Recovery of arm movement after stroke

To characterize the time-course of change in motor impairment, we examined voluntary elbow movement in stroke survivors over a period of one year post-stroke. We quantified several kinematic features of voluntary rapid elbow extension, by measuring the movement trajectory and its derivatives. The subjects were examined five times, at 1-, 2-, 3-, 6- and 12-months post-stroke. The data analyses had two steps. First we used the "growth mixture" model to characterize the recovery patterns of these kinematic parameters. Based on the observed measurements over 1 year, we found two classes of recovery patterns for each kinematic parameter. Subjects in class 1 started with a low value for each parameter and these values increased over time, while subjects in class 2 tended to start with higher value and showed widely divergent recovery patterns. Second, we used the logistic regression analysis to predict these recovery patterns based on Fugl Mayer Scale (FMS) of upper extremity measured on the first visit (i.e. 1 month after stroke). Based on the clinical evaluation of motor function (i.e. FMS) within the first month after stroke, these findings enable us to predict the recovery of arm impaired voluntary movement in hemiparetic stroke subjects at different times during the first year, and potentially beyond.

Comparison of neuromuscular abnormalities between upper and lower extremities in hemiparetic stroke

We studied the neuromuscular mechanical properties of the elbow and ankle joints in chronic, hemiparetic stroke patients and healthy subjects. System identification techniques were used to characterize the mechanical abnormalities of these joints and to identify the contribution of intrinsic and reflex stiffness to these abnormalities. Modulation of intrinsic and reflex stiffness with the joint angle was studied by applying PRBS perturbations to the joint at different joint angles. The experiments were performed for both spastic (stroke) and contralateral (control) sides of stroke patients and one side of healthy (normal) subjects. We found reflex stiffness gain (GR) was significantly larger in the stroke than the control side for both elbow and ankle joints. GR was also strongly position dependent in both joints. However, the modulation of GR with position was slightly different in two joints. GR was also larger in the control than the normal joints but the differences were significant only for the ankle joint. Intrinsic stiffness gain (K) was also significantly larger in the stroke than the control joint at elbow extended positions and at ankle dorsiflexed positions. Modulation of K with the ankle angle was similar for stroke, control and normal groups. In contrast, the position dependency of the elbow was different. K was larger in the control than normal ankle whereas it was lower in the control than normal elbow. However, the differences were not significant for any joint. The findings demonstrate that both reflex and intrinsic stiffness gain increase abnormally in both upper and lower extremities. However, the major contribution of intrinsic and reflex stiffness to the abnormalities is at the end of ROM and at the middle ROM, respectively. The results also demonstrate that the neuromuscular properties of the contralateral limb are not normal suggesting that it may not be used as a suitable control at least for the ankle study.

Periodically relieving ischial sitting load to decrease the risk of pressure ulcers

OBJECTIVE

To investigate the relieving effect on interface pressure of an alternate sitting protocol involving a sitting posture that reduces ischial support.

DESIGN

Repeated measures in 2 protocols on 3 groups of subjects.

SETTING

Laboratory.

PARTICIPANTS

Twenty able-bodied persons, 20 persons with paraplegia, and 20 persons with tetraplegia.

INTERVENTIONS

Two 1-hour protocols were used: alternate and normal plus pushup. In the alternate protocol, sitting posture was alternated every 10 minutes between normal (sitting upright with ischial support) and with partially removed ischial support (WO-BPS) postures; in the normal plus pushup protocol, sitting was in normal posture with pushups (lifting the subject off the seat) performed every 20 minutes.

MAIN OUTCOME MEASURE

Interface pressure on seat and backrest.

RESULTS

In WO-BPS posture, the concentrated interface pressure observed around the ischia in normal posture was significantly repositioned to the thighs. By cyclically repositioning the interface pressure, the alternate protocol was superior to the normal plus pushup protocol in terms of a significantly lower average interface pressure over the buttocks.

CONCLUSIONS

A sitting protocol periodically reducing the ischial support helps lower the sitting load on the buttocks, especially the area close to ischial tuberosities.

Adaptive assistance for guided force training in chronic stroke

This work describes a novel form of robotic therapy for the upper extremity in chronic stroke. Based on previous results, we hypothesized that a training task that encourages subjects to consciously guide endpoint forces generated by the hemiparetic arm will result in significant gains in functional ability of the arm, superior to more conventional methods of therapy. In addition, since stroke survivors present with varying degrees of arm movement ability, we developed an adaptive algorithm that tailors the amount of assistance provided in completing the guided force training task. The algorithm adapts a coefficient for velocity-dependent assistance based on measured movement speed, on a trial-to-trial basis. The training algorithm has been implemented with a simple linear robotic device called the ARM Guide. One participant completed a two month training program with the adaptive algorithm, resulting in significant improvements in the performance of functional tasks.

Neuromuscular abnormalities associated with spasticity of upper extremity muscles in hemiparetic stroke

Our objective was to assess the mechanical changes associated with spasticity in elbow muscles of chronic hemiparetic stroke survivors and to compare these changes with those recorded in the ankle muscles of a similar cohort. We first characterized elbow dynamic stiffness by applying pseudorandom binary positional perturbations to the joints at different initial angles, over the entire range of motion, with subjects relaxed. We separated this stiffness into intrinsic and reflex components using a novel parallel cascade system identification technique. In addition, for controls, we studied the nonparetic limbs of stroke survivors and limbs of age-matched healthy subjects as primary and secondary controls. We found that both reflex and intrinsic stiffnesses were significantly larger in the stroke than in the nonparetic elbow muscles, and the differences increased as the elbow was extended. Reflex stiffness increased monotonically with the elbow angle in both paretic and nonparetic sides. In contrast, the modulation of intrinsic stiffness with elbow position was different in nonparetic limbs; intrinsic stiffness decreased sharply from full- to mid-flexion in both sides, then it increased continuously with the elbow extension in the paretic side. It remained invariant in the nonparetic side. Surprisingly, reflex stiffness was larger in the nonparetic than in the normal control arm, yet intrinsic stiffness was smaller in the nonparetic arm. Finally, we compare the angular dependence of paretic elbow and ankle muscles and show that the modulation of reflex stiffness with position was strikingly different.

Model based sensitivity analysis of EMG-force relation with respect to motor unit properties: applications to muscle paresis in stroke

The sensitivity of the electromyogram (EMG)-force relation to changes in motoneuron and muscle properties was explored using a simulation approach, and by applying existing motoneuron pool, muscle force, and surface EMG models. The simulation results indicate that several factors contribute potently to known changes in the EMG-force relation in paretic stroke muscles. First, compression of the motor unit recruitment range with respect to the injected current tends to generate greater EMG amplitude at a given force, and to produce a highly nonlinear EMG-force relation. The overall mean slope of the EMG-force relation tends to be flatter, primarily because of this non-linear behavior. Second, with reductions of the mean motor unit firing rates, the slope of the EMG-force relation also tends to increase especially as the mean firing rates dropped substantially below the motor unit fusion frequency. Finally, similar effects were observed with a reduction in the number of motor units, and with variation in motor unit contractile properties, which also altered the EMG-force relation. These findings provide new insight toward our understanding of experimental EMG-force relations in both normal and pathological states, such as the abnormal EMG-force relations of paresis muscles in stroke.

Analysis of the effects of firing rate and synchronization on spike-triggered averaging of multidirectional motor unit torque

Spike-triggered averaging (STA) of muscle force transients has often been used to estimate motor unit contractile properties, using the discharge of a motor unit within the muscle as the triggering events. For motor units that exert torque about multiple degrees-of-freedom, STA has also been used to estimate motor unit pulling direction. It is well known that motor unit firing rate and weak synchronization of motor unit discharges with other motor units in the muscle can distort STA estimates of contractile properties, but the distortion of STA estimates of motor unit pulling direction has not been thoroughly evaluated. Here, we derive exact equations that predict that STA decouples firing rate and synchronization distortion when used to estimate motor unit pulling direction. We derive a framework for analyzing synchronization, consider whether the distortion due to synchronization can be removed from STA estimates of pulling direction, and show that there are distributions of motor unit pulling directions for which STA is insensitive to synchronization. We conclude that STA may give insight into how motoneuronal synchronization is organized with respect to motor unit pulling direction.

Evolution of reflexive and muscular mechanical properties in stroke-induced spasticity

We studied the natural history of reflexive and mechanical properties in hemiparetic spastic stroke subjects. System identification techniques were used to characterize the mechanical abnormalities of the elbow joint and to identify the contribution of intrinsic and reflex stiffness to these abnormalities over one year post-injury. Modulation of intrinsic and reflex stiffness of the elbow joint was studied by applying PRBS perturbations to the elbow at different joint angles at five intervals following stroke. We found that both reflex and intrinsic stiffness were larger in the stroke than in the control arms. They were also strongly position dependent; they both increased with increasing elbow extension but reflex stiffness declined at full extension in some subjects. This position dependency was consistent during stroke recovery. Both intrinsic and reflex abnormally increased over time after stroke. These findings help better understanding of the origins of mechanical abnormalities associated with spasticity and document the time course of these abnormalities during stroke recovery.

Abnormal intrinsic and reflex stiffness related to impaired voluntary movement

We studied the relationship between mechanical abnormalities associated with spasticity and impairments in voluntary movements of the spastic joint in chronic, hemiparetic stroke subjects. System identification techniques were used to characterize the mechanical abnormalities of the elbow joint and to identify the contribution of intrinsic and reflex stiffness to these abnormalities. Repeated voluntary movements of the elbow from full flexion to extension at maximum speed were also conducted. These movements were quantified by measuring their kinematics parameters. The correlation coefficient was measured to determine the relationship between abnormal modulation of intrinsic and reflex stiffness as function of joint position with the kinematics parameters. We found that both intrinsic and reflex stiffness were significantly larger in stroke than control sides and were strongly position dependent, increasing with elbow extension. Abnormal modulation of intrinsic and reflex stiffness with position (slope) was correlated with an increase in duration of movement (DM), and a decrease in peak-velocity (Pv), peak-acceleration (Pa) and maximum voluntary contraction (MVC). Weakness, quantified as a decrease in MVC, was also correlated with the reduction in Pv, Pa and active range of motion (AROM). These findings demonstrate that abnormal modulation of both intrinsic and reflex stiffness with position are related to antagonist muscle weakness that may cause stroke patients to move slower and take longer to complete reaching tasks.

A comparison of the effects of imposed extension and flexion movements on Parkinsonian rigidity

OBJECTIVE

To test a hypothesis that Parkinsonian rigidity is more pronounced in imposed extension than flexion movement.

METHODS

Twelve Parkinsonian subjects (both "Off" and "On" medication states) and seven control subjects participated in the protocol, in which a servomotor imposed wrist flexion and extension. Rigidity was quantitatively evaluated by the rectified torque integral with time, i.e., temporal score, and by the torque integral with joint angle, i.e., work score, for extension and flexion, respectively.

RESULTS

In the "Off" state, the imposed extension induced a significantly higher resistance than did flexion. Dopaminergic medication significantly reduced the temporal score associated with imposed extension, and significantly decreased the work score of both movements. Compared with controls, the scores were higher for patients in the "On" state.

CONCLUSIONS

Rigidity is more readily elicited in extension movement. The distinction is not evident in clinical practice, whereas it can be clearly revealed with the application of biomechanical analyses.

SIGNIFICANCE

This distinction may prove to be a standard feature of rigidity. The procedures may be helpful in diagnosis and useful in evaluating new treatments and developing rehabilitation programs.

Weakness Is the Primary Contributor to Finger Impairment in Chronic Stroke

OBJECTIVE

To assess the relative contributions of several neurologic and biomechanic impairment mechanisms to overall finger and hand impairment in chronic hemiparetic stroke survivors.

DESIGN

Repeated-measures design.

SETTING

Clinical research laboratory.

PARTICIPANTS

Thirty stroke survivors with chronic hemiparesis. Fifteen subjects had severe hand motor impairment and 15 had moderate impairment, as measured with the Chedoke-McMaster Stroke Assessment.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

The biomechanic factors stiffness and resting flexion torque, together with the neurologic factors spasticity, strength, and coactivation, were quantified by using a custom hand manipulator, a dynamometer, and electromyographic recordings. Both passive and active rotations of the metacarpophalangeal joints of the fingers were examined.

RESULTS

Although subjects in the severely impaired group exhibited statistically greater passive stiffness and resting flexion torque than their moderately impaired counterparts (P<.05), the overall effect of these biomechanic changes appeared small in relation to the deficits attributable to neurologic changes such as spasticity and, especially, weakness. In fact, weakness in grip strength and isometric extension accounted for the greatest portion of the variance between the 2 groups (eta(2)=.40 and eta(2)=.23, respectively).

CONCLUSIONS

Thus, deficits in hand motor control after stroke seem to derive mainly from weakness, which may be attributable to the loss of descending corticospinal pathway activation of motoneurons.

The effect of motor imagery on spinal segmental excitability

The purpose of this study was to investigate the effect of motor imagery on spinal segmental excitability by recording the reflex responses to externally applied stretch of the extrinsic finger flexors and extensors during the performance of an imaginary task. Nine young healthy subjects performed a series of imagined flexion-extension movements of the fingers. Muscle stretch was imposed concurrently by applying rotations of the metacarpophalangeal joints at 100, 300, or 500 degrees /sec. Three of the nine tested subjects also generated 0.2 Newton meter voluntary flexion torque in preloading tasks before stretch. At 300 degrees /sec stretch, electromyogram (EMG) and torque reflex responses, which were observed in the finger flexors in four of nine subjects during motor imagery, were activated at a short latency (38.6 +/- 10.6 msec). This latency was similar to that recorded during a stretch of preactivated flexor muscles (34.4 +/- 3.6 msec), in which motoneurons are already suprathreshold and in which monosynaptic effects of muscle afferents are likely to be discernable. In a similar manner, for stretches imposed at 500 degrees /sec, responses to stretch of the flexors were observed in all five tested subjects in imaginary flexion tasks at very short latencies (26.4 +/- 3.7 msec), again similar to those induced by tendon taps (22.8 +/- 1.2 msec). No EMG response was observed at rest during stretches. These observations support the view that effects must have been mediated by imagery-related subthreshold activation of spinal motoneurons and/or interneurons, rather than by long-latency transcortical reflex responses. We conclude that motor imagery has a potent effect on the excitability of spinal reflex pathways.

Neuromuscular Reflexes Contribute to Knee Stiffness During Valgus Loading

We have previously shown that abduction angular perturbations applied to the knee consistently elicit reflex responses in knee joint musculature. Although a stabilizing role for such reflexes is widely proposed, there are as of yet no studies quantifying the contribution of these reflex responses to joint stiffness. In this study, we estimate the mechanical contributions of muscle contractions elicited by mechanical excitation of periarticular tissue receptors to medial-lateral knee joint stiffness. We hypothesize that these reflex muscle contractions will significantly increase knee joint stiffness in the adduction/abduction direction and enhance the overall stability of the knee. To assess medial-lateral joint stiffness, we applied an abducting positional deflection to the fully extended knee using a servomotor and recorded the torque response using a six degree-of-freedom load-cell. EMG activity was also recorded in both relaxed and preactivated quadriceps and hamstrings muscles with surface electrodes. A simple, linear, second-order, delayed model was used to describe the knee joint dynamics in the medial/lateral direction. Our data indicate that excitation of reflexes from periarticular tissue afferents results in a significant increase of the joint’s adduction-abduction stiffness. Similar to muscle stretch reflex action, which is modulated with background activation, these reflexes also show dependence on muscle activation. The potential significance of this reflex stiffness during functional tasks was also discussed. We conclude that reflex activation of knee muscles is sufficient to enhance joint stabilization in the adduction/abduction direction, where knee medial-lateral loading arises frequently during many activities.

Reflex reciprocal facilitation of antagonist muscles in spinal cord injury

STUDY DESIGN

Electromyographic study in complete and incomplete spinal cord injury (SCI).

OBJECTIVE

To examine the changes in the pattern of reciprocal inhibition between agonist and antagonist muscles in SCI.

SETTINGS

Sensory Motor Performance Program, Rehabilitation Institute of Chicago, IL, USA.

METHODS

Tendon taps were delivered manually with an instrumented hammer to the tendons of the tibialis anterior and soleus muscle in positions of full-ankle dorsiflexion and plantarflexion in eight subjects with complete SCI and eight subjects with incomplete SCI. Electromyographic activity (EMG) was recorded from ankle dorsiflexor and plantarflexor muscles. Tapping force was also recorded by a force sensor mounted to the tendon hammer, indicating the stimulus onset. Measures of reflex EMG magnitude and reflex latency were obtained for both agonist and antagonist muscles. The ratio of antagonist to agonist EMG was computed based on normalized EMG.

RESULTS

Substantial reflex responses occurred in both the stretched muscle and in its antagonist. The reflex in antagonist, which we term 'reciprocal facilitation (RF)', was most evident in subjects with incomplete SCI. The magnitude of RF was consistently greater than reflex responses in agonist muscles under all test conditions. The latency of the RF was comparable to that of monosynaptic reflex response.

CONCLUSIONS

Following SCI, reciprocal organization of segmental reflexes at the ankle is often partially or completely suppressed, allowing reflex activation in antagonist muscles to be manifested. Possible mechanisms underlying these changes in neural organization are discussed.

SPONSORSHIP

This study was supported by Spinal Cord Research Foundation, the Paralyzed Veterans of America.

The Movement-Specific Effect of Motor Imagery on the Premotor Time

The purpose of this study was to investigate the effect of motor imagery on the premotor time (PMT). Twelve healthy adults performed reaction time movements in response to external visual signals at rest, when holding an object (muscle activation), or performing different background imagined movements (motor imagery). When compared to rest, muscle activation reduced the PMT; imagined finger extension of the right hand and imagined finger flexion of the left hand elongated the PMT; imagined finger flexion of the right hand had no effect on the PMT. This movement-specific effect is interpreted as the sum of the excitatory effect caused by enhanced corticospinal excitability specifically for the primary mover of the imagined movement and an overall inhibition associated with increased task complexity during motor imagery. Our results clearly demonstrate that motor imagery has movement-specific effects on the PMT.

MUAP Number Estimates in Surface EMG: Template-Matching Methods and Their Performance Boundaries

Estimates of the number of motor unit action potential (MUAP)s appearing in the surface electromyogram (EMG) signal, which offers potentially valuable information about motor unit recruitment and firing rates, are likely to provide a more accurate reflection of the neural command to muscle than are current EMG quantification methods. In this paper, we show that the basic shapes of surface MUAPs recorded from the first dorsal interosseous (FDI) muscle can ideally be represented by a small number of waveforms. On the basis of this, we seek to estimate the number of MUAPs present in standard surface EMG records, using template-matching techniques to identify MUAP occurrences. Our simulation study indicates that the performance of template-matching methods for MUAP number estimation is mainly constrained by the MUAP superposition in the signal, and the maximum number of MUAPs allowed in the signal for a good estimation is determined by the duration of MUAPs. To further explore this from experimental surface EMG signals, we compare the recordings from a selective multiple concentric ring electrode against those derived from a standard differential EMG electrode situated over the same muscle. We conclude that the ring surface electrode only slightly reduces the MUAP duration and the less MUAP superposition rate contained in the signal is mainly achieved by reducing the pick up area of the electrode. Using a template-matching method, although the number of MUAPs can be approximately estimated based on a very selective surface EMG recording at low force levels, the maximum number of MUAPs correctly estimated from the surface EMG is constrained by the MUAP duration.

Robotic devices for movement therapy after stroke: current status and challenges to clinical acceptance

Robotic devices for movement therapy are moving closer to becoming commercially available tools for aiding in stroke rehabilitation. Robotic technology offers a range of functions that will augment current clinical practice by leveraging therapists' time, cost effectively extending therapy programs, providing new measures of impairment, and offering new therapy protocols. In this article, we review work from several research laboratories that supports the clinical value of stroke therapy systems. A commercialization effort based on these results is described. We also discuss challenges to achieving clinical acceptance and practical implementation of these devices.

Relative contributions of neural mechanisms versus muscle mechanics in promoting finger extension deficits following stroke

The origins of impaired finger and hand function were examined in 10 stroke survivors with chronic spastic hemiparesis, with the intent of assessing whether mechanical restraint or altered neurophysiological control mechanisms are responsible for the well-known impairment of finger extension. Simultaneous extension of all four metacarpophalangeal (MCP) joints of the impaired hand was either externally imposed using a rotary actuator or attempted voluntarily by the subject. Trials were conducted both before and after administration of a local anesthetic, blocking the median and ulnar nerves at the elbow. The anesthetic was administered to reduce the activity of the muscles flexing the MCP joints, in order to distinguish mechanical from neuronal resistance to imposed MCP rotation. We found that the nerve blockade resulted in a reduction in velocity-dependent torque (P = 0.01), thereby indicating significant joint impedance due to spasticity. Blockade also produced a posture-dependent reduction in static torque in declaratively relaxed subjects (P = 0.04), suggesting some tonic flexor activity for specific hand postures. No change in either extensor isometric (P = 0.33) or isokinetic (0.53) torque was apparent, but 3 of the 10 subjects did exhibit substantial (>10 degrees ) improvement in voluntary MCP extension following the blockade. This improvement seemed largely due to a decrease in inappropriate flexor activity during the movement, rather than an increase in extensor activity. We argue that persistent and inappropriate flexor activation plays a role in limiting voluntary finger extension, and that this activation is potentially a reflection of altered supraspinal control of key spinal pathways. In all cases, this inappropriate activation was compounded by weakness, apparent in both the extensor and flexor muscles.

Brain-computer interface technology: a review of the Second International Meeting

This paper summarizes the Brain-Computer Interfaces for Communication and Control, The Second International Meeting, held in Rensselaerville, NY, in June 2002. Sponsored by the National Institutes of Health and organized by the Wadsworth Center of the New York State Department of Health, the meeting addressed current work and future plans in brain-computer interface (BCI) research. Ninety-two researchers representing 38 different research groups from the United States, Canada, Europe, and China participated. The BCIs discussed at the meeting use electroencephalographic activity recorded from the scalp or single-neuron activity recorded within cortex to control cursor movement, select letters or icons, or operate neuroprostheses. The central element in each BCI is a translation algorithm that converts electrophysiological input from the user into output that controls external devices. BCI operation depends on effective interaction between two adaptive controllers, the user who encodes his or her commands in the electrophysiological input provided to the BCI, and the BCI that recognizes the commands contained in the input and expresses them in device control. Current BCIs have maximum information transfer rates of up to 25 b/min. Achievement of greater speed and accuracy requires improvements in signal acquisition and processing, in translation algorithms, and in user training. These improvements depend on interdisciplinary cooperation among neuroscientists, engineers, computer programmers, psychologists, and rehabilitation specialists, and on adoption and widespread application of objective criteria for evaluating alternative methods. The practical use of BCI technology will be determined by the development of appropriate applications and identification of appropriate user groups, and will require careful attention to the needs and desires of individual users.

Muscular torque generation during imposed joint rotation: torque-angle relationships when subjects' only goal is to make a constant effort

It is a reasonable expectation that voluntarily activated spinal motoneurons will be further excited by increases in spindle afferent activity produced by muscle stretch. Human motor behavior attributed to tonic stretch reflexes and to reflexes recruited by relatively slow joint rotation has been reported from several laboratories. We reinvestigated this issue by rotating the elbow joint over the central portion of its range while subjects focused on keeping their elbow flexion effort constant at one of three different levels and made no attempt to control the position, speed or direction of movement of their forearm. There is evidence that subjects' voluntary motor status is constant under these conditions so that any change in torque would be of involuntary origin. On average, torques rose somewhat and then fell as the elbow was flexed through a range of 80 degrees at 10, 20 and 60 degrees/s and a similar pattern occurred during elbow extension; i.e., both concentric and eccentric torque-angle profiles had roughly similar shapes and neither produced consistent stabilizing cross-range stiffness. The negative stiffness (rising torque) during the early part of a concentric movement and the negative stiffness (falling torque) during the later part of an eccentric movement would not have occurred if a stabilizing stretch reflex had been present. Positive stiffness rarely gave rise to torque changes greater than 20% in either individual or cross-subject averaged data. When angular regions of negative stiffness are combined with regions of low positive stiffness (torque change 10% or less), much of the range of motion was not well stabilized, especially during eccentric movements. The sum of the EMGs from biceps brachii, brachioradialis and brachialis showed a pattern opposite to that expected for a stretch reflex; there was an upward trend in the EMG as the elbow was flexed and a downward trend as the elbow was extended. There was little change in the shape of this EMG-angle relationship with either direction or velocity. The individual EMG-angle relationships were distinctive for each of these three elbow flexor muscles in four of the six subjects; in the remaining two, biceps was distinctive, but brachioradialis and brachialis appeared to be coupled. Although the EMGs of individual muscles were modulated over the angular range, no consistent stretch reflexes could be seen in the individual records. Thus, we could find no clear evidence for stretch reflex stabilization of human subjects maintaining a constant effort. Rather, muscle torque appears to be reflexly modulated across a much used portion of the elbow's angular range so that any appreciable stabilizing stiffness that is sustained for more than fractions of a second is associated with a change in effort.

Reflex mechanisms for motor impairment in spinal cord injury

Spasticity is common feature of human spinal cord injury. It contributes to motor impairment and it also promotes joint deformity in patients who have sustained such injury. The classical definition of spasticity highlights the increased resistance of a joint to externally imposed motion. This resistance is attributable largely to changes in stretch reflex excitability, and it is manifested primarily in those muscles being stretched by the motion. Under this definition, there would be little activity in muscles crossing other joints. In spinal cord injury, however, muscles innervated from distal spinal segments often exhibit little hypertonia, yet patients report the occurrence of disabling spasms. These spasms appear as coordinated patterns of muscle activation throughout the limb, involving either limb flexors or extensors. These patterns are therefore quite different from those of classical spasticity. The receptor origins and neural pathways responsible for the spasms in spinal cord injury will be addressed.

Reflex muscle contractions can be elicited by valgus positional perturbations of the human knee

Experimental evidence on the reflex responses of thigh muscles to valgus mechanical perturbations at the human knee are presented. Random step positional deflections, ranging from 5 degrees to 12 degrees at 60 degrees /s, were applied to the fully extended knees of seven healthy subjects. Subjects were instructed to maintain a constant background co-activation ( approximately 2-11% MVC) of the quadriceps and hamstring muscles prior to and during the mechanical stimulus. We found that the reflex response to sustained valgus joint deflection in the vasti muscles had longer onset latencies (range: 83-92ms) than did the stretch reflex in the same muscles (latencies: 29-31ms). This reflex EMG response consisted typically of a peak followed by sustained muscle activity throughout the step perturbation. The sustained EMG activity was dependent on the amplitude of the perturbing stimulus, but in a nonlinear manner. The long latency of the valgus response suggests that the reflex originates in nonmuscular sensory pathways, potentially from mechanoreceptors lying in periarticular tissues such as joint ligaments and capsule. Analysis of the spatial distribution of reflex responses showed an asymmetrical pattern with preferential activation of medial vs. lateral muscles of the knee. We assess whether these asymmetric reflex contractions could promote joint stability, either by inducing generalized joint stiffening, or by preferential activation of those muscles that are best suited to resist induced ligament strain.

Windup of flexion reflexes in chronic human spinal cord injury: a marker for neuronal plateau potentials?

The physiological basis of flexion spasms in individuals after spinal cord injury (SCI) may involve alterations in the properties of spinal neurons in the flexion reflex pathways. We hypothesize that these changes would be manifested as progressive increases in reflex response with repetitive stimulus application (i.e., "windup") of the flexion reflexes. We investigated the windup of flexion reflex responses in 12 individuals with complete chronic SCI. Flexion reflexes were triggered using trains of electrical stimulation of plantar skin at variable intensities and inter-stimulus intervals. For threshold and suprathreshold stimulation, windup of both peak ankle and hip flexion torques and of integrated tibialis anterior electromyographic activity was observed consistently in all patients at inter-stimulus intervals < or =3 s. For subthreshold stimuli, facilitation of reflexes occurred only at intervals < or =1 s. Similarly, the latency of flexion reflexes decreased significantly at intervals < or =1 s. Patients that were receiving anti-spasticity medications (e.g., baclofen) had surprisingly larger windup of reflex responses than those who did not take such medications, although this difference may be related to differences of spasm frequency between the groups of subjects. The results indicate that the increase in spinal neuronal excitability following a train of electrical stimuli lasts for < or =3 s, similar to previous studies of nociceptive processing. Such long-lasting increases in flexion reflex responses suggest that cellular mechanisms such as plateau potentials in spinal motoneurons, interneurons, or both, may partially mediate spinal cord hyperexcitability in the absence of descending modulatory input.

Extrinsic flexor muscles generate concurrent flexion of all three finger joints

The role of the forearm (extrinsic) finger flexor muscles in initiating rotation of the metacarpophalangeal (MCP) joint and in coordinating flexion at the MCP, the proximal interphalangeal (PIP), and distal interphalangeal (DIP) joints remains a matter of some debate. To address the biomechanical feasibility of the extrinsic flexors performing these actions, a computer simulation of the index finger was created. The model consisted of a planar open-link chain comprised of three revolute joints and four links, driven by the change in length of the flexor muscles. Passive joint characteristics, included in the model, were obtained from system identification experiments involving the application of angular perturbations to the joint of interest. Simulation results reveal that in the absence of passive joint torque, shortening of the extrinsic flexors results in PIP flexion (80 degrees ), but DIP (8 degrees ) and MCP (7 degrees ) joint extension. The inclusion of normal physiological levels of passive joint torque, however, results in simultaneous flexion of all three joints (63 degrees for DIP, 75 degrees for PIP, and 43 degrees for MCP). Applicability of the simulation results was confirmed by recording finger motion produced by electrical stimulation of the extrinsic flexor muscles for the index finger. These findings support the view that the extrinsic flexor muscles can initiate MCP flexion, and produce simultaneous motion at the MCP, PIP, and DIP joints.

Effect of muscle biomechanics on the quantification of spasticity

The impact of muscle biomechanics on spasticity was assessed by comparison of the reflex responses of the elbow and metacarpophalangeal (MCP) flexor muscles in individuals with chronic spastic hemiplegia following stroke. Specifically, methods were developed to quantify reflex responses and to normalize these responses for comparison across different muscle groups. Stretch reflexes were elicited in the muscles of interest by constant velocity ramp-and-hold stretches at the corresponding joint. The muscles were initially passive, with the joint placed in a midrange position. Estimates of biomechanical parameters were used to convert measured reflex joint torque and joint angle into composite flexor muscle stress and stretch. We found that the stretch reflex response for the MCP muscle group had a 74% greater mean stiffness modulus than that for the elbow muscle group, and that the reflex threshold was initiated at an 80% shorter mean muscle stretch. However, we determined that initial normalized fiber length was significantly greater for the experiments involving the MCP muscles than for those involving the elbow muscles. Increasing the initial composite fiber length of the elbow flexors produced significant reduction of the reflex threshold (p<0.001), while decreasing the initial length of the MCP flexors significantly reduced their measured reflex stiffness (p<0.001). Thus, biomechanical parameters of muscle do appear to have an important effect on the stretch reflex in individuals with impairment following stroke, and this effect should be accounted for when attempting to quantify spasticity.

Reflex and intrinsic changes induced by fatigue of human elbow extensor muscles

Fatigue-induced changes in intrinsic and reflex properties of human elbow extensor muscles and the underlying mechanisms for fatigue compensation were investigated. The elbow joint was perturbed using small-amplitude and pseudorandom movement patterns while subjects maintained steady levels of mean joint extension torque. Intrinsic and reflex properties were identified simultaneously using a nonlinear delay differential equation model. Intrinsic joint properties were characterized by measures of joint stiffness, viscous damping, and limb inertia and reflex properties characterized by measures of dynamic and static reflex gains. Fatigue was induced using 15 min of intermittent voluntary isometric (submaximal) exercise, and a rest period of 10 min was taken to allow the fatigued muscles to recover from acute fatigue effects. Identical experimental and data analysis procedures were used before and after fatigue. Our findings were that after fatigue, joint stiffness was significantly reduced at higher torque levels, presumably reflecting the reduced force-generating capacity of fatigued muscles. Conversely, joint viscosity was increased after fatigue potentially because of the reduced crossbridge detachment rate and prolonged relaxation associated with intracellular acidosis accompanying fatigue. Static stretch reflex gain decreased significantly at higher torque levels after fatigue, indicating that the isometric fatiguing exercise might be associated with a preferential change in properties of spindle chain fibers and bag(2) fibers. For matched pre- and postfatigue torque levels, dynamic reflexes contributed relatively more torque after fatigue, displaying higher dynamic reflex gains and larger dynamic electromyographic responses elicited by the controlled small-amplitude position perturbations. These changes appear to counteract the fatigue-induced reductions in joint stiffness and static reflex gain. The compensatory responses could be partly due to the effects of increasing the number of active motoneurons innervating the fatiguing muscles. This shift in operating point gave rise to significant compensation for the loss of contractile force. The compensation could also be due to fusimotor adjustment, which could make the dynamic reflex gain much less sensitive to fatigue than intrinsic stiffness. In short, the reduced contribution from intrinsic stiffness to joint torque was compensated by increased contribution from dynamic stretch reflexes after fatigue.

Impairment of voluntary control of finger motion following stroke: role of inappropriate muscle coactivation

Subjects with chronic hemiplegia following stroke attempted to perform voluntary isometric, isokinetic, and free contractions of the extensor muscles of the metacarpophalangeal (MCP) joints. We recorded torque, metacarpophalangeal joint angle and velocity, and electromyographic (EMG) activity of the extrinsic extensors and flexors and the first dorsal interosseous (FDI). We found that voluntary MCP joint extension in hemiparetic subjects was greatly impaired in comparison with control subjects: only two of the 11 stroke subjects were able to generate even 0.21 N-m of isometric extension torque, only two could produce positive finger extension with no load, and none could develop an isokinetic concentric extension. Deficits seemed to result from a combination of coactivation of the finger flexor and extensor muscles and decreased voluntary excitation of the extensors, as normalized flexor and FDI EMG activity were greater for stroke than for control subjects (P < 0.001), but normalized extensor activity was reduced (P < 0.001).

Identification of static and dynamic components of reflex sensitivity in spastic elbow flexors using a muscle activation model

Static and dynamic components of the stretch reflex were studied in elbow flexors of 13 hemiparetic brain-injured individuals. Constant-velocity joint rotations were applied to the elbow, and the resulting stretch reflex torque and electromyographic responses were recorded in the biceps brachii and brachioradialis muscles. Ten elbow extension velocities between 6 and 150 deg s(-1) were applied in random order. The resulting reflex torque response was plotted as a function of elbow angle and fitted with a mathematical model designed to depict elbow flexor activation. We found that four of the six model parameters were essentially independent of test velocity. Conversely, 73% (19/26) of cases involving the other two model parameters were dependent on velocity of joint extension (p<0.05). We conclude from these results that four of the model parameters reflect the static reflex response while the two remaining velocity-dependent parameters reflect the dynamic reflex response. To describe overall velocity dependence of stretch reflexes in spastic elbow muscles, the two dynamic reflex parameters were fitted to a fractional exponential function of velocity, similar to a model previously used to describe spindle firing rate in the cat hindlimb. We found that the mean velocity exponent of the dynamic reflex parameters was 0.24 + 0.17 (s.d.) (N = 13), a value similar to that for muscle spindle velocity sensitivity in reduced animal preparations. We conclude that both static and dynamic reflex sensitivities can be measured by examining different aspects of the torque/angle relation associated with the reflex response to a large-amplitude ramp stretch of the elbow.

Damping actions of the neuromuscular system with inertial loads: human flexor pollicis longus muscle

Our previous work in an animal model showed that neuromuscular damping properties help maintain limb posture by effectively dissipating mechanical energy arising from disturbances. The purpose of this study was to determine whether similar damping properties were expressed in intact, normal human muscles. To review briefly, when the reflexively active soleus muscle in a decerebrate cat is coupled to an inertial load, application of a force impulse to the load results in lightly damped oscillations. By calculating the logarithmic decrement in muscle velocity following the impulse (the decrement being related to the amount of energy dissipated from the inertia), we found that damping increased with oscillation amplitude, a nonlinear property. This nonlinearity represents an automatic compensation for larger perturbations. Our findings in parallel experiments on the interphalangeal joint of the human thumb were that the long thumb flexor, the flexor pollicis longus (FPL), displayed mechanical and reflex behavior closely comparable to that reported earlier for the cat soleus, despite differences in architectural and metabolic properties between these muscles. Specifically, by selecting experimental trials that did not include voluntary interventions, we observed amplitude-dependent differences in damping in which larger amplitude movements elicited larger damping than did smaller movements. In addition, even after accounting for amplitude-dependent differences in damping, damping was found to be larger in later cycles than in the first cycle. This nonlinearity indicates that both mechanical properties of muscle and reflex mechanisms are dependent on prior movement history. We propose that this history-dependent behavior arises from the effects of prior movement on stretch reflex gain, and these effects are mediated primarily via changes in muscle spindle properties. Recordings of electromyographic activity from the FPL, during the first and second cycles of oscillation supported this postulate of a reduced reflex gain following prior motion. The functional significance of these nonlinear damping properties is that during the initial muscle stretch, the stiffness is high, which helps to preserve the initial position (although at the expense of promoting oscillation). Subsequently, the ensuing increase in damping helps suppress continuing oscillation. This sequence of varying mechanical properties is broadly analogous to the features of a predictive, or feed-forward controller, designed to produce a response that initially maintains position, and subsequently dampens oscillations. These results show that the intrinsic properties of muscle and spinal reflexes automatically provide a complex time-varying response, appropriate for maintenance of stable limb posture.

Spinal interneurons that receive input from muscle afferents are differentially modulated by dorsolateral descending systems

The possibility that descending systems have differential actions on the spinal interneurons that receive input from muscle afferents was investigated. Prolonged, physiological inputs were generated by stretch of the triceps surae muscles. The resulting firing patterns of 25 lumbosacral interneurons were recorded before and during a reversible cold block of the dorsolateral white matter at the thoracic level in nonparalyzed, decerebrate preparations. The strength of group I muscle afferent input was assessed from the response to sinusoidal tendon vibration, which activated muscle spindle Ia afferents directly and tendon organ Ib afferents via the resulting reflex force. The stretch-evoked responses of interneurons with strong responses to vibration were markedly suppressed by dorsal cold block, whereas the stretch-evoked responses of interneurons with weak vibration input were enhanced. The cells most strongly activated by vibration received their primary input from Ia afferents and all of these cells were inhibited by the cold block. These results suggest that a disruption of the descending system, such as occurs in spinal cord injury, will lead to a suppression of the interneuronal pathways with group Ia input while enhancing excitability within interneuronal pathways transmitting actions from higher threshold afferents. One possible consequence of this suppression would be a decreased activity among the Ia inhibitory interneurons that mediate reciprocal inhibition, resulting in abnormal reciprocal relations between antagonists and promoting anomalous muscle cocontraction.

Limit cycle behavior in spasticity: analysis and evaluation

We examined ankle clonus in four spastic subjects to determine whether this oscillatory behavior has the properties of a limit cycle, and whether it is driven by peripheral sensory input or by a spinal generator. Using Floquet Theory and Poincare sections to assess reflex stability, we found that cycle-to-cycle variability was small, such that the Floquet multipliers were always less than unity. Furthermore, the steady-state periodic orbit was not dependent on the initial position of the ankle. Both of these findings, coupled with strong correlations between the size of the applied load and the frequency of ankle movements and electromyogram burst frequency suggests that clonus behaves as a locally stable limit cycle driven from peripheral receptors. To better understand how nonlinear elements might produce stable oscillatory motion, we simulated the ankle stretch reflex response. We found that delays in the pathway caused the reflex to come on during the shortening phase of movement, so the additional reflex torque required to sustain oscillatory ankle movements was quite small. Furthermore, because the resistance to stretch is largely due to passive mechanics whose properties are quite stationary, the system is robust to small perturbations within the reflex pathway.

End points of planar reaching movements are disrupted by small force pulses: an evaluation of the hypothesis of equifinality

A single force pulse was applied unexpectedly to the arms of five normal human subjects during nonvisually guided planar reaching movements of 10-cm amplitude. The pulse was applied by a powered manipulandum in a direction perpendicular to the motion of the hand, which gripped the manipulandum via a handle at the beginning, at the middle, or toward the end the movement. It was small and brief (10 N, 10 ms), so that it was barely perceptible. We found that the end points of the perturbed motions were systematically different from those of the unperturbed movements. This difference, dubbed "terminal error," averaged 14.4 +/- 9.8% (mean +/- SD) of the movement distance. The terminal error was not necessarily in the direction of the perturbation, although it was affected by it, and it did not decrease significantly with practice. For example, while perturbations involving elbow extension resulted in a statistically significant shift in mean end-point and target-acquisition frequency, the flexion perturbations were not clearly affected. We argue that this error distribution is inconsistent with the "equilibrium point hypothesis" (EPH), which predicts minimal terminal error is determined primarily by the variance in the command signal itself, a property referred to as "equifinality." This property reputedly derives from the "spring-like" properties of muscle and is enhanced by reflexes. To ensure that terminal errors were not due to mid-course voluntary corrections, we only accepted trials in which the final position was already established before such a voluntary response to the perturbation could have begun, that is, in a time interval shorter than the minimum reaction time (RT) for that subject. This RT was estimated for each subject in supplementary experiments in which the subject was instructed to move to a new target if perturbed and to the old target if no perturbation was detected. These RT movements were found to either stop or slow greatly at the original target, then re-accelerate to the new one. The average latency of this second motion was used to estimate the voluntary RT for each subject (316 ms mean). Additionally, we found that the hand neither exerted target-oriented force against the handle nor drifted toward the desired end point just before coming to rest, making it unlikely that the mechanical properties of the manipulandum prevented the hand from reaching its intended target.

Understanding and treating arm movement impairment after chronic brain injury: progress with the ARM guide

Significant potential exists for enhancing physical rehabilitation following neurologic injury through the use of robotic and mechatronic devices (or "rehabilitators"). We review the development of a rehabilitator (the "ARM Guide") to diagnose and treat arm movement impairment following stroke and other brain injuries. As a diagnostic tool, the ARM Guide provides a basis for evaluation of several key motor impairments, including abnormal tone, incoordination, and weakness. As a therapeutic tool, the device provides a means to implement and evaluate active assist therapy for the arm. Initial results with three stroke subjects demonstrate that such therapy can produce quantifiable benefits in the chronic hemiparetic arm. Directions for future research regarding the efficacy and practicality of rehabilitators are discussed.

Persistence of motor adaptation during constrained, multi-joint, arm movements

We studied the stability of changes in motor performance associated with adaptation to a novel dynamic environment during goal-directed movements of the dominant arm. Eleven normal, human subjects made targeted reaching movements in the horizontal plane while holding the handle of a two-joint robotic manipulator. This robot was programmed to generate a novel viscous force field that perturbed the limb perpendicular to the desired direction of movement. Following adaptation to this force field, we sought to determine the relative role of kinematic errors and dynamic criteria in promoting recovery from the adapted state. In particular, we compared kinematic and dynamic measures of performance when kinematic errors were allowed to occur after removal of the viscous fields, or prevented by imposing a simulated, mechanical "channel" on movements. Hand forces recorded at the handle revealed that when kinematic errors were prevented from occurring by the application of the channel, recovery from adaptation to the novel field was much slower compared with when kinematic aftereffects were allowed to take place. In particular, when kinematic errors were prevented, subjects persisted in generating large forces that were unnecessary to generate an accurate reach. The magnitude of these forces decreased slowly over time, at a much slower rate than when subjects were allowed to make kinematic errors. This finding provides strong experimental evidence that both kinematic and dynamic criteria influence motor adaptation, and that kinematic-dependent factors play a dominant role in the rapid loss of adaptation after restoring the original dynamics.

Hyperactive tendon reflexes in spastic multiple sclerosis: measures and mechanisms of action

OBJECTIVE

To develop new measures of tendon reflexes and evaluate hyperactive reflexes in patients with spastic multiple sclerosis (MS).

DESIGN

With the subject relaxed, a hand-held instrumented hammer was used to tap the patellar tendon and record the tapping force, while knee extension torque and quadriceps EMG were recorded isometrically as measures of the reflex response.

SETTING

Research laboratory in a rehabilitation hospital.

SUBJECTS

Ten spastic MS and 14 healthy subjects.

MAIN OUTCOME MEASURES

Tendon tapping force (designated as system input), reflex torque (as output), their dynamic relationship (characterized as system parameters tendon reflex gain, contraction rate, and reflex loop delay), Ashworth scale, and tendon reflex scale.

RESULTS

The system parameters provide more repeatable measures than do input or output parameters alone because they quantify the input and output simultaneously and dynamically. Compared with control subjects, MS patients had a significantly lower threshold in tapping force (p = .026), yet their evoked reflex torque was significantly higher (p = .033). Despite significant quadriceps weakness (p < .0001), MS patients had a significantly higher reflex gain (p = .0002) and contraction rate (p = .0002), and shorter reflex loop delay (p = .0046), indicating hyperexcitability of motoneurons and peripheral receptors, and indicating that relatively more of the muscle was activated reflexively, with greater recruitment of larger fast-twitch fibers. Both the reflex gain and rate measures correlated more closely with the Ashworth scale and tendon reflex scale than did the output measures, indicating their potential clinical value.

CONCLUSIONS

With appropriate simplification, the method may be used in clinical practice to quantify more precisely the tendon jerk than is currently feasible with standard clinical tests.

The use of basis functions in modelling joint articular surfaces: application to the knee joint

This article introduces a new method to represent bone surface geometry for simulations of joint contact. The method uses the inner product of two basis functions to provide a mathematical representation of the joint surfaces. This method guarantees a continuous transition in the direction of the surface normals, an important property for computation of joint contact. Our formulation handles experimental data that are not evenly distributed, a common characteristic of digitized data of musculoskeletal morphologies. The method makes it possible to represent highly curved surfaces, which are encountered in many anatomical structures. The accuracy of this method is demonstrated by modeling the human knee joint. The mean relative percentage error in the representation of the patellar track surface was 0.25% (range 0-1.56%) which corresponded to an absolute error of 0.17mm (range 0-0.16mm).

Quantitative features of the stretch response of extrinsic finger muscles in hemiparetic stroke

Despite its potential importance in hand dysfunction, spasticity in the finger muscles following stroke has not been well described. To explore this area, we assessed the role of finger flexor spasticity, along with that of passive mechanical forces, in resisting finger movement in 13 chronic stroke subjects. Subjects were tested with a device that stretched the extrinsic finger muscles through imposed rotation of the metacarpophalangeal (MCP) joints. Both maintained and constant-velocity stretches were imposed. For the constant-velocity stretches, eight of the 13 stroke subjects exhibited strong stretch reflexes, as determined by electromyography and net work. The net work of this reflex response, calculated from the integral of the torque-angle plots, increased proportionally with increasing velocity, indicating a contribution from flexor muscle spasticity. Conversely, nine of the 13 stroke subjects did not possess distinctly greater passive, mechanical resistance to MCP rotation than control subjects. While extensor spasticity was not observed, stretch of the extrinsic finger flexors also produced some reflex activity in the finger extensors concomitant with reflex excitation of the flexors. These findings suggest that resistance to muscle stretching following stoke is mediated primarily by neurological rather than biomechanical disturbances, although changes in muscle fiber length may exaggerate the resistance.

Flexor reflexes in chronic spinal cord injury triggered by imposed ankle rotation

Hypersensitivity of the flexor reflexes to input from force-sensitive muscle afferents may contribute to the prevalence and severity of muscle spasms in patients with spinal cord injuries. In the present study, we triggered flexor reflexes with constant-velocity ankle movements into end-range dorsiflexion and plantarflexion positions in 8 individuals with spinal cord injuries. We found that all 8 subjects had coordinated increases in flexion torque at the hip and ankle following externally imposed plantarflexion movements at the ankle. In addition, end-range dorsiflexion movements also triggered flexor reflexes in 3 subjects, although greater loads were required to trigger such reflexes using dorsiflexion movements (compared to plantarflexion movements). These three-joint reflex torque patterns triggered by ankle movement were broadly comparable to flexion withdrawal responses elicited by electrocutaneous stimuli applied to a toe, although the amplitude of the torque response was generally lower. We conclude that excitation of muscle and joint-related afferents induced by end-range movements may be responsible for exaggerated flexion reflex responses in spinal cord injury.

Deficits in the coordination of multijoint arm movements in patients with hemiparesis: evidence for disturbed control of limb dynamics

This study provides a detailed analysis of disturbances in the kinematics and dynamics of the acceleration phase of multijoint arm movements in six patients with chronic hemiparesis. Movements of the dominant and nondominant limbs were also examined in three control subjects. Subjects performed rapid movements from a central starting point to 16 targets located equidistantly around the circumference of a circle. Support of the upper limb was provided by an air-bearing apparatus, which allowed very low friction movements in the horizontal plane. We found that patients retained the capacity to modulate, in response to target direction, the initial direction of movements performed with the paretic limb. However, in comparison to the nonparetic limb or control subjects, movements of the paretic limb were misdirected systematically. An inverse dynamics analysis revealed an abnormal spatial tuning of the muscle torque at the elbow used to initiate movements of the paretic limb. Based on electromyographic recordings, similar spatial abnormalities were also apparent in the initial activations of elbow muscles. We argue that these spatial abnormalities result from a systematic disturbance in the control signal to limb muscles that cannot be attributed to previously identified mechanisms such as weakness, spasticity mediated restraint, or stereotypic muscle activation patterns (muscle synergies). Instead, our analysis of movement dynamics and simulation studies demonstrate that the spatial abnormalities are consistent with an impaired feedforward control of the passive interaction torques which arise during multijoint movements. This impaired control is hypothesized to reflect a degradation of the internal representation of limb dynamics that occurs either as a primary consequence of brain injury or secondary to disuse.

Mechatronic assessment of arm impairment after chronic brain injury

Significant potential exists for mechatronic devices to improve assessment and treatment of individuals with a movement disability following stroke, traumatic brain injury, or cerebral palsy. We report the use of a mechatronic device for evaluation of the arm after chronic brain injury. We performed a series of experiments with the device in order to identify the relative contribution of three different motor impairments to decreased active range of motion of reaching in five brain-injured subjects. Our findings were that passive tissue restraint and agonist weakness, rather than antagonist restraint, were the most common contributors to decreased active range of motion. These results demonstrate the feasibility of objective assessment of functional movement using a mechatronic device, and could provide the basis for improved, individualized treatment planning and monitoring following brain injury.

Stretch reflex adaptation in elbow flexors during repeated passive movements in unilateral brain-injured patients

OBJECTIVE

To evaluate the effects of repeated, externally imposed, flexion-extension movements of the elbow on the resulting stretch reflex response in hemiparetic spastic brain-injured patients. These effects were compared within a recording session and across sessions for the same subject to determine the impact of movement history on the quantification of spastic hypertonia using the stretch reflex response.

DESIGN

Twenty to 30 sequential, constant velocity flexion-extension movements were applied to the impaired elbow of our cohort, with a 10-second hold interposed between flexion and extension. Movements were applied regularly at 1-minute intervals. Changes in stretch reflex responses were monitored during the applied movements.

PARTICIPANTS

We examined a convenience sample of seven hemiparetic brain-injured subjects between the ages of 26 and 60 yrs, with moderate-to-severe spastic hypertonia of elbow muscles (Ashworth score 2-4/4). Subjects participated in 2 to 9 sessions.

MEASURES

Elbow torque, position, velocity, and electromyograms of the biceps, brachioradialis, and triceps muscles were recorded for each flexion and extension movement. Stretch reflex torque was calculated by subtracting passive torque from total elbow torque, recorded over large amplitude movements. A linear regression analysis quantified both the initial torque response of the stretch reflex and the ensuing adaptation of the stretch reflex during sequential movements. Intersession variability was characterized both for spastic hypertonia measures and for stretch reflex adaptation.

RESULTS

Repeated, externally imposed, sequential flexion-extension movements of the elbow decreased the elbow flexor stretch reflex in six of seven subjects. The mean reduction in reflex torque after 30 movements was 50% of the initial torque values (p = .001, t test vs. 0% change). Intersession stretch reflex responses for each subject were found to vary greatly (SDs of reflex torque ranged from 0.1 to 4.0 Nm), and there were also significant variations in the degree of adaptation between subjects.

CONCLUSIONS

Stretch reflex adaptation must be taken into consideration when spastic hypertonia is quantified using repeated joint motion, as is often the case. The magnitude of intersession variation in spastic hypertonia measures suggests that ideally, such measurements should be made across multiple sessions before conclusions are made regarding the efficacy of spastic hypertonia interventions. This study provides quantitative evidence that repeated joint movements may have a significant short-term beneficial effect on spastic hypertonia.

Damping actions of the neuromuscular system with inertial loads: soleus muscle of the decerebrate cat

A transient perturbation applied to a limb held in a given posture can induce oscillations. To restore the initial posture, the neuromuscular system must provide damping, which is the dissipation of the mechanical energy imparted by such a perturbation. Despite their importance, damping properties of the neuromuscular system have been poorly characterized. Accordingly, this paper describes the damping characteristics of the neuromuscular system interacting with inertial loads. To quantitatively examine damping, we coupled simulated inertial loads to surgically isolated, reflexively active soleus muscles in decerebrate cats. A simulated force impulse was applied to the load, causing a muscle stretch, which elicited a reflex response. The resulting deviation from the initial position gave rise to oscillations, which decayed progressively. Damping provided by the neuromuscular system was then calculated from the load kinetics. To help interpret our experimental results, we compared our kinetic measurements with those of an analogous linear viscoelastic system and found that the experimental damping properties differed in two respects. First, the amount of damping was greater for large oscillation amplitudes than for small (damping is independent of amplitude in a linear system). Second, plots of force against length during the induced movements showed that damping was greater for shortening than lengthening movements, reflecting greater effective viscosity during shortening. This again is different from the behavior of a linear system, in which damping effects would be symmetrical. This asymmetric and nonlinear damping behavior appears to be related to both the intrinsic nonlinear mechanical properties of the soleus muscle and to stretch reflex properties. The muscle nonlinearities include a change in muscle force-generating capacity induced by forced lengthening, akin to muscle yield, and the nonlinear force-velocity property of muscle, which is different for lengthening versus shortening. Stretch reflex responses are also known to be asymmetric and amplitude dependent. The finding that damping is greater for larger amplitude motion represents a form of automatic gain adjustment to a larger perturbation. In contrast, because of reduced damping at small amplitudes, smaller oscillations would tend to persist, perhaps contributing to normal or "physiological" tremor. This lack of damping for small amplitudes may represent an acceptable compromise for postural regulation in that there is substantial damping for larger movements, where energy dissipation is more critical. Finally, the directional asymmetry in energy dissipation provided by muscle and reflex properties must be reflected in the neural mechanisms for a stable posture.

Control strategies for the transition from multijoint to single-joint arm movements studied using a simple mechanical constraint

Changes were studied in neuromotor control that were evoked by constraining the motion of the elbow joint during planar, supported movements of the dominant arm in eight normal human subjects. Electromyograph (EMG) recordings from shoulder and arm muscles were used to determine whether the normal multijoint muscle activity patterns associated with reaching to a visual target were modified when the movement was reduced to a single-joint task, by pinning the elbow to a particular location in the planar work space. Three blocks of 150 movements each were used in the experiments. Subjects were presented with the unconstrained task in the first and third blocks with an intervening block of constrained trials. Kinematic, dynamic, and EMG measures of performance were compared across blocks. The imposition of the pin constraint caused predictable changes in kinematic performance, in that near-linear motions of the hand became curved. This was followed by changes in limb dynamic performance at the elbow. However, changes in EMG activity at the shoulder lagged the kinematic changes substantially (by about 15 trials). The gradual character of the changes in EMG timing does not support a primary role for segmental reflex action in mediating the transition between multijoint and single-joint control strategies. Furthermore, the scope and magnitude of these changes argues against the notion that human motor performance is driven by the optimization of muscle- or joint-related criteria alone. The findings are best described as reflecting the actions of a feedforward adaptive controller that has properties that are modified progressively according to the environmental state.

Reflex torque response to movement of the spastic elbow: theoretical analyses and implications for quantification of spasticity

A parametric model of the human reflex torque response to a large-amplitude, constant angular velocity elbow extension was developed in order to help quantify spasticity in hemiparetic stroke patients, and to better understand its pathophysiology. The model accounted for the routinely observed leveling of torque (i.e., a plateau) at a mean angular increment of 51 degrees +/- 10 degrees s.d. (n = 98) after the initial rise. This torque "plateau" was observed in all eight subjects, and in 98 of 125 trials across 25 experimental sessions. The occurrence of this plateau cannot be explained by decreases in elbow flexor moment arms during elbow extension. Rather, the plateau is attributable to a consistent leveling in muscle activation as confirmed both qualitatively from recordings of rectified, smoothed electromyograph (EMG) activity, and quantitatively using an EMG coefficient model. A parametric model was developed in which the pattern of muscle activation in the stretch reflex response of elbow flexors was described as a cumulative normal distribution with respect to joint angle. Two activation functions, one related to biceps and the other to brachioradialis/brachialis, were incorporated into the model in order to account for observations of a bimodal angular stiffness profile. The resulting model yielded biologically plausible parameters of the stretch reflex response which may prove useful for quantifying spasticity. In addition, the model parameters had clear pathophysiological analogs, which may help us understand the nature of the stretch reflex response in spastic muscles.

Assessment of active and passive restraint during guided reaching after chronic brain injury

We report the use of a mechatronic device for assessing arm movement impairment after chronic brain injury. The device, called the "Assisted Rehabilitation and Measurement Guide," is designed to guide reaching movements across the workspace, to measure movement and force generation, and to apply controlled forces to the arm along linear reaching paths. We performed a series of experiments using the device in order to identify the contribution of active muscle and passive tissue restraint to decreased active range of motion of guided reaching (i.e., "workspace deficits") in a group of five chronic, spastic hemiparetic, brain-injured subjects. Our findings were that passive tissue restraint was increased in the spastic arms, as compared to the contralateral, nonparetic arms. Active muscle restraint, on the other hand, was typically comparable in the two arms, as quantified by measurements of active arm stiffness at the workspace boundary during reaching. In all subjects, there was evidence of movement-generated weakness, consistent with a small contribution of spasticity to workspace deficits. These results demonstrate the feasibility of mechatronic assessment of the causes of decreased functional movement, and could provide a basis for enhanced treatment planning and monitoring following brain injury.

Reorganization of flexion reflexes in the upper extremity of hemiparetic subjects

We examined spatiotemporal abnormalities in the flexor reflex response in the impaired upper extremity of hemiparetic subjects. Electrical stimulation was used to elicit flexion reflexes in both upper extremities of 8 hemiparetic brain-injured and 6 control subjects. Electromyograms (EMGs) were recorded from 12 arm muscles, and reflex forces and moments were recorded at the wrist with a load cell, and converted to shoulder and elbow torques. We found that the onset of reflex torque and EMG was delayed in the impaired arm and delays were greater at the shoulder than at the elbow. The normal reflex torque response consisted of elbow flexion, shoulder extension, and shoulder adduction. In contrast, in the impaired limb shoulder, flexion torque was observed in 7 subjects and shoulder abduction in 3. The delays in reflex onset and altered torque patterns in the impaired arm may be related to the abnormal movement synergies observed following stroke. &copy 1999 John Wiley & Sons, Inc.

A simulation study of reflex instability in spasticity: origins of clonus

Clonus is defined as an involuntary rhythmic muscle contraction that generally occurs in people who have sustained lesions involving descending motor pathways in the neuraxis, and is usually accompanied by other signs of reflex hyperexcitability such as spasticity. This paper hypothesizes that clonus arises when two conditions occur simultaneously: 1) the reflex pathway contains long delay times (implying innervation of distal limb muscles, exacerbated when these muscles display slow twitch properties) and 2) the excitability of the motoneurons is enhanced. This paper tested this dual hypothesis by developing a computer model representing the ankle reflex pathway. This model included the ankle muscles, afferent and efferent pathways, and a monosynaptic spinal link between spindle afferents and motoneurons. Simulations show that as the motoneuron current threshold was reduced (reflecting increased excitability of spinal motoneurons), normal reflex responses became unstable and oscillations developed similar to those observed in spastic patients. In parallel, when we choose reflex delay times typical for distal leg muscles in man, system stability is poor, and oscillations occur readily with increasing motoneuron excitability. As simulated pathway delays are reduced, oscillatory behavior is also reduced, and usually damps out. Conversely, as simulated reflex delays are increased, oscillations increase in amplitude and do not decay. Finally, these two phenomena interact, so that increasing motoneuron excitability will induce reflex oscillations for intermediate loop delays. These findings support the hypothesis that unstable oscillatory behavior, such as the oscillations observed in clonus, will occur when the motoneuron excitability increases in a reflex pathway containing long delays. This change in excitability is mediated by a reduction in motoneuron firing threshold, rather than by an increase in feedback gain. Furthermore, we demonstrate that sustained oscillations occur readily through self reexcitation, which reduces the need to propose that a "central oscillator" must be involved in generating clonus.

System identification of tendon reflex dynamics

Patellar tendon reflexes were evaluated in 12 healthy adult subjects using several measures of the reflex responses and of the system input-output relationship. A hand-held instrumented hammer was used to tap the patellar tendon and to elicit the reflex response. Tendon reflex dynamics were estimated using the recorded tapping force (as input) and the quadriceps muscle electromyogram and knee joint extension torque signals (as output). A dome-shaped rubber pad was mounted onto the most sensitive spot on the patellar tendon, where it served as a tapping target, and helped to reduce the reflex variability significantly (p < 0.01). The input-output properties of the system relating the reflex torque to the tapping force were characterized using several measures: the tendon reflex gain (Gtr), contraction rate (Rc), and half-relaxation rate (Rhr). Reflex loop delay (t(d)) was estimated using the delay from the onset of tapping force to the onset of reflex torque. We determined that these system parameters provided significantly more repeatable and consistent characterization of tendon reflexes than did reflexive torque or EMG signals alone (p < 0.025). The input-output relationship relating the EMG signals of the stretched muscle to the tapping force was also identified to help characterize neuromuscular dynamics of tendon reflexes. The observed sensitivity and consistency of the reflex system measures suggest that with appropriate simplification of the instrumentation, these methods may prove useful in routine clinical practice, and may allow more precise quantification of the tendon jerk than is currently feasible with standard clinical tests.

Guidance-based quantification of arm impairment following brain injury: a pilot study

This paper reports the design and preliminary testing of a device for evaluating arm impairment after brain injury. The assisted rehabilitation and measurement (ARM) Guide is capable of mechanically guiding reaching and retrieval movements across the workspace and of measuring constraint forces and range of motion during guidance. We tested the device on four hemiplegic brain-injured individuals and four unimpaired control subjects. During guided movement, the brain-injured subjects generated distinct spatial patterns of constraint force with their impaired arms that were consistent with the standard flexion and extension "synergies" described in the clinical literature. In addition, the impaired arms exhibited well-defined workspace deficits as measured by the ARM Guide. These results suggest that constraint force and range of motion measurements during mechanically guided movement may prove useful for precise monitoring of arm impairment and of the effects of treatment techniques targeted at abnormal synergies and workspace deficits.

Damping in reflexively active and areflexive lengthening muscle evaluated with inertial loads

Damping in reflexively active and areflexive lengthening muscle evaluated with inertial loads. J. Neurophysiol. 80: 3369-3372, 1998. Studies of active areflexive muscle have shown that during a constant velocity stretch the increment in force elicited by an incremental length change falls dramatically after a few hundred micrometers of stretch, a finding labeled as "muscle yield." The mechanical behavior after the yield was like a viscous damper, in that force varied only with velocity. In light of these observations, our aims were to determine whether viscous properties are also evident under more physiological conditions, specifically under inertial loading, and to evaluate the damping action of reflexively intact compared with that of deafferented muscle. The active soleus muscle in a decerebrate cat was forcibly stretched by a simulated inertia with a specified initial velocity. We compared muscle length changes when afferent pathways were intact with those recorded after cutting the dorsal roots. Our findings were that areflexive muscle showed highly damped responses, with large changes in mean muscle length, indicative of high viscosity relative to stiffness. In contrast, reflexively active muscle produced lightly damped oscillations, with minimal changes in mean length, reflecting low viscosity and high stiffness. It appears that the stretch reflect modifies the relative contributions of elastic and viscous-like forces, maintaining elasticity, which in turn sustains oscillations. These differences highlight tradeoffs between positional and velocity regulation, in that elastic properties of reflexively active muscle promote oscillations with modest change in mean muscle length, whereas viscous-like properties of areflexive muscle produce damped responses, with poor positional regulation.