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Biomedical subjects

T Sinkjaer

Publications and source records attributed to T Sinkjaer.

At least 73 records · Page 4Linked to original sources
PubMed1997

Restoration of lateral hand grasp using natural sensors.

A closed-loop control system for controlling the key grip of a C6 tetraplegic patient was developed. Natural sensors served as the source of the feedback signal. The neural signals from cutaneous receptors were picked up by an implanted cuff electrode placed around the radial branch of the median nerve innervating the lateral part of the index finger. Mechanical stress applied to the skin, like pressure and slips, resulted in an increase in amplitude of the recorded neural signal. The goal of the study was to determine whether the recorded neural signals were able to indicate the slip of an object during lateral grasp and whether the slip could be stopped by increasing the grasp force through functional electrical stimulation of the thumb adductor and flexor.

Action Potentials↗
PubMed1997

Long-lasting depression of soleus motoneurons excitability following repetitive magnetic stimuli of the spinal cord in multiple sclerosis patients.

The effect of repetitive magnetic stimulation at the spinal level on the soleus H-reflex amplitude was evaluated in II MS patients with lower limb spasticity and in nine healthy subjects. In MS patients stimulation with a train of 16 stimuli at 25 Hz induced a decrease in amplitude to 61.2 +/- 25.7% of the unconditioned H-reflex amplitude at interstimulus interval (ISI) of 10-1000 ms (P < 0.01). The amount of decrease in H-reflex amplitude was highly dependent on the stimulation intensity and the placement of the coil, and to a lesser extent influenced by the stimulation frequency. No decrease in motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation was seen following trains of 16 stimuli at mid-thoracic in contrast to the post-stimulation depression in H-reflex amplitude which could imply that mechanisms acting at presynaptic level were involved. In response to repetitive magnetic stimuli for 5 min, a long-lasting decrease in H-reflex amplitude to a level of about 70% of the pre-stimulation H-reflex amplitude occurred in MS patients (P < 0.01). A similar although not significant decrease was observed in healthy subjects. We propose that long-lasting depression of the soleus H-reflex amplitude after repetitive magnetic stimuli is due to long-term depression of the synaptic transmission.

Adult↗
PubMed1997

Effect of long-term implanted nerve cuff electrodes on the electrophysiological properties of human sensory nerves.

During a long-term implantation (307 days) of a tripolar split cuff electrode around the palmar digital nerve to the radial side of the left index finger, branching off the median nerve in a medullary lesioned C6 patient, the physiological state of the nerve was intensively monitored. The resulting sensory nerve action potential (SNAP) amplitude was recorded, using both near-nerve electrodes and the implanted cuff electrode. The SNAP amplitude declined within 10 days to approximately 50% of the first SNAP cuff amplitude measured on Day 2 after implantation and recovered to the initial amplitude within 3 months. The SNAP amplitude measurements made with near-nerve electrodes were consistent with the cuff results; the SNAP conduction velocity (CV) recorded by the near-nerve electrodes and the cuff electrode was constant during the whole implantation period. This is in agreement with the results from two other patients: one with a cuff implanted around the sural nerve, and the other with a cuff implanted around a branch of the tibial nerve. These results and animals studies show that the cuff electrode is an electrically stable neural-electrical transducer.

Action Potentials↗
PubMed1997

Muscle, reflex and central components in the control of the ankle joint in healthy and spastic man.

In understanding the control of the ankle joint during different motor tasks, we have to investigate at least three components, namely the influence of i) the passive and intrinsic properties of the intact and active muscle system around the joint (termed the non-reflex component), ii) the mechanical importance of the stretch reflex in the stretched and unloaded muscles, and iii) the supraspinal control of the stretch reflex. This thesis is dealing with the importance of the three components in healthy and spastic persons during sitting, standing, and walking. The results are based on stretch reflex and H-reflex measurements from the ankle extensor muscles. During stretch reflex experiments the foot was mounted to a platform (portable during walking) from which the ankle joint torque and the position were measured. To elicit a stretch reflex, the ankle joint was rotated by a strong motor connected to the platform. The mechanical importance of the stretch reflex was investigated by measuring the changes in joint torque. Electrically, the stretch reflex was recorded as the compound muscle action potential through bipolar surface EMG electrodes placed over the soleus muscle. During H-reflex experiments, the tibial nerve was stimulated at the popliteal fossa and the H-reflex recorded over the soleus muscle as during stretch reflex experiments. To investigate how the contractile properties of a muscle in humans depend on the history of activation, we investigated the intrinsic stiffness of the ankle extensors in healthy subjects. At matched background contraction in sitting subjects, a prolonged contraction increased the intrinsic muscle stiffness by 49%. Muscle yielding has been considered especially important for understanding the reflex compensation. We found a general lack of muscle yield and a mechanically important non-reflex stiffness of the ankle extensors showing that non-reflex stiffness is a prominent factor in normal movements of the ankle joint. In both healthy and spastic persons, we found a mechanically strong stretch reflex in the isometric, contracted muscles during sitting. This posed the question; how is the reflex regulated during more functional motor tasks. This was dealt with by studying the H-reflex during isometric ramp contractions and during walking in healthy and spastic persons. In the healthy subjects the H-reflex was modulated in consistency with a task dependent control. In the spastic patients the H-reflex lacked a task dependent modulation. In consistency with earlier findings it was suggested that the decreased modulation could have been caused by decreased control of the pre-synaptic inhibition of the Ia terminals or a change in recruitment gain. To test if the stretch reflex behaved as the H-reflex, the short latency stretch reflex was investigated during walking. Here we found that the stretch reflex was strongly modulated during a step in healthy subjects as seen for the H-reflex, but when comparing the stretch reflex at matched excitation levels (same background EMGs) during standing and walking, no task-specific reflex modulation was found except the one relating to the excitation level. Therefore, the results emphasise that at least during walking and standing it is not always possible to draw conclusions about the stretch reflex based on observations of the H-reflex. When investigating the modulation of the short latency stretch reflex during walking in spastic patients, we found that the stretch reflex modulation was impaired in spastic patients at least to the extent demonstrated earlier for the H-reflex. The passive stiffness of the ankle joint was at the same time increased in the patients. At matched ankle extensor contraction levels, stretch responses were compared before and after reversible block of the common peroneal nerve and during an attempted, voluntary, fictive dorsiflexion after common peroneal nerve block. (ABSTRACT TRUNCATED)

Afferent Pathways↗
PubMed1996

Fiber type composition of articular branches of the tibial nerve at the knee joint in man.

BACKGROUND: We examined the fiber type composition of the articular branches of the tibial nerve in human cadavers. Our primary motivation was to assess the suitability of these nerve branches for making neural recordings by using an interface, such as a nerve cuff electrode or a microelectrode array. Articular branches of the tibial nerve innervate primarily the posterior joint capsule of the knee (Gardner, 1948. Anat. Rec, 101:109-130); the main branch corresponds anatomically to the posterior articular nerve (PAN), which has been studied extensively in animals such as the cat. MATERIALS AND METHODS: By using light microscopy, we counted the numbers of myelinated fibers in articular branches of the tibial nerve removed from eight cadavers. Unmyelinated fibers were counted in the same specimens by using electron microscopy, and the percentage of unmyelinated fibers was calculated. RESULTS: We found on average 2,280 myelinated fibers in the main articular branch and 279 fibers in individual capsular ramifications. Myelinated fiber diameter histograms showed either bimodal (peaks at 3-4 and 9 microns) or unimodal (peak at 3-4 microns) distributions, depending on the specimen. These histograms were similar in appearance for both the individual capsular ramifications and the main articular branch of the tibial nerve. Numbers of unmyelinated fibers ranged from 4,176 to 5,200 in the main tibial nerve branch (average percentage of unmyelinated fibers = 69.6%) and from 750 to 2,250 in the individual capsular ramifications (average percentage of unmyelinated fibers = 78.5%). CONCLUSIONS: The percentage of unmyelinated fibers is comparable to that found in articular nerves in other species. We discovered that the main articular branch of the tibial nerve contains a branch projecting distal to the knee joint capsule; therefore, the best location for placement of a neural recording interface to record from capsular afferents appears to be the well-defined ramifications of the articular branch that penetrate the joint capsule. Branches that contain only these ramifications are 0.5-1.0 mm in diameter and, on average, have 658 myelinated axons, which should be a sufficient number from which to record.

Aged↗
PubMed1996

Modeling of the H-reflex facilitation during ramp and hold contractions.

Healthy subjects were asked to make a voluntary ramp and hold contraction. The duration of the ramp stage was 500 ms, and the torque increment in this period was set to 15 Nm. The contraction was made from a relaxed and from a 5 Nm background torque situation. Hoffmann (H-) reflexes were elicited during the voluntary contraction, mostly with 100 ms intervals. These experiments showed an increase (facilitation) in the H-reflex before the torque or the EMG started to increase. This facilitation of the H-reflex remained during all the stages of the voluntary movement and declined to normal levels again only at the very end of the hold phase, which lasted for one second. This specific pattern of facilitation during a voluntary contraction was modeled using a modeling language, that is specifically designed to calculate neuronal systems with a high degree of reality (Ekeberg et al., 1991). Our model consisted of a motoneuron pool with 200 neurons connected to an EMG-model of the human soleus muscle and an extra group of higher-level neurons for controlling the amount of decrease of presynaptic inhibition. The model was used to simulate the observed modulation of the H-reflex with both a presynaptic and a postsynaptic mechanism. Simulations showed that a continuous change in the descending control signals is needed to make the model based on postsynaptic mechanism fit with the experimental data, whereas no extra control from the CNS over the excitatory drive to the motoneuron pool is needed when the decrease of presynaptic inhibition mechanism is applied.

Adult↗
PubMed1996

Impaired stretch reflex and joint torque modulation during spastic gait in multiple sclerosis patients.

The modulation of the short latency stretch reflex of the soleus muscle during walking was investigated in seven spastic multiple sclerosis (MS) patients and nine healthy control subjects. Ankle joint stretches were applied by a system which can rotate that ankle joint in any phase of the step cycle during treadmill walking. The torque related to the muscle fibres contracting prior to the stretch and the passive tissues around the ankle joint were measured as the "non-reflex torque". At the same time the short latency stretch reflex-mediated EMG response was measured. The findings show that the stretch reflex modulation was impaired in spastic patients during walking. The stretch reflex modulation was quantified by a modulation index of an average 50% (range -5 to 100%) in the patients and 93% (78-100%) in the control subjects (P < 0.05). The passive stiffness of the ankle joint was at the same time increased in the patients (P < 0.05). It is proposed that the impaired modulation of the stretch reflex along with increased ankle joint stiffness contribute to the impaired walking ability in spastic MS patients.

Adult↗
PubMed1996

Soleus stretch reflex modulation during gait in humans.

1. The modulation of the short-latency stretch reflex during walking at different walking speeds was investigated and compared with the stretch reflex during standing in healthy human subjects. 2. Ankle joint stretches were applied by a system able to rotate the human ankle joint during treadmill walking in any phase of the step cycle. The system consisted of a mechanical joint attached to the subject's ankle joint and connected to a motor placed beside the treadmill by means of bowden wires. The weight of the total system attached to the leg of the subject was 900 g. 3. The short-latency soleus stretch reflex was modulated during a step. In the stance phase, the amplitude equaled that found during standing at matched soleus background electromyogram (EMG). In the transition from stance to swing, the amplitude was 0 in all subjects. In late swing, the stretch reflex amplitude increased to 45 +/- 27% (mean +/- SD) of the maximal amplitude in the stance phase (stretch amplitude 8 degrees, stretch velocity 250 degrees/s). 4. The onset (42 +/- 3.2 ms) and peak latencies (59 +/- 2.5 ms) of the stretch reflex did not depend on the phase in the step cycle at which the reflex was elicited. 5. When the ankle joint is rotated, a change in torque can be measured. The torque measured over the first 35 ms after stretch onset (nonreflex torque) was at a maximum during late stance, when the leg supported a large part of the body's weight, and at a minimum during the swing phase. At heel contact the nonreflex torque was 50% of its maximal value. 6. During the stance phase the maximal EMG stretch reflex had a phase lead of approximately 120 ms with respect to the maximal background EMG and a phase lead of approximately 250 ms with respect to the maximal nonreflex torque. 7. The constant latency of the stretch reflex during a step implied that the ankle extensor muscle spindles are always taut during walking. 8. The relatively high amplitude of the stretch reflex in late swing and at heel contact made it likely that the stretch reflex contributed to the activation of the ankle extensor muscles in early stance phase.

Adult↗
PubMed1996

A comparison of clinical and laboratory measures of spasticity.

Clinical evaluation of spasticity was performed in lower extremities in 35 ambulatory multiple sclerosis patients and compared with the soleus stretch reflex and the Hoffman reflex. There was no relation between the muscle tone score of dorsiflexion of the foot and the biomechanical/electrophysiological parameters. In contrast, the Achilles tendon reflex score was significantly related to the amplitude (rho = 0.411, P < 0.05) and the slope of the stretch reflex (rho = 0.523, P < 0.01). The clinical examination at the ankle joint revealed 33% normal reflex examinations but only 7% normal muscle tone examinations. In contrast, the number of normal examinations of patellar reflex and muscle tone at the knee joint were similar. It is concluded that the muscle tone score overestimates the amount of spasticity because of changes in the non-reflex properties of the spastic extremity and that a reflex score should be used as a clinical measure of spasticity. In addition, biomechanical/electrophysiological evaluation of spasticity at the ankle joint relates to the over-all total muscle tone and reflex scores of lower extremities in this group of MS patients.

Adult↗
PubMed1996

Treatment of spasticity with repetitive magnetic stimulation; a double-blind placebo-controlled study.

The effect of repetitive magnetic stimulation on spasticity was evaluated in 38 patients with multiple sclerosis in a double-blind placebo-controlled study. One group was treated with repetitive magnetic stimulation (n = 21) and the other group with sham stimulation (n = 17). Both groups were treated twice daily for 7 consecutive days. Primary end-points of the study were changes in the patients self-score, in clinical spasticity score, and in the stretch reflex threshold. The self-score of ease of daily day activities improved by 22% (P = 0.007) after treatment and by 29% (P = 0.004) after sham stimulation. The clinical spasticity score improved -3.3 +/- 4.7 arbitrary unit (AU) in treated patients and 0.7 +/- 2.5 AU in sham stimulation (P = 0.003). The stretch reflex threshold increased 4.3 +/- 7.5 deg/s in treated patients and -3.8 +/- 9.7 deg/s in sham stimulation (P = 0.001). The data presented in this study supports the idea that repetitive magnetic stimulation has an antispastic effect in multiple sclerosis. Future studies should clarify the optimal treatment regimen.

Activities of Daily Living↗
PubMed1996

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

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

Electric Stimulation Therapy↗
PubMed1995

Central control of reciprocal inhibition during fictive dorsiflexion in man.

The size of the soleus Hoffmann reflex (H-reflex) was measured in six healthy human subjects before and after transmission in the common peroneal nerve (CPN) was blocked reversibly by local injection of lidocaine. It was found that the H-reflex at rest increased after the block. When the subjects attempted to perform a dorsiflexion while the CPN was blocked ("fictive dorsi flexion"), the soleus H-reflex was strongly depressed. Stimulation of the CPN proximal to the block elicited a short-latency inhibition of the soleus H-reflex, which was probably mediated by reciprocal Ia interneurones. This inhibition never increased during dorsiflexion relative to rest prior to the CPN block, but after the block a significant increase in inhibition was seen during fictive dorsiflexion in three subjects. Stimulation of the femoral nerve (FN) elicited a short-latency monosynaptic Ia facilitation of the soleus H-reflex. This facilitation was found to decrease during dorsiflexion relative to rest before the block as well as during fictive dorsiflexion after the block. The decrease in the H-reflex during fictive dorsiflexion demonstrates that although an increased Ia-afferent feedback via the gamma-loop may contribute to reciprocal inhibition when transmission in the CPN is intact, a noticeable, centrally mediated inhibition may occur in the absence of this feedback. It is suggested that this inhibition is caused by central facilitation of interneurones mediating disynaptic Ia inhibition as well as interneurones mediating presynaptic inhibition of Ia afferents.

Adult↗
PubMed1995

Synaptic linkages between red nucleus cells and limb muscles during a multi-joint motor task.

The magnocellular red nucleus (RNm) becomes highly active when a monkey reaches to grasp an object. However, the only spike-triggered averaging studies of the RNm to date have been restricted to a simple wrist tracking paradigm and electromyographic (EMG) measurements of muscles of the forearm. We have now measured EMG signals from a large number of muscles throughout the shoulder, arm, forearm, and hand during a variety of tasks, including unconstrained reaching and grasping movements. Relations between these EMG signals and single-unit activity were assessed by on-line spike-triggered averaging and revealed significant post-spike effects among muscles of the shoulder and proximal arm, as well as intrinsic hand muscles. Although there remained a strong bias toward the extensor muscles of the forearm, as has been shown earlier, these results reinforce the importance of the RNm in the control of coordinated, whole-limb reaching movements.

Animals↗
PubMed1995

H-reflex modulation during gait in multiple sclerosis patients with spasticity.

The aim of the present study was to quantitate the Hoffmann (H-) reflex modulation in the soleus muscle of spastic multiple sclerosis patients during walking at different stimulus strengths, and to quantitate the influence of the excitation level of the soleus muscle on the H-reflex. The H-reflex modulation was quantified by the minimum H-reflex in the swing phase and the modulation index introduced by Yang et al. (1). Stimulus strengths were matched between subjects, and only stimuli intensities, which reflect modulation in the H-reflex with true neurophysiological origin, were used. Based on this approach, we found that the reflex modulation is impaired in moderate spastic patients at both a strong and a weak afferent input to the motoneuron pool in consistency with studies on severe spastic patients. This study concludes that the H-reflex modulation in the spastic patients is only related to the excitation level of the muscles, whereas both the excitation level and other factors than the excitation level are known to influence the H-reflex modulation in healthy subjects.

Adult↗
PubMed1995

Simultaneous 31P-NMR spectroscopy and EMG in exercising and recovering human skeletal muscle: a correlation study.

A large number of studies have shown amplitude and spectral changes of the electromyogram during exercise, leading to several theories of how these changes might be related to the underlying metabolic changes. The amplitude and spectral changes are generally interpreted as changes in motor unit recruitment and a reduction of the muscle fiber conduction velocity due to proton or lactate accumulation. This study focuses on the causality of spectral changes of the surface electromyogram and proton or lactate accumulation and how the changes in motor unit recruitment are related to the metabolic status of the muscle. Simultaneous 31P-nuclear magnetic resonance spectroscopy and surface electromyography were performed during sustained static exercise and recovery in healthy volunteers and a patient with McArdle's disease. A clear dissociation between the median power frequency of the surface electromyogram and pH was seen in the healthy volunteers during recovery and during exercise in the patient with McArdle's disease. The results indicate that proton or lactate accumulation is not primarily responsible for the spectral changes of the surface electromyogram as previously suggested. The motor unit recruitment (as judged by the root mean square of the surface electromyogram) increased hyperbolically during the submaximal static exercise, with decreasing phosphocreatine-to-P(i) ratio reaching maximum at 0.6 (exhaustion), and seems to constitute a consistent metabolic limit to the exercise. The increased myoelectrical activity seen after exercise is not caused by an incomplete recovery of phosphorous metabolism, pH, or lactate but could probably be an impairment of the excitation-contraction coupling.

Adult↗
PubMed1995

Mechanical and electromyographic analysis of reciprocal inhibition at the human ankle joint.

1. The purpose of the present study is to investigate how reciprocal inhibition influences the mechanical and electromyographic (EMG) properties of the ankle plantar flexors in humans during a voluntary contraction. 2. At different levels of maintained plantar flexion contractions ranging from 0 to 20 Nm, the size of the soleus EMG stretch reflex and the ankle joint stiffness (ration between the torque increment and the amplitude of the stretch) were measured in response to an imposed dorsiflexion. At matched plantar flexion contraction levels, stretch responses were compared before and after reversible block of the common peroneal nerve (CPN). Stretch responses were also measured during an attempted voluntary fictive dorsiflexion after CPN block. 3. In the preactivated soleus muscles, the phasic EMG response to stretch consisted of two peaks labeled M1 and M2. After CPN block, the M1 short-latency stretch reflex on average increased by 25 +/- 5.7%, mean +/- SD (P < 0.001), and the M2 stretch reflex increased on average by 29 +/- 13.0% (P = 0.002). 4. The total stiffness of the ankle joint during a stretch is the sum of the nonreflex and the reflex mediated stiffness. The total stiffness after CPN block increased on average by 13 +/- 2.7% (P = 0.002) and the estimated reflex stiffness by 33 +/- 6.5% (P < 0.001). 5. When the subjects were asked to make a strong dorsiflexion after CPN block, the soleus stretch reflex was depressed to the extent that the reflex mediated mechanical effect around the ankle joint was abolished.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1995

31P NMR spectroscopy and electromyography during exercise and recovery in patients with fibromyalgia.

OBJECTIVE: To investigate whether patients with fibromyalgia (FM) have normal motor unit recruitment in relation to muscle metabolism during exhausting exercise and recovery, and whether the reduced voluntary muscle force normally seen is related to a smaller muscle size. METHODS: Female patients with FM and sedentary controls were examined using simultaneous 31P nuclear magnetic resonance spectroscopy (NMR) and surface electromyography (SEMG) during exhaustive static exercise of the anterior tibial muscle and during recovery. The maximum voluntary contraction force was estimated, and the maximum cross sectional muscle area was evaluated using 1H NMR imaging. The sedentary controls were matched to patients for sex, age and, as far as possible, daily physical activity levels. RESULTS: Patients with FM had reduced maximum voluntary contraction force in relation to the sedentary controls, despite having similar muscle size. In general the myoelectrical-metabolic relation during exercise and recovery was normal in patients with FM. CONCLUSION: The less extreme changes in motor unit recruitment and metabolism during exhaustive exercise indicated a lower exercise tolerance that could be connected with the lower physical activity levels.

Adult↗