Synchronization of lower limb motor unit activity during walking in human subjects

Reduced corticospinal drive and inflexible temporal adaptation during visually guided walking in older adults

Corticospinal drive during walking is reduced in older adults compared with young adults, but it is not clear how this decrease might compromise one's ability to adjust stepping, particularly during visuomotor adaptation. We hypothesize that age-related changes in corticospinal drive could predict differences in older adults' step length and step time adjustments in response to visual perturbations compared with younger adults. Healthy young (n = 21; age 18-33 yr) and older adults (n = 20; age 68-80 yr) were tested with a treadmill task, incorporating visual feedback of the foot position and stepping targets in real-time. During adaptation, the visuomotor gain was reduced on one side, causing the foot cursor and step targets to move slower on that side of the screen (i.e., split-visuomotor adaptation). Corticospinal drive was quantified by coherence between electromyographic signals in the beta-gamma frequency band (15-45 Hz). The results showed that 1) older adults adapted to visuomotor perturbations during walking, with a similar reduction in error asymmetry compared with younger adults; 2) however, older adults showed reduced adaptation in step time symmetry, despite demonstrating similar adaptation in step length asymmetry compared with younger adults; and 3) smaller overall changes in step time asymmetry was associated with reduced corticospinal drive to the tibialis anterior in the slow leg during split-visuomotor adaptation. These findings suggest that changes in corticospinal drive may affect older adults' control of step timing in response to visual challenges. This could be important for safe navigation when walking in different environments or dealing with unexpected circumstances.NEW & NOTEWORTHY Corticospinal input is essential for visually guided walking, especially when the walking pattern must be modified to accurately step on safe locations. Age-related changes in corticospinal drive are associated with inflexible step time, which necessitates different locomotor adaptation strategies in older adults.

Intramuscular coherence enables robust assessment of modulated supra-spinal input in human gait: an inter-dependence study of visual task and walking speed

Intramuscular high-frequency coherence is increased during visually guided treadmill walking as a consequence of increased supra-spinal input. The influence of walking speed on intramuscular coherence and its inter-trial reproducibility need to be established before adoption as a functional gait assessment tool in clinical settings. Here, fifteen healthy controls performed a normal and a target walking task on a treadmill at various speeds (0.3 m/s, 0.5 m/s, 0.9 m/s, and preferred) during two sessions. Intramuscular coherence was calculated between two surface EMG recordings sites of the Tibialis anterior muscle during the swing phase of walking. The results were averaged across low-frequency (5-14 Hz) and high-frequency (15-55 Hz) bands. The effect of speed, task, and time on mean coherence was assessed using three-way repeated measures ANOVA. Reliability and agreement were calculated with the intra-class correlation coefficient and Bland-Altman method, respectively. Intramuscular coherence during target walking was significantly higher than during normal walking across all walking speeds in the high-frequency band as obtained by the three-way repeated measures ANOVA. Interaction effects between task and speed were found for the low- and high-frequency bands, suggesting that task-dependent differences increase at higher walking speeds. Reliability of intramuscular coherence was moderate to excellent for most normal and target walking tasks in all frequency bands. This study confirms previous reports of increased intramuscular coherence during target walking, while providing first evidence for reproducibility and robustness of this measure as a requirement to investigate supra-spinal input.Trial registration Registry number/ClinicalTrials.gov Identifier: NCT03343132, date of registration 2017/11/17.

Is there frequency-specificity in the motor control of walking? The putative differential role of alpha and beta oscillations

Alpha and beta oscillations have been assessed thoroughly during walking due to their potential role as proxies of the corticoreticulospinal tract (CReST) and corticospinal tract (CST), respectively. Given that damage to a descending tract after stroke can cause walking deficits, detailed knowledge of how these oscillations mechanistically contribute to walking could be utilized in strategies for post-stroke locomotor recovery. In this review, the goal was to summarize, synthesize, and discuss the existing evidence on the potential differential role of these oscillations on the motor descending drive, the effect of transcranial alternate current stimulation (tACS) on neurotypical and post-stroke walking, and to discuss remaining gaps in knowledge, future directions, and methodological considerations. Electrophysiological studies of corticomuscular, intermuscular, and intramuscular coherence during walking clearly demonstrate that beta oscillations are predominantly present in the dorsiflexors during the swing phase and may be absent post-stroke. The role of alpha oscillations, however, has not been pinpointed as clearly. We concluded that both animal and human studies should focus on the electrophysiological characterization of alpha oscillations and their potential role to the CReST. Another approach in elucidating the role of these oscillations is to modulate them and then quantify the impact on walking behavior. This is possible through tACS, whose beneficial effect on walking behavior (including boosting of beta oscillations in intramuscular coherence) has been recently demonstrated in both neurotypical adults and stroke patients. However, these studies still do not allow for specific roles of alpha and beta oscillations to be delineated because the tACS frequency used was much lower (i.e., individualized calculated gait frequency was used). Thus, we identify a main gap in the literature, which is tACS studies actually stimulating at alpha and beta frequencies during walking. Overall, we conclude that for beta oscillations there is a clear connection to descending drive in the corticospinal tract. The precise relationship between alpha oscillations and CReST remains elusive due to the gaps in the literature identified here. However, better understanding the role of alpha (and beta) oscillations in the motor control of walking can be used to progress and develop rehabilitation strategies for promoting locomotor recovery.

Intramuscular coherence during challenging walking in incomplete spinal cord injury: Reduced high-frequency coherence reflects impaired supra-spinal control

Individuals regaining reliable day-to-day walking function after incomplete spinal cord injury (iSCI) report persisting unsteadiness when confronted with walking challenges. However, quantifiable measures of walking capacity lack the sensitivity to reveal underlying impairments of supra-spinal locomotor control. This study investigates the relationship between intramuscular coherence and corticospinal dynamic balance control during a visually guided Target walking treadmill task. In thirteen individuals with iSCI and 24 controls, intramuscular coherence and cumulant densities were estimated from pairs of Tibialis anterior surface EMG recordings during normal treadmill walking and a Target walking task. The approximate center of mass was calculated from pelvis markers. Spearman rank correlations were performed to evaluate the relationship between intramuscular coherence, clinical parameters, and center of mass parameters. In controls, we found that the Target walking task results in increased high-frequency (21–44 Hz) intramuscular coherence, which negatively related to changes in the center of mass movement, whereas this modulation was largely reduced in individuals with iSCI. The impaired modulation of high-frequency intramuscular coherence during the Target walking task correlated with neurophysiological and functional readouts, such as motor-evoked potential amplitude and outdoor mobility score, as well as center of mass trajectory length. The Target walking effect, the difference between Target and Normal walking intramuscular coherence, was significantly higher in controls than in individuals with iSCI [F(1.0,35.0) = 13.042, p < 0.001]. Intramuscular coherence obtained during challenging walking in individuals with iSCI may provide information on corticospinal gait control. The relationships between biomechanics, clinical scores, and neurophysiology suggest that intramuscular coherence assessed during challenging tasks may be meaningful for understanding impaired supra-spinal control in individuals with iSCI.

Motoneurone synchronization for intercostal and abdominal muscles: interneurone influences in two different species

The contribution of branched-axon monosynaptic inputs in the generation of short-term synchronization of motoneurones remains uncertain. Here, synchronization was measured for intercostal and abdominal motoneurones supplying the lower thorax and upper abdomen, mostly showing expiratory discharges. Synchronization in the anaesthetized cat, where the motoneurones receive a strong direct descending drive, is compared with that in anaesthetized or decerebrate rats, where the direct descending drive is much weaker. In the cat, some examples could be explained by branched-axon monosynaptic inputs, but many others could not, by virtue of peaks in cross-correlation histograms whose widths (relatively wide) and timing indicated common inputs with more complex linkages, e.g., disynaptic excitatory. In contrast, in the rat, correlations for pairs of internal intercostal nerves were dominated by very narrow peaks, indicative of branched-axon monosynaptic inputs. However, the presence of activity in both inspiration and expiration in many of the nerves allowed additional synchronization measurements between internal and external intercostal nerves. Time courses of synchronization for these often consisted of combinations of peaks and troughs, which have never been previously described for motoneurone synchronization and which we interpret as indicating combinations of inputs, excitation of one group of motoneurones being common with either excitation or inhibition of the other. Significant species differences in the circuits controlling the motoneurones are indicated, but in both cases, the roles of spinal interneurones are emphasised. The results demonstrate the potential of motoneurone synchronization for investigating inhibition and have important general implications for the interpretation of neural connectivity measurements by cross-correlation.

Muscle Synergies and Coherence Networks Reflect Different Modes of Coordination During Walking

When walking speed is increased, the frequency ratio between the arm and leg swing switches spontaneously from 2:1 to 1:1. We examined whether these switches are accompanied by changes in functional connectivity between multiple muscles. Subjects walked on a treadmill with their arms swinging along their body while kinematics and surface electromyography (EMG) of 26 bilateral muscles across the body were recorded. Walking speed was varied from very slow to normal. We decomposed EMG envelopes and intermuscular coherence spectra using non-negative matrix factorization (NMF), and the resulting modes were combined into multiplex networks and analyzed for their community structure. We found five relevant muscle synergies that significantly differed in activation patterns between 1:1 and 2:1 arm-leg coordination and the transition period between them. The corresponding multiplex network contained a single module indicating pronounced muscle co-activation patterns across the whole body during a gait cycle. NMF of the coherence spectra distinguished three EMG frequency bands: 4-8, 8-22, and 22-60 Hz. The community structure of the multiplex network revealed four modules, which clustered functional and anatomical linked muscles across modes of coordination. Intermuscular coherence at 4-22 Hz between upper and lower body and within the legs was particularly pronounced for 1:1 arm-leg coordination and was diminished when switching between modes of coordination. These findings suggest that the stability of arm-leg coordination is associated with modulations in long-distant neuromuscular connectivity.

Increased intramuscular coherence is associated with temporal gait symmetry during split-belt locomotor adaptation

When walking on a split-belt treadmill where one belt moves faster than the other, the nervous system consistently attempts to maintain symmetry between legs, quantified as deviation from double support time or step length symmetry. It is known that the cerebellum plays a critical role in locomotor adaptation. Less is known about the role of corticospinal drive in maintaining this type of proprioceptive-driven locomotor adaptation. The objective of this study was to examine the functional role of oscillatory drive in relation to changes in spatiotemporal gait parameters during split-belt walking adaptation. Eighteen healthy participants adapted and deadapted on a split-belt treadmill; 13 out of 18 participants repeated the paradigm two more times to examine the effects of reexposure. Coherence analysis was used to quantify the coupling between electromyography (EMG) from the proximal (TAprox) and distal tibialis anterior (TAdist) muscle during the swing phase of walking. EMG-EMG coherence was examined within the alpha (8–15 Hz), beta (15–30 Hz), and gamma (30–45 Hz) frequencies. Our results showed that 1) beta- and gamma-band coherence (markers of corticospinal drive) increased during early split-belt walking compared with baseline walking in the slow leg, 2) beta-band coherence decreased from early to late split-belt adaptation in the fast leg, 3) alpha-, beta-, and gamma-band coherence decreased from first to third split-belt exposure in the fast leg, and 4) there was a relationship between higher beta coherence in the slow leg TA and smaller double support asymmetry. Our results suggest that corticospinal drive may play a functional role in the temporal control of split-belt walking adaptation.

NEW & NOTEWORTHY This is the first study to examine the functional role of intramuscular coherence in relation to changes in spatiotemporal gait parameters during split-belt walking adaptation. We found that the corticospinal drive measured by intramuscular coherence in tibialis anterior changes with adaptation and that the corticospinal drive is related to temporal but not spatial parameters. This study may give insight as to the specific role of the motor cortex during gait.

EEG time-frequency analysis provides arguments for arm swing support in human gait control

BACKGROUND

Human gait benefits from arm swing, which requires four-limb co-ordination. The Supplementary Motor Area (SMA) is involved in multi-limb coordination. With its location anterior to the leg motor cortex and the pattern of its connections, this suggests a distinct role in gait control.

RESEARCH QUESTION

Is the SMA functionally implicated in gait-related arm swing?

METHODS

Ambulant electroencephalography (EEG) was employed during walking with and without arm swing in twenty healthy subjects (mean age: 64.9yrs, SD 7.2). Power changes across the EEG frequency spectrum were assessed by Event Related Spectral Perturbation (ERSP) analysis over both the putative SMA at electrode position Fz and additional sensorimotor regions.

RESULTS

During walking with arm swing, midline electrodes Fz and Cz showed a step-related pattern of Event Related Desynchronization (ERD) followed by Event Related Synchronization (ERS). Walking without arm swing was associated with significant ERD-ERS power reduction in the high-beta/low-gamma band over Fz and a power increase over Cz. Electrodes C3 and C4 revealed a pattern of ERD during contralateral- and ERS during ipsilateral leg swing. This ERD power decreased in gait without arm swing (low-frequency band). The ERSP pattern during walking with arm swing was similar at CP1 and CP2: ERD was seen during double support and the initial swing phase of the right leg, while a strong ERS emerged during the second half of the left leg's swing. Walking without arm swing showed a significant power reduction of this ERD-ERS pattern over CP2, while over CP1, ERS during left leg's swing turned into ERD.

CONCLUSION

The relation between arm swing in walking and a step-related ERD-ERS pattern in the high-beta/low-gamma band over the putative SMA, points at an SMA contribution to integrated cyclic anti-phase movements of upper- and lower limbs. This supports a cortical underpinning of arm swing support in gait control.

Gait characteristics of CKD patients: a systematic review

BACKGROUND

People with Chronic Kidney Disease (CKD) often present with prevalent gait impairment and high fall rates, particularly in advanced CKD stages. Gait impairment and its consequences is associated with increased hospital admission, institutionalization, and greater need for health care. The objective of this systematic review was to evaluate the quality of studies investigating CKD patients' gait characteristics at different CKD stages, to highlight areas of agreement and contradiction between studies reporting aspects of gait in CKD, and to discuss and emphasize gait parameters associated with fall risk.

METHODS

We performed a literature search of trials in CINAHL (EBSCO), Cochrane Library, EMBASE, Medline (EBSCO), PEDro, PubMed, and Scopus databases from their inception to June 30th 2018 using a two-stage process for the identification of studies. We retrieved English-, German-, Italian-, Spanish-, Portuguese and Dutch-language articles for review. Methodological quality of randomized and non-randomized studies was assessed with an adapted version of the Downs and Black checklist.

RESULTS

Thirty-one studies (22 cross-sectional with 3901 participants) and 9 longitudinal intervention studies (1 randomized control trial, 5 controlled clinical trials and 3 one-group pre-post-test; with 659 participants) were considered. The studies revealed a primary emphasis on gait speed measures within clinical tests, and a neglect of spatiotemporal gait variables. Most of the studies showed that CKD progression is associated with slowing of walking speed. No studies analysed the relation between gait parameters and fall risk.

CONCLUSIONS

There was a paucity of studies investigating aspects of gait quality in patients with CKD. In the majority of studies, only gait speed is analysed as a performance indicator. The relation between gait parameters and fall risk in CKD is not investigated. We formulate several recommendations to fill the current research gap, encourage the use of standardized gait analysis protocols that include assessment of spatiotemporal parameters in clinical care of patients with CKD, aimed at prevention of mobility decline and falls risk.

The development of functional and directed corticomuscular connectivity during tonic ankle muscle contraction across childhood and adolescence

In adults, oscillatory activity in the sensorimotor cortex is coherent with contralateral muscle activity at beta frequencies (15-35 Hz) during tonic contraction. This functional coupling reflects the involvement of the sensorimotor cortex, the corticospinal pathway, and likely also ascending sensory feedback in the task at hand. However, little is known about the developmental trajectory of task-related corticomuscular connectivity relating to the voluntary control of the ankle muscles. To address this, we recorded electroencephalography (EEG) from the vertex (Cz) and electromyography (EMG) from ankle muscles (proximal and distal anterior tibial, TA; soleus, SOL; gastrocnemius medialis, GM) in 33 participants aged 7-23 yr during tonic dorsi- and plantar flexion requiring precise maintenance of a submaximal torque level. Coherence was calculated for Cz-TA, Cz-SOL, TA-TA, and SOL-GM signal pairs. We found strong, positive associations between age and beta band coherence for Cz-TA, Cz-SOL, and TA-TA, suggesting that oscillatory corticomuscular connectivity is strengthened during childhood development and adolescence. Directionality analysis indicated that the primary interaction underlying this age-related increase was in the descending direction. In addition, performance during dorsi- and plantar flexion tasks was positively associated with age, indicating more precise control of the ankle joint in older participants. Performance was also positively associated with beta band coherence, suggesting that participants with greater coherence also exhibited greater precision. We propose that these results indicate an age-related increase in oscillatory corticospinal input to the ankle muscle motoneuron pools during childhood development and adolescence, with possible implications for maturation of precision force control. Within the theoretical framework of predictive coding, we suggest that our results may reflect an age-related increase in reliance on feedforward control as the developing nervous system becomes better at predicting the sensory consequences of movement. These findings may contribute to the development of novel intervention strategies targeting improved sensorimotor control in children and adolescents with central motor disorders.

Changes in lower limb muscle synchronisation during walking on high-heeled shoes

The goal of this research was to investigate the effect of wearing high-heeled shoes (HHS) on lower limb muscle synchronisation during walking, using beta band (15-30 Hz) coherence analysis. Fifteen females with no previous neuromuscular disorders volunteered in this study. Surface electromyography in frequency domain was studied from rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM) and semitendinosus (ST) muscles during walking by subjects wearing HHS of three different heel heights (low - 4 cm, medium - 6 cm and high - 10 cm). Average coherence values were calculated for RF-VL, RF-VM and RF-ST muscles in beta band to analyse muscle pair synchronisation. In this study, significant increase in beta band coherence was found in all three muscle pairs during walking on HHS of different heel heights (p<0.05). Increased beta band coherence obtained from this study suggested that walking on HHS demands higher muscle pair synchronisation, to maintain stability around the knee joint.

Assessing Brain-Muscle Connectivity in Human Locomotion through Mobile Brain/Body Imaging: Opportunities, Pitfalls, and Future Directions

Assessment of the cortical role during bipedalism has been a methodological challenge. While surface electroencephalography (EEG) is capable of non-invasively measuring cortical activity during human locomotion, it is associated with movement artifacts obscuring cerebral sources of activity. Recently, statistical methods based on blind source separation revealed potential for resolving this issue, by segregating non-cerebral/artifactual from cerebral sources of activity. This step marked a new opportunity for the investigation of the brains' role while moving and was tagged mobile brain/body imaging (MoBI). This methodology involves simultaneous mobile recording of brain activity with several other body behavioral variables (e.g., muscle activity and kinematics), through wireless recording wearable devices/sensors. Notably, several MoBI studies using EEG-EMG approaches recently showed that the brain is functionally connected to the muscles and active throughout the whole gait cycle and, thus, rejecting the long-lasting idea of a solely spinal-driven bipedalism. However, MoBI and brain/muscle connectivity assessments during human locomotion are still in their fledgling state of investigation. Mobile brain/body imaging approaches hint toward promising opportunities; however, there are some remaining pitfalls that need to be resolved before considering their routine clinical use. This article discusses several of these pitfalls and proposes research to address them. Examples relate to the validity, reliability, and reproducibility of this method in ecologically valid scenarios and in different populations. Furthermore, whether brain/muscle connectivity within the MoBI framework represents a potential biomarker in neuromuscular syndromes where gait disturbances are evident (e.g., age-related sarcopenia) remains to be determined.

A critical period of corticomuscular and EMG-EMG coherence detection in healthy infants aged 9-25 weeks

KEY POINTS

The early postnatal development of functional corticospinal connections in human infants is not fully clarified. Corticospinal drive to upper and lower limb muscle shows developmental changes with an increased functional coupling in infants between 9 and 25 weeks in the beta frequency band. The changes in functional coupling coincide with the developmental period where fidgety movements are present in healthy infants. Data support a possible sensitive period where functional connections between corticospinal tract fibres and spinal motoneurones undergo activity-dependent reorganization.

ABSTRACT

The early postnatal development of functional corticospinal connections in human infants is not fully clarified. We used EEG and EMG to investigate the development of corticomuscular and intramuscular coherence as indicators of functional corticospinal connectivity in healthy infants aged 1-66 weeks. EEG was recorded over leg and hand area of motor cortex. EMG recordings were made from right ankle dorsiflexor and right wrist extensor muscles. Quantification of the amount of corticomuscular coherence in the 20-40 Hz frequency band showed a significantly larger coherence for infants aged 9-25 weeks compared to younger and older infants. Coherence between paired EMG recordings from tibialis anterior muscle in the 20-40 Hz frequency band was also significantly larger for the 9-25 week age group. A low-amplitude, broad-duration (40-50 ms) central peak of EMG-EMG synchronization was observed for infants younger than 9 weeks, whereas a short-lasting (10-20 ms) central peak was observed for EMG-EMG synchronization in older infants. This peak was largest for infants aged 9-25 weeks. These data suggest that the corticospinal drive to lower and upper limb muscles shows significant developmental changes with an increase in functional coupling in infants aged 9-25 weeks, a period which coincides partly with the developmental period of normal fidgety movements. We propose that these neurophysiological findings may reflect the existence of a sensitive period where the functional connections between corticospinal tract fibres and spinal motoneurones undergo activity-dependent reorganization. This may be relevant for the timing of early therapy interventions in infants with pre- and perinatal brain injury.

How we Walk: Central Control of Muscle Activity during Human Walking

Recent advances in noninvasive electrophysiological and brain imaging techniques have made investigation of the central control of human walking possible. We are thus now able to ask in what way the motor control circuitries in the human brain and spinal cord have been modified in order to control bipedal walking. This information is of importance not only for our understanding of basic control strategies and paradigms but also for future attempts at rehabilitating the gait ability of patients after lesions of the brain and spinal cord.

A common neural element receiving rhythmic arm and leg activity as assessed by reflex modulation in arm muscles

Neural interactions between regulatory systems for rhythmic arm and leg movements are an intriguing issue in locomotor neuroscience. Amplitudes of early latency cutaneous reflexes (ELCRs) in stationary arm muscles are modulated during rhythmic leg or arm cycling but not during limb positioning or voluntary contraction. This suggests that interneurons mediating ELCRs to arm muscles integrate outputs from neural systems controlling rhythmic limb movements. Alternatively, outputs could be integrated at the motoneuron and/or supraspinal levels. We examined whether a separate effect on the ELCR pathways and cortico-motoneuronal excitability during arm and leg cycling is integrated by neural elements common to the lumbo-sacral and cervical spinal cord. The subjects performed bilateral leg cycling (LEG), contralateral arm cycling (ARM), and simultaneous contralateral arm and bilateral leg cycling (A&L), while ELCRs in the wrist flexor and shoulder flexor muscles were evoked by superficial radial (SR) nerve stimulation. ELCR amplitudes were facilitated by cycling tasks and were larger during A&L than during ARM and LEG. A low stimulus intensity during ARM or LEG generated a larger ELCR during A&L than the sum of ELCRs during ARM and LEG. We confirmed this nonlinear increase in single motor unit firing probability following SR nerve stimulation during A&L. Furthermore, motor-evoked potentials following transcranial magnetic and electrical stimulation did not show nonlinear potentiation during A&L. These findings suggest the existence of a common neural element of the ELCR reflex pathway that is active only during rhythmic arm and leg movement and receives convergent input from contralateral arms and legs.

Task-Dependent Intermuscular Motor Unit Synchronization between Medial and Lateral Vastii Muscles during Dynamic and Isometric Squats

PURPOSE

Motor unit activity is coordinated between many synergistic muscle pairs but the functional role of this coordination for the motor output is unclear. The purpose of this study was to investigate the short-term modality of coordinated motor unit activity-the synchronized discharge of individual motor units across muscles within time intervals of 5ms-for the Vastus Medialis (VM) and Lateralis (VL). Furthermore, we studied the task-dependency of intermuscular motor unit synchronization between VM and VL during static and dynamic squatting tasks to provide insight into its functional role.

METHODS

Sixteen healthy male and female participants completed four tasks: Bipedal squats, single-leg squats, an isometric squat, and single-leg balance. Monopolar surface electromyography (EMG) was used to record motor unit activity of VM and VL. For each task, intermuscular motor unit synchronization was determined using a coherence analysis between the raw EMG signals of VM and VL and compared to a reference coherence calculated from two desynchronized EMG signals. The time shift between VM and VL EMG signals was estimated according to the slope of the coherence phase angle spectrum.

RESULTS

For all tasks, except for singe-leg balance, coherence between 15-80Hz significantly exceeded the reference. The corresponding time shift between VM and VL was estimated as 4ms. Coherence between 30-60Hz was highest for the bipedal squat, followed by the single-leg squat and the isometric squat.

CONCLUSION

There is substantial short-term motor unit synchronization between VM and VL. Intermuscular motor unit synchronization is enhanced for contractions during dynamic activities, possibly to facilitate a more accurate control of the joint torque, and reduced during single-leg tasks that require balance control and thus, a more independent muscle function. It is proposed that the central nervous system scales the degree of intermuscular motor unit synchronization according to the requirements of the movement task at hand.

The optimal neural strategy for a stable motor task requires a compromise between level of muscle cocontraction and synaptic gain of afferent feedback

Increasing joint stiffness by cocontraction of antagonist muscles and compensatory reflexes are neural strategies to minimize the impact of unexpected perturbations on movement. Combining these strategies, however, may compromise steadiness, as elements of the afferent input to motor pools innervating antagonist muscles are inherently negatively correlated. Consequently, a high afferent gain and active contractions of both muscles may imply negatively correlated neural drives to the muscles and thus an unstable limb position. This hypothesis was systematically explored with a novel computational model of the peripheral nervous system and the mechanics of one limb. Two populations of motor neurons received synaptic input from descending drive, spinal interneurons, and afferent feedback. Muscle force, simulated based on motor unit activity, determined limb movement that gave rise to afferent feedback from muscle spindles and Golgi tendon organs. The results indicated that optimal steadiness was achieved with low synaptic gain of the afferent feedback. High afferent gains during cocontraction implied increased levels of common drive in the motor neuron outputs, which were negatively correlated across the two populations, constraining instability of the limb. Increasing the force acting on the joint and the afferent gain both effectively minimized the impact of an external perturbation, and suboptimal adjustment of the afferent gain could be compensated by muscle cocontraction. These observations show that selection of the strategy for a given contraction implies a compromise between steadiness and effectiveness of compensations to perturbations. This indicates that a task-dependent selection of neural strategy for steadiness is necessary when acting in different environments.

Adult rat motor neurons do not re-establish electrical coupling during axonal regeneration and muscle reinnervation

Gap junctions (GJs) between neurons are present in both the newborn and the adult nervous system, and although important roles have been suggested or demonstrated in a number of instances, in many other cases a full understanding of their physiological role is still missing. GJs are expressed in the rodent lumbar cord at birth and mediate both dye and electrical coupling between motor neurons. This expression has been proposed to mediate: (i) fast synchronization of motoneuronal spike activity, in turn linked to the process of refinement of neuromuscular connections, and (ii) slow synchronization of locomotor-like oscillatory activity. Soon after birth this coupling disappears. Since in the adult rat regeneration of motor fibers after peripheral nerve injury leads to a recapitulation of synaptic refinement at the target muscles, we tested whether GJs between motor neurons are transiently re-expressed. We found that in conditions of maximal responsiveness of lumbar motor neurons (such as no depression by anesthetics, decerebrate release of activity of subsets of motor neurons, use of temporal and spatial summation by antidromic and orthodromic stimulations, testing of large ensembles of motor neurons) no firing is observed in ventral root axons in response to antidromic spike invasion of nearby counterparts. We conclude that junctional coupling between motor neurons is not required for the refinement of neuromuscular innervation in the adult.

Tibialis Anterior muscle coherence during controlled voluntary activation in patients with spinal cord injury: diagnostic potential for muscle strength, gait and spasticity

Background

Coherence estimation has been used as an indirect measure of voluntary neurocontrol of residual motor activity following spinal cord injury (SCI). Here intramuscular Tibialis Anterior (TA) coherence estimation was performed within specific frequency bands for the 10-60 Hz bandwidth during controlled ankle dorsiflexion in subjects with incomplete SCI with and without spasticity.

Methods

In the first cohort study 15 non-injured and 14 motor incomplete SCI subjects were recruited to evaluate TA coherence during controlled movement. Specifically 15-30 Hz EMG was recorded during dorsiflexion with: i) isometric activation at 50, 75 and 100% of maximal voluntary torque (MVT), ii) isokinetic activation at 60 and 120°/s and iii) isotonic dorsiflexion at 50% MVT. Following identification of the motor tasks necessary for measurement of optimal TA coherence a second cohort was analyzed within the 10-16 Hz, 15-30 Hz, 24-40 Hz and 40-60 Hz bandwidths from 22 incomplete SCI subjects, with and without spasticity.

Results

Intramuscular 40-60 Hz, but not 15-30 Hz TA, coherence calculated in SCI subjects during isometric activation at 100% of MVT was lower than the control group. In contrast only isometric activation at 100% of MVT 15-30 Hz TA coherence was higher in subjects with less severe SCI (AIS D vs. AIS C), and correlated functionally with dorsiflexion MVT. Higher TA coherence was observed for the SCI group during 120°/s isokinetic movement. In addition 15-30 Hz TA coherence calculated during isometric activation at 100% MVT or 120°/s isokinetic movement correlated moderately with walking function and time from SCI, respectively. Spasticity symptoms correlated negatively with coherence during isometric activation at 100% of MVT in all tested frequency bands, except for 15-30 Hz. Specifically, 10-16 Hz coherence correlated inversely with passive resistive torque to ankle dorsiflexion, while clinical measures of muscle hypertonia and spasm severity correlated inversely with 40-60 Hz.

Conclusion

Analysis of intramuscular 15-30 Hz TA coherence during isometric activation at 100% of MVT is related to muscle strength and gait function following incomplete SCI. In contrast several spasticity symptoms correlated negatively with 10-16 Hz and 40-60 Hz TA coherence during isometric activation at 100% MVT. Validation of the diagnostic potential of TA coherence estimation as a reliable and comprehensive measure of muscle strength, gait and spasticity should facilitate SCI neurorehabilation.

Reliability and agreement of intramuscular coherence in tibialis anterior muscle

Background

Neuroplasticity drives recovery of walking after a lesion of the descending tract. Intramuscular coherence analysis provides a way to quantify corticomotor drive during a functional task, like walking and changes in coherence serve as a marker for neuroplasticity. Although intramuscular coherence analysis is already applied and rapidly growing in interest, the reproducibility of variables derived from coherence is largely unknown. The purpose of this study was to determine the test-retest reliability and agreement of intramuscular coherence variables obtained during walking in healthy subjects.

Methodology/Principal Findings

Ten healthy participants walked on a treadmill at a slow and normal speed in three sessions. Area of coherence and peak coherence were derived from the intramuscular coherence spectra calculated using rectified and non-rectified M. tibialis anterior Electromyography (EMG). Reliability, defined as the ability of a measurement to differentiate between subjects and established by the intra-class correlation coefficient, was on the limit of good for area of coherence and peak coherence when derived from rectified EMG during slow walking. Yet, the agreement, defined as the degree to which repeated measures are identical, was low as the measurement error was relatively large. The smallest change to exceed the measurement error between two repeated measures was 66% of the average value. For normal walking and/or other EMG-processing settings, not rectifying the EMG and/or high-pass filtering with a high cutoff frequency (100 Hz) the reliability was only moderate to poor and the agreement was considerably lower.

Conclusions/significance

Only for specific conditions and EMG-processing settings, the derived coherence variables can be considered to be reliable measures. However, large changes (>66%) are needed to indicate a real difference. So, although intramuscular coherence is an easy to use and a sufficiently reliable tool to quantify intervention-induced neuroplasticity, the large effects needed to reveal a real change limit its practical use.

Involvement of the corticospinal tract in the control of human gait

Given the inherent mechanical complexity of human bipedal locomotion, and that complete spinal cord lesions in human leads to paralysis with no recovery of gait, it is often suggested that the corticospinal tract (CST) has a more predominant role in the control of walking in humans than in other animals. However, what do we actually know about the contribution of the CST to the control of gait? This chapter will provide an overview of this topic based on the premise that a better understanding of the role of the CST in gait will be essential for the design of evidence-based approaches to rehabilitation therapy, which will enhance gait ability and recovery in patients with lesions to the central nervous system (CNS). We review evidence for the involvement of the primary motor cortex and the CST during normal and perturbed walking and during gait adaptation. We will also discuss knowledge on the CST that has been gained from studies involving CNS lesions, with a particular focus on recent data acquired in people with spinal cord injury.

Childhood development of common drive to a human leg muscle during ankle dorsiflexion and gait

Corticospinal drive has been shown to contribute significantly to the control of walking in adult human subjects. It is unknown to what extent functional change in this drive is important for maturation of gait in children. In adults, populations of motor units within a muscle show synchronized discharges during walking with pronounced coherence in the 15-50 Hz frequency band. This coherence has been shown to depend on cortical drive. Here, we investigated how this coherence changes with development. Forty-four healthy children aged 4-15 years participated in the study. Electromyographic activity (EMG) was recorded from pairs of electrodes placed over the right tibialis anterior (TA) muscle during static dorsiflexion and during walking on a treadmill (speed from 1.8 to 4.8 km h(-1)). A significant increase of coherence with increasing age was found in the 30-45 Hz frequency band (gamma) during walking and during static ankle dorsiflexion. A significant correlation with age was also found in the 15-25 Hz frequency band (beta) during static foot dorsiflexion. χ(2) analysis of differences of coherence between different age groups of children (4-6, 7-9, 10-12 and 13-15 years of age) revealed a significantly lower coherence in the gamma band for recordings during walking in children aged 4-6 years as compared to older children. Recordings during static dorsiflexion revealed significant differences in both the beta and gamma bands for children in the 4-6 and 7-9 years age groups as compared to the older age groups. A significant age-related decrease in step-to-step variability of toe position during the swing phase of walking was observed. This reduction in the step-to-step variability of gait was correlated with increased gamma band coherence during walking. We argue that this may reflect an increased ability to precisely control the ankle joint position with age, which may be contingent on maturation of corticospinal control of the foot dorsiflexor muscles.

Impaired transmission in the corticospinal tract and gait disability in spinal cord injured persons

Rehabilitation following spinal cord injury is likely to depend on recovery of corticospinal systems. Here we investigate whether transmission in the corticospinal tract may explain foot drop (inability to dorsiflex ankle) in persons with spinal cord lesion. The study was performed in 24 persons with incomplete spinal cord lesion (C1 to L1) and 15 healthy controls. Coherence in the 10- to 20-Hz frequency band between paired tibialis anterior muscle (TA) electromyographic recordings obtained in the swing phase of walking, which was taken as a measure of motor unit synchronization. It was significantly correlated with the degree of foot drop, as measured by toe elevation and ankle angle excursion in the first part of swing. Transcranial magnetic stimulation was used to elicit motor-evoked potentials (MEPs) in the TA. The amplitude of the MEPs at rest and their latency during contraction were correlated to the degree of foot drop. Spinal cord injured participants who exhibited a large foot drop had little or no MEP at rest in the TA muscle and had little or no coherence in the same muscle during walking. Gait speed was correlated to foot drop, and was the lowest in participants with no MEP at rest. The data confirm that transmission in the corticospinal tract is of importance for lifting the foot during the swing phase of human gait.

Effect of cancellation on triggered averaging used to determine synchronization between motor unit discharge in separate muscles

Synchronization between single motor unit (SMU) discharges in separate muscles has been estimated from peaks in averaged electromyographic (EMG) recordings from one muscle triggered from SMU discharge in another. This study evaluated the effect of EMG signal cancellation on this measure of synchronization. SMU activity was recorded with 8 fine-wire electrodes in vastus medialis obliquus (VMO) and vastus lateralis (VL) during gentle isometric knee extension in 7 subjects. Data from 5 VL recordings were summed then rectified, or rectified then summed, to produce multi-unit recordings with and without cancellation, respectively. Averages of summed VL data were triggered from VMO SMUs. Synchronization, defined as a peak >3 SD above the triggered average mean, occurred in 73.68% and 78.95% of recordings with and without cancellation, respectively. To further investigate the effect of cancellation on synchronization, 250 "virtual" EMG recordings were created from VL data. VL SMUs were sorted and modified with respect to discharge rate, amplitude and polarity to create a collection of possible SMU discharge patterns. Virtual recordings were added one-by-one to VL recordings that showed synchronization. Virtual channels were rectified then added or added then rectified, to create data with and without cancellation. Identification of synchronization decreased similarly in both conditions with addition of virtual data. Our data show estimation of synchronization from triggered averages is more likely to detect synchronization in recordings with fewer SMUs, but cancellation has little effect. Synchronization must be interpreted with caution if number of SMUs changes between conditions.

Bilateral motor unit synchronization of leg muscles during a simple dynamic balance task

To handle the rich repertoire of behavioural goals, the CNS has to control the many degrees of freedom of the musculoskeletal system in a flexible manner. This problem can be drastically simplified if muscle synergies serve as the to-be-controlled building blocks of motor performance, instead of the individual degrees of freedom. Muscle synergies have been identified as coherent activation patterns of a group of muscles in space or time, but the neural mechanisms underlying their formation remain largely unknown. Here we evaluated the hypothesis that synergies are reflected in common input to different contributing muscles, and investigated modulations in motor unit (MU) synchronization of homologous muscles during a rhythmic balance task. If common input is related to muscle synergies, the resultant MU synchronization should not be static but task dependent and, in the present context, vary in time. Coherence between surface electromyographic signals of bilateral leg muscles revealed MU synchronization in two distinct frequency bands. MU synchronization was not constant but modulated within a movement cycle, and its time course resembled the activation patterns of the muscles. These results are congruent with a linkage between MU synchronization and muscle synergies, and suggest that MU synchronization provides an expedient method for studying synergy-related neural mechanisms.

Neuromechanics of muscle synergies during cycling

Muscle synergies have been proposed as building blocks that could simplify the construction of motor behaviors. However, the muscles within synergistic groups may have different architectures, mechanical linkages to the skeleton, and biochemical properties, and these put competing demands on the most appropriate way to activate them for different mechanical tasks. This study identifies the extent to which synergistic patterns of muscle activity vary when the mechanical demands on a limb were altered, and additionally identifies how consistent the spectral profiles of the electromyographic (EMG) intensities were across the different movement tasks. The muscle activities were measured with surface EMG across 10 muscles in the leg during cycling at a range of loads and velocities. The EMGs were quantified by their intensities in time-frequency space using wavelet analysis; the instantaneous patterns of activity identified using principal component analysis, statistically compared and further visualized using the varimax rotation. Variability (35.7%) in the patterns of activity between the muscles were correlated with the torque and velocity of the pedal crank. Anatomic groups of muscles share a common mechanical action across a joint; uncoupling between such muscles was identified in 68.8% of the varimax patterns that encompassed all 10 muscles and 20.8-29.5% of the activity patterns when the anatomic groups were analyzed separately. The EMG spectra showed greatest heterogeneity for the gastrocnemii. These results show that the activity of muscles within anatomic groups is partially uncoupled in response to altered mechanical demands on the limb.

Soleus H-reflex modulation during body weight support treadmill walking in spinal cord intact and injured subjects

The soleus H-reflex modulation pattern was investigated in ten spinal cord intact subjects during treadmill walking at varying levels of body weight support (BWS), and nine spinal cord injured (SCI) subjects at a BWS level that promoted the best stepping pattern. The soleus H-reflex was elicited by tibial nerve stimulation with a single 1-ms pulse at an intensity that the M-waves ranged from 4 to 8% of the maximal M-wave (M(max)). During treadmill walking, the H-reflex was elicited every four steps, and stimuli were randomly dispersed across the gait cycle which was divided into 16 equal bins. EMGs were recorded with surface electrodes from major left and right hip, knee, and ankle muscles. M-waves and H-reflexes at each bin were normalized to the M(max) elicited at 60-100 ms after the test reflex stimulus. For every subject, the integrated EMG area of each muscle was established and plotted as a function of the step cycle phase. The H-reflex gain was determined as the slope of the relationship between H-reflex and soleus EMG amplitudes at 60 ms before H-reflex elicitation for each bin. In spinal cord intact subjects, the phase-dependent H-reflex modulation, reflex gain, and EMG modulation pattern were constant across all BWS (0, 25, and 50) levels, while tibialis anterior muscle activity increased with less body loading. In three out of nine SCI subjects, a phase-dependent H-reflex modulation pattern was evident during treadmill walking at BWS that ranged from 35 to 60%. In the remaining SCI subjects, the most striking difference was an absent H-reflex depression during the swing phase. The reflex gain was similar for both subject groups, but the y-intercept was increased in SCI subjects. We conclude that the mechanisms underlying cyclic H-reflex modulation during walking are preserved in some individuals after SCI.

Masticatory muscle activity during deliberately performed oral tasks

The aim of this study was to investigate masticatory muscle activity during deliberately performed functional and non-functional oral tasks. Electromyographic (EMG) surface activity was recorded unilaterally from the masseter, anterior temporalis and suprahyoid muscles in 11 subjects (5 men, 6 women; age = 34.6 +/- 10.8 years), who were accurately instructed to perform 30 different oral tasks under computer guidance using task markers. Data were analyzed by descriptive statistics, repeated measurements analysis of variance (ANOVA) and hierarchical cluster analysis. The maximum EMG amplitude of the masseter and anterior temporalis muscles was more often found during hard chewing tasks than during maximum clenching tasks. The relative contribution of masseter and anterior temporalis changed across the tasks examined (F 5.2; p < or = 0.001). The masseter muscle was significantly (p < or = 0.05) more active than the anterior temporalis muscle during tasks involving incisal biting, jaw protrusion, laterotrusion and jaw cupping, the difference being statistically significant (p < or = 0.05). The anterior temporalis muscle was significantly (p < or = 0.01) more active than the masseter muscle during tasks performed in intercuspal position, during tooth grinding, and during hard chewing on the working side. Based upon the relative contribution of the masseter, anterior temporalis, and suprahyoid muscles, the investigated oral tasks could be grouped into six separate clusters. The findings provided further insight into muscle- and task-specific EMG patterns during functional and non-functional oral behaviors.

Spinal and brain control of human walking: implications for retraining of walking

In this update, the authors will discuss evidence for both spinal and brain regulation of walking in humans. They will consider the sensory control of walking in young babies and spinal cord-injured adults, two models with weak descending input from the brain, to suggest that subcortical structures are important in shaping walking behavior. Based on evidence from development, the authors suggest that the primitive pattern of walking seen in babies forms the base upon which additional features are added by supraspinal input as independent walking develops. Increasing evidence suggests the motor cortex is important in the control of level-ground walking in adults, in contrast to quadrupeds. This brain input seems particularly important for distal flexors in the leg. Finally, the authors will consider evidence that the recovery of walking after incomplete spinal cord injuries is dependent on the presence of descending input from the motor cortex and our ability to strengthen that input. These findings imply that training methods for improving walking after injury to the nervous system must promote the involvement of both spinal and brain circuits.

Sensitivity of the cross-correlation between simulated surface EMGs for two muscles to detect motor unit synchronization

The purpose of the study was to evaluate the use of cross-correlation analysis between simulated surface electromyograms (EMGs) of two muscles to quantify motor unit synchronization. The volume conductor simulated a cylindrical limb with two muscles and bone, fat, and skin tissues. Models of two motor neuron pools were used to simulate 120 s of surface EMG that were detected over both muscles. Short-term synchrony was established using a phenomenological model that aligned the discharge times of selected motor units within and across muscles to simulate physiological levels of motor unit synchrony. The correlation between pairs of surface EMGs was estimated as the maximum of the normalized cross-correlation function. After imposing four levels of motor unit synchrony across muscles, five parameters were varied concurrently in the two muscles to examine their influence on the correlation between the surface EMGs: 1) excitation level (5, 10, 15, and 50% of maximum); 2) muscle size (350 and 500 motor units); 3) fat thickness (1 and 4 mm); 4) skin conductivity (0.1 and 1 S/m); and 5) mean motor unit conduction velocity (2.5 and 4 m/s). Despite a constant and high level of motor unit synchronization among pairs of motor units across the two muscles, the cross-correlation index ranged from 0.08 to 0.56, with variation in the five parameters. For example, cross-correlation of EMGs from pairs of hand muscles, each having thin layers of subcutaneous fat and mean motor unit conduction velocities of 4 m/s, may be relatively insensitive to the level of synchronization across muscles. In contrast, cross-correlation of EMGs from pairs of leg muscles, with larger fat thickness, may exhibit a different sensitivity. These results indicate that cross correlation of the surface EMGs from two muscles provides a limited measure of the level of synchronization between motor units in the two muscles.

Changes in cortically related intermuscular coherence accompanying improvements in locomotor skills in incomplete spinal cord injury

In human spinal cord injury, the neuronal mechanisms mediating the improvement of locomotor function in response to intensive treadmill training are not well understood. In this study, we examined if such recovery is mediated, in part, by increases in residual corticospinal drive to muscles of the leg during walking. To do this, we measured the coherence of electromyogram (EMG) activity between two antagonist muscles (intermuscular coherence), specifically at frequencies between 24 and 40 Hz, which is thought to indicate common drive to two muscles from corticospinal inputs. In 12 subjects with incomplete spinal cord injury, intermuscular coherence was measured between hamstrings and vastus lateralis EMG that was activated during walking on a motorized treadmill. Before training, appreciable coherence in the 24-40 Hz frequency band was only present in subjects with moderate volitional motor strength in their leg muscles (n = 8 subjects) compared with subjects with little or no leg muscle strength (n = 4 subjects), reconfirming that 24-40 Hz frequency coherence is likely mediated by common supraspinal inputs. After training, increases in 24-40 Hz coherence only occurred in the eight subjects with moderate leg muscle strength who also exhibited improvements in locomotor recovery as assessed by the 21 point WISCI II scale (termed responders). In contrast, development of intermuscular coherence in the 24-40 Hz frequency band did not occur in the four subjects with absent or weak muscle strength. These subjects also did not improve in their locomotor ability as reflected in unchanging WISCI II scores (termed nonresponders). Lower-frequency coherence (5-18 Hz), which is thought to contain common drive from spinal inputs, did not change in either group. In a subset of subjects that were previously assessed with transcranial magnetic stimulation (TMS) before and after training (n = 5 responders and 3 nonresponders), there was a significant and positive relationship between increases in 24-40 Hz coherence and increases in evoked muscle responses to TMS of the primary motor cortex. Taken together, increases in higher-frequency EMG coherence in subjects with residual voluntary muscle strength and its parallel relation to changes in TMS-evoked responses provides further evidence that improvements in locomotor function from treadmill training are mediated, in part, by increases in corticospinal drive to muscles of the leg during walking.

Organization of common synaptic drive to motoneurones during fictive locomotion in the spinal cat

The basic locomotor rhythm in the cat is generated by a neuronal network in the spinal cord. The exact organization of this network and its drive to the spinal motoneurones is unknown. The purpose of the present study was to use time (cumulant density) and frequency domain (coherence) analysis to examine the organization of the last order drive to motoneurones during fictive locomotion (evoked by application of nialamide and dihydroxyphenylalanine (DOPA)) in the spinal cat. In all cats, narrow central synchronization peaks (half-width < 3 ms) were observed in cumulants estimated between electroneurograms (ENGs) of close synergists, but not between nerves belonging to muscles acting on different joints or to antagonistic muscles. Coherence was not observed at frequencies above 100 Hz and was mainly observed between synergists. Intracellular recording was obtained from a population of 70 lumbar motoneurones. Significant short-term synchronization was observed between the individual intracellular recordings and the ENGs recorded from nerves of the same pool and of close synergists. Recordings from 34 pairs of motoneurones (10 pairs belonged to the same motor pool, 11 pairs to close synergists and 13 pairs to antagonistic pools) failed to reveal any short-lasting synchronization. These data demonstrate that short-term synchronization during fictive locomotion is relatively weak and is restricted to close synergists. In addition, coherence analysis failed to identify any specific rhythmic component in the locomotor drive that could be associated with this synchronization. These results resemble findings obtained during human treadmill walking and imply that the spinal interneurones participating in the generation of the locomotor rhythm are themselves weakly synchronized. The restricted synchronization within the locomotor drive to motoneuronal pools may be a feature of the locomotor generating networks that is related to the ability of these networks to produce highly adaptive patterns of muscle activity during locomotion.

Motor unit synchronization between medial and lateral vasti muscles

OBJECTIVE

Accurate neuromuscular control of the patellofemoral joint is important in knee joint mechanics. Strategies to coordinate the vasti muscles, such as motor unit synchronization, may simplify control of patellar tracking. This study investigated motor unit synchronization between vastus medialis (VM) and lateralis (VL).

METHODS

Electromyographic (EMG) recordings of single motor unit action potentials (MUAPs) were made from VM and single- and multi-unit recordings were made from VL. Synchronization was quantified from peaks in the cross-correlogram generated from single MUAP pairs in VL and VM. The proportion of motor units in VM with synchronized firing in VL was also quantified from peaks in averages of multiunit VL EMG triggered from the VM MUAP.

RESULTS

A high degree of synchronization of motor unit firing between VM and VL was identified. Results were similar for cross-correlation ( approximately 45% of cases) and triggered averages (approximately 41% of cases).

CONCLUSIONS

The data suggest that synchronization between VM and VL is higher than expected. Agreement between traditional cross-correlation and triggered averaging methods suggest that this new technique may provide a more clinically viable method to quantify synchronization.

SIGNIFICANCE

High synchronization between VM and VL may provide a solution to simplify control of the mechanically unstable patellofemoral joint.

Motor unit synchronization of the vasti muscles in closed and open chain tasks

OBJECTIVES

To investigate motor unit synchronization between medial and lateral vasti and whether such synchronization differs in closed and open chain tasks.

DESIGN

Electromyographic recordings of single motor unit action potentials were made from the vastus medialis obliquus (VMO) and multiunit recordings from vastus lateralis during isometric contractions at 30 degrees of knee flexion in closed and open chain conditions.

SETTING

Laboratory.

PARTICIPANTS

Five volunteers with no history of knee pain (age, 30+/-3.32 y).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURE

The degree of synchronization between motor unit firing was evaluated by identifying peaks in the electromyographic averages of the vastus lateralis, triggered from motor unit action potentials in the VMO, and the proportion of power in the power spectral density of the triggered average at the firing frequency of the reference motor unit. The proportion of cases in which there was significant power and peaks in the triggered averages was calculated.

RESULTS

The proportion of trials with peaks in the triggered averages of the vastus lateralis electromyographic activity was greater than 61.5% in all tasks, and there was a significantly greater proportion of cases where power in the spectrum was greater than 7.5% ( P =.01) for the closed chain condition.

CONCLUSIONS

There was a high proportion of synchronized motor units between the 2 muscles during isometric contractions, with evidence for greater common drive between the VMO and vastus lateralis in closed chain tasks. This has implications for rehabilitation because it suggests that closed chain tasks may generate better coordination between the vasti muscles.

Reduction of common synaptic drive to ankle dorsiflexor motoneurons during walking in patients with spinal cord lesion

It is possible to obtain information about the synaptic drive to motoneurons during walking by analyzing motor-unit coupling in the time and frequency domains. The purpose of the present study was to compare motor-unit coupling during walking in healthy subjects and patients with incomplete spinal cord lesion to obtain evidence of differences in the motoneuronal drive that result from the lesion. Such information is of importance for development of new strategies for gait restoration. Twenty patients with incomplete spinal cord lesion (SCL) participated in the study. Control experiments were performed in 11 healthy subjects. In all healthy subjects, short-term synchronization was evident in the discharge of tibialis anterior (TA) motor units during the swing phase of treadmill walking. This was identified from the presence of a narrow central peak in cumulant densities constructed from paired EMG recordings and from the presence of significant coherence between these signals in the 10- to 20-Hz band. Such indicators of short-term synchrony were either absent or very small in the patient group. The relationship between the amount of short-term synchrony and the magnitude of the 10- to 20-Hz coherence in the patients is discussed in relation to gait ability. It is suggested that supraspinal drive to the spinal cord is responsible for short-term synchrony and coherence in the 10- to 20-Hz frequency band during walking in healthy subjects. Absence or reduction of these features may serve as physiological markers of impaired supraspinal control of gait in SCL patients. Such markers could have diagnostic and prognostic value in relation to the recovery of locomotion in patients with central motor lesions.

Synchronization of motor units in human soleus muscle during standing postural tasks

During standing posture, the soleus muscles acts to control sway in the anteroposterior (AP) direction. The soleus muscles bilaterally share a common function during standing tasks. We sought to determine whether common descending inputs, as evidenced by the synchronization of bilateral motor unit pairs, were employed as a strategy to control this common function. Single motor units were recorded from the soleus muscles in subjects who stood on adjacent force platforms for 5 min with their eyes open or closed. While standing with the eyes open, only 4/39 bilateral motor unit pairs showed significant synchronization. Similarly, only 3/36 motor unit pairs were significantly synchronized during the eyes closed task. The low incidence of synchronization was observed despite a high correlation in the amount of sway in the AP direction between legs in both the eyes open and eyes closed tasks (rho = 0.80 and rho = 0.83, respectively). When the extent of synchronization was assessed between pairs of motor units within the same leg with the eyes open, 10/12 pairs were synchronized. Furthermore, when pairs of soleus motor units were recorded both bilaterally and unilaterally during voluntary isometric ankle plantarflexion, only 4/30 bilateral pairs showed significant synchronization, whereas 19/24 unilateral pairs had significant synchronization. In this study, there was little evidence of the existence of synchronization between bilateral soleus motor unit pairs in either postural tasks or voluntary isometric contractions. In cases in which bilateral synchronization was observed, it was considerably weaker than the synchronization of motor units within a single soleus muscle. The results of this study reveal that it is rather uncommon for bilateral soleus motoneurons to receive common descending synaptic inputs, whereas two motoneurons within a single soleus muscle do.

Influence of amplitude cancellation on the simulated surface electromyogram

The purpose of the study was to quantify the influence of selected motor unit properties and patterns of activity on amplitude cancellation in the simulated surface electromyogram (EMG). The study involved computer simulations of a motor unit population with physiologically defined recruitment and rate coding characteristics that activated muscle fibers whose potentials were recorded on the skin over the muscle. Amplitude cancellation was quantified as the percent difference in signal amplitude when motor unit potentials were summed before and after rectification. The simulations involved varying the level of activation for the motor unit population, the recording configuration, the upper limit of motor unit recruitment, peak discharge rates, the amount of motor unit synchronization, muscle fiber length, the thickness of the subcutaneous tissue, and the motor unit properties that change with advancing age. The results confirmed a previous experimental report (Day SJ and Hulliger M, J Neurophysiol 86: 2144–2158, 2001) that amplitude cancellation in the surface EMG can reach 62% at maximal activation. A decrease in the range of amplitudes of the motor unit potentials, as can occur during fatiguing contractions, increased amplitude cancellation up to ∼85%. Differences in the amount of amplitude cancellation were observed across all simulated conditions, and resulted in substantial changes in the absolute magnitude of the EMG signal. The most profound factors influencing amplitude cancellation were the number of active motor units and the duration of the action potentials. The effects of amplitude cancellation were minimal (<5%) when the EMG amplitude was normalized to maximal values, with the exception of variations in peak discharge rate and recruitment range, which resulted in differences up to 17% in the normalized EMG signal across conditions. These results indicate the amount of amplitude cancellation that can occur in various experimental conditions and its influence on absolute and relative measures of EMG amplitude.

Patterns of EMG-EMG coherence in limb dystonia

Dystonia of the limbs may be due to a wide range of aetiologies and may cause major functional limitation. We investigated whether the previously described pathological 4 to 7 Hz drive to muscles in cervical dystonia is present in patients with aetiologically different types of dystonia of the upper and lower limbs. To this end, we studied 12 symptomatic and 4 asymptomatic carriers of the DYT1 gene, 6 patients with symptomatic dystonia due to focal basal ganglia lesions, and 11 patients with fixed dystonia, a condition assumed to be mostly psychogenic in aetiology. We evaluated EMG-EMG coherence in the tibialis anterior (TA) of these and 15 healthy control subjects. Ten of 12 (83%) of symptomatic DYT1 patients had an excessive 4 to 7 Hz common drive to TA, evident as an inflated coherence in this band. This drive also involved the gastrocnemius, leading to co-contracting electromyographic bursts. In contrast, asymptomatic DYT1 carriers, patients with symptomatic dystonia, patients with fixed dystonia, and healthy subjects showed no evidence of such a drive or any other distinguishing electrophysiological feature. Moreover, the pathological 4 to 7 Hz drive in symptomatic DYT1 patients was much less common in the upper limb, where it was only present in 2 of 6 (33%) patients with clinical involvement of the arms. We conclude that the nature of the abnormal drive to dystonic muscles may vary according to the muscles under consideration and, particularly, with aetiology.

PCA in studying coordination and variability: a tutorial

OBJECTIVE

To explain and underscore the use of principal component analysis in clinical biomechanics as an expedient, unbiased means for reducing high-dimensional data sets to a small number of modes or structures, as well as for teasing apart structural (invariant) and variable components in such data sets.

DESIGN

The method is explained formally and then applied to both simulated and real (kinematic and electromyographic) data for didactical purposes, thus illustrating possible applications (and pitfalls) in the study of coordinated movement.

BACKGROUND

In the sciences at large, principal component analysis is a well-known method to remove redundant information in multidimensional data sets by means of mode reduction. At present, principal component analysis is starting to penetrate the fundamental and clinical study of human movement, which amplifies the need for an accessible explanation of the method and its possibilities and limitations. Besides mode reduction, we discuss principal component analysis in its capacity as a data-driven filter, allowing for a separation of invariant and variant properties of coordination, which, arguably, is essential in studies of motor variability.

METHODS

Principal component analysis is applied to kinematic and electromyographic time series obtained during treadmill walking by healthy humans.

RESULTS

Common signal structures or modes are identified in the time series that turn out to be readily interpretable. In addition, the identified coherent modes are eliminated from the data, leaving a filtered, residual pattern from which useful information may be gleaned regarding motor variability.

CONCLUSIONS

Principal component analysis allows for the detection of modes (information reduction) in both kinematic and electromyographic data sets, as well as for the separation of invariant structure and variance in those data sets.

RELEVANCE

Principal component analysis can be successfully applied to movement data, both as feature extractor and as data-driven filter. Its potential for the (clinical) study of human movement sciences (e.g., diagnostics and evaluation of interventions) is evident but still largely untapped.

Cycling as a novel approach to resistance training increases muscle strength, power, and selected functional abilities in healthy older women

Cycling on a mechanically braked cycle ergometer was used as a novel approach to compare the effects of three different 16-wk resistance-training programs on isometric force, power output, and selected functional abilities in 31 healthy 65- to 74-yr-old women. Training was conducted three times per week. During each session, individuals of the speed group performed 8 sets of 16 pedal revolutions at 40% of the maximal resistance to complete two revolutions (2 RM); strength group performed 8 sets of 8 revolutions at 80% of 2 RM; and combination group performed 4 sets of 16 revolutions at 40% and 4 sets of 8 revolutions at 80% of 2 RM. During each set, all participants were required to pedal as fast as possible with a 2-min interval between sets. All training groups significantly increased force, power, and functional abilities (maximal treadmill walking speed, vertical jumping, and box stepping) at week 8 (in the range from 6.5 to 20.8%) with no further improvement at week 16 (except maximal treadmill walking speed), but no significant differences were observed between the three groups. The novel approach to performing both low- and high-resistance training, based on the use of a cycle ergometer, has been shown to be effective in improving strength, power, and functional abilities in a group of healthy women. Even fit older women can still improve in functional abilities. Interestingly, the "high-speed" and "low-speed" programs induced an increase in both power and strength of similar magnitude.

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.

Motor unit synchronisation is enhanced during slow lengthening contractions of a hand muscle

This study examined the strength of motor unit synchronisation based on time- and frequency-domain measures during postural, shortening and lengthening contractions of a hand muscle in young adults. Single motor unit activity was recorded with intramuscular electrodes in the left first dorsal interosseus muscle as the subject held the index finger at a constant position while supporting a light load for 2-5 min. The subject then performed slow (1.7 deg s(-1)) shortening and lengthening contractions to lift and lower the load. The movement required subjects to perform 10-25 constant-velocity contractions with the index finger over a 10 deg range of motion by using 6 s shortening and lengthening contractions. Individual discharge times were obtained from 23 pairs of motor units in 14 subjects to assess the strength of motor unit synchronisation and coherence during the three tasks. The strength of motor unit synchronisation was approximately 50 % greater during the lengthening contractions compared with the postural and shortening contractions, and the width of the central synchronous peak in the cross-correlation histogram was approximately 4 ms narrower during shortening contractions. These findings reveal that there is an increase in common input to motoneurones during lengthening contractions and a greater relative contribution of direct common inputs to motoneurones during shortening contractions compared with postural tasks. Furthermore, the amount of motor unit coherence in the low-frequency band (2-12 Hz) was reduced during shortening contractions compared with postural and lengthening contractions. These data indicate that the timing of inputs received by the motoneurones innervating the first dorsal interosseus of young adults differs during postural, shortening and lengthening contractions against a light load.

Synchronization of motor neurons during locomotion in the neonatal rat: predictors and mechanisms

We describe here the robust synchronization of motor neurons at a millisecond time scale during locomotor activity in the neonatal rat. Action potential activity of motor neuron pairs was recorded extracellularly using tetrodes during locomotor activity in the in vitro neonatal rat spinal cord. Approximately 40% of motor neuron pairs recorded in the same spinal segment showed significant synchronization, with the duration of the central peak in cross-correlograms between motor neurons typically ranging between approximately 30 and 100 msec. The percentage of synchronized motor neuron pairs was considerably higher for pairs with similar locomotor-related activity and strong rhythmic modulation. We also found synchronization between the activities of different motor pools, even if located several segments apart. Such distant synchronization was abolished in the absence of chemical synapses, although local coupling between motor neurons persisted. On the other hand, both local and distant coupling between motor neurons were preserved after antagonism of gap junction coupling between motor neurons. These results demonstrate that motor neuron activity is strongly synchronized at a millisecond time scale during the production of locomotor activity in the neonatal rat. These results also demonstrate that chemical synaptic inputs, in addition to electrical synapses, contribute to this synchronization, suggesting the existence of multiple mechanisms underlying motor neuron synchronization in the neonatal rat. The fast synchronization described here might be involved in activity-dependent processes during development or in the coordination of individual motor neurons into a functional population underlying behavior.