Biomedical subjects

T George Hornby

Publications and source records attributed to T George Hornby.

16 recordsLinked to original sources

PubMed2006

Temporal facilitation of spastic stretch reflexes following human spinal cord injury.

Recent evidence suggests that alterations in ionic conductances in spinal motoneurones, specifically the manifestation of persistent inward currents, may be partly responsible for the appearance of hyperexcitable reflexes following spinal cord injury (SCI). We hypothesized that such alterations would manifest as temporal facilitation of stretch reflexes in human SCI. Controlled, triangular wave, ankle joint rotations applied at variable velocities (30-120 deg s(-1)) and intervals between stretches (0.25-5.0 s) were performed on 14 SCI subjects with velocity-dependent, hyperexcitable plantarflexors. Repeated stretch elicited significant increases in plantarflexion torques and electromyographic (EMG) activity from the soleus (SOL) and medial gastrocnemius (MG). At higher velocities (> or = 90 deg s(-1)), reflex torques declined initially, but subsequently increased to levels exceeding the initial response, while mean EMG responses increased throughout the joint perturbations. At lower velocities (< or = 60 deg s(-1)), both joint torques and EMGs increased gradually. Throughout a range of angular velocities, reflex responses increased significantly only at intervals < or = 1 s between stretches and following at least four rotations. Ramp-and-hold perturbations used to elicit tonic stretch reflexes revealed significantly prolonged EMG responses following one or two triangular stretches, as compared to single ramp-and-hold excursions. Post hoc analyses revealed reduced reflex facilitation in subjects using baclofen to control spastic behaviours. Evidence of stretch reflex facilitation post-SCI may reflect changes in underlying neuronal properties and provide insight into the mechanisms underlying spastic reflexes.

Adult↗

PubMed2006

Hip joint position modulates volitional knee extensor muscle activity after stroke.

Evidence from animal and human models has demonstrated the importance of hip proprioceptors and vestibular inputs in modulating lower-extremity muscle activity through reflex pathways. Comprehension of the role of these sensory inputs following stroke may be important in understanding pathological muscle activity during functional activities. We therefore examined the influence of both hip and head/trunk position on volitional quadriceps activity in chronic stroke and control subjects. With the knee held at 60 degrees, maximal voluntary isometric quadriceps contractions were elicited with trunk orientation (head position) and hip angle systematically positioned at 0 degrees, 45 degrees, and 90 degrees. Integrated electromyographic activity from the quadriceps was compared between groups and conditions. Vasti activity in the stroke group was greater in a seated upright posture (hip flexed) than supine (hip neutral). Controlling for vestibular input, the stroke group demonstrated greater quadriceps activity (VL and RF) with a neutral hip compared to flexion. Such findings may have implications for understanding inappropriate muscle activity during walking after stroke, as hip extension occurs immediately prior to toe off, when inappropriate quadriceps activity is commonly observed.

Adult↗

PubMed2006

Metabolic costs and muscle activity patterns during robotic- and therapist-assisted treadmill walking in individuals with incomplete spinal cord injury.

BACKGROUND AND PURPOSE: Robotic devices that provide passive guidance and stabilization of the legs and trunk during treadmill stepping may increase the delivery of locomotor training to subjects with neurological injury. Lower-extremity guidance also may reduce voluntary muscle activity as compared with compliant assistance provided by therapists. The purpose of this study was to investigate differences in metabolic costs and lower-limb muscle activity patterns during robotic- and therapist-assisted treadmill walking. SUBJECTS: Twelve ambulatory subjects with motor incomplete spinal cord injury participated. METHODS: In 2 separate protocols, metabolic and electromyographic (EMG) data were collected during standing and stepping on a treadmill with therapist and robotic assistance. During robotic-assisted walking, subjects were asked to match the kinematic trajectories of the device and maximize their effort. During therapist-assisted walking, subjects walked on the treadmill with manual assistance provided as necessary. RESULTS: Metabolic costs and swing-phase hip flexor EMG activity were significantly lower when subjects were asked to match the robotic device trajectories than with therapist-assisted walking. These differences were reduced when subjects were asked to maximize their effort during robotic-assisted stepping, although swing-phase plantar-flexor EMG activity was increased. In addition, during standing prior to therapist- or robotic-assisted stepping, metabolic costs were higher without stabilization from the robotic device. DISCUSSION AND CONCLUSION: Differences in metabolic costs and muscle activity patterns between therapist- and robotic-assisted standing and stepping illustrate the importance of minimizing passive guidance and stabilization provided during step training protocols.

Adolescent↗

PubMed2005

Flexor reflex responses triggered by imposed knee extension in chronic human spinal cord injury.

Hypersensitivity of the flexor reflex pathways to input from force-sensitive muscle afferents may contribute to the prevalence and severity of muscle spasms in patients with spinal cord injury (SCI). In this study, we triggered flexor reflexes with constant velocity knee movements in 15 subjects with SCI. Ramp and hold knee extension perturbations were imposed on one leg while the hip and ankle were held in an isometric position using an instrumented leg brace. Knee, ankle and hip torque responses and electromyograms from six muscles of the leg were recorded following controlled knee extension at four different velocities. Tests were conducted with the hip in both flexed and extended positions. During the movement into knee extension, a velocity-dependent stretch reflex, represented by a progressively increasing knee flexion torque, was observed. In addition, another type of reflex that resembled a flexor reflex (flexion of the hip and ankle) was also triggered by the imposed knee extension. The magnitude of the ankle dorsiflexion torque responses was significantly correlated to the stretch reflex torque at the knee in 9 of the 15 subjects. We concluded that stretch reflexes initiate a muscle contraction that then can contribute to a flexor reflex response, possibly through muscle group III/IV afferent pathways. These results suggest that spasticity in SCI consists of a myriad of complex reflex responses that extend beyond stretch reflexes.

Adult↗

PubMed2005

Stimulation-induced changes in lower limb corticomotor excitability during treadmill walking in humans.

Magnetic stimulation of human primary motor cortex (M1) paired with electrical stimulation of a peripheral motor nerve has been used to produce a lasting modulation of corticomotor (CM) excitability. This 'paired associative stimulation' (PAS) protocol has been used to induce bidirectional changes in excitability in upper limb CM pathways. The present study tested the hypothesis that temporally dependent PAS applied to the common peroneal nerve during the swing phase of walking would induce bidirectional changes in CM excitability consistent with the Hebbian principle of activity-dependent plasticity. Fourteen subjects with no known neurological disorder participated in two data collection sessions each. PAS was delivered as a single block of 120 pairs of stimuli delivered in a 10 min period during treadmill walking at 4.0 km h(-1). Changes in CM excitability were assessed by examining the size of motor potentials evoked by transcranial magnetic stimulation prior to and following PAS. Tibialis anterior motor-evoked potentials amplitude increased to 121% over baseline when the magnetic stimulus was delivered over M1 after the estimated arrival time of the afferent volley in sensorimotor cortex and decreased to 83% of baseline when the magnetic stimulus was delivered prior to the estimated afferent volley arrival. This extent of modulation was undiminished following a further 10 min period of walking without stimulation. The temporal nature of the bidirectional effects following PAS, their rapid evolution and subsequent persistence supported the study's hypothesis and were similar to the effects described by others in quiescent muscles of the upper limb.

Adult↗

PubMed2005

The intralimb coordination of the flexor reflex response is altered in chronic human spinal cord injury.

The current study compared the intralimb coordination of flexor reflex responses in spinal intact and complete chronic spinal cord injured (SCI) individuals. Noxious electrocutaneous stimulation was applied at the apex of the medial arch of the foot (50 mA, 500 Hz, 1 ms pulse width, 20 ms) in 21 complete chronic SCI and 19 spinal intact volunteers and the flexor reflex response was quantified by measuring the isometric joint torques at the ankle, knee and hip. The results showed that SCI individuals had significantly smaller peak knee and hip joint flexion torques, often exhibited a net knee extension torque, and produced a much smaller hip joint flexion torque during the flexor reflex response in contrast to the spinal intact individuals. The latency of the reflex response, measured from the tibialis anterior electromyogram, was comparable in both test populations. These findings indicate that the intralimb coordination of the flexor reflex response of chronic complete SCI individuals is altered, possibly reflecting a functional reorganization of the flexion pathways of the spinal cord.

Chronic Disease↗

PubMed2005

A physiologically based clinical measure for spastic reflexes in spinal cord injury.

OBJECTIVE: To test the validity of the Spinal Cord Assessment Tool for Spastic reflexes (SCATS), a clinical tool intended to rate spastic motor behavior after spinal cord injury (SCI). DESIGN: By using correlational analyses, the SCATS was validated using concurrent measurements of kinematics and electromyograms and traditional assessments of spasms and spastic hypertonia. SETTING: Research laboratory (kinematics and electromyography) and outpatient medical clinic (traditional measures of spastic hypertonia). PARTICIPANTS: Eleven people with SCI were used for kinematic and electromyographic measurements. Seventeen people with SCI were used for comparison with other clinical scales. INTERVENTIONS: Not applicable. Main outcome measures Kinematic and surface electromyographic measurements of the tested lower extremity were used to quantify magnitude and/or duration of motor behaviors, and the Penn Spasm Frequency Scale (PSFS) and the Ashworth Scale were used to measure spasm frequency and resistance to joint movement for the hip flexors, knee flexors, and ankle plantarflexors, respectively. Concurrently, the SCATS was used to assess the clonus response to an imposed ankle dorsiflexion, the flexion response to a stimulus to the foot, and the knee extensor activity in response to an imposed leg extension. Each component of the SCATS was compared with the Ashworth Scale, the PSFS, and kinematic and electromyographic measurements by using the Spearman rank correlation test. RESULTS: Clonus, flexor spasm, and extensor spasm responses measured by using the SCATS correlated significantly with kinematic and electromyographic recordings (P<.01). Significant correlations were also observed between the SCATS extensor spasms and the Ashworth scores for hip and knee flexors and for ankle plantarflexors (rho=.98, .88, .61, respectively). Also, SCATS flexor spasms and SCATS clonus scores correlated significantly with some of the Ashworth scores. Only SCATS clonus scores correlated significantly with spasm frequency measures (rho=.59, P<.05). CONCLUSIONS: The SCATS produced a valid measure of 3 distinct types of spastic motor behaviors in SCI and may provide a complementary tool for measuring spastic hypertonia. Such a measure is valuable because current assessment tools do not differentiate between the different types of spastic motor behaviors that manifest after SCI. Distinguishing the 3 spastic reactions using an efficient and valid clinical tool may help guide management of spastic hypertonia in SCI.

Adolescent↗

PubMed2005

Effectiveness of automated locomotor training in patients with chronic incomplete spinal cord injury: a multicenter trial.

OBJECTIVE: To determine whether automated locomotor training with a driven-gait orthosis (DGO) can increase functional mobility in people with chronic, motor incomplete spinal cord injury (SCI). DESIGN: Repeated assessment of the same patients or single-case experimental A-B design. SETTING: Research units of rehabilitation hospitals in Chicago; Heidelberg, Germany; and Basel and Zurich, Switzerland. PARTICIPANTS: Twenty patients with a chronic (>2 y postinjury), motor incomplete SCI, classified by the American Spinal Injury Association (ASIA) Impairment Scale with ASIA grades C (n=9) and D (n=11) injury. Most patients (n=16) were ambulatory before locomotor training. INTERVENTION: Locomotor training was provided using robotic-assisted, body-weight-supported treadmill training 3 to 5 times a week over 8 weeks. Single training sessions lasted up to 45 minutes of total walking time, with gait speed between .42 and .69 m/s and body-weight unloading as low as possible (mean +/- standard deviation, 37%+/-17%). MAIN OUTCOME MEASURES: Primary outcome measures included the 10-meter walk test, the 6-minute walk test, the Timed Up & Go test, and the Walking Index for Spinal Cord Injury-II tests. Secondary measures included lower-extremity motor scores and spastic motor behaviors to assess their potential contribution to changes in locomotor function. All subjects were tested before, during, and after training. RESULTS: Locomotor training using the DGO resulted in significant improvements in the subjects' gait velocity, endurance, and performance of functional tasks. There were no significant changes in the requirement of walking aids, orthoses, or external physical assistance. There was no correlation between improvements in walking speed or changes in muscle strength or spastic motor behaviors. CONCLUSIONS: Intensive locomotor training on a treadmill with the assistance of a DGO results in improved overground walking.

Adolescent↗

PubMed2005

Robotic-assisted, body-weight-supported treadmill training in individuals following motor incomplete spinal cord injury.

BACKGROUND AND PURPOSE: Performance of therapist-assisted, body-weight-supported treadmill training (BWSTT) to enhance walking ability of people with neurological injury is an area of intense research. Its application in the clinical setting, however, is limited by the personnel and labor requirements placed on physical therapists. Recent development of motorized ("robotic") rehabilitative devices that provide assistance during stepping may improve delivery of BWSTT. CASE DESCRIPTION: This case report describes the use of a robotic device to enhance motor recovery and ambulation in 3 people following motor incomplete spinal cord injury. INTERVENTIONS: Changes in motor impairment, functional limitations, and locomotor disability were monitored weekly during robotic-assisted BWSTT and following transition to therapist-assisted BWSTT with the assistance of one therapist. OUTCOMES: Following this training, 2 patients recovered independent over-ground walking and another improved his gait speed and endurance. DISCUSSION: The use of robotic devices may assist physical therapists by providing task-specific practice of stepping in people following neurological injury.

Adolescent↗

PubMed2004

Extensor spasms triggered by imposed knee extension in chronic human spinal cord injury.

Extensor spasms, which are a significant component of spasticity in spinal cord injury (SCI), are still incompletely understood. In this study, contributions of knee proprioceptors to the origination of extensor spasms were examined in fifteen subjects with SCI. Ramp and hold knee extension perturbations were imposed to one leg while the hip and ankle were held in an isometric position using an instrumented leg brace. Isometric joint torques of knee, ankle and hip, and electromyograms (EMGs) from six muscles of the leg were recorded following controlled knee extension at four different velocities. Tests were conducted with the hip in both flexed and extended positions. A stereotypical torque response consisting of hip flexion, knee extension, and ankle plantar flexion was observed following knee perturbations, although not all components were demonstrated in every subject. During the hold periods with the knee extended, EMG activity recorded from the vastus medialis, medial gastrocnemius and rectus femoris demonstrated patterns consistent with clinical observations of extensor spasms. Furthermore, larger responses were observed with the hip in the extended vs. flexed position (p<0.05). Such behaviors emphasize the role of knee and hip proprioceptors in the initiation of extensor spasms in human SCI. This knowledge may be especially helpful in identifying rehabilitation strategies for producing functional movements in human SCI.

Adult↗

PubMed2004

Changes in voluntary torque and electromyographic activity following oral baclofen.

The consequences of baclofen intake on voluntary motor behaviors remain unclear. We studied the effects of single oral doses of baclofen on voluntary, isometric knee extension torques and surface and single motor unit (MU) electromyographic (EMG) activity from the vastus lateralis in 11 individuals without neurological injury. Examination of submaximal to maximal contractions of varying duration performed pre- and post-baclofen ingestion revealed significant decreases in maximal knee torques and EMG magnitude, accompanied by an increase in slope of the torque-EMG relation. A decreased slope of the torque-MU firing rate relation was also demonstrated post-baclofen, but without changes in minimal firing rates or recruitment forces. During sustained contractions at < or =25% of maximal voluntary torque elicited after baclofen ingestion, increased EMG activity was observed without significant differences in MU firing rates. Our results demonstrate a clear reduction in the maximal torque-generating ability following baclofen. Specific changes in MU firing patterns indicate that weakness may be due partly to reduced motoneuronal excitability, although use of MU discharge patterns to assess these effects is limited in its sensitivity.

Administration, Oral↗

PubMed2003

Absence of local sign withdrawal in chronic human spinal cord injury.

Local sign withdrawal, a reflex to direct the limb away from noxious cutaneous stimuli, is thought to be indicative of a modular organization of the spinal cord. To assess the integrity of such an organization of the spinal cord in chronic human spinal cord injury (SCI), we tested the electromyogram (EMG) and joint torque responses to cutaneous stimuli applied to 6 locations of the leg in 10 SCI volunteers and 3 spinal-intact controls. The 6 locations included the medial arch of the foot, the second metatarsal, the dorsum, the region over the sural nerve at the lateral malleolus, and the anterior and posterior aspects of the lower leg. Although spinal-intact subjects demonstrated local sign withdrawal, the data from SCI subjects indicated that an invariant flexion response pattern was produced regardless of stimulus location. Ankle dorsiflexion and hip flexion were produced in all subjects at all locations and no difference in the ratio of hip:ankle torques could be detected for the 6 test locations. A windup-crossover test, employing a sequence of 6 stimuli at 1-s intervals was used to assess whether common neuronal pathways were responsible for the loss of modular organization. An additional 10 SCI volunteers were tested using stimuli in which the stimulus location was switched between the 2nd and 3rd stimulus of the test sequence. The response to the crossover stimulus more closely resembled the response to the 3rd stimulus of a windup sequence than a response without conditioning stimuli. These results indicate that increased excitability produced by windup at one stimulus site is maintained at the 2nd site. This observation suggests that deep dorsal horn neurons, typically associated with musculotopic mapping, may be reorganized in chronic spinal cord injury.

Adult↗

PubMed2002

Associations between the morphology and physiology of ventral-horn neurons in the adult turtle.

This study compared some morphologic and physiological properties of adult turtle spinal motoneurons (MNs) vs. interneurons (INs). Reconstructions were made of 20 biocytin-stained cells, which had been previously studied physiologically in 2-mm-thick slices of lumbosacral spinal cord. The intracellularly measured physiological properties included resting potential, input resistance (R(N)), threshold (rheobase, I(Rh)), and slope of the stimulus current (I) -spike frequency (f) relation. The seven morphologic properties that were quantified for each cell included three indices of somal size (diameter, area, volume), and four of dendritic size: the number of first- and last-order branches, rostrocaudal extent, and sigma individual lengths. Significant differences were shown between all seven morphologic parameters for MNs vs. INs. Despite the small sample size, significant differences were also shown for five of seven parameters for high-threshold vs. low-threshold MNs, and three of seven for low-threshold MNs vs. INs. These latter three parameters were the number of terminal dendritic branches, their rostrocaudal extent, and the sigma dendritic lengths. Linear associations for the MN + IN and the MN samples were stronger between the four dendritic parameters than between soma-dendritic ones. Exponential associations between morphologic and physiological properties were mostly significant (28 of 30), and their strength was in the order I(Rh) < R(N) < f/I slope for the MN +IN sample and I(Rh) < R(N) = f/I slope for the MN sample. There is discussion of the relevance of the above findings to the provisional classification of turtle ventral-horn neurons on the basis of electrophysiology alone.

Action Potentials↗

PubMed2002

Motoneurons: A preferred firing range across vertebrate species?

The term "preferred firing range" describes a pattern of human motor unit (MU) unitary discharge during a voluntary contraction in which the profile of the spike-frequency of the MU's compound action potential is dissociated from the profile of the presumed depolarizing pressure exerted on the unit's spinal motoneuron (MN). Such a dissociation has recently been attributed by inference to the presence of a plateau potential (PP) in the active MN. This inference is supported by the qualitative similarities between the firing pattern of human MUs during selected types of relatively brief muscle contraction and that of intracellularly stimulated, PP-generating cat MNs in a decerebrate preparation, and turtle MNs in an in vitro slice of spinal cord. There are also similarities between the stimulus-response behavior of intracellularly stimulated turtle MNs and human MUs during the elaboration of a slowly rising voluntary contraction. This review emphasizes that there are a variety of open issues concerning the PP. Nonetheless, a rapidly growing body of comparative vertebrate evidence supports the idea that the PP and other forms of non-linear MN behavior play a major role in the regulation of muscle force, from the lamprey to the human.

Animals↗

PubMed2002

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.

Adult↗

PubMed2002

Effects of excitatory modulation on intrinsic properties of turtle motoneurons.

The purpose of this study was to quantify the effects of excitatory modulation on the intrinsic properties of motoneurons (MNs) in slices of spinal cord taken from the adult turtle. Responses were noted following application of an excitatory modulator: serotonin (5-HT), muscarine, trans-1-amino-1,3-cyclopentane dicarboxylic acid (tACPD), or all three combined. A sample of 44 MNs was divided into 2 groups, on the basis of whether MNs did (28/44) or did not (16/44) demonstrate a nifedipine-sensitive acceleration of discharge during a 2-s, intracellularly injected stimulus pulse. Such acceleration indicates the development of a plateau potential (PP). Excitatory modulation lowered the MNs' resting potential, increased input resistance, decreased rheobase, reduced several afterhyperpolarization values, and shifted the conventional, one-phase stimulus current-spike frequency (I-f) relation to the left. For both MN groups, the relative efficacy of excitatory modulation on both non-PP and PP MNs was generally in the following order: combined application > 5-HT approximately muscarine > tACPD. In many instances, the effects of modulation differed significantly for non-PP versus PP MNs, the most pronounced being in their I-f relation. To describe this difference, it was necessary to measure a two-phase relation. In PP MNs, excitatory modulation considerably increased the slope of the first (initial) phase and flattened the second (later) phase of this relation. The latter result bore similarities to that obtained in a previous study, which addressed MN firing behavior during fictive locomotion of the high-decerebrate cat.

Animals↗