Facilitation of soleus H-reflex amplitude evoked by cutaneous nerve stimulation at the wrist is not suppressed by rhythmic arm movement

Neural connections between the cervical and lumbosacral spinal cord may assist in arm and leg coordination during locomotion. Currently the extent to which arm activity can modulate reflex excitability of leg muscles is not fully understood. We showed recently that rhythmic arm movement significantly suppresses soleus H-reflex amplitude probably via modification of presynaptic inhibition of the IA afferent pathway. Further, during walking reflexes evoked in leg muscles by stimulation of a cutaneous nerve at the wrist (superficial radial nerve; SR) are phase and task dependent. However, during walking both the arms and legs are rhythmically active thus it is difficult to identify the locus of such modulation. Here we examined the influence of SR nerve stimulation on transmission through the soleus H-reflex pathway in the leg during static contractions and during rhythmic arm movements. Nerve stimulation was delivered with the right shoulder in flexion or extension. H-reflexes were evoked alone (unconditioned) or with cutaneous conditioning via stimulation of the SR nerve (also delivered alone without H-reflex in separate trials). SR nerve stimulation significantly facilitated H-reflex amplitude during static contractions with the arm extended and countered the suppression of reflex amplitude induced by arm cycling. The results demonstrate that cutaneous feedback from the hand on to the soleus H-reflex pathway in the legs is not suppressed during rhythmic arm movement. This contrasts with the observation that rhythmic arm movement suppresses facilitation of soleus H-reflex when cutaneous nerves innervating the leg are stimulated. In conjunction with other data taken during walking, this suggests that the modulation of transmission through pathways from the SR nerve to the lumbosacral spinal cord is partly determined by rhythmic activity of both the arms and legs.     

Rhythmic leg cycling modulates forearm muscle H-reflex amplitude and corticospinal tract excitability

Rhythmic arm cycling leads to suppression of H-reflexes in both leg and arm muscles, and a reduction in the excitability of corticospinal projections to the forearm flexors. It is unknown, however, whether leg cycling modulates excitability in neural projections to the arms. Here we studied the extent to which rhythmic movement of the legs alters reflex (Experiment 1) and corticospinal (Experiment 2) transmission to arm muscles. In experiment 1, flexor carpi radialis (FCR) H-reflex recruitment curves were recorded with the legs static, and during rhythmic leg movement, while the FCR was both contracted and relaxed. The results indicate that rhythmic leg movement suppresses reflex transmission, both when FCR is at rest and during tonic contraction, but that the effect is not phase-dependent. In experiment 2, we used transcranial magnetic stimulation (TMS) to elicit motor-evoked potentials in the contracted and relaxed FCR during static leg, and leg cycling conditions. Sub-threshold TMS was also used to condition H-reflexes in order to provide specific information about cortical excitability during leg cycling. Both resting and tonically contracting arm muscles showed a greater corticospinal excitability during leg cycling than during the static leg condition. The magnitude of TMS facilitation of H-reflexes was similar during leg cycling and rest, suggesting a considerable sub-cortical component to the increased corticospinal excitability. The results suggest a differential regulation of afferent and descending projections to the arms during leg cycling, and are consistent with the idea that there is a loose, but significant, neural coupling between the arms and legs during rhythmic movement.     

How does action resist visual illusion? Uncorrected oculomotor information does not account for accurate pointing in peripersonal space

The amplitudes and signs of cutaneous reflexes are modulated during rhythmic movements of the arms and legs (during walking and arm or leg cycling for instance). This reflex modulation is frequently independent of the background muscle activity and may involve central pattern generator (CPG) circuits. The purpose of the present study was to investigate the nature and degree of coupling between the upper limbs during arm cycling, with regard to the regulation of cutaneous reflexes. Responses to electrical stimulations of the right, superficial radial nerve (five 1 ms pulses, 300 Hz) were recorded bilaterally in six arm muscles of eight participants during arm cycling involving only the limb ipsilateral to the stimulation, only the limb contralateral to the stimulation, and bilateral movement when the limbs were both in-phase and 180° out of phase. The pattern of cutaneous reflex modulation throughout the arm cycle was independent of the functional state of the limb contralateral to the recording site, irrespective of whether recordings were made ipsilateral or contralateral to the stimulation. Furthermore, cutaneous reflexes were significantly (p<0.05) modulated with arm position in only 8% of cases in which the limb containing the responding muscle was either stationary or being moved passively by the experimenter. The results show that there is relatively weak coupling between the arms with regard to the regulation of cutaneous reflexes during rhythmic, cyclical arm movements. This suggests a loose connection between the CPGs for each arm that regulate muscle activity and reflex amplitude during rhythmic movement.     

Modulation of cutaneous reflexes in arm muscles during walking: further evidence of similar control mechanisms for rhythmic human arm and leg movements

Stimulation of cutaneous nerves innervating the hand evokes prominent reflexes in many arm muscles during arm cycling. We hypothesized that the mechanisms controlling reflex modulation during the rhythmic arm swing of walking would be similar to that documented during arm cycling. Thus, we expected cutaneous reflexes to be modulated by position in the walking cycle (phase dependence) and be different when walking compared to contraction while standing (task dependence). Subjects performed static postures similar to those occurring during walking and also walked on a treadmill while the superficial radial nerve was electrically stimulated pseudorandomly throughout the step cycle. EMG was recorded bilaterally from upper limb muscles and kinematic recordings were obtained from the elbow and shoulder joints. Step cycle information was obtained from force-sensing insoles. Analysis was conducted after averaging contingent upon the occurrence of stimulation in the step cycle. Phase-dependent modulation of cutaneous reflexes at early (~50–80 ms) and middle (~80–120 ms) latencies was observed. Coordinated bilateral reflexes were seen in posterior deltoid and triceps brachii muscles. Task dependency was seen in that reflex amplitude was only correlated with background EMG during static contraction (75% of comparisons for both early and middle latency reflexes). During walking, no significant relationship between reflex amplitude and background EMG level was found. The results show that cutaneous reflex modulation during rhythmic upper limb movement is similar to that seen during arm cycling and to that observed in leg muscles during locomotion. These results add to the evidence that, during cyclical movements of the arms and legs, similar neural mechanisms observed only during movement (e.g. central pattern generators) control reflex output. Electronic Publication     

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.     

Suppression of soleus H-reflex amplitude is graded with frequency of rhythmic arm cycling

In humans, rhythmic arm cycling has been shown to significantly suppress the soleus H-reflex amplitude in stationary legs. The specific nature of the relationship between frequency of arm cycling and H-reflex modulation in the legs has not been explored. We speculated that the effect of arm cycling on reflexes in leg muscles is related to the neural control of arm movement; therefore, we hypothesized that a graded increase in arm cycling frequency would produce a graded suppression of the soleus H-reflex amplitude. We also hypothesized that a threshold frequency of arm cycling would be identified at which the H-reflex amplitude significantly differed from static control trials (i.e., the arms were stationary). Soleus H-reflexes were evoked in stationary legs with tibial nerve stimulation during both control and rhythmic arm cycling (0.03–2.0 Hz) trials. The results show a significant inverse linear relation between arm cycling frequency and soleus H-reflex amplitude (P < 0.05). Soleus H-reflex amplitude significantly differed from control at an average threshold cycling frequency of 0.8 Hz. The results demonstrate that increased frequency of upper limb movement increases the intensity of interlimb influences on the neural activity in stationary legs. Further, a minimum threshold frequency of arm cycling is required to produce a significant effect. This suggests that achieving a threshold frequency of rhythmic arm movement may be important to incorporate in rehabilitation strategies to engage the appropriate interlimb neural pathways.     

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.     

Interlimb coupling from the arms to legs is differentially specified for populations of motor units comprising the compound H-reflex during “reduced” human locomotion

Recent experiments have identified neuromechanical interactions between the arms and legs during human locomotor movement. Previous work reported that during the rhythmic movement of all four limbs, the influence of the arms on reflex expression in the legs was superimposed on the dominant effect of the legs. This evidence was based upon studies using cutaneous and H-reflex modulation as indices of neuronal activity related to locomotion. The earlier H-reflex study was restricted to one phase of movement and to only a fixed H-reflex amplitude. Also, all four limbs were actively engaged in locomotor movement, and this led to the speculation that the effect from the arms could be underestimated by “swamping” of the conditioning during movement of the test limb. Work from the cat suggests that descending locomotor drive may be differentially specified for different motor unit populations in the hindlimb. Accordingly, details of interlimb coordination between the arms and legs in humans require further characterization and an examination of different populations of motor units as can be obtained from H-reflex recruitment curve (RC) parameters. Using modulation of H-reflex amplitudes across the entire ascending limb as neural probes for interlimb coupling, the present study evaluated the separated influences of rhythmic activity of the arms and leg on neuronal excitability of a stationary “test leg”. This three-limb “reduced” locomotion approach was applied using a stepping ergometer during the performance of three rhythmic movement tasks: arms (A); contralateral leg (L); and arms and contralateral leg (AL). Data were sampled at four different phases of the stepping cycle (using the moving leg as reference): start power (SP); end power (EP); start recovery (SR); and end recovery (ER). The main result was a large and significant influence of rhythmic AL activity on RC parameters of the H-reflex at EP and SP phases. However, the parameters (and thus motor unit populations) were differentially affected at each phase and task. For instance, a significant contribution of arms movement was noticed for H _max (largest motor units) at EP phase (P < 0.05), but no changes was observed for other parameters related to lower reflex amplitude (e.g., H-reflex evoked with an input that elicited 50% of maximum reflex response during static condition; H@50%). On the other hand, at SR phase, the parameter H@50% was significantly affected during AL compared to L. It is suggested that the remote effect from arms rhythmic activity has been differentially manifested across motor unit populations for each phase of movement. These findings provide definitive evidence for interlimb coupling between cervical and lumbar oscillators in gating the excitability of reflex pathways to a leg muscle for different populations of motorneurons within the pool. This further supports the contention of similar functional organization for locomotor networks in the human when compared to other animals. Additionally, these data provide additional confirmation of the significant role of the output of neural control for rhythmic arm movement in modulating reflex excitability of the legs that is specifically adjusted according to the phase and task.     

Forward and backward arm cycling are regulated by equivalent neural mechanisms

It was shown some time ago that cutaneous reflexes were phase-reversed when comparing forward and backward treadmill walking. Activity of central-pattern-generating networks (CPG) regulating neural activity for locomotion was suggested as a mechanism involved in this "program reversal." We have been investigating the neural control of arm movements and the role for CPG mechanisms in regulating rhythmic arm cycling. The purpose of this study was to evaluate the pattern of muscle activity and reflex modulation when comparing forward and backward arm cycling. During rhythmic arm cycling (forward and backward), cutaneous reflexes were evoked with trains (5 x 1.0 ms pulses at 300 Hz) of electrical stimulation delivered to the superficial radial (SR) nerve at the wrist. Electromyographic (EMG) recordings were made bilaterally from muscles acting at the shoulder, elbow, and wrist. Analysis was conducted on specific sections of the movement cycle after phase-averaging contingent on the timing of stimulation in the movement cycle. EMG patterns for rhythmic arm cycling are similar during both forward and backward motion. Cutaneous reflex amplitudes were similarly modulated at both early and middle latency irrespective of arm cycling direction. That is, at similar phases in the movement cycle, responses of corresponding sign and amplitude were seen regardless of movement direction. The results are generally parallel to the observations seen in leg muscles after stimulation of cutaneous nerves in the foot during forward and backward walking and provide further evidence for CPG activity contributing to neural activation and reflex modulation during rhythmic arm movement.     

Rhythmic arm cycling modulates Hoffmann reflex excitability differentially in the ankle flexor and extensor muscles

Rhythmic arm movement significantly suppresses H-reflex amplitude in the soleus (SOL) muscle. This is evidence of neural linkages between the arms and legs which can be exploited during locomotion and have been ascribed to the descending effects of CPGs for arm cycling. However, the generalizability of the effects of arm movement on reflex excitability within the lower leg musculature has not been confirmed, as findings have been limited solely to the ankle extensor group. Here we tested the hypothesis that rhythmic arm movement similarly modulates H-reflex amplitude in both the ankle flexors and extensors by observing responses in the SOL and tibialis anterior (TA) muscles. SOL and TA H-reflex recruitment curves were recorded bilaterally during control and 1 Hz arm cycling conditions. Our results showed significant suppression in H-reflex amplitude (H_max) in the SOL muscle in both the dominant and non-dominant legs during arm movement. However, results also revealed an unpredicted bidirectional (i.e. either suppression or facilitation) modulation of TA reflex amplitude that was not present in the SOL muscle. These findings suggest a differential regulation of ankle flexor and extensor H-reflex responses during rhythmic arm movement. This may be the result of differences in CPG output to the flexors and extensors during rhythmic movement, as well as increased involvement of cortical drive to the flexors relative to the extensors during rhythmic movement. These findings may be pertinent to future investigation of rehabilitative therapies that involve facilitative modulation of ankle flexor motor responses.     

Enhancement of arm and leg locomotor coupling with augmented cutaneous feedback from the hand

Cutaneous feedback from the hand could assist with coordination between the arms and legs during locomotion. Previously we used a reduced walking model of combined arm and leg (ARM&LEG) cycling to examine the separate effects of rhythmic arm (ARM) and leg (LEG) movement. Here we use this same paradigm to test the modulation H-reflexes with and without interlimb cutaneous conditioning evoked by stimulating a nerve innervating the hand (superficial radial, SR). It was hypothesized that both ARM and LEG would contribute significantly to suppression of H-reflex amplitude during ARM&LEG. We also predicted a conservation of interlimb cutaneous conditioning during movement and an interaction between arm and leg rhythmic movement control. Subjects were seated in a recumbent ARM&LEG cycle ergometer and maintained a low-level soleus contraction for all tasks. H-reflex amplitude was facilitated by cutaneous conditioning evoked by stimulation of the SR nerve. H-reflex amplitudes were taken from recruitment curves and included modulation of 50% H max and H max. The suppressive effect of arm was less than that for LEG and ARM&LEG, while suppression during LEG and ARM&LEG were generally equivalent. For H-reflexes conditioned by cutaneous input, amplitudes during ARM&LEG instead were in between those for ARM and LEG modulation. Multiple regression analysis revealed a significant contribution for arm only in trials when SR stimulation was used to condition H-reflex amplitudes. We suggest that there is a measurable interaction between neural activity regulating arm and leg movement during locomotion that is specifically enhanced when cutaneous input from the hand is present.     

Neural control of rhythmic human arm movement: phase dependence and task modulation of hoffmann reflexes in forearm muscles

Although we move our arms rhythmically during walking, running, and swimming, we know little about the neural control of such movements. Our working hypothesis is that neural mechanisms controlling rhythmic movements are similar in the human lumbar and cervical spinal cord. Thus reflex modulation during rhythmic arm movement should be similar to that seen during leg movement. Our main experimental hypotheses were that the amplitude of H-reflexes in the forearm muscles would be modulated during arm movement (i.e., phase-dependent) and would be inhibited during cycling compared with static contraction (i.e., task-dependent). Furthermore, to determine the locus of any modulation, we tested the effect that active and passive movement of the ipsilateral (relative to stimulated arm) and contralateral arm had on H-reflex amplitude. Subjects performed rhythmic arm cycling on a custom-made hydraulic ergometer in which the two arms could be constrained to move together (180 degrees out of phase) or could rotate independently. Position of the stimulated limb in the movement cycle is described with respect to the clock face. H-reflexes were evoked at 12, 3, 6, and 9 o'clock positions during static contraction as well as during rhythmic arm movements. Reflex amplitudes were compared between tasks at equal M wave amplitudes and similar levels of electromyographic (EMG) activity in the target muscle. Surface EMG recordings were obtained bilaterally from flexor carpi radialis as well as from other muscles controlling the wrist, elbow, and shoulder. Compared with reflexes evoked during static contractions, movement of the stimulated limb attenuated H-reflexes by 50.8% (P < 0.005), 65.3% (P < 0.001), and 52.6% (P < 0.001) for bilateral, active ipsilateral, and passive ipsilateral movements, respectively. In contrast, movement of the contralateral limb did not significantly alter H-reflex amplitude. H-reflexes were also modulated by limb position (P < 0.005). Thus task- and phase-dependent modulation were observed in the arm as previously demonstrated in the leg. The data support the hypothesis that neural mechanisms regulating reflex pathways in the moving limb are similar in the human upper and lower limbs. However, the inhibition of H-reflex amplitude induced by contralateral leg movement is absent in the arms. This may reflect the greater extent to which the arms can be used independently.     

Voluntary changes in leg cadence modulate arm cadence during simultaneous arm and leg cycling

Although there is some evidence showing that neural coupling plays an important role in regulating coordination between the upper and lower limbs during walking, it is unclear how tightly the upper and lower limbs are linked during rhythmic movements in humans. The present study was conducted to investigate how coupling of both limbs is coordinated during independent rhythmic movement of the upper and lower limbs. Ten subjects performed simultaneous arm and leg cycling (AL cycling) at their preferred cadences without feedback for 10 s, and then were asked to voluntarily change the cadence (increase, decrease, or stop) of arm or leg cycling. Leg cycling cadence was not affected by voluntary changes in arm cadence. By contrast, arm cycling cadence was significantly altered when leg cycling cadence was changed. These results suggest the existence of a predominant lumbocervical influence of leg cycling on arm cycling during AL cycling.     

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.     

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.     

Rhythmic arm cycling produces a non-specific signal that suppresses Soleus H-reflex amplitude in stationary legs

Rhythmic arm cycling significantly suppresses Hoffmann (H-) reflex amplitude in Soleus muscles of stationary legs. The specific parameters of arm cycling contributing to this suppression, however, are unknown. Between the arms or legs, movement results in suppression of the H-reflex that is specifically related to the phase of movement and the locus of limb movement. We speculated that the effects of arm movement features on H-reflexes in the leg would be similar and hypothesized that the Soleus H-reflex suppression evoked by arm movement would therefore be specifically related to: (1) phase of the movement; (2) the locus of the movement (i.e., ipsilateral or contralateral arm); (3) range of arm motion; and (4) frequency of arm cycling. Participants performed bilateral arm cycling at 1 and 2 Hz with short and long-crank lengths. Ipsilateral and contralateral arm cycling was also performed at 1 Hz with a long-crank length. Soleus H-reflexes were evoked at four equidistant phases and comparisons were made while maintaining similar evoked motor waves and Soleus activation. Our results show that comparable suppressive effects were seen at all phases of the arm movement: there was no phase-dependence. Further, bilateral or unilateral (whether ipsi- or contralateral arm) cycling yielded equivalent suppression of the H-reflex amplitude. Cycling at 2 Hz resulted in a significantly larger suppression than with 1 Hz cycling. We conclude that a general, rather than a specific, signal related to the command to produce rhythmic arm muscle activity mediates the suppression of Soleus H-reflex during arm cycling.     

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.     

Context-dependent modulation of interlimb cutaneous reflexes in arm muscles as a function of stability threat during walking

Cutaneous reflexes evoked in the muscles of the arms with electrical stimulation of nerves of the foot ("interlimb reflexes") are observed during walking. These reflexes have been suggested to coordinate the actions of the legs and arms when walking is disturbed. Recently, we showed that cutaneous reflexes evoked in the leg muscles after stimulation at the foot are modulated according to the level of postural threat during walking. We hypothesized that the amplitude of interlimb cutaneous reflexes would similarly be modulated when subjects walk in unstable environments. Subjects walked on a treadmill under four walking conditions: 1) normal; 2) normal with unpredictable anterior-posterior (AP) perturbations; 3) arms crossed; and 4) arms crossed with unpredictable AP perturbations. Interlimb reflexes evoked from electrical stimulation of the right superficial peroneal or sural nerves were recorded bilaterally, at four points of the step cycle. These reflexes were compared between conditions in which the arms were moving in a similar manner: 1) normal versus AP walking and 2) arms crossed versus arms crossed with AP perturbations. Differences in reflex amplitudes between arms-crossed conditions were observed in most upper limb muscles when subjects were perturbed while walking compared with undisturbed walking. This effect was less apparent when the arms were swinging freely. The results indicate that the strength of interlimb connections is influenced by the level of postural threat (i.e., the context of the behavior), thereby suggesting that these reflexes serve a functional link between the legs and arms during locomotion.     

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.     

Cutaneous reflexes during rhythmic arm cycling are insensitive to asymmetrical changes in crank length

The neural control of a movement depends upon the motor task performed. To further understand the neural regulation of different variations of the same type of movement, we created three dissimilar bilateral rhythmic arm cycling tasks by unilaterally manipulating crank length (CL). Modulation in the amplitude and sign of cutaneous reflexes was used as an index of neural control. Neurologically intact subjects performed three bilateral cycling trials at ∼1 Hz with the ipsilateral crank arm at one of three different lengths. Cutaneous reflexes were evoked during each trial with trains (5 × 1.0 ms pulses at 300 Hz) of electrical stimulation delivered to the superficial radial nerve at the ipsilateral wrist. EMG recordings were made bilaterally from muscles acting at the shoulder, elbow, and wrist. Analysis was conducted after phase-averaging contingent upon the timing of stimulation in the movement cycle. CL variation created an asymmetrical cycling pattern and produced significant changes in the range of motion at the ipsilateral shoulder and elbow. Background EMG amplitude in muscles of the contralateral arm generally increased significantly as CL decreased. Therefore at a given phase in the movement cycle, the background EMG was different between the three cycling trials. In contrast, cutaneous reflex amplitudes in muscles of both arms were similar at each phase of the movement cycle between the different CLs trials at both early and middle latencies. This was particularly evident in muscles ipsilateral to nerve stimulation. We suggest that variations of arm cycling that primarily yield significant changes in the amplitude of muscle activity do not require significant task-specific change in neural control.