Low-Profile, Shoe-Type Ankle-Foot Orthosis with Active Variable Ankle Stiffness via Wire-Fabric Compression Mechanism

Abstract

Acute ankle sprains frequently lead to chronic ankle instability and muscle atrophy by causing immobilization, which necessitates real-time stiffness modulation for ankle-foot orthoses (AFOs). This paper proposes Active Variable Compression Shoes (AVC-Shoes), an ankle support system inspired by the "heel-lock taping" technique, which employs a wire-fabric compression mechanism to selectively stiffen ankle joints at crucial points in the gait cycle. The experimental results confirmed that AVC-Shoes achieve variable ankle stiffness in all directions, demonstrating dorsiflexion and plantarflexion stiffness ranges of up to 8.3 and 5.9 Nm/rad, respectively. Additionally, preliminary human testing involving three healthy participants revealed that the gastrocnemius muscle activity during the push-off phase in the active compression mode was significantly higher (by 19%) than that in the brace mode. By selectively increasing stiffness at heel strikes, AVC-Shoes represent a promising advancement toward next-generation AFOs capable of stabilizing the ankle while preventing muscle atrophy, which is associated with prolonged brace use.

Keywords: ankle–foot orthoses; variable stiffness; wearable robots; wire–fabric compression mechanism.

Figure 1

AVC-Shoes: a shoe-integrated AFO with variable stiffness modulation based on a wire–fabric compression mechanism. The light blue shaded area represents the upper part of the boot around the ankle, the green line is the wire, and the black box behind the ankle represents the actuator. The side and rear views of the ankle show how the wire moves when spooled in and out by the actuator.

Figure 2

(a) Configuration of AVC-Shoes. (b) Design of upper part of AVC-Shoes, which is divided into two parts to generate a firm compression force on the talocrural and subtalar joints. (c) Design of actuation component. Spooling the fishing line generates a compression force on the ankle joint along the wire path (right inset: complete configuration of the actuator unit).

Figure 3

Concept of wire–fabric compression mechanism. (a) Top view of the ankle, showing how a compression force is applied to the ankle by the tension in the wire. (b) Mannequin-based testbed and data flow for validation of wire–fabric compression mechanism. (c) Data measured from the testbed, which include changes in wire tension and circumferential ankle pressure based on the wire position. The yellow gradient represents the compression level based on the wire position.

Figure 4

(a) Configuration of hardware and data flow on testbed for validating stiffness modulation performance of AVC-Shoes. (b) Wind rose graph showing the stiffness variation in AVC-Shoes, with the peak stiffness shown across the seven compression levels.

Figure 5

Experimental setup and protocol for human test under walking conditions. (a) Photograph of experimental setup. (b) Experimental protocol.

Figure 6

Human experiment results. The left-side graph shows the activity levels of the four muscles on the shank in terms of the GCP. The dashed line represents the toe-off event. The right-side bar graph shows the differences in the average peak activity of the gastrocnemius muscle between brace mode (red bar) and active compression mode (green bar) for each participant. The circle, triangle, and star symbols represent the three participants.