The online adaptation of exoskeleton control based on muscle activity sensing offers a promising approach to personalizing exoskeleton behavior based on the user's biosignals. While electromyography (EMG)-based methods have demonstrated improvements in joint torque estimation, EMG sensors require direct skin contact and extensive post-processing. In contrast, force myography (FMG) measures normal forces resulting from changes in muscle volume due to muscle activity. We propose an FMG-based method to estimate knee and ankle joint torques by integrating joint angles and velocities with muscle activity data. We learn a model for joint torque estimation using Gaussian process regression (GPR). The effectiveness of the proposed FMG-based method is validated on isokinetic motions performed by ten participants. The model is compared to a baseline model that uses only joint angle and velocity as well as a model augmented by EMG data. The results indicate that incorporating FMG into exoskeleton control can improve the estimation of joint torque for the ankle and knee joints in novel task characteristics within a single participant. Although the findings suggest that this approach may not improve the generalizability of estimates between multiple participants, they highlight the need for further research into its potential applications in exoskeleton control.

Ankle exoskeletons can assist the ankle joint and reduce the metabolic cost of walking. However, many existing ankle exoskeletons constrain the natural 3 degrees of freedom (DoF) of the ankle to limit the exoskeleton’s weight and mechanical complexity, thereby compromising comfort and kinematic compatibility with the user.This paper presents a novel ankle exoskeleton frame design that allows for 3 DoF ankle motion using a symmetric parallel frame design principle resulting in a strong frame while weighing 1.8 kg. Furthermore, a cable routing method is proposed to actuate the plantarflexion of the ankle. The kinematic compatibility of the proposed exoskeleton frame is evaluated in straight- and curve-walking scenarios with four users. The study demonstrates that the exoskeleton frame adapts to the natural 3 DoF ankle motion and the range of motion (RoM) during walking. The actuation in plantarflexion is evaluated in a stationary torque experiment demonstrating the ability of the frame to transfer large torque loads of up to 57.4 Nm. This work contributes to the design and development of more flexible and adaptable ankle exoskeletons for walking assistance.