Proprioceptive control of human wrist extensor motor units during an attention-demanding task

Spinal and corticospinal excitability changes with voluntary modulation of motor cortex oscillations

AIM

The aim of this study was to investigate the effects of EEG neurofeedback (NF)-induced modulation of sensorimotor alpha (i.e., mu) rhythm on spinal and corticospinal tract (CST) excitability.

METHODS

Forty-three healthy volunteers participated in 3 sessions of EEG-NF for upregulation (N=24) or downregulation (N=19) of individual alpha oscillations at central location Cz. Spinal excitability was studied before and during NF using H-reflex of the soleus muscle, and CST excitability was tested before and after NF, through Motor-Evoked Potential (MEP) of the tibialis anterior muscle. Mu rhythm was extracted using current source density. Differences in MEP and H-reflex before and during/after NF were analysed using repeated measures analysis. The relationship with motor cortexcortical excitability was estimated through linear regression between change in MEP/H-reflex, and change in power of mu rhythm and the upper portion of mu rhythm, muh.

RESULTS

CST excitability changes were significantly correlated to change in muh (p-value < 0.044, |r|>0.42), while spinal excitability changes were correlated to broad mu power modulation (p-value < 0.04, |r|> 0.43). While no distinct effect of NF on spinal versus CST excitability was found, the correlations indicate an inverted U-shape relationship between cortical and subcortical excitability. The trends of the correlations between spinal/CST excitability change and EEG power change were preserved when participants were grouped by success at NF task, and by mu modulation outcome, indicating that the net effect of power change at Cz weighs more than the task the participants attempted to accomplish.

CONCLUSIONS

The consistent direction of mu power correlation with both MEP, tested after NF, and H-reflex, tested during NF, indicates that modifications in mu activity are associated with spinal and CST adaptations lasting beyond the NF session, evidencing neuroplasticity. Together with the inverted U-shape relationship found between amplitude of mu modulation and spinal/CST excitability change, the results provide support for further research and clinical implementation of NF to induce CNS plasticity, a prerequisite for effective neural rehabilitation.

Visual feedback-dependent modulation of arousal, postural control, and muscle stretch reflexes assessed in real and virtual environments

Introduction

The mechanisms regulating neuromuscular control of standing balance can be influenced by visual sensory feedback and arousal. Virtual reality (VR) is a cutting-edge tool for probing the neural control of balance and its dependence on visual feedback, but whether VR induces neuromodulation akin to that seen in real environments (eyes open vs. closed or ground level vs. height platform) remains unclear.

Methods

Here we monitored 20 healthy young adults (mean age 23.3 ± 3.2 years; 10 females) during four conditions of quiet standing. Two real world conditions (eyes open and eyes closed; REO and REC) preceded two eyes-open virtual 'low' (ground level; VRL) and 'high' (14 m height platform; VRH) conditions. We measured arousal via electrodermal activity and psychosocial questionnaires rating perceived fear and anxiety. We recorded surface electromyography over the right soleus, medial gastrocnemius, and tibialis anterior, and performed force plate posturography. As a proxy for modulations in neural control, we assessed lower limb reflexive muscle responses evoked by tendon vibration and electrical stimulation.

Results

Physiological and perceptual indicators of fear and anxiety increased in the VRH condition. Background soleus muscle activation was not different across conditions; however, significant increases in muscle activity were observed for medial gastrocnemius and tibialis anterior in VRH relative to REO. The mean power frequency of postural sway also increased in the VRH condition relative to REO. Finally, with a fixed stimulus level across conditions, mechanically evoked reflexes remained constant, while H-reflex amplitudes decreased in strength within virtual reality.

Discussion

Notably, H-reflexes were lower in the VRL condition than REO, suggesting that these ostensibly similar visual environments produce different states of reflexive balance control. In summary, we provide novel evidence that VR can be used to modulate upright postural control, but caution that standing balance in analogous real and virtual environments may involve different neural control states.

The effect of visuospatial resolution on discharge variability among motor units and force-discharge relation

Although force steadiness varies with visuospatial information, accountable motor unit (MU) behaviors are not fully understood. This study investigated the modulation of MU discharges and force-discharge relation due to variations in the spatial resolution of visual feedback, with a particular focus on discharge variability among MUs. Fourteen young adults produced isometric force at 10% of maximal voluntary contraction (MVC) through index abduction, under the conditions of force trajectory displayed with low visual gain (LVG) and high visual gain (HVG). Together with smaller and more complex force fluctuations, HVG resulted in greater variabilities of the mean interspike interval and discharge irregularity among MUs than LVG did. Estimated via smoothening of a cumulative spike train of all MUs, global discharge rate was tuned to visual gain, with a more complex global discharge rate and a lower force-discharge relation in the HVG condition. These higher discharge variabilities were linked to larger variance of the common drive received by MUs for regulation of muscle force with higher visuospatial information. In summary, higher visuospatial information improves force steadiness with more complex force fluctuations, underlying joint effects of low-pass filter property of the musculotendon complex and central modulation of discharge variability among MUs.

Functional properties of human muscle spindles

Muscle spindles are ubiquitous encapsulated mechanoreceptors found in most mammalian muscles. There are two types of endings, primary and secondary, and both are sensitive to changes in muscle length and velocity, with the primary endings having a greater dynamic sensitivity. Unlike other mechanoreceptors in the somatosensory system, muscle spindles are unique in possessing motor innervation, via γ-motoneurons (fusimotor neurons), that control their sensitivity to stretch. Much of what we know about human muscles spindles comes from studying the behavior of their afferents via intraneural microelectrodes (microneurography) inserted into accessible peripheral nerves. We review the functional properties of human muscle spindles, comparing and contrasting with what we know about the functions of muscle spindles studied in experimental animals. As in the cat, many human muscle spindles possess a background discharge that is related to the degree of muscle stretch, but mean firing rates are much lower (~10 Hz). They can faithfully encode changes in muscle fascicle length in passive conditions, but higher level extraction of information is required by the central nervous system to measure changes in muscle length during muscle contraction. Moreover, although there is some evidence supporting independent control of human muscle spindles via fusimotor neurons, any effects are modest compared with the clearly independent control of fusimotor neurons observed in the cat.

Relation Between the Frequency of Short-Pulse Electrical Stimulation of Afferent Nerve Fibers and Evoked Muscle Force

Objective: Functional electrical stimulation (FES) is conventionally performed by the stimulation of motor axons causing the muscle fibers innervated by these axons to contract. An alternative strategy that may evoke contractions with more natural motor unit behavior is to stimulate afferent fibers (primarily type Ia) to excite the motor neurons at the spinal level. The aim of the study was to investigate the range of forces that can be evoked in this way and the degree to which the torque can be controlled. Methods: We stimulated the tibial nerve of ten healthy participants at amplitudes at which the highest H-reflex with minimal M-wave was present. The evoked plantar flexion torque was recorded following short stimulation pulses (0.4 ms) with frequencies ranging from 20 to 200 Hz. Results: Across all subjects, the median highest evocable torque was 38.3% (quartiles: 16.9-51.0) of the maximum voluntary contraction torque (MVC). The average torque variability (standard deviation) was 1.7 +/- 0.7% MVC. For most subjects, the relation between stimulation frequency and evoked torque was well characterized by sigmoidal curves (median root mean square error: 6.4% MVC). The plateau of this sigmoid curve (indicating the range of frequencies over which torque amplitude could be modulated) was reached at 56.0 (quartiles: 29.4-81.9) Hz. Conclusion: Using the proposed method for FES, substantial evoked torques that could be controlled by stimulation frequency were achieved. Significance: Stimulation of afferent fibers could be a useful and fatigue-resistant strategy for several applications of FES.Objective: Functional electrical stimulation (FES) is conventionally performed by the stimulation of motor axons causing the muscle fibers innervated by these axons to contract. An alternative strategy that may evoke contractions with more natural motor unit behavior is to stimulate afferent fibers (primarily type Ia) to excite the motor neurons at the spinal level. The aim of the study was to investigate the range of forces that can be evoked in this way and the degree to which the torque can be controlled. Methods: We stimulated the tibial nerve of ten healthy participants at amplitudes at which the highest H-reflex with minimal M-wave was present. The evoked plantar flexion torque was recorded following short stimulation pulses (0.4 ms) with frequencies ranging from 20 to 200 Hz. Results: Across all subjects, the median highest evocable torque was 38.3% (quartiles: 16.9-51.0) of the maximum voluntary contraction torque (MVC). The average torque variability (standard deviation) was 1.7 +/- 0.7% MVC. For most subjects, the relation between stimulation frequency and evoked torque was well characterized by sigmoidal curves (median root mean square error: 6.4% MVC). The plateau of this sigmoid curve (indicating the range of frequencies over which torque amplitude could be modulated) was reached at 56.0 (quartiles: 29.4-81.9) Hz. Conclusion: Using the proposed method for FES, substantial evoked torques that could be controlled by stimulation frequency were achieved. Significance: Stimulation of afferent fibers could be a useful and fatigue-resistant strategy for several applications of FES.

Emergence of gamma motor activity in an artificial neural network model of the corticospinal system

Muscle spindle discharge during active movement is a function of mechanical and neural parameters. Muscle length changes (and their derivatives) represent its primary mechanical, fusimotor drive its neural component. However, neither the action nor the function of fusimotor and in particular of γ-drive, have been clearly established, since γ-motor activity during voluntary, non-locomotor movements remains largely unknown. Here, using a computational approach, we explored whether γ-drive emerges in an artificial neural network model of the corticospinal system linked to a biomechanical antagonist wrist simulator. The wrist simulator included length-sensitive and γ-drive-dependent type Ia and type II muscle spindle activity. Network activity and connectivity were derived by a gradient descent algorithm to generate reciprocal, known target α-motor unit activity during wrist flexion-extension (F/E) movements. Two tasks were simulated: an alternating F/E task and a slow F/E tracking task. Emergence of γ-motor activity in the alternating F/E network was a function of α-motor unit drive: if muscle afferent (together with supraspinal) input was required for driving α-motor units, then γ-drive emerged in the form of α-γ coactivation, as predicted by empirical studies. In the slow F/E tracking network, γ-drive emerged in the form of α-γ dissociation and provided critical, bidirectional muscle afferent activity to the cortical network, containing known bidirectional target units. The model thus demonstrates the complementary aspects of spindle output and hence γ-drive: i) muscle spindle activity as a driving force of α-motor unit activity, and ii) afferent activity providing continuous sensory information, both of which crucially depend on γ-drive.

The vestibular system does not modulate fusimotor drive to muscle spindles in relaxed leg muscles of subjects in a near-vertical position

It has been shown that sinusoidal galvanic vestibular stimulation (sGVS) has no effect on the firing of spontaneously active muscle spindles in either relaxed or voluntarily contracting human leg muscles. However, all previous studies have been conducted on subjects in a seated position. Given that independent vestibular control of muscle spindle firing would be more valuable during postural threat, we tested the hypothesis that this modulation would become apparent for subjects in a near-vertical position. Unitary recordings were made from 18 muscle spindle afferents via tungsten microelectrodes inserted percutaneously into the common peroneal nerve of awake human subjects laying supine on a motorized tilt table. All recorded spindle afferents were spontaneously active at rest, and each increased its firing rate during a weak static contraction. Sinusoidal bipolar binaural galvanic vestibular stimulation (±2 mA, 100 cycles) was applied to the mastoid processes at 0.8 Hz. This continuous stimulation produced a sustained illusion of "rocking in a boat" or "swinging in a hammock." The subject was then moved into a near-vertical position (75°), and the stimulation repeated. Despite robust vestibular illusions, none of the fusimotor-driven spindles exhibited phase-locked modulation of firing during sinusoidal GVS in either position. We conclude that this dynamic vestibular stimulus was insufficient to modulate the firing of fusimotor neurons in the near-vertical position. However, this does not mean that the vestibular system cannot modulate the sensitivity of muscle spindles via fusimotor neurons in free unsupported standing, when reliance on proprioceptive feedback is higher.

Physiological recruitment of motor units by high-frequency electrical stimulation of afferent pathways

Neuromuscular electrical stimulation (NMES) is commonly used in rehabilitation, but electrically evoked muscle activation is in several ways different from voluntary muscle contractions. These differences lead to challenges in the use of NMES for restoring muscle function. We investigated the use of low-current, high-frequency nerve stimulation to activate the muscle via the spinal motoneuron (MN) pool to achieve more natural activation patterns. Using a novel stimulation protocol, the H-reflex responses to individual stimuli in a train of stimulation pulses at 100 Hz were reliably estimated with surface EMG during low-level contractions. Furthermore, single motor unit recruitment by afferent stimulation was analyzed with intramuscular EMG. The results showed that substantially elevated H-reflex responses were obtained during 100-Hz stimulation with respect to a lower stimulation frequency. Furthermore, motor unit recruitment using 100-Hz stimulation was not fully synchronized, as it occurs in classic NMES, and the discharge rates differed among motor units because each unit was activated only after a specific number of stimuli. The most likely mechanism behind these observations is the temporal summation of subthreshold excitatory postsynaptic potentials from Ia fibers to the MNs. These findings and their interpretation were also verified by a realistic simulation model of afferent stimulation of a MN population. These results suggest that the proposed stimulation strategy may allow generation of considerable levels of muscle activation by motor unit recruitment that resembles the physiological conditions.

Task-related changes in sensorimotor integration influence the common synaptic input to motor neurones

AIM

The purpose of this investigation was to understand how visual information, when used to guide muscle activity, influences the frequency content of the neural drive to muscles and the gain of afferent feedback.

METHODS

Subjects maintained static, isometric contractions of the tibialis anterior muscle by matching a visual display of their ankle dorsiflexion force to a target set at 10% of their maximum voluntary contraction level. Two visual feedback conditions were studied. The first was a high-sensitivity feedback, in which small changes in force were of large on-screen visual magnitude. The second was a low-sensitivity feedback, in which the on-screen scaling of feedback was reduced by a factor of 10, making small force fluctuations difficult to perceive. Force tremor and Hoffmann reflex (H-reflex) amplitudes were compared between the two conditions, as well as coherence among single motor unit spike trains derived from high-density EMG recordings.

RESULTS

The high-sensitivity feedback condition was associated with lower error, larger force tremor (4-12 Hz) and larger H-reflex amplitudes relative to the low-sensitivity feedback condition. In addition, the use of high-sensitivity feedback was associated with lower 1-5 Hz coherence among pairs of motor units, but larger coherence at high frequencies (6-12, approx. 20, >30 Hz).

CONCLUSION

Alteration of visual feedback influences nearly the entire frequency spectrum of common input to motor neurones, as well the gain of afferent feedback. We speculate that task-related modulation of afferent feedback could be the origin of many of the observed changes in the neural drive to muscles.

Effects of postural threat on spinal stretch reflexes: evidence for increased muscle spindle sensitivity?

Standing balance is often threatened in everyday life. These threats typically involve scenarios in which either the likelihood or the consequence of falling is higher than normal. When cats are placed in these scenarios they respond by increasing the sensitivity of muscle spindles imbedded in the leg muscles, presumably to increase balance-relevant afferent information available to the nervous system. At present, it is unknown whether humans also respond to such postural threats by altering muscle spindle sensitivity. Here we present two studies that probed the effects of postural threat on spinal stretch reflexes. In study 1 we manipulated the threat associated with an increased consequence of a fall by having subjects stand at the edge of an elevated surface (3.2 m). In study 2 we manipulated the threat by increasing the likelihood of a fall by occasionally tilting the support surface on which subjects stood. In both scenarios we used Hoffmann (H) and tendon stretch (T) reflexes to probe the spinal stretch reflex circuit of the soleus muscle. We observed increased T-reflex amplitudes and unchanged H-reflex amplitudes in both threat scenarios. These results suggest that the synaptic state of the spinal stretch reflex is unaffected by postural threat and that therefore the muscle spindles activated in the T-reflexes must be more sensitive in the threatening conditions. We propose that this increase in sensitivity may function to satisfy the conflicting needs to restrict movement with threat, while maintaining a certain amount of sensory information related to postural control.

A pilot study to measure upper extremity H-reflexes following neuromuscular electrical stimulation therapy after stroke

Upper extremity (UE) hemiparesis persists after stroke, limiting hand function. Neuromuscular electrical stimulation (NMES) is an effective intervention to improve UE recovery, although the underlying mechanisms are not fully understood. Our objective was to establish a reliable protocol to measure UE agonist-antagonist forearm monosynaptic reflexes in a pilot study to determine if NMES improves wrist function after stroke. We established the between-day reliability of the H-reflex in the extensor carpi radialis longus (ECRL) and flexor carpi radialis (FCR) musculature for individuals with prior stroke (n=18). The same-day generation of ECRL/FCR H-reflex recruitment curves was well tolerated, regardless of age or UE spasticity. The between-day reliability of the ECRL H-reflex was enhanced above FCR, similar to healthy subjects [20], with the Hmax the most reliable parameter quantified in both muscles. H-reflex and functional measures following NMES show the potential for NMES-induced increases in ECRL Hmax, but confirmation requires a larger clinical study. Our initial results support the safe, easy, and efficacious use of in-home NMES, and establish a potential method to measure UE monosynaptic reflexes after stroke.

Modulation of human motoneuron activity by a mental arithmetic task

This study aimed to determine whether the performance of a mental task affects motoneuron activity. To this end, the tonic discharge pattern of wrist extensor motor units was analyzed in healthy subjects while they were required to maintain a steady wrist extension force and to concurrently perform a mental arithmetic (MA) task. A shortening of the mean inter-spike interval (ISI) and a decrease in ISI variability occurred when MA task was superimposed to the motor task. Aloud and silent MA affected equally the rate and variability of motoneuron discharge. Increases in surface EMG activity and force level were consistent with the modulation of the motor unit discharge rate. Trial-by-trial analysis of the characteristics of motor unit firing revealed that performing MA increases activation of wrist extensor SMU. It is suggested that increase in muscle spindle afferent activity, resulting from fusimotor drive activation by MA, may have contributed to the increase in synaptic inputs to motoneurons during the mental task performance, likely together with enhancement in the descending drive. The finding that a mental task affects motoneuron activity could have consequences in assessment of motor disabilities and in rehabilitation in motor pathologies.

Development of abnormal gait detection and vibratory stimulation system on lower limbs to improve gait stability

The purpose of this study is to develop an abnormal gait detection algorithm and a vibratory stimulation system on a lower limb to improve gait stability and prevent falls. The system consists of a gait measurement module, an abnormal gait detection module, and a vibratory stimulation module. The gait measurement module measures the vertical acceleration of the ankle during walking using an accelerometer. The measured acceleration values are sent to a portable microcontroller, which controls vibratory stimulations to the ankles based on an algorithm that detects the peak acceleration values. If the acceleration peaks are found to occur irregularly, the abnormal gait detection algorithm activates the vibratory stimulation module. To determine the effect of vibratory stimulations under dynamic condition, this study investigated the contribution of ankle muscle proprioception on the control of dynamic stability and lower limb kinematics while walking using vibratory stimulation to alter the muscle spindle output of individuals' left lower limb. Vibrators were attached to the left ankle joint (tibialis anterior, triceps surae). Participants were required to walk along a travel path and step over an obstacle placed in their way. There were four task conditions; an obstacle (10%, 20%, and 30% of the participants' height) was positioned at the midpoint of the walkway, or the participants' walking path remained clear. For each obstacle condition, participants experienced either no vibration, or vibration of the tibialis anterior muscle and the triceps surae muscle of the left lower limb. Vibration began upon detection of an abnormal gait and continued for one second. Vibrating the ankle muscles of the left lower limb while stepping over an obstacle resulted in significant changes in COM behavior on both the anterior/posterior (A/P) and medial/lateral (M/L) planes. The results provide strong evidence that the primary endings of the ankle muscle spindles play a significant role in the control of posture and balance during the swing phase of locomotion by providing information on the movement of the body's COM with respect to the support foot.

Between-day reliability of upper extremity H-reflexes

H-reflexes are useful for evaluating the group Ia monosynaptic reflex excitability in the lower and upper extremities (UEs). However, there is no established between-day protocol for measuring H-reflex excitability in the UE extensor carpi radialis longus (ECRL). The purpose of this study was to develop a reliable protocol to measure the H-reflex excitability between-days for the ECRL, and the antagonist muscle, the flexor carpi radialis (FCR). H-reflex recruitment curves were recorded from eight healthy young subjects over 3 consecutive days in both muscles. Variables associated with the H-reflex excitability were measured: (a) maximum amplitude (Hmax); (b) gain (HGN); (c) threshold (HTH, visHTH, and sdHTH). All variables were normalized with respect to the M-wave. Within individual muscles, there were no statistically significant differences between-days for the group (p>0.05) and variables showed fair to good reliability (ICC=0.57-0.99). This method of reliably measuring H-reflex excitability within UE muscles will be useful for investigating the effects of pathology and rehabilitation on monosynaptic reflexes.

Computational motor control: feedback and accuracy

Speed/accuracy trade-off is a ubiquitous phenomenon in motor behaviour, which has been ascribed to the presence of signal-dependent noise (SDN) in motor commands. Although this explanation can provide a quantitative account of many aspects of motor variability, including Fitts' law, the fact that this law is frequently violated, e.g. during the acquisition of new motor skills, remains unexplained. Here, we describe a principled approach to the influence of noise on motor behaviour, in which motor variability results from the interplay between sensory and motor execution noises in an optimal feedback-controlled system. In this framework, we first show that Fitts' law arises due to signal-dependent motor noise (SDN(m)) when sensory (proprioceptive) noise is low, e.g. under visual feedback. Then we show that the terminal variability of non-visually guided movement can be explained by the presence of signal-dependent proprioceptive noise. Finally, we show that movement accuracy can be controlled by opposite changes in signal-dependent sensory (SDN(s)) and SDN(m), a phenomenon that could be ascribed to muscular co-contraction. As the model also explains kinematics, kinetics, muscular and neural characteristics of reaching movements, it provides a unified framework to address motor variability.

Anticipatory changes in human motoneuron discharge patterns during motor preparation

The influence of motor preparation on human motoneuron activity was studied by combining single motor unit recording techniques with reaction-time (RT) methods. The tonic activity of wrist extensor motor units associated with voluntary isometric contractions was analysed during preparation for a ballistic wrist extensor muscle contraction, using a time preparation procedure. Two durations of the preparatory period elapsing between the warning signal and the response signal were used in separate blocks of trials: a short preparatory period (1 s) allowing optimum time preparation, and a longer, non-optimum one (3 s). Changes in motoneuron tonic discharge patterns not associated with any changes in the force output were observed during the preparatory period, which suggests that these changes were subtle enough to prevent any changes in muscle contraction from occurring before the forthcoming movement. The changes observed were a lengthening of the mean interspike interval (ISI) and a decrease in the ISI variability. These data confirm that inhibitory mechanisms are activated during motor preparation and suggest that spinal inhibitory mechanisms are involved in the preparatory processes. The mechanisms possibly involved, such as presynaptic inhibition, disfacilitation processes or AHP conductance changes, are discussed. The fact that the preparation-induced effects on motoneuron activity were particularly prominent during the last part of the 3 s preparatory period suggests that they were probably related to the neural processes underlying temporal estimation. The anticipatory changes in motoneuron activity observed here during preparation for action provide evidence that central influences act on spinal motoneurons well before it is time to act.

Aging, visuomotor correction, and force fluctuations in large muscles

PURPOSE

To determine the contribution of visuomotor correction to increased force fluctuations in the elbow flexor and knee extensor muscles of elderly adults.

METHODS

Young (N = 22, 23 +/- 3 yr) and elderly (N = 23, 74 +/- 7 yr) adults performed constant-force contractions at target forces of 2.5, 30, and 65% MVC. Visual feedback was provided (6-8 s) and then removed (6-8 s). After removal of drift (< 0.5 Hz) from the force, the standard deviation (SD) and coefficient of variation (CV) of force were calculated from vision and no-vision data.

RESULTS

Maximal voluntary contraction (MVC) force was 19% lower for elbow flexors and 37% lower for knee extensors in elderly adults than in young adults. Overall, the CV of force was 27% greater in the vision condition compared with the no-vision condition. The CV of force for vision was greater for elderly adults than for young adults at the 2.5% MVC target force and lower at 30 and 65% MVC. For the 2.5% MVC target force, the decline in CV of force from vision to no vision was greater for elderly adults than for young adults. At 30 and 65% MVC, the decline was significant but similar for young and elderly adults. For elbow flexors, the change in power from vision to no vision was greater for 0- to 4-Hz (reduced power) and 8- to 12-Hz (increased power) frequencies for elderly adults compared with young adults.

CONCLUSION

Visuomotor correction contributed to force fluctuations in large proximal muscles. The contribution was greater for healthy elderly adults at low forces. Visuomotor processes thus contributed to the age-related increase in force fluctuations.

Vestibular inputs do not influence the fusimotor system in relaxed muscles of the human leg

Descending vestibular pathways have been shown to influence recruitment thresholds of alpha motoneurones in both human and cat. However, whereas parallel connections to the fusimotor system have been shown in the cat, such connections have not yet been demonstrated in humans. In the present study we investigated whether vestibular inputs can influence the firing of spontaneously active muscle spindles in the leg via activation of gamma motoneurones. Unitary recordings were made from 30 muscle spindle afferents via tungsten microelectrodes inserted percutaneously into the common peroneal nerve of seated awake human subjects. Sinusoidal bipolar binaural galvanic vestibular stimulation (GVS; frequency 0.2, 0.5, 0.8 Hz, amplitude +/-2 mA, 100 cycles) was applied to the mastoid processes. This continuous stimulation produced a sustained frequency-dependent illusion of "rocking in a boat" or "swinging in a hammock". Despite these robust illusions none of the spontaneously active muscle spindles exhibited phase-locked modulation of firing during sinusoidal GVS. We conclude that this dynamic vestibular input was not sufficient to recruit gamma motoneurones, which are known to have little spontaneous activity in relaxed human muscles.

Proprioception and myoclonus

This review focuses on sensory information originating from muscle spindles and its role in proprioception and motor control. The first part reminds of the structural and functional properties of these muscle mechanoreceptors, with arguments for an independent fusimotor command, i.e. the gamma-motoneurons, that would regulate spindle mechanical sensitivity in keeping with the requirements of ongoing motor action. The possibility that dysfunction of the fusimotor system might be responsible for clinical signs is discussed with respect to the hyperexcitability of the sensorimotor cortex that is observed in myoclonus of cortical origin. What is known about the spindle afferents projections into the spinal cord and about the dysfunction of the spinal sensorimotor networks in patients with neurological disorders, is put together in the second part. It is stressed on the significant complexity of the monosynaptic reflex in spite of its "simple" organization. The monosynaptic reflex constitutes the only possible way for testing the excitability of motoneurons and spinal networks. This method is extensively used clinically to examine changes in the nervous system with diseases. When studying changes from the norm, it is important to understand how the reflex functions in neurologically normal conditions. Different mechanisms such as pre-synaptic inhibition, post-activation depression and motoneuronal intrinsic properties are reviewed as they may induce changes in reflex amplitude and have therefore consequences for interpretation of spinal excitability.

Time to task failure varies with the gain of the feedback signal for women, but not for men

Varying the gain of the feedback signal during a target-matching task alters the synaptic input onto the motor neuron pool. The purpose was to determine the influence of the gain of the feedback signal on the time to failure for men and women when maintaining arm position while supporting a submaximal inertial load with the elbow flexor muscles. While seated with the upper arm vertical, 15 women and 14 men maintained a constant elbow angle (1.57 rad) and supported a load equal to 15% of maximal voluntary contraction (MVC) force until failure. The task was performed on separate days with either a low gain or a high gain for the joint-angle signal. The percent decline in MVC force after the fatiguing contraction was similar for the low- and high-gain conditions (P = 0.24), and did not differ for men and women (P = 0.11). The discharge of motor units in biceps brachii declined at a greater rate during the high-gain condition for men and women, but only the women experienced a briefer time to failure for the high-gain session (8.7 +/- 2.3 min) compared with the low-gain session (11.9 +/- 4.8; P = 0.003). The men had similar times to failure for the low- (6.0 +/- 2.2 min) and high-gain conditions (5.9 +/- 2.1 min; P = 0.35). Linear and stepwise, multiple-regression analyses revealed that the time to failure for the men was associated with the absolute target force, the standard deviation (SD) for the resultant wrist acceleration, and the brachialis aEMG (P <or= 0.02), whereas the time to failure for the women was associated with the rate of decline in motor unit discharge, the SD for the resultant wrist acceleration, and the changes in mean arterial pressure and heart rate (P <or= 0.001). Despite each subject exerting the same net muscle torque during the two gain conditions and a similar effect of feedback gain on the discharge rate of motor units for all subjects, the time to failure for the fatiguing contractions was limited by different mechanisms for the men and women.

Information processing limitations with aging in the visual scaling of isometric force

The experiment examined if age-related increases in force variability were due to decreases in visual acuity and/or visual-motor information processing deficits. Visual information scale was manipulated over a 250-fold range as young (20-29 years old) and old (60-79 years old) participants produced isometric force output to a visually presented target. Older adults were found to have a very small decrement in visual acuity, but there was no relation between visual acuity and force variability. Force variability exhibited a U-shaped trend as a function of visual information scale. Young adults had less relative variability and higher visual information transfer than the oldest old and these age differences increased with visual information scale. It is concluded that the age-related declines in visual-motor information processing influence changes in neuromuscular function and the emergent differences in force variability at the behavioral level.

How attentional focus on body sway affects postural control during quiet standing

The purpose of this study was to investigate how attentional focus on body sway affects postural control during quiet standing. To address this issue, sixteen young healthy adults were asked to stand upright as immobile as possible on a force platform in both Control and Attention conditions. In the latter condition, participants were instructed to deliberately focus their attention on their body sways and to increase their active intervention into postural control. The critical analysis was focused on elementary motions computed from the centre of pressure (CoP) trajectories: (1) the vertical projection of the centre of gravity (CoG(v)) and (2) the difference between CoP and CoG(v) (CoP-CoG(v)). The former is recognised as an index of performance in this postural task, whilst the latter constitutes a fair expression of the ankle joint stiffness and is linked to the level of neuromuscular activity of the lower limb muscles required for controlling posture. A frequency-domain analysis showed increased amplitudes and frequencies of CoP-CoG(v) motions in the Attention relative to the Control condition, whereas non-significant changes were observed for the CoG(v) motions. Altogether, the present findings suggest that attentional focus on body sway, induced by the instructions, promoted the use of less automatic control process and hampered the efficiency for controlling posture during quiet standing.

Differential effects of mental load on proximal and distal arm muscle activity

Work-related upper extremity disorders (WRUEDs) that result from keyboarding tasks are prevalent and costly. Although the precise mechanisms causing the disorder are not yet fully understood, several risk factors have been proposed. These include the repetitive nature of the motor task and the associated sustained static working postures, but also more psychological factors such as mental load. Epidemiological surveys have shown that WRUEDs are more prone to develop in the postural muscles of the neck/shoulder area than in the executive muscles controlling the hand. The present study investigated whether the activation patterns of these two muscle types are differentially affected by an additional mental load during the performance of a repetitive tapping task. Participants tapped various keying patterns with their dominant index finger at two prescribed tempi. Mental load was manipulated by means of an auditory short-term memory task. We recorded the EMG activity of two neck/shoulder muscles (trapezius and deltoid), two upper arm muscles (biceps and triceps), and four forearm muscles (flexor digitorum superficialis, extensor digitorum, extensor carpi radialis longus and extensor carpi ulnaris) and analyzed the kinematics and impact forces of the index finger. The results confirmed that the upper limb has two functions. Specifically, activity of the executive distal musculature was increased during tapping at the higher pace, while the activity of the postural upper limb musculature was elevated due to the memory task. We argue that continuously increased muscular activity can lead to fatigue and thus eventually cause musculoskeletal complaints. The results are discussed with respect to biomechanical adaptation strategies that deal with the consequences of increased noise in the neuromotor system due to enhanced mental processing.

Prolonged muscle vibration increases stretch reflex amplitude, motor unit discharge rate, and force fluctuations in a hand muscle

The purpose of this study was to compare the influence of prolonged vibration of a hand muscle on the amplitude of the stretch reflex, motor unit discharge rate, and force fluctuations during steady, submaximal contractions. Thirty-two young adults performed 10 isometric contractions at a constant force (5.0 +/- 2.3% of maximal force) with the first dorsal interosseus muscle. Each contraction was held steady for 10 s, and then stretch reflexes were evoked. Subsequently, 20 subjects had vibration applied to the relaxed muscle for 30 min, and 12 subjects received no vibration. The muscle vibration induced a tonic vibration reflex. The intervention (vibration or no vibration) was followed by 2 sets of 10 constant-force contractions with applied stretches (After and Recovery trials). The mean electromyogram amplitude of the short-latency component of the stretch reflex increased by 33% during the After trials (P < 0.01) and by 38% during the Recovery trials (P < 0.01). The standard deviation of force during the steady contractions increased by 21% during the After trials (P < 0.05) and by 28% during the Recovery trials (P < 0.01). The discharge rate of motor units increased from 10.3 +/- 2.7 pulses/s (pps) before vibration to 12.2 +/- 3.1 pps (P < 0.01) during the After trials and to 11.9 +/- 2.6 pps during the Recovery trials (P < 0.01). There was no change in force fluctuations or stretch reflex magnitude for the subjects in the Control group. The results indicate that prolonged vibration increased the short-latency component of the stretch reflex, the discharge rate of motor units, and the fluctuations in force during contractions by a hand muscle. These adjustments were necessary to achieve the target force due to the vibration-induced decrease in the force capacity of the muscle.

Changes in the Inhibitory Control Exerted by the Antagonist Ia Afferents on Human Wrist Extensor Motor Units During an Attention-Demanding Motor Task

The aim of this study was to determine the extent to which an attention-demanding visuomotor task affects the strength of the inhibitory control exerted by the wrist flexor group Ia afferents on the wrist extensor motoneurons. Effects of median nerve stimulation on the tonic activity of wrist extensor single motor units were analyzed in terms of the interspike interval (ISI) lengthening. Results show that the inhibitory effects exerted by the antagonistic group Ia afferents were significantly enhanced when the wrist extensor motoneurons were involved in an attention-demanding task. Enhanced inhibition from antagonist afferents may reflect task-related changes in the excitability of the di- and/or polysynaptic pathways mediating reciprocal inhibition due to either the action of descending inputs and/or an increase in the efficiency of the Ia inputs to the premotoneuronal inhibitory interneurons. Modulation of the inhibition exerted by proprioceptive antagonist afferents may be one of the processes which contribute to the fine adjustment of the wrist muscle force output required in fine handling tasks.