## 1 Introduction  
Walking gait assistance is essential, especially for people with abnormal gaits such as a weak ankle, spasticity, or foot drop [1]. These symptoms are common in post-stroke patients doing rehabilitation. In such cases, therapists and ancillary devices such as Ankle Foot Orthosis (AFO) may be used in gait assistance [2]. The AFO is an L-shape brace that covers the lower limb from the foot to the calf [3]. Figure 1 illustrates the gait assistance for foot drop patient using AFO. The reduction of dorsiflexion (e.g., blue circle) resulted in unwanted plantarflexion, which may lead to abnormal gait [4] toe-drags when walking [5]. The patient should avoid toe-dragging, as this disturbs walking stability and increases the probability of a fall [6]. Here, the stiffness of the AFO joint restricts the unwanted plantarflexion, which then prevents the foot drop symptom occurrence, thus increasing walking stability. Because of this function, the post-stroke patients with foot drop symptom are usually suggested to use AFO to address their gait abnormalities in rehabilitation [7]. The level of AFO restriction or AFO joint stiffness should be adjusted accordingly to achieve the maximum benefits [8]. For instance, sufficient work replacement by actuator will reduce muscle activity [9; 10]. Different types of joints, such as rigid joint, flexible joint, and articulated joint, have different joint stiffnesses [11]. Rigid AFO has the highest possible joint stiffness, as the joint is fix and cannot rotate all. This AFO type is not suitable for treatment as it decreases the forward propulsion of the user by limiting the range of motion (ROM) [7; 12]. A flexible AFO made from materials such as polypropylene [13] and carbon fiber [14] allows more ROM than a rigid AFO. However, the problem of a lack of forwarding propulsion remains [15]. Lastly, the articulated AFO allows maximum ROM as compared with the other two. It has electronic actuators (passive or active) and gait controller to control not only the joint stiffness but also ankle joint movement [11]. Therefore, an articulated AFO offers greater flexibility compared to rigid and flexible AFOs, and is capable of normal walking replication for intensive training purposes [16]. The previous study has developed a passive-controlled AFO (i.e., PICAFO in short). PICAFO is equipped with magnetorheological (MR) brake as a passive actuator for controlling the ankle joint stiffness using an electromyography (EMG) based Fuzzy Logic Controller (FLC) [17; 18; 19]. Surface electromyography or sEMG was used to classify the gait phases into two phases with a uniform threshold rather than a single threshold [20; 21]. The two phases namely, Phase I runs from foot flat (FF) to heel off (HO), continuing to toe-off (TO), while phase II runs from TO to initial contact (IC) to FF. As shown in Figure 2, FLC also used additional information from ankle positions for controlling the MR brake stiffness. The result was that the controller could generate different joint stiffness in distinct gait phases [17], such as low and medium stiffness during the stance phase and high stiffness during the swing phase. The PICAFO successfully demonstrated foot drop prevention due to the high stiffness during the swing phase [18]. Despite that, the exact amount of joint stiffness, which was the control reference parameter, was pre-determined to be similar, even though different subjects and walking styles may require different joint stiffnesses during the stance and swing. Locking the ankle position during the swing phase could be done very straightforwardly by applying high stiffness, regardless of the subject and walking style [6]. However, the stiffness level during the stance phase, which prevents over dorsiflexion, was necessary to be adjusted accordingly [9]. Too much stiffness will distract the movement like rigid  
Figure 1: Illustration of foot drop treatment using Ankle Foot Orthosis (AFO). and flexible AFO did, but insufficient stiffness will result in insignificant gait assistance [8].