Patrick E. Crago

A DYNAMIC MODEL FOR SIMULATING MOVEMENTS OF THE ELBOW, FOREARM, AND WRIST

We developed a dynamic model of the upper extremity to simulate forearm and wrist movements. The model is based on the skeletal structure of the arm and is capable of elbow flexion/extension, forearm pronosupination, and wrist flexion/extension and radial/ulnar deviation movements. Movements are produced by activation of a Hill-type model of muscle, and limits on joint motion are imposed by passive moments modeled after experimental results. We investigated the muscle output force sensitivity, as well as wrist flexion/extension motion sensitivity to parameter variations. The tendon slack length and muscle fiber length were found to have the greatest influence on muscle output and flexion/extension wrist motion. The model captured the direction of the moment vectors at the wrist well, but predicted much higher moments than were measured by stimulating the paralyzed muscles of one tetraplegic subject.

Modulation of Muscle Force by Recruitment During Intramuscular Stimulation

The input¿output relationships for modulation of force by recruitment during intramuscular electrical stimulation were examined for cat sleus muscles and human finger and thumb muscles. Recruitment was modulated by varying either the pulsewidth or amplitude of a monophasic, rectangular, cathodal current pulse train. Force was a nonlinear function of either pulsewidth or amplitude, and the shape of the nonlinearity was the same regardless of which parameter was modulated. The charge per stimulus pulse was lowest if pulsewidth was modulated with a fixed, high amplitude stimulus. The shape of the nonlinear relationship between pulsewidth and force (recruitment characteristic) depended on stimulus amplitude, electrode location in the muscle and muscle length. In most applications the amplitude and location would be fixed, so force would be a two-dimensional nonlinear function of pulsewidth and muscle length. The results are discussed with respect to possible mechanisns of recruitment during intramuscular stimulation, and the implications of the nonlinearities on the proportional control of orthoses employing electrically stimulated muscles.

Sensors for Use with Functional Neuromuscular Stimulation

Functional neuromuscular stimulation (FNS) designates artificially applied electrical activation of muscles to restore function lost as a result of neurological lesions. FNS prostheses are currently being designed to restore urinary bladder control, standing, walking, and hand function. All of these prostheses need sensors for interaction with the human users and the environment. This paper discusses each of these prostheses with special regard to the use of sensors and the design specifications that the sensors must meet.

Effects of voluntary force generation on the elastic components of endpoint stiffness

Abstract. The goal of this work was to determine how force loads applied at the hand change the elastic mechanical properties of the arm. Endpoint stiffness, which characterizes the relationship between hand displacements and the forces required to effect those displacements, was estimated during the application of planar, stochastic displacement perturbations to the human arm. A nonparametric system identification algorithm was used to estimate endpoint stiffness from the measured force and displacement data. We found that changes in the elastic component of arm stiffness during isometric force regulation tasks were due primarily to the actions of the single-joint muscles spanning the shoulder and elbow. This was shown to result in a nearly posture-independent regulation of joint torque-stiffness relationships, suggesting a simplified strategy that is used to regulate arm mechanics during these tasks.

An elbow extension neuroprosthesis for individuals with tetraplegia

Functional electrical stimulation (FES) of the triceps to restore control of elbow extension was integrated into a portable hand grasp neuroprosthesis for use by people with cervical level spinal cord injury. An accelerometer mounted on the upper arm activated triceps stimulation when the arm was raised above a predetermined threshold angle. Elbow posture was controlled by the subjects voluntarily flexing to counteract the stimulated elbow extension. The elbow moments created by the stimulated triceps were at least 4 N.m, which was sufficient to extend the arm against gravity. Electrical stimulation of the triceps increased the range of locations and orientations in the workspace over which subjects could grasp and move objects. In addition, object acquisition speed was increased. Thus elbow extension enhances a persons ability to grasp and manipulate objects in an unstructured environment.

Closed-Loop Control of Force During Electrical Stimulation of Muscle

The control of contractions elicited by electrical stimulation of muscle could be improved if there was a linear repeatable input-output relationship. The input is the command to the controlled stimulator and the output is the evoked contraction. Systems employing closed-loop force feedback to provide regulation of contractions were investigated in these studies. Force was modulated by both recruitment and temporal summation during intramuscular stimulation. Closed-loop systems with combined proportional and integral control were found to be stable and linear and to have good compensation for variations in muscle properties. A low proportional loop gain (approximately unity) was found necessary to prevent oscillation when modulating recruitment. Ratios of integral to proportional gain of about 10 gave the fastest response without compromising stability. The response time of the closed-loop system was as fast as or faster than the open-loop system.

The choice of pulse duration for chronic electrical stimulation via surface, nerve, and intramuscular electrodes

The peak current, peak voltage, charge transfer and energy dissipation necessary for equivalent stimulation were measured for several pulse durations in the range from 0.01 to 1.0 msec. The unidirectional, regulated current, rectangular waveform was studied for subcutaneous nerve and intramuscular stimulation in animals and for surface stimulation in humans. In addition, the unidirectional, regulated current, exponential waveform was studied in humans and was compared with the rectangular waveform. The question of the relationship between charge transfer and energy dissipation and possible tissue damage due to the electrochemical formation of toxic compounds or a temperature rise in the surrounding tissue was examined. The optimal pulse duration for reducing the possibility of tissue damage was concluded to be less than or equal to 0.01 msec for intramuscular stimulation in the test situation. No conclusion was made as to the optimal duration for nerve or surface stimulation. Excitation of muscle fibers was found to take place indirectly by was of muscle nerves during intramuscular stimulation. The exponential waveform required less charge transfer and energy dissipation than the rectangular waveform, but higher peak currents.

Feedback regulation of hand grasp opening and contact force during stimulation of paralyzed muscle

A fixed-parameter, discrete-time, first-order, feedback control system is described for regulating grasp during electrical stimulation of paralyzed muscles of the hand. The stiffness of the grasp (the relationship between grasp force and grasp opening) is kept constant by linearly combining force and position feedback signals. Thus, a single continuous command signal can control the size of the grasp opening prior to object acquisition and both grasp and opening after contact. The controller achieves this change in controlled variables by scaling and summing the force and position feedback signals, rather than by a discrete switch in control strategy. Experimental tests of the control system in quadriplegic subjects show that control can be obtained over conditions ranging from unloaded position regulation is isometric force regulation as well as in the transition between these conditions. The robustness of the control system was evaluated during force regulation with isometric loads. Step response rise time and overshoot were much more dependent on system gain than on the location of the controller zero. Responses with a rise time of less than and 2 s an overshoot of less than 30% were obtained over a gain range up to 10, indicating good robustness to muscle gain reductions such as might be caused by fatigue.<<ETX>>

A Discrete-Time Model of Electrcally Stimulated Muscle

A model describing the input/output properties of electrically stimulated isometric muscle is developed and experimentally tested. A discrete-time model gives the force output at the times of stimulation during pulse width modulation of recruitment at fixed stimulus amplitudes and periods. Two elements are necessary in the model: a static nonlinear element followed by a linear dynamic element. The static nonlinearity describes the relationship between pulse width and steady-state force. The dynamic properties are described with less than 10 percent error by a second-order discrete-time deterministic autoregressive moving average (DARMA) model. Exponentially weighted recursive least squares methods allow efficient parameter estimation. Model parameters are found to vary systematically with muscle length and stimulus frequency. Tests comparing actual and simulated feedback control of electrically stimulated muscle indicate that the model is adequate for digital controller design for applications in functional electrical stimulation.

Multijoint dynamics and postural stability of the human arm

The goal of this study was to examine how the mechanical properties of the human arm are modulated during isometric force regulation tasks. Specifically, we examined whether the dynamic stability of the limb remained nearly invariant across a range of voluntarily generated endpoint forces and limb postures. Previous single joint studies have demonstrated that dynamic joint stability, as quantified via estimates of the joint damping ratio, is nearly invariant during isometric torque regulation tasks. However, the relevance of these findings to the control of multijoint posture has not been investigated previously. A similar degree of invariance at the multijoint level could suggest a fundamental property of the motor system that could be incorporated into the planning and execution of multijoint tasks. In this work, limb mechanics were quantified using estimates of dynamic endpoint stiffness, which characterizes the relationship between imposed displacements of limb posture and the forces opposing those displacements. Endpoint stiffness was estimated using a two-link robot operating in the horizontal plane at the height of each subject’s glenohumeral joint. The robot was used to apply stochastic position perturbations to the arm and to measure the resulting forces. Endpoint stiffness dynamics were estimated nonparametrically and subsequently summarized using inertial, viscous and elastic parameters. We found that in the tasks studied, there was a differential modulation of endpoint elasticity and endpoint viscosity. Elasticity increased nearly linearly with increases in voluntary force generation while viscosity increased nonlinearly. This differential regulation resulted in limb dynamics that had a remarkably consistent damping ratio across all subjects and all tested conditions. These results emphasize the importance of considering the full dynamic response of a limb when investigating multijoint stability, and suggest that a minimal degree of limb stability is maintained over a wide range of force regulation tasks.