The complex interplay of neural and muscular activity underlying involuntary rhythmic wrist tremors poses significant challenges for effective symptom management, particularly with identifying key tremor characteristics such as dominant frequency and amplitude. This study introduces an ultrasound (US)-enhanced data-driven modeling framework that uses wrist angle kinematics as state variables, which are augmented with muscle-specific ultrasound data inputs. We explore three modeling configurations: a wrist kinematics-only baseline model, a fixed-parameter US-augmented model, and a real-time adaptive US-enhanced model updated using a recursive least squares (RLS) algorithm. Comprehensive validation using time- and frequency-domain analyses was conducted using experimental data from six patients with tremor. Results show that integrating US input improves modeling specificity and accuracy by capturing internal muscle dynamics and temporal evolution patterns that are not accessible through IMU alone. Specifically, the time-domain modeling error (nRMSE) decreased substantially from 49.36% in the baseline model to 24.53% in the US-augmented model. The adaptive model further reduced the error to 0.48%, demonstrating its ability to account for the variability of tremor behaviors dynamically. Moreover, this work introduces, for the first time in tremor research, a comprehensive eigenvalue analysis of the data-driven model to extract clinically relevant tremor characteristics. The method enables accurate estimation of dominant tremor frequencies (average across patients nRMSE = 12.7%), quantification of dominant state contributions (mean $\Delta _{\text{dominance}}$ = 18.73%), and reliable tremor event detection (F1-score = 0.796). These findings highlight the framework's ability to not only reproduce tremor trajectories but also uncover how tremor behavior evolves over time in response to underlying neuromuscular activity. This work establishes a foundation for real-time tremor tracking and model-based control strategies, such as closed-loop afferent stimulation. By leveraging the unique sensing capabilities of ultrasound, the proposed framework offers a promising path toward personalized tremor modeling and intervention.

The nonlinear dynamics required to model walking with multi-joint lower limb exoskeleton assistance results in high computational burden. To address this, we derive a Koopman-based linearized model of the human-exoskeleton system using electromyography and ultrasound-derived metrics of volitional muscle activity during exoskeleton-assisted walking. Data are collected from one participant with spinal cord injury (SCI) and two participants with no disabilities. Various electromyography and ultrasound-derived features in addition to normalized motor currents are used to derive predictive models, and we identify which muscle activation metrics produce the most accurate model for each subject. For both subjects without disabilities, the most accurate model uses only ultrasound-derived echogenicity as a metric of muscle activity, while the most accurate model for the subject with SCI uses only EMG wave length. Furthermore, the inclusion of ground reaction force increases the prediction accuracy of all models for one participant with no disabilities while decreasing the accuracy of most models for the participant with SCI. For all subjects, the most accurate subject-speclfic linear model has a root-mean-square error (averaged across limb segment angles) of < 8°.

A hybrid exoskeleton is a class of wearable robotic technology that simultaneously uses a powered exoskeleton and functional electrical stimulation (FES) to generate assistive joint torques for people with impaired mobility due to neurological disorders such as spinal cord injury (SCI). The hybrid assistive technology benefits from FES that actively elicits force from paralyzed muscles via their neural excitation, leading to muscle strengthening. The main technical barrier to realizing the hybrid technology is to attain stable coordination between FES and the exoskeleton despite the quick onset of FES-induced muscle fatigue, which causes a rapid decline in the muscle force. Current methods to measure the induced fatigue lack direct muscle state measurements and may be ineffective at capturing the muscle force decay due to FES. Instead, ultrasound (US) imaging accurately quantifies FES-related muscle contractility and fatigue due to the direct visualization of muscle fibers. In this article, we use real-time US imaging-derived muscle strain changes as biomarkers of FES-induced fatigue in an optimal controller that modulates exoskeleton assistance and FES dosage. To demonstrate that real-time US imaging is a promising muscle–machine interface technology that can optimize shared control in a hybrid exoskeleton, we perform experiments involving continuous seated knee extension and over-ground walking tasks on two participants with SCI and four participants without disabilities. Furthermore, this work helps design a novel and unprecedented robotic gait technology with the capability to impart FES-associated therapeutic benefits while assisting the gait of neurologically impaired individuals, including those with SCI, stroke, multiple sclerosis, etc.

This article presents an assist-as-needed (AAN) control framework for exoskeleton assistance based on human volitional effort prediction via a Hill-type neuromuscular model. A sequential processing algorithm-based multirate observer is applied to continuously estimate muscle activation levels by fusing surface electromyography (sEMG) and ultrasound (US) echogenicity signals from the ankle muscles. An adaptive impedance controller manipulates the exoskeleton's impedance for a more natural behavior by following a desired intrinsic impedance model. Two neural networks provide robustness to uncertainties in the overall ankle joint-exoskeleton model and the prediction error in the volitional ankle joint torque. A rigorous Lyapunov-based stability analysis proves that the AAN control framework achieves uniformly ultimately bounded tracking for the overall system. Experimental studies on five participants with no neurological disabilities walking on a treadmill validate the effectiveness of the designed ankle exoskeleton and the proposed AAN approach. Results illustrate that the AAN control approach with fused sEMG and US echogenicity signals maintained a higher human volitional effort prediction accuracy, less ankle joint trajectory tracking error, and less robotic assistance torque than the AAN approach with the sEMG-based volitional effort prediction alone. The findings support our hypotheses that the proposed controller increases human motion intent prediction accuracy, improves the exoskeleton's control performance, and boosts voluntary participation from human subjects. The new framework potentially paves a foundation for using multimodal biological signals to control rehabilitative or assistive robots.

Neuromuscular electrical stimulation (NMES) is a prescribed treatment for various neuromuscular disorders, where an electrical stimulus is provided to elicit a muscle contraction. Barriers to the development of NMES controllers exist because the muscle response to an electrical stimulation is nonlinear and the muscle model is uncertain. Efforts in this paper focus on the development of an adaptive inverse optimal NMES controller. The controller yields desired limb trajectory tracking while simultaneously minimizing a cost functional that is positive in the error states and stimulation input. The development of this framework allows tradeoffs to be made between tracking performance and control effort by putting different penalties on error states and control input, depending on the clinical goal or functional task. The controller is examined through a Lyapunov-based analysis. Experiments on able-bodied individuals are provided to demonstrate the performance of the developed controller.

An optimal extremum seeking approach is developed in this paper to identify frequency and voltage modulation parameters for a neuromuscular electrical stimulation control objective. The control objective is to externally apply optimally varied voltage or frequency modulation parameters to a human quadriceps muscle to generate a desired knee joint angle. Experimental results are provided to illustrate the limb positioning performance of a real-time extremum seeking routine (i.e., Brent's Method).