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Biomedical subjects

W K Durfee

Publications and source records attributed to W K Durfee.

14 recordsLinked to original sources
PubMed2006

Force field effects on cerebellar Purkinje cell discharge with implications for internal models.

The cerebellum has been hypothesized to provide internal models for limb movement control. If the cerebellum is the site of an inverse dynamics model, then cerebellar neural activity should signal limb dynamics and be coupled to arm muscle activity. To address this, we recorded from 166 task-related Purkinje cells in two monkeys performing circular manual tracking under varying viscous and elastic loads. Hand forces and arm muscle activity increased with the load, and their spatial tuning differed markedly between the viscous and elastic fields. In contrast, the simple spike firing of 91.0% of the Purkinje cells was not significantly modulated by the force nor was their spatial tuning affected. For the 15 cells with a significant force effect, changes were small and isolated. These results do not support the hypothesis that Purkinje cells represent the output of an inverse dynamics model of the arm. Instead these neurons provide a kinematic representation of arm movements.

Algorithms↗
PubMed1996

Design of a controlled-brake orthosis for FES-aided gait.

Functional electrical stimulation (FES) is a means of restoring gait to individuals with spinal cord injury, but the performance of most FES-aided gait systems is hampered by the rapid muscle fatigue which results from stimulated muscle contraction and the inadequate control of joint torques necessary to produce desired limb trajectories. The controlled-brake orthosis (CBO) addresses these limitations by utilizing FES in combination with a long-leg brace that contains controllable friction brakes at the knees and hips. A laboratory version of the CBO utilizing computer-controlled magnetic particle brakes at the joints was designed and constructed, and preliminary results with a single spinal cord injury (SCI) subject have demonstrated reduced fatigue and more repeatable gait trajectories when compared to FES-aided gait without the brace. Significant work remains to demonstrate the efficacy of the concept across a wide range of SCI subjects and to design a system which meets appropriate user requirements of size, weight, cosmesis, ease of use and cost. The primary purpose of the paper is to detail the design of the CBO.

Braces↗
PubMed1996

Neural signals for command control and feedback in functional neuromuscular stimulation: a review.

In current functional neuromuscular stimulation systems (FNS), control and feedback signals are usually provided by external sensors and switches, which pose problems such as donning and calibration time, cosmesis, and mechanical vulnerability. Artificial sensors are difficult to build and are insufficiently biocompatible and reliable for implantation. With the advent of methods for electrical interfacing with nerves and muscles, natural sensors are being considered as an alternative source of feedback and command signals for FNS. Decision making methods for higher level control can perform equally well with natural or artificial sensors. Recording nerve cuff electrodes have been developed and tested in animals and demonstrated to be feasible in humans for control of dorsiflexion in foot-drop and grasp in quadriplegia. Electromyographic signals, being one thousand times larger than electroneurograms, are easier to measure but have not been able to provide reliable indicators (e.g., of muscle fatigue) that would be useful in FNS systems. Animal studies have shown that information about the shape and movement of arm trajectories can be extracted from brain cortical activity, suggesting that FNS may ultimately be directly controllable from the central nervous system.

Electric Stimulation Therapy↗
PubMed1995

Reducing muscle fatigue in FES applications by stimulating with N-let pulse trains.

Applications of electrical stimulation for restoration of functional movements such as standing, gait, and grasp have always been hindered by the rapid fatigue of stimulated muscle. This paper describes an experimental investigation of stimulation with N-lets (a set of N closely spaced stimulation pulses) as a means of producing contractions with improved fatigue characteristics. Experiments were conducted on 27 able-bodied and four SCI human subjects using surface stimulation of the quadriceps muscle to produce isometric knee joint torque. Based upon evidence from the literature on muscle fatigue, parameters of the N-let trains for N = 1-6 were optimized to produce the most force per pulse. The results demonstrated that: 1) nonlinear summation of the twitch response occurs in human subjects with N-let surface stimulation; 2) for most subjects, doublet stimulation (N = 2) with a pulse interval of about 5 ms produced the maximum torque-time integral per pulse of the resulting twitch; and 3) on average, optimal N-let stimulation resulted in a 36% increase in isometric torque tracking when compared to traditional singlet stimulation. The results have immediate implications for alleviating the problem of premature fatigue during functional electrical stimulation.

Adult↗
PubMed1994

Control of prosthetic gait.

Recent advances in two types of prosthetic gait are particularly noteworthy, namely work on limb and neural prostheses. Current work on artificial limbs has been oriented towards improving devices, with commercialization as the driving force. Progress has been made in understanding how the compliant properties of the foot, ankle and knee joints of artificial legs affect the energetics and kinematics of gait. Work is continuing on automated systems for fabricating sockets with improved fit to increase the comfort of artificial limbs. Neural prostheses use electrical stimulation to activate paralyzed muscle. Advances have been made in understanding how to model the patterns of neural prosthetic gait and how neural prostheses respond to disturbances. Work in real-time control of stimulated muscle has progressed in the area of system identification and in using natural sensors for feedback signals. There still remains a wide gap, however, between able-bodied gait and the gait that can be achieved using current neural prosthesis systems.

Extremities↗
PubMed1994

Estimation of force-activation, force-length, and force-velocity properties in isolated, electrically stimulated muscle.

Designing advanced controllers for motor neural prosthesis applications requires appropriate models for electrically stimulated muscle. A nonlinear nonisometric muscle model based on a Hill-type structure is presented. Estimation algorithms were derived to parameterize the passive force-length, the passive force-velocity, the active force-length, and the active force-velocity properties, the isometric recruitment curve, and the linear contraction dynamics of the model. All parameters were based on experimental measurements rather than on values taken from the literature. The estimation methods were validated experimentally using isolated hind-limb muscles in two acute animal model preparations. The results demonstrated that the parameterized model is capable of predicting force output with reasonable accuracy for a wide range of simultaneously varying kinematic and stimulation inputs.

Algorithms↗
PubMed1993

Control of standing and gait using electrical stimulation: influence of muscle model complexity on control strategy.

Three approaches to controlling standing and gait in paraplegics through functional electrical stimulation are presented. The approaches differ in their requirements for modeling the muscle actuator. The first approach describes a detailed muscle model and presents methods for rapid experimental parameterization of the model. The second requires a less detailed model and knowledge of model error bounds to design advanced, non-linear controllers which guarantee stability. The third approach needs no muscle model since it controls limb trajectories through combining stimulation with an orthosis containing controllable friction brakes at the joints. Future clinical systems may use one or a combination of these approaches to restore useful function.

Electric Stimulation↗
PubMed1991

Open-loop position control of the knee joint using electrical stimulation of the quadriceps and hamstrings.

The clinical acceptability of functional electrical stimulation (FES) as an aid for restoration of paraplegic gait is limited by the inability to accurately and repeatedly position the lower extremity. To gain insight into the causes of and possible solutions to this problem, the responses of the quadriceps and hamstrings to FES were studied in able-bodied subjects. Isometric torque was dependent on knee angle and changed unpredictably with time. An open-loop feedforward knee-joint position controller was also tested. The results demonstrated that it is beneficial to account for the dependence of torque on position, that modifications to this open-loop controller might improve accuracy and that closed-loop control may be essential for functional restoration of gait.

Adolescent↗
PubMed1991

Simulator for evaluating shoulder motion as a command source for FES grasp restoration systems.

A simulator has been developed to evaluate the command channels through which a quadriplegic patient controls an upper limb neural prosthesis. The simulator consists of an animated grasping task implemented on a video screen. The patient controls the motion of an animated hand on the screen by moving his or her own hand while the animated hand opens and closes under control of the tested command channel. Experiments were performed using both able-bodied and quadriplegic subjects to evaluate shoulder motion as a command-channel source. The results demonstrated that optimal combinations of shoulder command-channel parameters are subject specific, which suggests using the simulator in the prescription of upper limb neural prostheses and in training quadriplegic persons who use them. Additionally, the experimental results quantified the reduction in performance that came from using ipsilateral vs contralateral shoulder control and the enhancement in performance realized when using substitute sensory force-feedback displays.

Computers↗
PubMed1990

Regulating knee joint position by combining electrical stimulation with a controllable friction brake.

Hybrid FES gait restoration systems which combine stimulation with controllable mechanical damping elements at the joints show promise for providing good control of limb motion despite variations in muscle properties. In this paper we compared three controllers for position tracking of the free swinging shank in able-bodied subjects. The controllers were open-loop (OL), proportional-derivative closed-loop (PD), and bang-bang plus controlled-brake control (CB). Both OL and PD controllers contained a forward path element, which inverted a model of the electrically stimulated muscle and limb system. The CB control was achieved by maximally activating the appropriate muscle group and controlling the brake to be a "moving-wall" against which the limb pushed. The CB control resulted in superior tracking performance for a wide range of position tracking tasks and muscle fatigue states but required no calibration or knowledge of muscle properties. The disadvantages of CB control include excess mechanical power dissipation in the brake and impact forces applied to the skeletal system.

Biomechanical Phenomena↗
PubMed1989

Task-based methods for evaluating electrically stimulated antagonist muscle controllers.

Single-joint motor neural prosthesis control algorithms were tested in a novel animal model. The model consisted of a human subject who provided joystick inputs to a controller. The controller output determined the stimulus activation levels of two antagonist muscles which manipulated the ankle joint of an intact, anesthetized cat. Using visual feedback, the subject manipulated the system to perform positioning tasks which simulated normal activity of an intact limb. Three controllers were evaluated, open-loop reciprocal control, P-D closed-loop reciprocal control, and open-loop cocontraction control. The results demonstrated that in the presence of visual feedback, open-loop cocontraction control compared favorably in performance to a P-D closed loop controller. This has a practical value for the implementation of clinical neural prostheses since it suggests that in some cases, feedback transducers may not be required for fine control.

Animals↗
PubMed1989

Methods for estimating isometric recruitment curves of electrically stimulated muscle.

Four methods for estimating the recruitment curve of isometric, electrically stimulated muscle are described. Three of the methods were tested experimentally in isolated tibialis anterior and medial gastrocnemius muscles of cats. The three methods are steady-state step response, peak impulse response, and deconvolved ramp response. The fourth method, described but not tested, is a stochastic iteration technique. The results demonstrate that estimations of recruitment curves depend on the method used and that all methods are sensitive to short-term and long-term time-variations in muscle properties. While the step response technique is the traditional method for estimating recruitment curves, the ramp deconvolution method appears to offer acceptable accuracy with much shorter testing times.

Animals↗