Review of Methods and Control Algorithms in Functional Electric Stimulation

 Functional electrical stimulation (FES) is a promising technique for rehabilitation and increasing the level of movement in paraplegic subjects. In this method, the movement in the hindlimbs can be restored by controlling the electrical current pulses and stimulating the intraspinal motor neurons or muscular fibers below the spinal lesion. In FES, functional control signals are received from a controller that creates a motor function for the paraplegic subject. By changing the pulse width or pulse amplitude of the current pulses (control input), the level of the contraction for generating the functional movement of the hindlimb joints can be altered. The electrodes are inserted into the spinal cord to stimulate the desired movement by stimulating the flexor and extensor muscles and motor neurons connected to the desired joint. Therefore, the superposition of torque in flexor and extensor muscles is applied to the musculoskeletal system and joint moves.

 In FES, a model of the musculoskeletal system that acts on simultaneous movement of the hip and knee joints (multi–joint) is used as the virtual patient. In general, the skeletal segment of the model consists of a planar swinging pendulum model of a two–link rigid robotic manipulator with nonlinear constraints on the hip and knee joint movements. A biarticular model of six muscles is used to generate the torques of the skeletal part of the model. Four muscles are considered for flexing and extending the hip and knee joints (one for flexing the joint and one for extending), and two biarticular muscles work on flexing the hip (knee) joint and extending the knee (hip) joint synergically. On the other hand, excellent tracking performance can be obtained with high precision, and the controllers can automatically switch between control inputs and the muscles by regulating the stimulation pulse width. The stimulation pulse width of muscles is continuous, nonsingular, with low chattering. The model dynamics can be assumed unknown and identified online without the controller's requirement for offline calibration. The controller robustly and rapidly switched the activation between the muscles to track the desired trajectory of the knee and hip joint. Various control methods, such as combining fuzzy adaptive or neural networks with classic sliding mode control, have generated FES control signals. Such hybrid methods have led to the chattering phenomenon, the control signal's singularity, and the controller's low speed outside and on the sliding surface.

 Rehabilitation movement in paralyzed limbs of paraplegic subjects based on FES in their muscles is associated with problems such as reflection in the spinal cord and unwanted movements, joint disturbances, fatigue, etc.

 The controller could automatically regulate the stimulation pulse width without considering the effect of the applied ground reaction force so that, by switching between muscles synergically, excellent tracking results were obtained in the presence of external unit step disturbance and muscle fatigue. The major challenges for developing an appropriate control method for stimulating paralyzed limbs include high–order nonlinear and time–varying properties of the neuromusculoskeletal system, spasms, and muscle fatigue. These drawbacks limit using prespecified stimulation patterns and open–loop control systems in the FES application.