetitive of thetasks, patient the recovery will befaster. of the Roboticpatient will technologies, be faster. Robotic such as orthotictechnologies, devices such and as exoskeletons, orthotic devices offer and intensive, exoskeletons, controlled, offer and intensive, monitored controlled, post-stroke and rehabilitation monitored topost-stroke help patients rehabilitation to perform to tasks help [ 3patients]. In addition, to perfor them pace tasks of [3]. robot-assisted In addition, training the pace is of faster robot-assisted than with justtraining the therapist is faster alone,than with thereby just reducing the therapist the therapist’s alone, thereby burden reducing [4]. Therefore, the therapist’s robotic technologies burden [4]. haveTherefore, been developedrobotic technologies over the pasthave decade been develop for theed assistance over the and past enhancement decade for the of theassistance training and or rehabilitationenhancement ofof post-strokethe training patients or rehabi [5litation]. of post-stroke patients [5]. One dailydaily activityactivity task is locomotion,locomotion, which definesdefines the way in whichwhich humanshumans move fromfrom oneone placeplace toto another. another. The The ability ability of locomotion,of locomotion, such such as walking, as walking, is crucial is forcrucial avoiding for avoiding anti-social anti-social behavior andbehavior anxiety and over anxiety the lossover ofthe mobility loss of mobility [6]. Therefore, [6]. Therefore, locomotion locomotion is one ofis one the primaryof the primary tasks totasks be acquiredto be acquired by a post-strokeby a post-stroke patient, patient, in addition in addition tohand to hand motions motions [7] [7] and and speech speech [8 [8].]. The The basicbasic formform of locomotionlocomotion isis walking,walking, besidesbesides running,running, jumping,jumping, and soso on.on. TheThe walkingwalking gaitgait isis aa quasiperiodicquasiperiodic activity, asas depicteddepicted inin FigureFigure1 1.. It It starts starts from from the the initial initial contact contact (IC) (IC) on on the the heel. heel. The The foot foot goes goes down down until thethe toe touches the ground. This is known as the foot flat flat (FF). From here, the leg moves forward along withwith thethe body. body. At At the the end end of of the the FF, FF, the the foot foot pushes pushes the groundthe ground with with a sequence a sequence of heel-off of heel-off (HO), then,(HO), toe-off then, toe-off (TO) movements. (TO) movements. The body The moves body forward moves forward further because further ofbecause the push of the from push the foot.from Thethe footfoot. is The swung foot in is theswung air before in the the air next before initial the contact. next initial However, contact. the However, walking gait the may walking be disturbed gait may due be todisturbed stroke symptoms. due to stroke For instance,symptoms. foot For drop instance, causes fo anot inappropriate drop causes an foot ina positionppropriate during foot the position swing phase,during whichthe swing leads phase, to the which IC being leads on to the the toe IC insteadbeing on of the the toe heel. instead In the of worst the heel. case, In the the patient worst case, may eventhe patient suffer may from even an inability suffer from to swing an inability the foot to atswing all. Therefore,the foot at all. training Therefore, for a normaltraining walkingfor a normal gait mustwalking be donegait must to recover be done the to patient’s recover walkingthe patient’s gait. walking gait. Figure 1. Walking gait classification.classification. Initial Contact (IC), Foot Flat (FF), Heel-Off (HO), Toe-OffToe-Off (TO).(TO). Conventionally, walkingwalking gaitgait trainingtraining isis performedperformed manuallymanually with the assistanceassistance of a strokestroke therapist.therapist. Two toto threethree therapiststherapists maymay bebe involvedinvolved inin helpinghelping thethe patientpatient toto maintainmaintain balancebalance andand toto performperform walkingwalking by by moving moving the foot.the foot. Ancillary Ancillary equipment, equipment, such as such a cane, as orthosis,a cane, walker,orthosis, theraband, walker, andtheraband, so on, canand also so on, be can used also to supportbe used theto support patient the during patient the during training the [9 ].training The equipment [9]. The equipment should fit (notshould be over-sizedfit (not be over-sized nor too tight) nor thetoo user’s tight) anatomythe user’s to anatomy ensure theto ensure user’s comfort,the user’s especially comfort, especially the fixing ofthe the fixing foot of and the shank foot toand the shank footplate to the and footplate the AFO and strut the [10 AFO,11]. strut Nowadays, [10,11]. the Nowadays, ancillary equipmentthe ancillary is enhancedequipment with is enhanced robotic technologies, with robotic such technologies, as an orthosis such and as exoskeleton, an orthosis toand make exoskeleton, it more sophisticated to make it formore rehabilitation sophisticated sessions. for rehabilitation Not only can sessions. the bio-physical Not only can data the be bio-physical monitored, butdata the be training