Evidence for transcortical reflex pathways in the lower limb of man

Cortical and spinal adaptations induced by balance training: correlation between stance stability and corticospinal activation

AIM

To determine the sites of adaptation responsible for improved stance stability after balance (=sensorimotor) training, changes in corticospinal and spinal excitability were investigated in 23 healthy subjects.

METHODS

Neural adaptations were assessed by means of H-reflex stimulation, transcranial magnetic stimulation (TMS) and conditioning of the H-reflex by TMS (Hcond) before and after 4 weeks of balance training. All measurements were performed during stance perturbation on a treadmill. Fast posterior translations induced short- (SLR), medium- and long-latency responses (LLR) in the soleus muscle. Motor-evoked potential- (MEP) and Hcond-amplitudes as well as Hmax/Mmax ratios were determined at SLR and LLR. Postural stability was measured during perturbation on the treadmill.

RESULTS

Balance training improved postural stability. Hmax/Mmax ratios were significantly decreased at LLR. MEPs and Hcond revealed significantly reduced facilitation at LLR following training. A negative correlation between adaptations of Hcond and changes in stance stability was observed (r = -0.87; P < 0.01) while no correlation was found between stance stability and changes in Hmax/Mmax ratio. No changes in any parameter occurred at the spinally organized SLR and in the control group.

CONCLUSION

The decrease in MEP- and Hcond-facilitation implies reduced corticospinal and cortical excitability at the transcortically mediated LLR. Changes in cortical excitability were directly related to improvements in stance stability as shown by correlation of these parameters. The absence of such a correlation between Hmax/Mmax ratios and stance stability suggests that mainly supraspinal adaptations contributed to improved balance performance following training.

Positive force feedback in human walking

The objective of this study was to determine if load receptors contribute to the afferent-mediated enhancement of ankle extensor muscle activity during the late stance phase of the step cycle. Plantar flexion perturbations were presented in late stance while able-bodied human subjects walked on a treadmill that was declined by 4%, inclined by 4% or held level. The plantar flexion perturbation produced a transient, but marked, presumably spinally mediated decrease in soleus EMG that varied directly with the treadmill inclination. Similarly, the magnitude of the control step soleus EMG and Achilles' tendon force also varied directly with the treadmill inclination. In contrast, the ankle angular displacement and velocity were inversely related to the treadmill inclination. These results suggest that Golgi tendon organ feedback, via the group Ib pathway, is reduced when the muscle-tendon complex is unloaded by a rapid plantar flexion perturbation in late stance phase. The changes in the unload response with treadmill inclination suggest that the late stance phase soleus activity may be enhanced by force feedback.

Facilitation of the soleus stretch reflex induced by electrical excitation of plantar cutaneous afferents located around the heel

Previous studies have demonstrated that plantar cutaneous afferents can adjust motoneuronal excitability, which may contribute significantly to the control of human posture and locomotion. However, the role of plantar cutaneous afferents with respect to their location specificity in modulating the mechanically induced stretch reflex still remains unclear. In the present study, it was hypothesized that electrical stimulation of the ipsilateral heel region of the foot is followed by a modulation of spinal excitability, leading to a facilitation of the soleus motor output. The study was performed to investigate the effect of excitation of plantar cutaneous afferents located around the heel on the soleus stretch reflex. The soleus stretch reflex was evoked by rotating the ankle joint in dorsiflexion direction at two different angular velocities of 50 and 200 degrees s(-1). A conditioning pulse train of non-noxious electrical stimulation was delivered to the plantar surface of the heel at different conditioning test intervals ranging from 5 to 100 ms. Excitation of plantar cutaneous afferents around the heel resulted in a pronounced facilitation of the soleus stretch reflex with magnitude and time course comparable for both velocities. This facilitation was manifested by a significant increase of reflex size for conditioning test intervals from 30 to 70 ms. The observed effect implies a potential functional role of cutaneous afferents in balance control conditions where the ankle is naturally disturbed, e.g., during step reactions to external perturbations.

Muscle activation following sudden ankle inversion during standing and walking

Dynamic response characteristics of ankle musculature following sudden ankle inversion have traditionally been tested in a static, standing position. However, this model does not take into consideration muscle activity and loading characteristics associated with active gait. This study compared muscle reaction times and amplitudes from sudden ankle inversion during standing (standing group) and walking (walking group) using one of two similar devices for each of these conditions. Surface EMG was collected from the peroneus longus (PL), brevis (PB), and tibialis anterior (TA) of the dominant leg from 25 subjects (age 20 +/- 1 years, height 174.0 +/- 10.2 cm, mass 74.3 +/- 12.9 kg) for each condition (walking and standing). Time to total inversion ROM (28 degrees ) was greater in the walking group (114.9 +/- 15.0 ms) than the standing group (65.6 +/- 17.8 ms, P < 0.05), whereas reaction time was less in the peroneals in the walking group (PL 56.9 +/- 8.4 ms, PB 60.1 +/- 10.6 ms, TA 65.0 +/- 14.9 ms) compared to the standing group (PL 74.3 +/- 8.5 ms, PB 73.5 +/- 8.2 ms, TA 73.3 +/- 8.3, P < 0.05). Additionally, Peak normalized EMG (% MVIC) for the walking condition (PL 367 +/- 254, PB 405 +/- 359, TA 84 +/- 39) exceeded that of the standing condition (PL 310 +/- 239, PB 328 +/- 215, TA 76 +/- 39, P < 0.05), and average normalized EMG (% MVIC) was greater in the peroneals for the walking condition (PL 233 +/- 171, PB 280 +/- 255) than the standing condition (PL 164 +/- 131, PB 193 +/- 137, P < 0.05). The differences noted between the conditions provide evidence that a dynamic response to ankle injury mechanisms is much different in a walking model compared to a traditional standing model. A walking model may be a more functional approach for evaluating dynamic response characteristics of ankle musculature due to sudden ankle inversion.

Direct corticospinal pathways contribute to neuromuscular control of perturbed stance

The antigravity soleus muscle (Sol) is crucial for compensation of stance perturbation. A corticospinal contribution to the compensatory response of the Sol is under debate. The present study assessed spinal, corticospinal, and cortical excitability at the peaks of short- (SLR), medium- (MLR), and long-latency responses (LLR) after posterior translation of the feet. Transcranial magnetic stimulation (TMS) and peripheral nerve stimulation were individually adjusted so that the peaks of either motor evoked potential (MEP) or H reflex coincided with peaks of SLR, MLR, and LLR, respectively. The influence of specific, presumably direct, corticospinal pathways was investigated by H-reflex conditioning. When TMS was triggered so that the MEP arrived in the Sol at the same time as the peaks of SLR and MLR, EMG remained unaffected. Enhanced EMG was observed when the MEP coincided with the LLR peak ( P < 0.001). Similarly, conditioning of the H reflex by subthreshold TMS facilitated H reflexes only at LLR ( P < 0.001). The earliest facilitation after perturbation occurred after 86 ms. The TMS-induced H-reflex facilitation at LLR suggests that increased cortical excitability contributes to the augmentation of the LLR peaks. This provides evidence that the LLR in the Sol muscle is at least partly transcortical, involving direct corticospinal pathways. Additionally, these results demonstrate that ∼86 ms after perturbation, postural compensatory responses are cortically mediated.

Modulations of interlimb and intralimb cutaneous reflexes during simultaneous arm and leg cycling in humans

OBJECTIVE

We investigated to what extent intralimb and interlimb cutaneous reflexes are altered while simultaneously performing arm and leg cycling (AL cycling) under different kinematic and postural conditions.

METHODS

Eleven subjects performed AL cycling under conditions in which the arm and leg crank ipsilateral to the stimulation side were moved synchronously (in-phase cycling) or asynchronously (anti-phase cycling) while sitting or standing. Cutaneous reflexes following superficial radial or superficial peroneal nerve stimulation (2.0-2.5 times radiating threshold, 5 pulses at 333 Hz) were recorded at 4 different pedal positions from 12 muscles in the upper and lower limbs. Cutaneous reflexes with a peak latency of 80-120 ms were then analyzed.

RESULTS

The magnitude of interlimb and intralimb cutaneous reflexes in the arm and leg muscles was significantly modulated depending on the crank position for the relevant limb (phase-dependent modulation). A significant correlation between the magnitude of the cutaneous reflex and background EMG was observed in the majority of muscles during static contraction, but not during AL cycling (task-dependent modulation). No significant difference was found in comparisons of the magnitude of intralimb and interlimb cutaneous reflexes obtained during in- and anti-phase AL cycling. Qualitatively, the same results were obtained during AL cycling while sitting or standing. In addition, the modulation of cutaneous reflexes in arm muscles was identical among in-phase, anti-phase and isolated arm cycling. Results were the same for leg muscles.

CONCLUSIONS

Cutaneous reflexes in arm muscles are little influenced by rhythmic movement of the legs and vice versa during AL cycling. It is likely that neural components that control interlimb reflexes are loosely coupled during AL cycling while sitting or standing.

SIGNIFICANCE

Our results provide a better understanding of the coordination between the upper and lower limbs during rhythmic movement.

Dynamic sensorimotor interactions in locomotion

Locomotion results from intricate dynamic interactions between a central program and feedback mechanisms. The central program relies fundamentally on a genetically determined spinal circuitry (central pattern generator) capable of generating the basic locomotor pattern and on various descending pathways that can trigger, stop, and steer locomotion. The feedback originates from muscles and skin afferents as well as from special senses (vision, audition, vestibular) and dynamically adapts the locomotor pattern to the requirements of the environment. The dynamic interactions are ensured by modulating transmission in locomotor pathways in a state- and phase-dependent manner. For instance, proprioceptive inputs from extensors can, during stance, adjust the timing and amplitude of muscle activities of the limbs to the speed of locomotion but be silenced during the opposite phase of the cycle. Similarly, skin afferents participate predominantly in the correction of limb and foot placement during stance on uneven terrain, but skin stimuli can evoke different types of responses depending on when they occur within the step cycle. Similarly, stimulation of descending pathways may affect the locomotor pattern in only certain phases of the step cycle. Section ii reviews dynamic sensorimotor interactions mainly through spinal pathways. Section iii describes how similar sensory inputs from the spinal or supraspinal levels can modify locomotion through descending pathways. The sensorimotor interactions occur obviously at several levels of the nervous system. Section iv summarizes presynaptic, interneuronal, and motoneuronal mechanisms that are common at these various levels. Together these mechanisms contribute to the continuous dynamic adjustment of sensorimotor interactions, ensuring that the central program and feedback mechanisms are congruous during locomotion.

Hamstring injury management--Part 2: Treatment

The management of hamstring injuries can be described as vexed at best. One reason for this may be because of a lack of high-quality research into the methods of treatment, rehabilitation and prevention. As a result, an evidence-based approach to injury management does not exist. Management is based on clinical experience, anecdotal evidence and the knowledge of the biological basis of tissue repair. Previous hamstring injury is the most recognized risk factor for injury, which indicates that treatment approaches may be suboptimal under certain conditions. The identification of these risk factors and the methods best designed to manage them should be addressed with future research efforts. Much anecdotal and indirect evidence exists to suggest that several non-local factors contribute to injury. Despite the knowledge that these factors may exist, the literature appears almost devoid of research investigating their possible identification and treatment. Treatment has traditionally been in the form of altering the muscle repair process through the application of electrophysical therapy and various soft-tissue-based and exercise-based techniques. Little research has investigated the role of other forms of manual therapy particularly when directed at non-local structures. This paper will explore and speculate on this potential connection and offer some new contributive factors for hamstring injury management.

Modulation of motor cortex excitability by sustained peripheral stimulation: The interaction between the motor cortex and the cerebellum

The excitability of cortical neurons in the motor cortex is determined by their membrane potential and by the level of intracortical inhibition. The excitability of the motor cortex as a whole is a function of single cell excitability, synaptic strength, and the balance between excitatory cells and inhibitory cells. It is now established that a sustained period of somatosensory stimulation increases the excitability of motor cortex areas controlling muscles in those body parts that received the stimulation prior to excitability testing. So far, it has been supposed that the sensorimotor cortex was the anatomical substrate of these excitability changes, which could represent an early change in cortical network function before structural plasticity occurs. Recent experimental studies highlight that the cerebellum, especially the interpositus nucleus, plays a key role in the adaptation of the motor cortex to repeated trains of stimulation. Interpositus neurons, which receive inputs from both sensorimotor cortex and the spinal cord, are involved in somesthetic reflex behaviors and assist the cerebral cortex in transforming sensory signals to motor-oriented commands by acting via the cerebello-thalamo-cortical projections. Moreover, climbing fibers originating in the inferior olivary complex and innervating the nucleus interpositus mediate highly integrated sensorimotor information derived from spinal modules. It appears that the interpositus nucleus is a main subcortical modulator of the excitability changes occurring in the motor cortex, which may be a substrate of early plasticity effective in motor learning and recovery from lesion.

The management of hamstring injury—Part 1: Issues in diagnosis

Hamstring injuries are the most prevalent muscle injury in sports involving rapid acceleration and maximum speed running. Injury typically occurs in an acute manner through an eccentric mechanism at the terminal stages of the swing phase of gait. Biceps femoris is most commonly injured. Re-injury rates are high and management is a challenge given the complex multi-factorial aetiology. The high rates of hamstring injury and re-injury may result from a lack of high-quality research into the aetiological factors underlying injury. Re-injury may also result from inaccuracy in diagnosis that results from the potential multi-factorial causes of these conditions. Inaccuracy in diagnosis could lead to multiple potential diagnoses that may result in the implementation of variable management protocols. Whilst potentially useful, such variability may also lead to the implementation of sub-optimal management strategies. Previous hamstring injury is the most recognized risk factor for injury, which indicates that future research should be directed at preventative measures. Much anecdotal and indirect evidence exists to suggest that several non-local factors contribute to injury, which may be addressed through the application of manual therapy. However, this connection has been neglected in previous research and literature. This paper will explore and speculate on this potential connection and offer some new contributive factors for hamstring injury management. This first paper of a two part series on hamstring injury will explore diagnostic issues relevant to hamstring injury and the second will investigate various established and speculative management approaches.

Neuromuscular Reflexes Contribute to Knee Stiffness During Valgus Loading

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

Gait acts as a gate for reflexes from the foot

During human gait, electrical stimulation of the foot elicits facilitatory P2 (medium latency) responses in TA (tibialis anterior) at the onset of the swing phase, while the same stimuli cause suppressive responses at the end of swing phase, along with facilitatory responses in antagonists. This phenomenon is called phase-dependent reflex reversal. The suppressive responses can be evoked from a variety of skin sites in the leg and from stimulation of some muscles such as rectus femoris (RF). This paper reviews the data on reflex reversal and adds new data on this topic, using a split-belt paradigm. So far, the reflex reversal in TA could only be studied for the onset and end phases of the step cycle, simply because suppression can only be demonstrated when there is background activity. Normally there are only 2 TA bursts in the step cycle, whereas TA is normally silent during most of the stance phase. To know what happens in the stance phase, one needs to have a means to evoke some background activity during the stance phase. For this purpose, new experiments were carried out in which subjects were asked to walk on a treadmill with a split-belt. When the subject was walking with unequal leg speeds, the walking pattern was adapted to a gait pattern resembling limping. The TA then remained active throughout most of the stance phase of the slow-moving leg, which was used as the primary support. This activity was a result of coactivation of agonistic and antagonistic leg muscles in the supporting leg, and represented one of the ways to stabilize the body. Electrical stimulation was given to a cutaneous nerve (sural) at the ankle at twice the perception threshold. Nine of the 12 subjects showed increased TA activity during stance phase while walking on split-belts, and 5 of them showed pronounced suppressions during the first part of stance when stimuli were given on the slow side. It was concluded that a TA suppressive pathway remains open throughout most of the stance phase in the majority of subjects. The suggestion was made that the TA suppression increases loading of the ankle plantar flexors during the loading phase of stance.

Motor cortical modulation of cutaneous reflex responses in the hindlimb of the intact cat

We have used the technique of spatial facilitation to examine the interactions between the signals conveyed by the corticospinal tract and those of cutaneous afferents in the hindlimb of the intact, walking cat. Microstimulation was applied to 20 cortical sites in the hindlimb representation of the motor cortex and to three different cutaneous nerves innervating the hindpaw in four cats. Conditioning stimuli to the motor cortex induced both facilitation and depression of cutaneous reflexes evoked by stimulation of nerves in the hindlimb contralateral to the cortical stimulation site. Facilitation was most frequently evoked by conditioning stimuli in the range of 10-30 ms before the cutaneous stimulation; depression was normally evoked by shorter and longer conditioning delays. Similar changes were observed after conditioning stimuli to the pyramidal tract, suggesting that the changes were independent of any changes in cortical excitability. Modulation of reflex activity varied according to the muscle under study, the cutaneous nerve used to evoke the reflex and the cortical site used to condition the reflex. Together, these results suggest that there is spatial convergence of corticospinal and cutaneous afferent activity and that this convergence is mediated by distinct subpopulations of spinal interneurons.

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° shoulder flexion or 10° 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.

Joint-afferent-mediated muscle activations yield a near-maximum torque response of the quadriceps

Previous work from our laboratory has shown that reflex activity is systematically evoked in a number of major knee muscles by large (>5 degrees ) (non-physiological) abduction angular perturbations of the human knee. This reflex action was shown to originate from periarticular tissue afferents. Furthermore, it was demonstrated that specific muscle activation patterns exist in knee muscles with preferential activation in medial muscles in response to the lateral perturbations. This study examines the hypothesis that in response to the mechanical stimulus, the sensory information mediated by these afferents results in activation patterns that provide the largest resisting moment by the knee muscles. It is further hypothesized that this near maximum resistance cannot be achieved by the selective activation of medial muscles alone. To examine this, the previously reported mechanically induced reflex EMG activation patterns, a stochastic 3D musculoskeletal patello-femoral joint model, and new data from selective electrical stimulation experiments were used. Using the model, the knee adduction-abduction moment in response to an applied abduction load at the knee joint for every possible random set of quadriceps activity was computed. These adduction moments were then compared to the adduction moment computed by the model when the mechanically induced muscle activation patterns were used. The data presented here illustrated that selective activation of a medial muscle alone would result in an abduction moment, regardless of the knee flexion angle. Furthermore, the findings of this study revealed that the recorded combinations of muscle activity provide a near maximum capability of the quadriceps muscles to resist externally applied abducting stimuli. It was concluded that stabilization in the abduction direction could only be achieved by a control strategy that involves activation of both medial and lateral muscles at the knee. It was also concluded that this control strategy was near optimal when mediated by joint afferents.

On the reflex coactivation of ankle flexor and extensor muscles induced by a sudden drop of support surface during walking in humans

Recent studies have revealed that the stretch reflex responses of both ankle flexor and extensor muscles are coaugmented in the early stance phase of human walking, suggesting that these coaugmented reflex responses contribute to secure foot stabilization around the heel strike. To test whether the reflex responses mediated by the stretch reflex pathway are actually induced in both the ankle flexor and extensor muscles when the supportive surface is suddenly destabilized, we investigated the electromyographic (EMG) responses induced after a sudden drop of the supportive surface at the early stance phase of human walking. While subjects walked on a walkway, the specially designed movable supportive surface was unexpectedly dropped 10 mm during the early stance phase. The results showed that short-latency reflex EMG responses after the impact of the drop (<50 ms) were consistently observed in both the ankle flexor and extensor muscles in the perturbed leg. Of particular interest was that a distinct response appeared in the tibialis anterior muscle, although this muscle showed little background EMG activity during the stance phase. These results indicated that the reflex activities in the ankle muscles certainly acted when the supportive surface was unexpectedly destabilized just after the heel strike during walking. These reflex responses were most probably mediated by the facilitated stretch reflex pathways of the ankle muscles at the early stance phase and were suggested to be relevant to secure stabilization around the ankle joint during human walking.

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 (approximately 50-80 ms) and middle (approximately 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.

Facilitation of both stretch reflex and corticospinal pathways of the tibialis anterior muscle during standing in humans

Excitability of both stretch reflex (SR) and motor evoked potential (MEP) elicited in the tibialis anterior (TA) muscle by transcranial magnetic stimulation were tested in standing humans. The results demonstrated significantly greater values for both SR and MEP in the TA while standing than while in the supine posture, although background electromyographic activity was silent in the two conditions. Taken together with previous reports that both pathways are facilitated in the TA at the early stance phase of human walking, our findings suggest that a common neural mechanism underlies both observations, one that might be functionally relevant for securing ankle joint stabilization during upright standing.

Functional coupling of motor units is modulated during walking in human subjects

Time- and frequency-domain analysis of the coupling between pairs of electromyograms (EMG) recorded from leg muscles was investigated during walking in healthy human subjects. For two independent surface EMG signals from the tibialis anterior (TA) muscle, coupling estimated from coherence measurements was observed at frequencies </=50 Hz, with identifiable peaks occurring in two frequency bands ranging approximately from 8 to 15 and 15 to 20 Hz. The coherence between TA recordings was greatest toward the end of swing, reduced in early swing, and largely absent in midswing. In time-domain estimates constructed from paired TA EMG recordings, a short-lasting central peak indicative of motor-unit synchronization was observed. This feature of motor-unit coupling was also reduced in mid swing. In paired recordings made among triceps surae, quadriceps, and hamstring muscles, a similar pattern of correlation to that for paired TA recordings was observed. However, no significant coupling was observed in recordings for which one EMG recording was made from an ankle flexor/extensor muscle and the other from a knee extensor/flexor muscle. These results demonstrate that for TA a modulation exists in the functional coupling of motor units recruited during swing. The data also indicate that human motoneurons belonging to different muscles are only weakly coupled during walking. This absence of widespread short-term synchronization between the activities of muscles of the leg may provide a basis for the highly adaptive nature of human gait patterns.

[The appearance manner of C-response in healthy individuals]

The existence of long-latency responses following M and H waves in the complex muscular action potential elicited by stimulation of peripheral nerves was reported by Upton et al. This electrical potential, called the C-response, is applied to examinations in central nervous system diseases. However, the pathway and details on fundamental types of waveforms have not yet been clarified. The main purpose of this study is to classify the waveforms of the C-response, based on the analysis of waveforms. To investigate the type of C-responses, we developed a modified superimposing method. We also investigated the types in different age groups. Fifty-seven healthy individuals (30 males and 27 females) aged between 20 and 73 (average age: 43.1 years old) were enrolled as subjects. All subjects were instructed to oppose the thumb against the little finger of their dominant hands so that the abductor pollicis brevis was in voluntary isometric contraction; subsequently, electrical stimuli were repeatedly applied to the median nerve at the wrist. The stimuli had a strength of 110% of the threshold value of the M wave. The electrical potential was recorded with surface electrodes placed on the muscle belly of the abductor pollicis brevis. In each measurement, 200 waveforms were averaged. A Neuropack 8 (Nihon Kohden, Co., Ltd.) was used for recording and analysis of electromyograms. Measured negative peak latencies (ms) were divided by the subjects' heights (m) to obtaining the corrected latencies per unit height. The time axis of a waveform was also corrected with each height, and shifted so that the latency of the negative peak of the H wave was observed at the same position (modified superimposing method). Then, the position where the negative peak of the C-response appeared most frequently was examined. We clarified that C-responses have three types. C-responses have two major negative peaks basically, C1 and C2 (the latency of C1 is shorter than that of C2). Type 1 has only C2; Type 2 has C1 and C2; Type 3 has both C1 and C2 but the latency of C2 is shorter than that of Type 2. Type 1 was observed in 37 cases (64.9%), Type 2 in 15 cases (26.3%), and Type 3 in 5 cases (8.8%). The incidence of each C-response type depended on the age of the subjects, Type 1 was observed frequently in young subjects, and Types 2 and 3 were observed more frequently as the age of the subjects increased.

Changes in intracortical excitability induced by stimulation of wrist afferents in man

1. Inhibitory and facilitatory neuronal circuits may be explored in the human motor cortex by double pulse transcranial magnetic stimulation (TMS). At short interstimulus intervals (2-5 ms), conditioned motor-evoked potentials (MEPs) are reduced (intracortical inhibition, ICI), whereas they are facilitated at longer interstimulus intervals (8-25 ms; intracortical facilitation, ICF). The aim of this study was to investigate the effects of homonymous and antagonist nerve stimulation on the intracortical inhibition and facilitation in the cortical areas that control the wrist extensor and flexor radialis muscles. 2. Sixteen subjects were asked to contract either their wrist extensor or flexor muscles. The MEP evoked by a test TMS (at 1.2 x MEP threshold) and recorded in the target muscle was then conditioned by subthreshold TMS (at 0.8 x MEP threshold) 2 and 14 ms before the test TMS. The median and radial nerves were stimulated at 0.8 x motor threshold (MT). 3. In both flexor and extensor muscles, antagonist nerve stimulation 40 ms before the test TMS decreased ICI and increased ICF. In contrast, homonymous nerve stimulation had no effect on ICI and ICF. 4. The intensity of the antagonist nerve stimulation required to alter ICI and ICF was as low as 0.6 x MT, which suggests that thick diameter afferents may be involved. The nerve stimulation had to be applied 35-45 ms prior to the test TMS to alter significantly the intracortical excitability. 5. Cutaneous afferents were probably not responsible for the alterations of intracortical excitability, since cutaneous stimulation had no effect on either ICI or ICF at the investigated intervals. 6. The present data suggest that antagonist muscular afferent inputs may evoke reciprocal facilitation or disinhibition at the cortical level. This pattern of antagonist sensory afferent effects may be of significance for control of the wrist extensor and flexor muscles when used as synergists during manipulatory finger movements and gripping tasks.

Neural control: novel evaluation of stretch reflex sensitivity

We evaluated the stretch reflex activities of the elbow flexor and extensor muscles considering the relationship between the reflex electromyographic (EMG) responses and their corresponding standardized muscle stretch velocities. Specifically, muscular stretch velocity was estimated by using ultrasonograms. Stretch reflex EMG responses were elicited in the biceps brachii, brachioradialis and triceps brachii with a ramp-and-hold rotation at the elbow joint, which consisted of various angular velocities for the extension- or flexion-direction. The whole muscle stretch velocity induced by each ramp-and-hold rotation was calculated on the basis of fibre length changes associated with the elbow joint angle. A linear regression equation was fitted to the relation between the whole muscle stretch velocity and the reflex EMG responses, and the variables from the equation were used to quantify sensitivity of each reflex EMG component. The reflex EMG responses were increased as the ramp-and-hold rotational velocity increased. There were no significant differences in the recorded magnitudes of reflex EMG responses with equivalent joint rotational velocity between the brachioradialis and the triceps brachii medial head. These muscles showed the highest reflex responses in the flexor and extensor muscles, respectively. To the contrary, the reflex EMG response elicited by the standardized muscle stretches was significantly greater in the extensor muscles, indicating a higher reflex sensitivity. This was because of the lower muscle stretch velocity of the triceps brachii with an equivalent elbow joint rotation. The stretch reflex sensitivity in both the elbow flexor and extensor muscles might be regulated so as to make the reflex responses the same when the equivalent joint rotational velocity is applied to these muscles.

State-dependent modulation of sensory feedback

By tradition - and for historical reasons - reflex pathways and interneurones have been named by their dominating sensory input. Later studies have demonstrated that each individual interneurone, as a rule, receives a broad convergence from a large variety of sensory modalities, as well as inputs from one or more descending tracts. It is thus possible that the traditional nomenclature inadvertently has served as a 'straightjacket' for conceptual development in this field. Indeed, there is now much evidence in favour of the view that the many classes of spinal interneurones may be seen as 'functional units' representing different levels of muscle synergies, parts of movements, or even more integrated motor behaviour. Such 'functional units' may be used by (different) descending pathways to mediate the motor commands from the brain and integrate the appropriate (multimodal) sensory feedback into the central command. A given sensory stimulus would then be able to affect the motor output through a number of parallel, or alternative, segmental pathways belonging to different 'functional units'. If this were correct it would indeed be predicted, rather than coming as a surprise, that a given sensory stimulus can result in different outputs - even with a different sign - depending on the preceding selection of active 'functional units', i.e. the type of motor activity initiated by the brain.