Synthesis of optimal electrical stimulation patterns for functional motion restoration: applied to spinal cord-injured patients

Flatness of musculoskeletal systems under functional electrical stimulation

Control of musculoskeletal yy system through functional electrical stimulation (FES) still remains a complex and a challenging process. Indeed, the used musculoskeletal models are often complex and highly nonlinear, which makes their control and inversion (getting appropriate inputs from a desired outputs) very difficult. On the other hand, the system flatness has been proved to be an efficient method for nonlinear system control, since in this technique, the nonlinear system can be controlled more easily through its flat outputs. Therefore, it is very promising to apply this control technique on the musculoskeletal system, to overcome its problems, which has never been explored so far. The aim of this work is to explore the flatness technique and its feasibility on the knee joint musculoskeletal system in dynamic condition, controlled by electrically stimulated quadriceps muscle. A mathematical proof developed in the current work highlights that the two-input musculoskeletal system is flat, where two flat outputs are the muscle stiffness and the knee joint angle. It also shows that the single-input musculoskeletal system is not flat. These results are crucial for flatness-based control of musculoskeletal systems, since this model in literature deals with a single input. Simulation results in open-loop control of two-input system highlight the consistency of the mathematical proof, and the applicability of this technique on the musculoskeletal system, where its simulated outputs fit perfectly with the desired ones if the model is considered perfect. When, one parameter of the system is not well estimated (10% of error), simulations show limits of open-loop control, with a joint angle rms deviation of 4%; hence, the closed-loop control should be considered. Graphical Abstract Flatness Study and control of Musculoskeletal systems.

[Neuroprostheses to compensate for motor impairments]

Neuroprostheses are medical devices which, interfaced with the nervous system, are able to provoke the artificial generation of nerve signals. These signals, correctly coded, can then be interpreted by target organs such as the muscles.

Real-Time Closed-Loop Functional Electrical Stimulation Control of Muscle Activation with Evoked Electromyography Feedback for Spinal Cord Injured Patients

Functional electrical stimulation (FES) is a neuroprosthetic technique to help restore motor function of spinal cord-injured (SCI) patients. Through delivery of electrical pulses to muscles of motor-impaired subjects, FES is able to artificially induce their muscle contractions. Evoked electromyography (eEMG) is used to record such FES-induced electrical muscle activity and presents a form of [Formula: see text]-wave. In order to monitor electrical muscle activity under stimulation and ensure safe stimulation configurations, closed-loop FES control with eEMG feedback is needed to be developed for SCI patients who lose their voluntary muscle contraction ability. This work proposes a closed-loop FES system for real-time control of muscle activation on the triceps surae and tibialis muscle groups through online modulating pulse width (PW) of electrical stimulus. Subject-specific time-variant muscle responses under FES are explicitly reflected by muscle excitation model, which is described by Hammerstein system with its input and output being, respectively, PW and eEMG. Model predictive control is adopted to compute the PW based on muscle excitation model which can online update its parameters. Four muscle activation patterns are provided as desired control references to validate the proposed closed-loop FES control paradigm. Real-time experimental results on three able-bodied subjects and five SCI patients in clinical environment show promising performances of tracking the aforementioned reference muscle activation patterns based on the proposed closed-loop FES control scheme.

Wavelet analysis of skin perfusion to assess the effects of FREMS therapy before and after occlusive reactive hyperemia

Laser Doppler Fluxmetry is used to evaluate the hemodynamics of skin microcirculation. Laser Doppler signals contain oscillations due to fluctuations of microvascular perfusion. By performing spectral analysis, six frequency intervals from 0.005 to 2 Hz have been identified and assigned to distinct cardiovascular structures: heart, respiration, vascular myocites, sympathetic terminations and endothelial cells (dependent and independent on nitric oxide). Transcutaneous electrical pulses are currently applied to treat several diseases, i.e. neuropathies and chronic painful leg ulcers. Recently, FREMS (Frequency Rhythmic Electrical Modulation System) has been applied to vasculopathic patients, too. In this study Laser Doppler signals of skin microcirculation were measured in five patients with intermittent claudication, before and after the FREMS therapy. Changes in vascular activities were assessed by wavelet transform analysis. Preliminary results demonstrate that FREMS induces alterations in vascular activities.