Biomedical subjects

R F Kirsch

Publications and source records attributed to R F Kirsch.

At least 19 recordsLinked to original sources

PubMed2001

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

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.

Adult↗

PubMed2001

A neuroprosthesis for high tetraplegia.

BACKGROUND: This case report describes a neuroprosthesis that restored shoulder and elbow function in a 23-year-old man with chronic C3 complete tetraplegia. Before implementation of the neuroprosthesis, electrodiagnostic testing revealed denervation from C5 to T1, with the greatest degree of denervation in the C8 and T1 myotomes. Thirteen percutaneous intramuscular electrodes were implanted into muscles acting on the shoulder and elbow of one upper limb. Before functional testing, the subject underwent a conditioning regimen to maximize the strength and endurance of the implanted muscles. RESULTS: After completion of the 8-week exercise regimen, stimulated active range of motion against gravity included 60 degrees of shoulder abduction, 45 degrees of shoulder flexion, 10 degrees of shoulder external rotation with the shoulder passively abducted to 90 degrees, and 110 degrees of elbow flexion. Stimulated elbow extension lacked 20 degrees of full extension with gravity eliminated. After system setup, the subject was able to pick up mashed potatoes on a plate with a utensil and bring them to his mouth using the neuroprosthesis and a balanced forearm orthosis. A switch mounted on the headrest of the subject's wheelchair and a position sensor mounted on the contralateral shoulder allowed the subject to control movement of his upper limb.

Activities of Daily Living↗

PubMed2001

Modeling the postural disturbances caused by upper extremity movements.

This paper describes the design, validation, and application of a dynamic, three-dimensional (3-D) model of the upper extremity for the purpose of estimating postural disturbances generated by movements of the arms. The model consists of two links representing the upper and lower arms, with the shoulder and elbow modeled as gimbal joints to allow three rotational degrees of freedom. With individualized segment inertial parameters based on anthropometric measurements, the model performs inverse dynamic analysis of recorded arm movements to calculate reaction forces and moments acting on the body at the shoulder in three dimensions. The method was validated by comparing the output of the model to estimates obtained from ground reaction loads during stereotypical and free form unilateral movements at various velocities and with different loads carried by human subjects while seated on biomechanical force platforms. The correlation between predicted and measured reaction forces and moments was very good under all conditions and across all subjects, with average rms errors less than 8% of measured peak-to-peak values. The model was then applied to bimanual activities representative of functional movements that would typically be performed while standing at a counter. The resulting estimates were consistent and adequate for the purpose of evaluating postural disturbances caused by upper extremity movements.

Adult↗

PubMed2001

Postural arm control following cervical spinal cord injury.

This study used estimates of dynamic endpoint stiffness to quantify postural arm stability following cervical spinal cord injury (SCI) and to investigate how this stability was affected by functional neuromuscular stimulation (FNS). Measurements were made in the horizontal plane passing through the glenohumeral joint on three SCI-impaired arms, which ranged in functional level from a weak C5 to a strong C6. Endpoint stiffness, which characterizes the relationship between externally imposed hand displacements and the resultant forces, was estimated during the application of planar, stochastic perturbations to each arm. These estimates were used in conjunction with voluntary endpoint force measurements to quantify stability and strength during voluntary contractions and during voluntary contractions in the presence of triceps FNS. The primary findings were: 1) the differences in the force generating capabilities of these arms were due primarily to differences in shoulder strength; 2) measurements of strength alone could not be used to predict arm stability; and 3) triceps FNS improved postural arm stability for all tested conditions. These results suggest strategies for improved control of FNS systems designed to restore arm function following cervical SCI and underscore the importance of examining the effects of FNS on both strength and stability.

Algorithms↗

PubMed2001

Three-dimensional shoulder kinematics in individuals with C5-C6 spinal cord injury.

The shoulder kinematics of five able-bodied subjects and those of five arms in three subjects with spinal cord injuries at C5 or C6 levels were measured as the subjects elevated their arms in three different planes: coronal, scapular and sagittal. The range of humeral elevation was significantly reduced in all spinal cord injury (SCI) subjects relative to able-bodied subjects. Over this restricted range of humeral motion, the scapula of SCI subjects tended to be medially rotated, relative to able-bodied subjects, and the protraction and spinal tilt angles of the scapula of the SCI subjects indicated scapular winging. These results are consistent with paralysis or at least with significant weakness of the serratus anterior muscle. If further study confirms this hypothesis, functional neuromuscular stimulation of the serratus anterior muscle via a nerve cuff electrode may be an effective intervention for improving shoulder function in C5-C6 SCI.

Biomechanical Phenomena↗

PubMed2000

A robotic manipulator for the characterization of two-dimensional dynamic stiffness using stochastic displacement perturbations.

Experimental techniques for estimating the two-dimensional dynamic stiffness of the human arm over a wide range of conditions have been developed. A robotic manipulator has been developed to create loads against which subjects perform various tasks and also to impose perturbations onto the endpoint of the arm to allow estimation of its mechanical properties. The manipulator can produce static endpoint forces exceeding 220 N in any direction in its plane of motion, and this plane can be vertically translated and tilted over wide ranges to study arm dynamic stiffness in many functionally relevant planes. It can impose stochastic position and force perturbations whose bandwidth exceeds that of the arm. These random perturbations avoid undesirable volitional reactions and allow the efficient estimation of stiffness dynamics using experimental trials of short duration. The ability of this manipulator to characterize inertial-viscoelastic systems was tested using several two-dimensional physical systems whose properties were independently characterized. The endpoint dynamic stiffness properties of a human arm were estimated as an example of the use of the manipulator in studying upper limb mechanical properties. The system properties characterized by these methods will be useful in probing normal neural arm control strategies and in developing rehabilitation interventions to improve arm movements in disabled individuals.

Arm↗

PubMed2000

Neuroprosthetic applications of electrical stimulation.

Neural prostheses are a developing technology that use electrical activation of the nervous system to restore function to individuals with neurological impairment. Neural prostheses function by electrical initiation of action potentials in nerve fibers that carry the signal to an endpoint where chemical neurotransmitters are released, either to affect an end organ or another neuron. Thus, in principle, any end organ under neural control is a candidate for neural prosthetic control. Applications have included stimulation in both the sensory and motor systems and range in scope from experimental trials with single individuals to commercially available devices. Outcomes of motor system neural prostheses include restoration of hand grasp and release in quadriplegia, restoration of standing and stepping in paraplegia, restoration of bladder function (continence, micturition) following spinal cord injury, and electrophrenic respiration in high-level quadriplegia. Neural prostheses restore function and provide greater independence to individuals with disability.

Bionics↗

PubMed2000

EMG-based prediction of shoulder and elbow kinematics in able-bodied and spinal cord injured individuals.

We have evaluated the ability of a time-delayed artificial neural network (TDANN) to predict shoulder and elbow motions using only electromyographic (EMG) signals recorded from six shoulder and elbow muscles as inputs, both in able-bodied subjects and in subjects with tetraplegia arising from C5 spinal cord injury. For able-bodied subjects, all four joint angles (elbow flexion-extension and shoulder horizontal flexion-extension, elevation-depression, and internal-external rotation) were predicted with average root-mean-square (rms) errors of less than 20 degrees during movements of widely different complexities performed at different speeds and with different hand loads. The corresponding angular velocities and angular accelerations were predicted with even lower relative errors. For individuals with C5 tetraplegia, the absolute rms errors of the joint angles, velocities, and accelerations were actually smaller than for able-bodied subjects, but the relative errors were similar when the smaller movement ranges of the C5 subjects were taken into account. These results indicate that the EMG signals from shoulder and elbow muscles contain a significant amount of information about arm moVement kinematics that could be exploited to develop advanced control systems for augmenting or restoring shoulder and elbow movements to individuals with tetraplegia using functional neuromuscular stimulation of paralyzed muscles.

Adult↗

PubMed2000

Estimation of intrinsic and reflex contributions to muscle dynamics: a modeling study.

This work evaluated system identification-based approaches for estimating stretch reflex contributions to muscle dynamics. Skeletal muscle resists externally imposed stretches via both intrinsic stiffness properties of the muscle and reflexively mediated changes in muscle activation. To separately estimate these intrinsic and reflex components, system identification approaches must make several assumptions. We examined the impact of making specific structural assumptions about the intrinsic and reflex systems on the system identification accuracy. In particular, we compared an approach that made specific parametric assumptions about the reflex and intrinsic subsystems to another that assumed more general nonparametric subsystems. A simulation-based approach was used so that the "true" characters of the intrinsic and reflex systems were known; the identification methods were judged on their abilities to retrieve these known system properties. Identification algorithms were tested on three experimentally based models describing the stretch reflex system. Results indicated that the assumed form of the intrinsic and reflex systems had a significant impact on the stiffness separation accuracy. In general, the algorithm incorporating nonparametric subsystems was more robust than the fully parametric algorithm because it had a more general structure and because it provided a better indication of the appropriateness of the assumed structure.

Algorithms↗

PubMed1999

Multiple-input, multiple-output system identification for characterization of limb stiffness dynamics.

This study presents time-domain and frequency-domain, multiple-input, multiple-output (MIMO) linear system identification techniques that can be used to estimate the dynamic endpoint stiffness of a multijoint limb. The stiffness of a joint or limb arises from a number of physiological mechanisms and is thought to play a fundamental role in the control of posture and movement. Estimates of endpoint stiffness can therefore be used to characterize its modulation during physiological tasks and may provide insight into how the nervous system normally controls motor behavior. Previous MIMO stiffness estimates have focused upon the static stiffness components only or assumed simple parametric models with elastic, viscous, and inertial components. The method presented here captures the full stiffness dynamics during a relatively short experimental trial while assuming only that the system is linear for small perturbations. Simulation studies were performed to investigate the performance of this approach under typical experimental conditions. It was found that a linear MIMO description of endpoint stiffness dynamics was sufficient to describe the displacement responses to small stochastic force perturbations. Distortion of these linear estimates by nonlinear centripetal and Coriolis forces was virtually undetectable for these perturbations. The system identification techniques were also found to be robust in the presence of significant output measurement noise and input coupling. These results indicate that the approach described here will allow the estimation of endpoint stiffness dynamics in an experimentally efficient manner with minimal assumptions about the specific form of these properties.

Algorithms↗

PubMed1998

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 person's ability to grasp and manipulate objects in an unstructured environment.

Biomechanical Phenomena↗

PubMed1997

Identification of time-varying stiffness dynamics of the human ankle joint during an imposed movement.

The time-varying stiffness dynamics of the human ankle joint were identified during a large stretch imposed upon the active triceps surae muscles. Small stochastic position perturbations were superimposed upon many repetitions of the larger movement and an ensemble time-varying identification technique was then used to characterize the relationship between the small perturbation and the torque it evoked at each sample in time throughout the movement. This technique was found to provide an excellent description of the ankle stiffness dynamics throughout this movement, with the identified stiffness impulse response functions accounting for more than 80% of the torque variance at all times. The average low-frequency stiffness values (K(low)) derived from the stiffness impulse responses at each sample in time are believed to reflect primarily the instantaneous elastic properties of active crossbridges. These properties, which reflect the contractile state of the muscles more directly than force or torque measurements, have not been obtained previously from an intact muscle-joint system. We found that stiffness actually increased during the later portion of the large imposed stretch, indicating the triceps surae muscles did not yield significantly, and that the post-stretch steady-state stiffness level was approximately 60% higher than prior to the stretch. Reflex activity evoked by the large stretch did not produce a detectable change in K(low), even though this activity did produce a clear twitch-like response in joint torque beginning approximately 60 ms following stretch onset. A second-order mechanical model was found to provide an adequate characterization of stiffness dynamics for steady-state periods before and well after the imposed movement, but it could not adequately describe the observed changes in stiffness dynamics during the movement itself. However, the variation of model parameters indicated that the torque evoked by the stochastic displacement was predominantly elastic in nature. The stiffness behavior during stretch observed here for the intact human ankle joint is largely consistent with previous studies performed in isolated muscle preparations.

Adult↗

PubMed1996

Measurement of isometric elbow and shoulder moments: position-dependent strength of posterior deltoid-to-triceps muscle tendon transfer in tetraplegia.

This report describes an apparatus which has been developed to measure several isometric elbow and shoulder forces and moments simultaneously and also allows this characterization to be performed across a range of shoulder and elbow joint angles in a horizontal plane. This apparatus was used to characterize the elbow extension strength in individuals with tetraplegia resulting from cervical level spinal cord injury. In all of these individuals, voluntary elbow extension was provided exclusively by the posterior deltoid muscle, which had previously been surgically transferred to the tendon of the paralyzed triceps muscle. Elbow extension is essential for many daily activities, such as reaching above shoulder level and pushing objects away from the body; the widely used posterior deltoid-to-triceps muscle tendon transfer surgery restores some degree of voluntary control to this important function. The apparatus contained a six-axis force-moment transducer to which the arm of each subject was attached. The six outputs of the transducer were transformed to correspond to physiological elbow and shoulder moments and forces. A customized table allowed the shoulder and elbow angles of the subject to be varied over a wide range in a horizontal plane so that the effects of posterior deltoid muscle length could be characterized over the likely functional range of the subject within this plane. It was found that elbow extension strength varied widely across subjects with C5 or C6 tetraplegia, from quite weak to strong enough to propel a manual wheelchair. Furthermore, the elbow extension strength of most subjects showed a strong dependence on both elbow and shoulder angles. Elbow extension was typically weak when the upper arm was elevated to shoulder level at the side, which unfortunately corresponds to the position often adopted by these individuals due to shoulder weakness.

Elbow Joint↗

PubMed1995

Effect of maintained stretch on the range of motion of the human ankle joint.

The effectiveness of maintained stretch in expanding the range of motion of the human ankle joint was assessed in a population of normal adults. Controlled movements were imposed upon the ankle, and triceps surae and tibialis anterior electromyograms were monitored to ensure that only passive joint properties generated ankle torque. We found that a majority of subjects (7 of 12) showed evidence of muscle activity sufficient to distort a subjective assessment of changes in range of motion. For the remaining five subjects, a 60-s maintained stretch produced a small decrease in the torque subsequently generated by an imposed dorsiflexing movement, but this effect was transient and largely disappeared following 300 s of rest at a neutral position. This short-term effect is consistent with the viscoelastic properties of collagenous material stretched during such treatment and is unlikely to lead to long-term increases in range of motion. RELEVANCE: The results of these experiments indicate that subjective assessments of changes in joint range of motion may be distorted by voluntary and reflexive muscle activation. Moreover the presumed increases in range of motion produced by maintained stretch in our normal subjects were small and transient. These results suggest that future assessments of the long-term efficacy of this treatment must monitor muscle activity and take into account known viscoelastic properties of collagenous materials.

Journal Article↗

PubMed1994

Muscle stiffness during transient and continuous movements of cat muscle: perturbation characteristics and physiological relevance.

Continuous stochastic position perturbations are an attractive alternative to transient perturbations in muscle and reflex studies because they allow efficient characterization of system properties. However, the relevance of the results obtained from stochastic perturbations remains unclear because they may induce a state change in muscle properties. We addressed this concern by comparing the force and stiffness responses of isolated muscles of the decerebrate cat elicited by stochastic perturbations to those evoked by "step" stretches of similar amplitudes. Muscle stiffness during stochastic perturbations was found to be predominantly linear and elastic in nature for a given operating point, showing no evidence of instantaneous amplitude-dependent nonlinearities, even during large movements. In contrast, force responses evoked by step stretches were found to be mainly viscous in nature and nonlinear for larger stretches, with only a small maintained (elastic) component. Stiffness magnitude decreased with displacement amplitude for both stochastic and step perturbations. Our results are largely consistent with the crossbridge theory of muscle contraction, indicating that transient and continuous displacements evoke different, although functionally relevant, aspects of muscle behavior. These differences have several implications for the neural control of posture and movement, and for the design of perturbations appropriate for its study.

Animals↗

PubMed1993

Identification of time-varying dynamics of the human triceps surae stretch reflex. I. Rapid isometric contraction.

We have examined the time variations of stretch reflex dynamics throughout rapid voluntary changes in the isometric contraction level of the human triceps surae muscles. This was achieved by superimposing a small stochastic displacement upon many such changing contractions and then identifying the time-varying relationship between the perturbation and the evoked electromyograms (EMGs). An "ensemble" time-varying system identification technique was used to estimate these input-output dynamics as a set of impulse response functions, one for each time before, during, and after the change in contraction level, with a temporal resolution equal to the data acquisition rate. Three main findings resulted. First, stretch reflex gain (relating joint velocity to EMG) was significantly modulated during changes in voluntary contraction level, increasing as the subject contracted the muscles and decreasing as the subject relaxed. Second, stretch reflex dynamics did not change with contraction level, even when its gain varied substantially. Third, the time course of the gain changes closely followed the level of the EMG, even though the subjects used rather different activation and deactivation patterns. These results suggest that, for the behavior studied (i.e., rapid changes in isometric contraction level), stretch reflex gain and motoneuron pool activation level were controlled by a common descending command rather than being independently specified.

Adult↗

PubMed1993

Identification of time-varying dynamics of the human triceps surae stretch reflex. II. Rapid imposed movement.

We examined the time-varying dynamics of the human triceps surae stretch reflex before, during, and after a large stretch was imposed upon the ankle joint, during a constant voluntary contraction of 15% of maximum voluntary contraction. Stretch reflex dynamics were estimated by superimposing a small stochastic displacement on many such stretches and using an "ensemble-based" time-varying identification procedure to compute impulse response functions relating the perturbation to the evoked electromyogram (EMG) at each point throughout the task. We found that stretch reflex magnitude (relating joint velocity to EMG) varied directly with baseline EMG activity during steady-state conditions before and after the large imposed stretch. Following the large stretch and the reflex activity it evoked, both background EMG and stretch reflex magnitude declined for up to 100 ms; changes in the stretch reflex were substantially greater in magnitude and followed a different time course from the corresponding changes in background EMG, however, indicating that stretch reflex properties were modulated independently of motoneuron pool activation level. Based on timing and the invariance of stretch reflex dynamics across time, it is argued that this behavior is largely mediated via peripheral neural mechanisms. This peripheral modulation of the stretch reflex presumably supplements various descending influences to adjust reflex properties.

Adult↗

PubMed1992

Neural compensation for fatigue-induced changes in muscle stiffness during perturbations of elbow angle in human.

1. The contribution to muscle force regulation provided by reflex pathways was studied in the elbow flexor muscles of seven normal human subjects, with the use of voluntary fatigue to induce a deficit in the force-generating capability of these muscles. To estimate the changes in the mechanical state of the muscle and the compensatory actions taken by reflex pathways to minimize the impact of fatigue, stochastic and "step" angular perturbations were applied to the joint, and the resulting joint stiffness and electromyographic (EMG) responses were compared before and after fatigue. 2. The magnitude of contractile fatigue, induced by repeatedly lifting a weight via a pulley system, was quantified by comparing the slope of the isometric torque-EMG relationship before and after fatigue. The exercise routine was quite effective in producing severe and long-lasting fatigue, with average percentage changes in the isometric torque-EMG slope of 210-306% for biceps and 129-205% for brachioradialis, depending on the point in time examined. 3. The torque response to a rapid step stretch of the elbow joint was quite similar before and after fatigue for the time interval before reflex action (less than 20 ms after stretch onset), suggesting that intrinsic muscle stiffness for a given mean torque level was not changed by fatigue. The steady-state torque level attained after completion of the stretch was always decreased after fatigue, indicating a decrease in the reflex component of joint stiffness, but this decrease was small compared with the change in the isometric torque-EMG relationship and was accompanied by a significantly larger incremental EMG response after fatigue. This increase in incremental EMG after fatigue was found to be of reflex origin, with activation-related reflex gain changes apparently playing a significant role only at low contraction levels. 4. Torque and angle responses recorded during stochastic perturbations were used to identify elbow joint compliance impulse responses. A second-order mechanical model was fit to each impulse response, and the parameters representing joint inertia, elastic stiffness, and viscous stiffness were used to summarize changes in joint mechanical properties as the mean contraction level was varied. For a perturbation with a relatively wide bandwidth (0-25 Hz), fatigue had little or no effect on the form of the compliance impulse response, apparently because the stimulus disabled reflex force generation in elbow flexor muscles, whereas a perturbation with a more restricted bandwidth (0-10 Hz) demonstrated consistent decreases in joint stiffness after fatigue.(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗