Neural compensation for compliant loads during rhythmic movement

An experiment was performed to characterise the movement kinematics and the electromyogram (EMG) during rhythmic voluntary flexion and extension of the wrist against different compliant (elastic-viscous-inertial) loads. Three levels of each type of load, and an unloaded condition, were employed. The movements were paced at a frequency of 1 Hz by an auditory metronome, and visual feedback of wrist displacement in relation to a target amplitude of 100 degree was provided. Electromyographic recordings were obtained from flexor carpi radialis (FCR) and extensor carpi radialis brevis (ECR). The movement profiles generated in the ten experimental conditions were indistinguishable, indicating that the CNS was able to compensate completely for the imposed changes in the task dynamics. When the level of viscous load was elevated, this compensation took the form of an increase in the rate of initial rise of the flexor and the extensor EMG burst. In response to increases in inertial load, the flexor and extensor EMG bursts commenced and terminated earlier in the movement cycle, and tended to be of greater duration. When the movements were performed in opposition to an elastic load, both the onset and offset of EMG activity occurred later than in the unloaded condition. There was also a net reduction in extensor burst duration with increases in elastic load, and an increase in the rate of initial rise of the extensor burst. Less pronounced alterations in the rate of initial rise of the flexor EMG burst were also observed. In all instances, increases in the magnitude of the external load led to elevations in the overall level of muscle activation. These data reveal that the elements of the central command that are modified in response to the imposition of a compliant load are contingent, not only upon the magnitude, but also upon the character of the load.     

Neural control of rhythmic arm cycling after stroke

Disordered reflex activity and alterations in the neural control of walking have been observed after stroke. In addition to impairments in leg movement that affect locomotor ability after stroke, significant impairments are also seen in the arms. Altered neural control in the upper limb can often lead to altered tone and spasticity resulting in impaired coordination and flexion contractures. We sought to address the extent to which the neural control of movement is disordered after stroke by examining the modulation pattern of cutaneous reflexes in arm muscles during arm cycling. Twenty-five stroke participants who were at least 6 mo postinfarction and clinically stable, performed rhythmic arm cycling while cutaneous reflexes were evoked with trains (5 × 1.0-ms pulses at 300 Hz) of constant-current electrical stimulation to the superficial radial (SR) nerve at the wrist. Both the more (MA) and less affected (LA) arms were stimulated in separate trials. Bilateral electromyography (EMG) activity was recorded from muscles acting at the shoulder, elbow, and wrist. Analysis was conducted on averaged reflexes in 12 equidistant phases of the movement cycle. Phase-modulated cutaneous reflexes were present, but altered, in both MA and LA arms after stroke. Notably, the pattern was "blunted" in the MA arm in stroke compared with control participants. Differences between stroke and control were progressively more evident moving from shoulder to wrist. The results suggest that a reduced pattern of cutaneous reflex modulation persists during rhythmic arm movement after stroke. The overall implication of this result is that the putative spinal contributions to rhythmic human arm movement remain accessible after stroke, which has translational implications for rehabilitation.     

Ventilatory responses at the onset of passive movement and voluntary exercise with arms and legs

This study was undertaken to elucidate whether phase I appeared at the onset of voluntary and passive arm movements and to compare these results with those of similar leg movements. Instead of the conventional cranking exercise, seven male subjects performed alternately flexion-relaxation of both arms, extension-relaxation of both legs, and combined arm and leg exercise at the rate of about 60 min^-1 for four breaths in a sitting position. Similar movements were accomplished passively by the experimenters. In all experiments, minute ventilation increased rapidly within the first breath after the onset of exercise. The difference of ventilation (delta value) between the mean of the first two breaths at the onset of voluntary exercise and that of five breaths during rest was significantly (P < 0.05) greater in arm (7.751 min^-1) than in leg (5.191 min^-1). Passive movement showed a similar tendency. Arm delta ventilation correlated highly (r = 0.74 ± 0.91) with leg delta ventilation and the slope of the regression lines was about 1.2. Heart rate increased abruptly while cardiac output did not always increase rapidly at the onset of locomotion. Oxygen uptake in the voluntary leg exercise continued for 3 min was slightly but nonsignificantly higher than in the arm exercise, indicating the equality of the exercise intensity. In conclusion, ventilatory responses at the onset of the arm exercise are larger than those of the leg in both voluntary and passive conditions regardless of the muscle mass, suggesting the different neurogenic mechanism between arm and leg.     

基于快速节律性运动的皮层脑电分析

节律运动是人们生活中一种基本的运动形式,与单次运动相区别,快速节律运动在脑电中有特殊的表现形式.本文以两位植入皮层电极的癫痫病人作为受试,在1Hz与2Hz听觉节拍器提示下进行手指节律运动,同时记录皮层脑电数据.对脑电数据的能量和相关性进行离线分析,结果显示节律运动中运动感觉皮层脑电的能量在特定频段上呈下降趋势,相干性呈上升趋势,且运动相关能量与相干性在不同的运动速度下具有明显的统计性差异.对不同功能区之间相干性的分析表明辅助运动区可能是与运动速度有关的皮层功能区.     

Afterhyperpolarization modulation in lumbar motoneurons during locomotor-like rhythmic activity in the neonatal rat spinal cord in vitro

Motoneuron afterhyperpolarization (AHP) amplitude and somatic input conductance were monitored during pharmacologically induced, locomotorlike ventral root activity using an isolated neonatal rat spinal cord preparation (transected at the C1 level). Nonspontaneously firing motoneurons were selected for study. Single spikes were evoked at regular intervals by brief depolarizing current pulse injections, while somatic input conductance was monitored by hyperpolarizing current pulses. The induction of rhythmic ventral root activity was associated with tonic depolarization of motoneurons as well as superimposed rhythmically alternating membrane depolarization and hyperpolarization (locomotor drive potentials, LDPs). In 9 of 13 trials (six of eight cells) the peak amplitude of AHPs following current-evoked action potentials was reduced during both the hyperpolarized and the depolarized phases of the LDP, compared with the pre-locomotor condition. The peak AHP amplitude increased during the depolarized phase of the LDP in 4 of 13 trials (three of eight cells); however, in 3 of these 4 trials measurement of the AHP later in the course of its trajectory, using a half decay time (HDt) reference point, demonstrated AHP amplitude reduction during rhythmic activity compared with the prelocomotor condition. In seven of eight motoneurons the induction of rhythmic activity was associated with a decrease in input conductance. The pattern of AHP amplitude and conductance modulation during the two phases of the LDP was consistent for individual trials; however, there was considerable intertrial variation. The results suggest that AHP modulation during locomotor-like activity in this preparation can be mediated independently of supraspinal influences by intrinsic spinal cord mechanisms, and the observed AHP suppression does not appear to be the passive result of an increase in background conductance. The discrepancy between peak and HDt-based AHP amplitude measurements during the depolarized phase of the LDP in some trials may be due to competing effects of passively enhanced potassium currents and a mechanism that actively reduces the calcium-dependent potassium conductance. The possibility that both the AHP amplitude and the input conductance changes observed during locomotor-like activity reflect a regulation of potassium channels is discussed.     

Effect of rhythmic arm movement on reflexes in the legs: modulation of soleus H-reflexes and somatosensory conditioning

During locomotor tasks such as walking, running, and swimming, the arms move rhythmically with the legs. It has been suggested that connections between the cervical and lumbosacral spinal cord may mediate some of this interlimb coordination. However, it is unclear how these interlimb pathways modulate reflex excitability during movement. We hypothesized that rhythmic arm movement would alter the gain of reflex pathways in the stationary leg. Soleus H-reflexes recorded during arm cycling were compared with those recorded at similar positions with the arms stationary. Nerve stimulation was delivered with the right arm at approximately 70 degrees shoulder flexion or 10 degrees shoulder extension. H-reflexes were evoked alone (unconditioned) or with sural or common peroneal nerve (CP) conditioning to decrease or increase soleus IA presynaptic inhibition, respectively. Both conditioning stimuli were also delivered with no H-reflex stimulation. H-reflex amplitudes were compared at similar M-wave amplitudes and activation levels of the soleus. Arm cycling significantly reduced (P < 0.05) unconditioned soleus H-reflexes at shoulder flexion by 21.7% and at shoulder extension by 8.8% compared with static controls. The results demonstrate a task-dependent modulation of soleus H-reflexes between arm cycling and stationary trials. Sural nerve stimulation facilitated H-reflexes at shoulder extension but not at shoulder flexion during static and cycling trials. CP nerve stimulation significantly reduced H-reflex amplitude in all conditions. Reflexes in soleus when sural and CP nerve stimulation were delivered alone, were not different between cycling and static trials; thus the task-dependent change in H reflex amplitude was not due to changes in motoneuron excitability. Therefore modulation occurred at a pre-motoneuronal level, probably by presynaptic inhibition of the IA afferent volley. Results indicate that neural networks coupling the cervical and lumbosacral spinal cord in humans are activated during rhythmic arm movement. It is proposed that activation of these networks may assist in reflex linkages between the arms and legs during locomotor tasks.     

Differentiation of sound rhythmic stimuli during free movement of the animal

Summary In conditions of free movement of animals (3 experimental dogs), it was impossible to achieve stable differentiation of the metronome frequencies-120 and 60 beats per minute-on the basis of the excitation process concentration. To elaborate stable differentiation in the given experimental conditions, the nervous process provoking differentiation of conditioned stimuli should be formed in the latent period of conditioned reaction.(Presented by Active Member AMN SSSR P. S Kupalov) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 51, No. 4, pp. 17–21, April, 1961.     

Multi-frequency arm cycling reveals bilateral locomotor coupling to increase movement symmetry

Upright stance has allowed for substantial flexibility in how the upper limbs interact with each other: the arms can be coordinated in alternating, synchronous, or asymmetric patterns. While synchronization is thought to be the default mode of coordination during bimanual movement, there is little evidence for any bilateral coupling during locomotor-like arm cycling movements. Multi-frequency tasks have been used to reveal bilateral coupling during bimanual movements, thus here we used a multi-frequency task to determine whether the arms are coupled during arm cycling. It was hypothesized that bilateral coupling would be revealed as changes in background EMG and cutaneous reflexes when temporal coordination was altered. Twelve subjects performed arm cycling at 1 and 2 Hz with one arm while the contralateral arm was either at rest, cycling at the same frequency, or cycling at a different frequency (i.e., multi-frequency cycling with one arm at 1 Hz and the other at 2 Hz). To evoke reflexes, the superficial radial nerve was stimulated at the wrist. EMG was collected continuously from muscles of both arms. Results showed that background EMG in the lower frequency arm was amplified while reflex amplitudes were unaltered during multi-frequency cycling. We propose that neural coupling between the arms aids in equalizing muscle activity during asymmetric tasks to permit stable movement. Conversely, such interactions between the arms would likely be unnecessary in determining a reflexive response to a perturbation of one arm. Therefore, bilateral coupling was expressed when it was relevant to symmetry.     

Perceptual coupling in rhythmic movement coordination: stable perception leads to stable action

Rhythmic movement coordination exhibits characteristic patterns of stability, specifically that movements at 0 degrees mean relative phase are maximally stable, 180 degrees is stable but less so than 0 degrees, and other coordinations are unstable without training. Recent research has demonstrated a role for perception in creating this pattern; perceptual variability judgments covary with movement variability results. This suggests that the movement results could be due in part to differential perceptual resolution of the target movement coordinations. The current study used a paradigm that enabled simultaneous access to both perception (between-trial) and movement (within-trial) stability measures. A visually specified 0 degrees target mean relative phase enabled participants to produce stable movements when the movements were at a non-0 degrees relationship to the target being tracked. Strong relationships were found between within-trial stability (the traditional movement measure) and between-trial stability (the traditional perceptual judgment measure), suggestive of a role for perception in producing coordination stability phenomena. The stabilization was incomplete, however, indicating that visual perception was not the sole determinant of movement stability. Rhythmic movement coordination is intrinsically a perception/action system.     

Corticospinal excitability is lower during rhythmic arm movement than during tonic contraction

Humans perform rhythmic, locomotor movements with the arms and legs every day. Studies using reflexes to probe the functional role of the CNS suggest that spinal circuits are an important part of the neural control system for rhythmic arm cycling and walking. Here, by studying motor-evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) of the motor cortex, and H-reflexes induced by electrical stimulation of peripheral nerves, we show a reduction in corticospinal excitability during rhythmic arm movement compared with tonic, voluntary contraction. Responses were compared between arm cycling and tonic contraction at four positions, while participants generated similar levels of muscle activity. Both H-reflexes and MEPs were significantly smaller during arm cycling than during tonic contraction at the midpoint of arm flexion (F = 13.51, P = 0.006; F = 11.83, P = 0.009). Subthreshold TMS significantly facilitated the FCR H-reflex during tonic contractions, but did not significantly modulate H-reflex amplitude during arm cycling. The data indicate a reduction in the responsiveness of cells constituting the fast, monosynaptic, corticospinal pathway during arm cycling and suggest that the motor cortex may contribute less to motor drive during rhythmic arm movement than during tonic, voluntary contraction. Our results are consistent with the idea that subcortical regions contribute to the control of rhythmic arm movements despite highly developed corticospinal projections to the human upper limb.     

Neural regulation of rhythmic arm and leg movement is conserved across human locomotor tasks

It has been proposed that different forms of rhythmic human limb movement have a common central neural control ('common core hypothesis'), just as in other animals. We compared the modulation patterns of background EMG and cutaneous reflexes during walking, arm and leg cycling, and arm-assisted recumbent stepping. We hypothesized that patterns of EMG and reflex modulation during cycling and stepping (deduced from mathematical principal components analysis) would be comparable to those during walking because they rely on similar neural substrates. Differences between the tasks were assessed by evoking cutaneous reflexes via stimulation of nerves in the foot and hand in separate trials. The EMG was recorded from flexor and extensor muscles of the arms and legs. Angular positions of the hip, knee and elbow joints were also recorded. Factor analysis revealed that across the three tasks, four principal components explained more than 93% of the variance in the background EMG and middle-latency reflex amplitude. Phase modulation of reflex amplitude was observed in most muscles across all tasks, suggesting activity in similar control networks. Significant correlations between EMG level and reflex amplitude were frequently observed only during static voluntary muscle activation and not during rhythmic movement. Results from a control experiment showed that strong correlation between EMG and reflex amplitudes was observed during discrete, voluntary leg extension but not during walking. There were task-dependent differences in reflex modulation between the three tasks which probably arise owing to specific constraints during each task. Overall, the results show strong correlation across tasks and support common neural patterning as the regulator of arm and leg movement during various rhythmic human movements.     

Modulation of human cutaneous reflexes during rhythmic cyclical arm movement

The organization and pattern of cutaneous reflex modulation is unknown during rhythmic cyclical movements of the human upper limbs. On the assumption that these cyclic arm movements are central pattern generator (CPG) driven as has been suggested for leg movements such as walking, we hypothesized that cutaneous reflex amplitude would be independent of electromyographic (EMG) muscle activation level during rhythmic arm movement (phase-dependent modulation, as is often the case in the lower limb during locomotion). EMG was recorded from eight muscles crossing the human shoulder, elbow, and wrist joints while whole arm rhythmic cyclical movements were performed. Cutaneous reflexes were evoked with trains of electrical stimulation delivered at non-noxious intensities (approximately 2 x threshold for radiating paresthesia) to the superficial radial nerve innervating the lateral portion of the back of the hand. Phasic bursts of rhythmic muscle activity occurred throughout the movement cycle. Rhythmic EMG and kinematic patterns were similar to what has been seen in the human lower limb during locomotor activities such as cycling or walking: there were extensive periods of reciprocal activation of antagonist muscles. For most muscles, cutaneous reflexes were modulated with the movement cycle and were strongly correlated with the movement-related background EMG amplitude. It is concluded that cutaneous reflexes are primarily modulated by the background muscle activity during rhythmic human upper limb movements, with only some muscles showing phase-dependent modulation.     

Electrical measurement of fluid distribution in legs and arms

Intra- and extra-cellular fluid distribution is very important to know the physiological and clinical state of living subjects. However, it is quite difficult to measure the distribution in vivo. Electrical impedance of living tissue is mainly affected by extra-cellular fluid at an applied frequency lower than the beta-dispersion frequency, and is affected by both extra- and intra-cellular fluid at higher applied frequencies than that. In this paper, we discuss the problems of measurement of intra- and extra-cellular fluid distribution in living tissues by means of electrical impedance. The intra- and extra-cellular fluid distribution is related to some physiological parameters, such as blood circulation, metabolism of tissues, and the electrolytic concentration of intra- and extra-cellular fluids. Therefore, the information about the distribution of fluids in tissues is quite useful for the diagnosis of various diseases, the monitoring of seriously ill patients, and in medical treatments such as artificial dialysis. We discuss the method of measurement and the results of experiments.     

Venous return from distal regions affects heat loss from the arms and legs during exercise-induced thermal loads

Summary To study the role of venous return from distal parts of the extremities in influencing heat loss from the more proximal parts, changes in mean skin temperature (ˉT _sk) of the non-exercising extremities were measured by color thermography during leg and arm exercise in eight healthy subjects. Thirty minutes of either leg or arm exercise at an ambient temperature (T _a) of 20° C or 30° C produced a greatly increased blood flow in the hand or foot and a great increase in venous return through the superficial skin veins of the extremities. During the first 10 min of recovery from the exercise, blood flow to and venous return from the hand or foot on the tested side was occluded with a wrist or ankle cuff at a pressure of 33.3 kPa (250 mm Hg), while blood flow to the control hand or foot remained undisturbed. During the 10-min wrist occlusion, ˉT _sk increased significantly from 28.3°±0.41° C to 30.1°±0.29° C in the control forearm, but remained at nearly the same level (28.0°±0.34° C to 28.2°±0.25° C) in the occluded forearm. In the legs, although ˉT _sk on both sides was virtually identical (32.0°±0.31° C, control vs 32.0°±0.36° C, tested) before occlusion, ˉT _sk on the control side (32.6°±0.27° C) was significantly higher than that on the tested side (32.2°±0.21° C) after ankle occlusion. As monitored by a laser-Doppler flowmeter, skin blood flow in both forearms and legs did not increase but rather decreased during the 30-min recovery. Thus, the increase in ˉT _skin the control forearm or leg cannot be explained by the change in forearmor leg-skin blood flow. We estimated the contribution of the heat transferred from the venous blood of the hand to be 89% of the increase in heat loss from the forearm during occlusion atT _a of 20° C, and to be 67% from the occluded leg atT _a of 30° C. The present results suggest that heat loss from proximal parts of the extremities is greatly affected by the change in blood flow through the distal parts of the extremities, and such venous blood flow plays an important role in the regulation of heat dissipation during exercise in cool (20° C) to warm (30° C) environments.     

Bimanual coordination between isometric contractions and rhythmic movements: an asymmetric coupling

 Interactions between rhythmically moving limbs typically result in attraction to a limited number of coordination modes, which are distinguished in terms of their stability. In addition, the stability of coordination typically decreases with elevations in movement frequency. To gain more insight into the neurophysiological mechanisms underlying these stability characteristics, the effects of phasic voluntary muscle activation onto the movement pattern of the contralateral limb as well as onto the stability of interlimb coordination were examined. This was done in circumstances in which a minimal degree of movement-elicited afferent information was available to mediate the coupling influences. The task involved rhythmic application of isometric torque by one hand, while the other hand was moving rhythmically with unconstrained amplitude. The effects of two levels of applied torque, two coordination patterns (inphase and antiphase), and two movement frequencies were determined, both at the behavioural level (movement kinematics and kinetics) and the neuromuscular level (EMG). The isometric applications of torque clearly influenced the muscle-activation profile and movement pattern of the other limb, affecting both temporal variability and amplitude. Surprisingly, there were no differences between the two coordination patterns or between the tempo conditions. As such, the results did not conform to the Haken-Kelso-Bunz model for rhythmic movement coordination. These data suggest that the archetypal differences in stability of rhythmic bimanual coordination are contingent upon a correspondence between the limbs in terms of their respective tasks. This interpretation is elaborated in terms of the role of sensory feedback and the functional specificity of motor unit recruitment in rhythmic interlimb coordination. Received: 6 November 1998 / Accepted: 7 July 1999     

Neural coupling between the upper and lower limbs in humans

The aim of this study was to establish the effects of active sinusoidal ipsilateral and contralateral upper limb flexion, extension, abduction, and adduction with elbows extended on the right soleus H-reflex with subjects seated and standing. Reflex effects were also established when both arms moved synchronously in a reciprocal pattern with elbows flexed in seated and standing subjects. Sinusoidal arm movements were timed to a metronome and performed at 0.2 Hz. Soleus H-reflexes were elicited only once (every 4s) in every movement cycle of the upper limbs. Position of arms, and activity of shoulder muscles were recorded through twin-axis goniometers and surface electromyography (EMG), respectively. We found that in seated subjects, regardless the direction of the active movement or the upper limb being moved, the soleus H-reflex was depressed. In standing subjects, a reflex depression was observed during extension, abduction, and adduction of the ipsilateral and contralateral upper limbs. Muscles were active during arm flexion and abduction in all directions of arm movement with subjects either seated or standing. It is suggested that arm movement might be incorporated in the rehabilitation training of people with a supraspinal or spinal cord lesion, since it can benefit motor recovery by decreasing spinal reflex excitability of the legs in these patients.     

Dynamic stability of locomotor respiratory coupling during cycling in humans

We explored the locomotor respiratory coupling (LRC) during a 50-min constant-load submaximal cycling exercise. A 4-week recombinant human erythropoietin (r-HuEPO) treatment improved participants' aerobic capabilities, but did not elicit significant changes in LRC. The distributions of the respiratory frequency over pedalling frequency ratios were systematically bimodal, with a preferred use of 1/3 and 1/2, and a progressive shift of the higher mode from 1/3 towards 1/2 with exercise duration. These results are interpreted in the framework of the sine circle map as the result of coordination dynamics between the physiological subsystems involved in the breathing pedalling cooperation.     

Cardiovascular responses to static extension and flexion of arms and legs

This study compared cardiovascular responses to static extension and flexion exercises at four upper and lower limb joints. Eight males performed a 2 min static contraction at 30% of maximal voluntary torque followed immediately by 2 min post-exercise muscle ischaemia (PEMI) using each of four joints: the wrist, elbow, ankle, and knee. In the PEMI, an occlusion cuff placed around the proximal portion of the exercising muscle was inflated to 250 mmHg immediately before the cessation of exercise. Mean arterial pressure (MAP), heart rate (HR), calf blood flow, and calf vascular conductance (CVC) in the non-exercised calf were measured. There was a significant interaction for direction of movement (extension vs. flexion) and limb (upper vs. lower) in HR and CVC during both exercise and PEMI; extension in the wrist and elbow evoked a greater increase in HR and a greater decrease in CVC than flexion, whereas flexion in the ankle and knee elicited a greater increase in HR and a greater decrease in CVC than extension. These results suggest that the cardiovascular responses to extension and flexion differ between arms and legs, partly arising from the activation of the muscle metaboreflex.