Mechanomyographic and electromyographic responses to eccentric muscle contractions

Little is known regarding the modulation of torque during eccentric muscle actions. Mechanomyographic (MMG) and electromyographic (EMG) signals have been used to examine motor control strategies. The purpose of this study was to examine the MMG and EMG amplitude and frequency in relation to torque during eccentric muscle contractions. Eight women performed eccentric leg extension muscle contractions at 10-100% of peak torque (PT). A piezoelectric crystal contact sensor and bipolar surface electrodes were placed on the vastus medialis to detect the MMG and EMG signals. Polynomial regression analyses indicated that EMG amplitude (r(2)=0.994) and MMG wavelet center frequency (CF) (r(2)=0.846) increased linearly to 100% eccentric PT, whereas there were no significant relationships for EMG wavelet CF or MMG amplitude and eccentric torque. These results suggested that eccentric torque is primarily modulated through changes in motor unit firing rate.

Isolating rule- versus evidence-based prefrontal activity during episodic and lexical discrimination: a functional magnetic resonance imaging investigation of detection theory distinctions

Dorsolateral and frontopolar prefrontal cortices (PFCs) are often implicated in neuroimaging studies of memory retrieval, with this activity ascribed to controlled monitoring processes indicative of difficult or demanding retrieval. Difficulty, however, is multiply determined, with success rates governed both by the available evidence and by the nature of decision rules applied to that evidence. Using event-related functional magnetic resonance imaging, we isolated these factors by 1) contrasting different decision rules across matched evidence and 2) manipulating the level of evidence within a fixed decision rule. For identically constructed retrieval probes (1 old and 1 new item), same-different (are these different?) compared with forced-choice (which one is old?) decision rules yielded bilateral dorsolateral and right frontopolar PFC increases. However, these regions were unaffected when the available evidence was greatly lowered within forced-choice decisions. Thus, the regions were simultaneously sensitive to the type of decision rule and yet insensitive to the level of evidence supporting those decisions. Analogous lexical tasks yielded similar patterns, demonstrating that the PFC responses were not episodic memory specific. We discuss the mechanistic differences between same-different versus forced-choice decisions and the implications of these data for current theories of PFC activity during episodic remembering and executive control.

Dynamics of frontal, striatal, and hippocampal systems during rule learning

We examined interactions between frontal, striatal, and hippocampal systems during a rule-learning task. Nineteen healthy young adults solved multiple rule-learning problems requiring hypothesis testing while functional magnetic resonance images were obtained. Activity in the head of the caudate peaked early after the beginning of each problem and then dropped rapidly. In contrast, activity in prefrontal cortex areas reached peak values later. These results are in accordance with theories suggesting that the striatum identifies the behavioral context necessary for the frontal lobe to select an appropriate strategy. Striatal and hippocampal systems showed antagonistic patterns of activity: Activation in the anterior hippocampus decreased, whereas caudate activity increased. Good learners showed higher activity in the body and tail of the caudate than poor learners, whereas learning success correlated negatively with activity in the hippocampus. Activation in the head of the caudate correlated negatively with hippocampal activation, indicating a potential mechanism for hippocampal activity reduction.

The level of spinal cord involvement influences the pattern of movement-associated cortical recruitment in patients with isolated myelitis

Using fMRI, an increased recruitment of the ipsilateral primary sensorimotor cortex (SMC), supplementary motor area, and middle frontal gyrus has been detected in patients with cervical cord myelitis of possible demyelinating origin. The aim of this study was to evaluate, using fMRI, whether the level of cord involvement influences cortical reorganization by comparing patients with isolated myelitis of the cervical and the dorsal portions of the cord, and to investigate whether the extent of cortical reorganization is associated with the extent of cervical cord pathology measured using magnetization transfer (MT) MRI. We studied 24 right-handed patients (14 with a previous involvement of the cervical cord and 10 with an involvement of the dorsal cord) in a chronic and clinically stable phase following an isolated myelitis of possible demyelinating origin and 15 sex- and age-matched healthy controls. During a single session, we obtained fMRI during repetitive flexion-extension of the last four fingers of the right and left hands and cervical cord MT MRI. Average cord MTR was lower in patients with cervical (P < 0.0001) and dorsal (P = 0.0001) myelitis than in controls. Compared to controls and for both tasks, patients with myelitis had an increased recruitment of the ipsilateral primary SMC, which was independent of the level of cord involvement. On the contrary, patients with cervical myelitis had a more widespread recruitment of frontal and parietal regions, whereas those with dorsal myelitis had a more widespread recruitment of temporal and cerebellar regions. Strong correlations (r values ranging from -0.72 to -0.88) were found between relative activations of cortical areas and the severity of cervical cord damage. Patients with isolated myelitis have different patterns of movement-associated cortical activations according to the level of cord involvement. This "level-dependent" functional reorganization of the cortex is likely to have an adaptive role in limiting the clinical outcome of cord damage and should be considered when designing rehabilitation strategies for these patients.

Effects of pelvic stabilization on lumbar muscle activity during dynamic exercise

Many commonly utilized low-back exercise devices offer mechanisms to stabilize the pelvis and to isolate the lumbar spine, but the value of these mechanisms remains unclear. The purpose of this study was to examine the effect of pelvic stabilization on the activity of the lumbar and hip extensor muscles during dynamic back extension exercise. Fifteen volunteers in good general health performed dynamic extension exercise in a seated upright position on a lumbar extension machine with and without pelvic stabilization. During exercise, surface electromyographic activity of the lumbar multifidus and biceps femoris was recorded. The activity of the multifidus was 51% greater during the stabilized condition, whereas there was no difference in the activity of the biceps femoris between conditions. This study demonstrates that pelvic stabilization enhances lumbar muscle recruitment during dynamic exercise on machines. Exercise specialists can use these data when designing exercise programs to develop low back strength.

Diurnal changes in the amplitude of the Hoffmann reflex in the human soleus but not in the flexor carpi radialis muscle

Changes in the reflex amplitude throughout the day have been observed in non-human mammals. The present experiment tested whether diurnal fluctuations also occur in humans. Hoffmann reflex (H-reflex) amplitude was measured in soleus and flexor carpi radialis (FCR) muscles from the data collected over a 12-h period between 7:00-9:00 a.m. and 7:00-9:00 p.m. At 4-h intervals, M/H recruitment curves were obtained, and two measures of H-reflex excitability were calculated. The maximal H-reflex (H (max)) was calculated as the average of the three largest H-reflexes. H-reflexes were also sampled from the ascending limb of the M/H recruitment curve (H (A), n=10), with a corresponding M-wave of 5% M (max). All values were normalized to the maximal M-wave (M (max)). Soleus H-reflex amplitude and plantar flexion maximal voluntary isometric contraction force (MVIC) were significantly smaller (p<0.05) in the morning (H (max)=57.2% M (max), H (A)=42.3%, M (max), MVIC=162.1 Nm) than in the evening (H (max)=69.1% M (max), a 20.1% increase, H (A)=54.1% M (max), a 27.4% increase and MVIC=195.8 Nm, a 20.8% increase). In contrast, FCR H-reflex amplitude and FCR MVIC were unchanged across all testing sessions. The data show that diurnal fluctuations are present in the amplitude of the human soleus but not in the FCR H-reflex. Diurnal fluctuation in the human soleus H-reflex amplitude must be considered when interpreting H-reflex data, especially when a repeated measures design spanning several days is utilized.

Recruitment of synapses in the neurosecretory process during long-term facilitation at the lobster neuromuscular junction

We investigated long-term facilitation at the lobster neuromuscular synapse employing a combination of FM1-43 staining of synaptic vesicles, electron microscopy analysis, and electrical recordings of synaptic activity. Synaptic terminals were loaded with the fluorescent dye FM1-43 producing clusters of activity-dependent fluorescent spots. Electron microscopy analysis of synaptic ultrastructure suggested that fluorescent spots represent compartments of synaptic terminals filled with vesicles. Excitatory postsynaptic currents were recorded from the stained synaptic terminals using focal macropatch electrodes. Terminals were stained during the nerve stimulation at a low stimulation frequency (2, 5 or 10 Hz) before and after long-term facilitation was elicited by high-frequency stimulation (20 or 30 Hz for 5 min). We found that staining after long-term facilitation results in the appearance of new fluorescent spots, as well as in the increase in fluorescence of the spots that appeared before long-term facilitation. This increase in fluorescence accounted for the increase in quantal release. Activation of individual fluorescent spots was found to be non-uniform. In spite of overall increase in fluorescence, some synaptic compartments decreased their staining after long-term facilitation. Thus, our study demonstrates that long-term facilitation produces non-uniform activation of FM1-43 uptake in synaptic compartments that correlates with the increase in quantal neurosecretion.

Change in Muscle Fascicle Length Influences the Recruitment and Discharge Rate of Motor Units During Isometric Contractions

This study examines the effect of fascicle length change on motor-unit recruitment and discharge rate in the human tibialis anterior (TA) during isometric contractions of various intensities. The torque produced during dorsiflexion and the surface and intramuscular electromyograms (EMGs) from the TA were recorded in eight subjects. The behavior of the same motor unit ( n = 59) was compared at two ankle joint angles (+10 and −10° around the ankle neutral position). Muscle fascicle length of the TA was measured noninvasively using ultrasonography recordings. When the ankle angle was moved from 10° plantarflexion to 10° dorsiflexion, the torque produced during maximal voluntary contraction (MVC) was significantly reduced [35.2 ± 3.3 vs. 44.3 ± 4.2 (SD) Nm; P < 0.001] and the average surface EMG increased (0.47 ± 0.08 vs. 0.43 ± 0.06 mV; P < 0.05). At reduced ankle joint angle, muscle fascicle length declined by 12.7% ( P < 0.01) at rest and by 18.9% ( P < 0.001) during MVC. Motor units were activated at a lower recruitment threshold for short compared with long muscle fascicle length, either when expressed in absolute values (2.1 ± 2.5 vs. 3.6 ± 3.7 Nm; P < 0.001) or relative to their respective MVC (5.2 ± 6.1 vs. 8.8 ± 9.0%). Higher discharge rate and additional motor-unit recruitment were observed at a given absolute or relative torque when muscle fascicles were shortened. However, the data indicate that increased rate coding was mainly present at low torque level (<10% MVC), when the muscle-tendon complex was compliant, whereas recruitment of additional motor units played a dominant role at higher torque level and decreased compliance (10–35% MVC). Taken together, the results suggest that the central command is modulated by the afferent proprioceptive information during submaximal contractions performed at different muscle fascicle lengths.

A comparison of symptomatic and asymptomatic office workers performing monotonous keyboard work—1: Neck and shoulder muscle recruitment patterns

Work-related neck and upper limb disorders (WRNULD) are common problems among office workers who use computers intensively and maintain prolonged static postures. These disorders have often been attributed to result from sustained muscle activity in the neck-shoulder musculature. The present study examined whether symptomatic subjects exhibited the same muscle activity patterns as asymptomatic controls when they performed a prolonged computer task under the same conditions. Surface electromyography (EMG) of four major neck-shoulder muscles were compared between a Case Group (n=23) and a Control Group (n=20) of female office workers. The Case Group had higher activity in the right upper trapezius (UT) while the Control Group had more symmetrical muscle activity between left and right UT. The Case subjects could also be differentiated into "High Discomfort" and "Low Discomfort" sub-groups based on their discomfort scores. The High Discomfort Group had significantly higher right UT activity compared to the Low Discomfort and Control Groups. Results suggested that symptomatic individuals had altered muscle recruitment patterns that persisted throughout the sustained occupational task, while discomfort increased with time-at-task. These findings indicate that altered muscle recruitment patterns observed in the symptomatic subjects preceded the onset of task discomfort, and this finding may have important implications for the etiology of WRNULD.

Intraspinal microstimulation preferentially recruits fatigue-resistant muscle fibres and generates gradual force in rat

Intraspinal microstimulation (ISMS), a novel rehabilitative therapy consisting of stimulation through fine, hair-like microwires targeted at the ventral spinal cord, has been proposed for restoring standing and walking following spinal cord injury. This study compared muscle recruitment characteristics of ISMS with those produced by peripheral nerve cuff stimulation (NCS). Thirty-three minutes of either ISMS or NCS at 1, 20 or 50 s(-1) and 1.2 x threshold (T) amplitude depleted glycogen from muscle fibres of vastus lateralis and rectus femoris. ISMS and NCS were also carried out at 20 s(-1) and 3.0T. Muscle serial sections were stained for glycogen and for myosin heavy chain (MHC)-based fibre types using a panel of monoclonal antibodies. The results of this study show that ISMS recruited fatigue-resistant (FR) fibres at 2.9, 1.9, 1.7 and 2.5 times their relative MHC content at 1, 20 and 50 s(-1) 1.2T and 20 s(-1) 3.0T, respectively. In contrast, NCS recruited FR fibres at 1.2, 1.0, 2.1 and 0.0 times their MHC content at 1, 20 and 50 s(-1) 1.2T and 20 s(-1) 3.0T, respectively. The proportion of FR fibres recruited by ISMS and NCS was significantly different in the 20 s(-1) 3.0T condition (P < 0.0001). We also report that force recruitment curves were 4.9-fold less steep (P < 0.019) for ISMS than NCS. The findings of this study provide evidence for the efficacy of ISMS and further our understanding of muscle recruitment properties of this novel rehabilitative therapy.

Resistance training: cortical, spinal, and motor unit adaptations

During the first few weeks of isometric resistance training there is an increase in maximal muscle force output that cannot be accounted for by muscle hypertrophy. Early on, researchers postulated the existence of neural adaptations to training primarily through the use of surface electromyographic recordings. More recent evidence also suggests that increased excitation may occur at the cortical levels following short-term resistance training. Alterations in synergistic activation and reductions in antagonist activation are neural factors that have been identified as changing during the early stages of resistance training which could contribute to maximal force generation. Neural adaptations that occur during the ramp-up phase of isometric contraction include decreases in motor unit recruitment thresholds, increased motor unit discharge rates, and increases in double discharges. An increase in the maximal rate of force development also occurs during the early stages of resistance training, but whether the neural mechanisms associated with the increase in the rate of rise are also associated with the increase in maximal force has not been elucidated. More work is needed to examine the integration of changes in cortical and spinal excitability with single motor unit firing patterns during this simple form of exercise before we can extend our understanding to different types of training.

MEP recruitment curves in multiple sclerosis and hereditary spastic paraplegia

OBJECTIVE

Axons remodel at multiple levels after a single inflammatory lesion in the spinal cord, which can contribute to recovery. The primary aim of this study was to investigate whether the MEP response as function of the excitatory strength, here called recruitment curves, may be used in discriminating demyelination from compensated axonal loss. Multiple sclerosis (MS) represents both demyelination and axonal degeneration. Hereditary Spastic Paraplegia (HSP) was included as a model of pure axonal loss.

METHODS

To investigate both spinal and cortical recruitment, the methods used for gradual recruitment were two different test paradigms of voluntary pre-activation and stimulus intensity. The MEP-recruitment curves were obtained by means of transcranial magnetic stimulation (TMS) in 29 MS patients, 9 patients with HSP and in 30 healthy controls.

RESULTS

Saturated recruitment curves were obtained in all subject groups, muscles and paradigms and were generally found to be identical. The two groups of patients had clinical signs, CMCT changes and reduced MEP amplitude reflecting relevant cortico-spinal disorder.

CONCLUSIONS

We conclude that both demyelination and axonal degeneration in the CNS leads to diminished MEP amplitudes and CMCT changes. The recruitment curves of MS and HSP was identical to controls and may not be used for diagnostic or monitoring purposes.

Propagation of amygdala-kindled seizures to the hippocampus in the rat: electroencephalographic features and behavioural correlates

BACKGROUND

The propagation of amygdala-kindled seizures to the dorsal and ventral hippocampus was examined, in rats. The relation of contralateral seizure propagation to the onset of generalized convulsions was also studied.

METHODS

In all subjects, electrodes were implanted in the amygdala. Two additional electrodes were implanted (bilaterally) in the dorsal (n = 6) or ventral (n = 8) hippocampus. Kindling stimulations were delivered twice daily (interval 4 h).

RESULTS

Initially, triggered after-discharges (ADs) were recorded only in the amygdala. With repeated stimulation, the AD propagated to the hippocampus. Rates of propagation were as follows (mean # of stimulations +/- SEM): ipsilateral ventral hippocampus, 4.0 +/- 0.9; ipsilateral dorsal hippocampus, 6.2 +/- 1.4; contralateral dorsal hippocampus, 7.5 +/- 1.4; contralateral ventral hippocampus, 8.5 +/- 1.0. AD propagation to contralateral sites was significantly slower than to ipsilateral sites. Ipsilateral hippocampal recruitment occurred between stages 1 and 2 (partial seizures), whereas contralateral hippocampal recruitment occurred between stages 2 and 3 (transition to generalized seizures).

SUMMARY

These results indicate that during amygdala-kindling, it takes several stimulations before discharge propagates to the hippocampus. The close link between contralateral hippocampus involvement and seizure generalization warrants further study, and may lead to a better understanding of the pathways involved in seizure spread.

Percutaneous electrical stimulation in strength training: an update

Numerous studies have used percutaneous electrical stimulation (PES) in the context of training programs to develop strength and physical performance in healthy populations (sedentary or trained). Significant increases in muscle and fiber cross-sectional area, isokinetic peak torque, maximal isometric and dynamic strength, and motor performance skills have been found after PES training. These strength gains are explained on the basis of the characteristics of PES motor units (MUs) recruitment: (a) a continuous and exhausting contractile activity in the same pool of MUs during the entire exercise period, (b) a supramaximal temporal recruitment imposed by the high frequency chosen (up to 40 Hz), and (c) a synchronous recruitment of neighboring fibers. The PES training method is complementary to voluntary training, mainly because the application of PES causes an unconventional spatial recruitment of MUs that, depending on the muscular topography, may entail the preferential recruitment of the fast-twitch MUs. In addition, the method does not specifically develop elasticity in skeletal muscle, and it must be accompanied by a technical workout.

Mechanomyographic and electromyographic responses of the vastus medialis muscle during isometric and concentric muscle actions

The purpose of this study was to examine the patterns for the mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) vs. torque relationships during submaximal to maximal isometric and isokinetic muscle actions. Seven men (mean +/- SD age, 22.4 +/- 1.3 years) volunteered to perform isometric and concentric isokinetic leg extension muscle actions at 20, 40, 60, 80, and 100% of maximal voluntary contraction (MVC) and peak torque (PT) on a Cybex II dynamometer. A piezoelectric MMG recording sensor was placed between bipolar surface EMG electrodes on the vastus medialis. Polynomial regression and separate 1-way repeated-measures analysis of variance were used to analyze the EMG amplitude, MMG amplitude, EMG MPF, and MMG MPF data for the isometric and isokinetic muscle actions. For the isometric muscle actions, EMG amplitude (R(2) = 0.999) and MMG MPF (R(2) = 0.946) increased to MVC, mean MMG amplitude increased to 60% MVC and then plateaued, and mean EMG MPF did not change (p > 0.05) across torque levels. For the isokinetic muscle actions, EMG amplitude (R(2) = 0.988) and MMG amplitude (R(2) = 0.933) increased to PT, but there were no significant mean changes with torque for EMG MPF or MMG MPF. The different torque-related responses for EMG and MMG amplitude and MPF may reflect differences in the motor control strategies that modulate torque production for isometric vs. dynamic muscle actions. These results support the findings of others and suggest that isometric torque production was modulated by a combination of recruitment and firing rate, whereas dynamic torque production was modulated primarily through recruitment.

Electromyographic activity of the pectoralis major and anterior deltoid muscles during three upper-body lifts

The purpose of this study was to examine the differences in activation levels and times of activation for the pectoralis major and anterior deltoid when performing the concentric phase of 3 upper-body lifts. Twelve college-age men and women with various degrees of lifting experience performed 3 repetitions using the 6 repetition maximum in a barbell bench press, dumbbell bench press, and dumbbell fly while being monitored for electromyographic activity in both muscles. Motor unit activation of both muscles was not significantly different during all 3 lifts. However, dumbbell flys had significantly less relative time of activation than did barbell or dumbbell bench presses. Therefore, dumbbell flys may be better suited as an auxiliary lift, whereas barbell and dumbbell bench presses may be used interchangeably in training programs. The compatibility of the barbell and dumbbell bench presses may aid lifters in overcoming training plateaus by alternating exercises for the same muscle groups.

Altered neural substrates of cognitive control in childhood ADHD: evidence from functional magnetic resonance imaging

OBJECTIVE

The study compared the neural bases of two cognitive control operations, interference suppression and response inhibition, between children with and children without attention deficit hyperactivity disorder (ADHD).

METHOD

Ten children (7-11 years of age) with combined-type ADHD and 10 comparison subjects matched for age and gender underwent rapid event-related functional magnetic resonance imaging (fMRI) during performance of a modified flanker task. Functional maps were generated through group averaging and performance-based correlational analyses.

RESULTS

Interference suppression in ADHD subjects was characterized by reduced engagement of a frontal-striatal-temporal-parietal network that subserved healthy performance. In contrast, response inhibition performance relied upon different regions in the two groups, frontal-striatal in comparison subjects but right superior temporal in ADHD children.

CONCLUSIONS

Alteration in the neural basis of two cognitive control operations in childhood ADHD was characterized by distinct, rather than unitary, patterns of functional abnormality. Greater between-group overlap in the neural network activated for interference suppression than in response inhibition suggests that components of cognitive control are differentially sensitive to ADHD. The ADHD children's inability to activate the caudate nucleus constitutes a core abnormality in ADHD. Observed functional abnormalities did not result from prolonged stimulant exposure, since most children were medication naive.

Neural correlates of superior intelligence: stronger recruitment of posterior parietal cortex

General intelligence (g) is a common factor in diverse cognitive abilities and a major influence on life outcomes. Neuroimaging studies in adults suggest that the lateral prefrontal and parietal cortices play a crucial role in related cognitive activities including fluid reasoning, the control of attention, and working memory. Here, we investigated the neural bases for intellectual giftedness (superior-g) in adolescents, using fMRI. The participants consisted of a superior-g group (n = 18, mean RAPM = 33.9 +/- 0.8, >99%) from the national academy for gifted adolescents and the control group (n = 18, mean RAPM = 22.8 +/- 1.6, 60%) from local high schools in Korea (mean age = 16.5 +/- 0.8). fMRI data were acquired while they performed two reasoning tasks with high and low g-loadings. In both groups, the high g-loaded tasks specifically increased regional activity in the bilateral fronto-parietal network including the lateral prefrontal, anterior cingulate, and posterior parietal cortices. However, the regional activations of the superior-g group were significantly stronger than those of the control group, especially in the posterior parietal cortex. Moreover, regression analysis revealed that activity of the superior and intraparietal cortices (BA 7/40) strongly covaried with individual differences in g (r = 0.71 to 0.81). A correlated vectors analysis implicated bilateral posterior parietal areas in g. These results suggest that superior-g may not be due to the recruitment of additional brain regions but to the functional facilitation of the fronto-parietal network particularly driven by the posterior parietal activation.

Cross-modal activation of visual cortex during depth perception using auditory substitution of vision

Previous neuroimaging studies identified multimodal brain areas in the visual cortex that are specialized for processing specific information, such as visual-haptic object recognition. Here, we test whether visual brain areas are involved in depth perception when auditory substitution of vision is used. Nine sighted volunteers were trained blindfolded to use a prosthesis substituting vision with audition both to recognize two-dimensional figures and to estimate distance of an object in a real three-dimensional environment. Using positron emission tomography, regional cerebral blood flow was assessed while the prosthesis was used to explore virtual 3D images; subjects focused either on 2D features (target search) or on depth (target distance comparison). Activation foci were found in visual association areas during both the target search task, which recruited the occipito-parietal cortex, and the depth perception task, which recruited occipito-parietal and occipito-temporal areas. This indicates that some brain areas of the visual cortex are relatively multimodal and may be recruited for depth processing via a sense other than vision.

Assessment of muscle fatigue in low level monotonous task performance during car driving

Knowledge of the muscle activation and the development of muscle fatigue may provide more inside in the effects of long-term driving in the occurrence of health problems in the neck/shoulder/back area. The basic assumption behind fatigue detection with electromyography (EMG) is an increase in the EMG amplitude and a decrease of the mean frequency (MF). This study aimed at checking this assumption in monotonous task performance with low level activity during car driving. Surface electromyography was captured from left and right trapezius and deltoid muscles, during a repetitive, non-continuous, driving task (gearing and steering) and the active parts were separated from the non-active parts. Muscle stiffness was reported by more than half of the subjects after a 1 h drive. Only for the active parts a significant decrease of the MF was seen. But also the EMG amplitude decreased significantly. Two possible mechanisms are posted in literature for this finding: no extra recruitment of motor units (MU) and potentiation of muscle fibers. Literature also hypothesizes that low-force occupational work engages only a fraction of the MU available for recruitment and that these units are selectively type I muscle fibers (Cinderella fibers). Initiators of this phenomenon are probably the time lag between activations and the stress from driving and vibration exposure.

Recruitment of the thigh muscles during sprint cycling by muscle functional magnetic resonance imaging

The purpose of the present study was to investigate recruitment patterns of the thigh muscles during maximal sprint cycling by muscle functional magnetic resonance imaging (mfMRI). Twelve healthy men participated in this study and performed 2, 5, and 10 sets of 6-s supramaximal cycling with a load of 7.5 % of their body weight with 0.5 min of rest between the sets. Before and immediately after the exercise, T2-weighted MR images, i.e. mfMRI, of the right-thigh were taken to calculate T2 of eleven thigh muscles. Vastus lateralis, semitendinosus, and sartorius were the highest activated, i. e. had the greatest T2 change, among the quadriceps, hamstring, and adductors, respectively, compared with other muscles. Total power output during 2, 5, and 10 sets of sprint cycling was correlated with percent change in T2 in the quadriceps correlated (r (2) = 0.507 to 0.696, p < 0.01), the hamstring (r (2) = 0.162 to 0.335, p < 0.05 approximately 0.001), and the adductor muscles (r (2) = 0.162 to 0.473, p < 0.05 approximately 0.0001). With use of stepwise regression analysis, total power output was significantly correlated with % change in T2 of the vastus medialis (VM) (p < 0.0001) and vastus intermedius (VI) (p < 0.05) (r (2) = 0.698, p < 0.0001). We concluded that eleven thigh muscles were activated non-uniformly, and that the VM and VI play a key role during maximal sprint cycling.

Dynamic and electromyographical analysis in variants of push-up exercise

The purpose of the study was to record dynamic and muscular modifications during push-up exercise variants (EV). Eight healthy men performed 6 EV of push-ups: normal, abducted, adducted, posterior, anterior, and on knees. Ground-reaction forces were recorded with a force plate while surface muscular activity with electrodes on triceps and pectoralis major. Significant differences (p < 0.05) existed for most vertical force variables but not for anteroposterior force and time variables. The initial load relative to body weight was 66.4% at the normal position, while only 52.9% at the on-knees EV. Muscle activity was less during the on-knees EV for both muscles. At the posterior EV, pectoralis major was activated higher than normal; however, triceps were activated lower than normal. Dynamic behavior and muscle activity were significantly altered between push-up EV. Instructions for push-up exercises should be followed carefully because dynamic and muscular challenge is altered when hands are differently positioned.

Trunk muscle recruitment patterns in specific chronic low back pain populations

BACKGROUND

It is hypothesized that injury or degeneration of osteoligamentous spinal structures would require compensation by trunk musculature and alterations in motor control to maintain spine stability. While, biomechanical modeling has supported this hypothesis, studies of muscle recruitment patterns in chronic low back pain patients both with and without significant osteoligamentous damage have been limited. This study utilized a non-randomized case-control design to investigate trunk muscle recruitment patterns around the neutral spine position between subgroups of patients with chronic mechanical low back pain and asymptomatic controls.

METHODS

Twenty subjects with chronic low back pain attributed to clinical lumbar instability were matched to 20 asymptomatic controls. In addition 12 patients with non-specific chronic low back pain were studied. Surface EMG from five trunk muscles was analyzed to determine activation levels and patterns of recruitment during a standing reach under two different loading conditions.

FINDINGS

The chronic low back pain group with symptoms attributed to clinical instability demonstrated significantly higher activation levels of the external oblique and rectus abdominus muscles and lower abdominal synergist ratios than the control group. No significant differences were found between patient subgroups.

INTERPRETATION

While these data demonstrate altered muscle recruitment patterns in patients with chronic low back pain, the changes are not consistent with Panjabi's theory suggesting that these alterations are driven by passive subsystem damage. However, the higher activation of global abdominal musculature and altered synergist patterns may represent a motor control pattern that has consequences for continued dysfunction and chronic pain.

The size principle: a rule describing the recruitment of motoneurons

This essay looks at the historical significance of five APS classic papers that are freely available online:

McPhedran AM, Wuerker RB, and Henneman E. Properties of motor units in a homogeneous red muscle (soleus) of the cat. J Neurophysiol 28: 71–84, 1965 ( http://jn.physiology.org/cgi/reprint/28/1/71 ).

Wuerker RB, McPhedran AM, and Henneman E. Properties of motor units in a heterogeneous pale muscle (m. gastrocnemius) of the cat. J Neurophysiol 28: 85–99, 1965 ( http://jn.physiology.org/cgi/reprint/28/1/85 ).

Henneman E, Somjen G, and Carpenter DO. Functional significance of cell size in spinal motoneurons. J Neurophysiol 28: 560–580, 1965 ( http://jn.physiology.org/cgi/reprint/28/3/560 ).

Henneman E and Olson CB. Relations between structure and function in the design of skeletal muscles. J Neurophysiol 28: 581–598, 1965 ( http://jn.physiology.org/cgi/reprint/28/3/581 ).

Henneman E, Somjen G, and Carpenter DO. Excitability and inhibitability of motoneurons of different sizes. J Neurophysiol 28: 599–620, 1965 ( http://jn.physiology.org/cgi/reprint/28/3/599 ).

H-reflex and motor responses to acute ischemia in apparently healthy individuals

The authors examined the effect of acute ischemia on peripheral nerve function in healthy subjects. It was hypothesized that acute ischemia would interfere with the ability of sensory and motor nerves to propagate an impulse. Twelve young, apparently healthy adults participated in the study. Soleus H-reflex and motor recruitment curves were determined for subjects during a control condition, after 5 minutes of ischemia by femoral artery occlusion, and after a 5-minute recovery. During ischemia, the stimulus intensity required to evoke an H-reflex or M-wave was reduced by 18.3% and 18.4%, respectively. Hmax/Mmax ratios were significantly reduced with acute ischemia (mean +/- standard error) 66.29% +/- 5.4% and 58.81% +/- 6.7% for control and ischemia, respectively, owing to a decrease in Hmax during acute ischemia with no change in Mmax. After ischemia, the Hmax/Mmax returned to control values, as did the M-threshold. However, although the H-threshold slightly recovered, it failed to return to control threshold after 5 minutes of recovery. The results suggest that acute ischemia decreases motor and H-reflex thresholds in healthy individuals with a longer lasting effect for the H-reflex. In addition, a decrease in Hmax/Mmax ratio was observed, suggesting that acute ischemia has differential effects on sensory nerve propagation and synapse transmission.

Predicting optimal electrical stimulation for repetitive human muscle activation

Functional electrical stimulation is the use of electrical currents to activate paralyzed muscles to produce functional movements. Muscle force output must meet or exceed the external load to maintain a posture or produce movements. A mathematical force-fatigue modeling system that predicts muscle force responses during repetitive electrical stimulation has been developed in our laboratory to help identify stimulation patterns that optimize force output for individual subjects. This study tests how well this model predicts the number of contractions that can be maintained above a required force level (successful contractions) during repetitive activation of a muscle. Healthy human quadriceps muscles were tested isometrically on 12 subjects. Data were first collected and used to parameterize the model. Next, the model was used to predict the number of successful contractions that were produced by trains with frequencies ranging from 5 to 100 Hz while the pulse durations and amplitudes were held constant. Finally, three clinically relevant stimulation frequencies were selected and tested to verify the model's predictions. Under these conditions, the model accurately predicted the number of successful contractions for clinically relevant stimulation frequencies. Furthermore, the model appears to have the potential to identify the stimulation frequency that maximizes muscle force output and minimizes fatigue for each subject.

From precipitation to inhibition of seizures: rationale of a therapeutic paradigm

Epileptic seizures can be triggered by both nonspecific facilitating factors such as sleep withdrawal, fever, or excessive alcohol intake, and specific reflex epileptic mechanisms. These consist of sensory or cognitive inputs activating circumscribed cortical areas or functional anatomic systems that, due to some functional instability, respond with an epileptic discharge. Interruption of seizure activity at the stage of the aura (i.e., locally restricted discharge) also can be achieved by nonspecific (e.g., relaxation or concentration techniques or vagal nerve stimulation) or by specific focus-targeted sensory or cognitive inputs. The latter, again, activate circumscribed cortical areas. Intriguingly, in some patients, the same stimulus can either precipitate or abort a seizure. The response depends on the state of cortical activation: seizure precipitation occurs in the resting condition, and seizure interruption occurs when the epileptic discharge has begun close to the activated area. These relations can be understood on the background of experimental data showing that an intermediate state of neuronal activation is a precondition for the generation of paroxysmal depolarization shifts, whereas a hyperpolarized neuron will remain subthreshold, and a depolarized neuron that already produces action potentials is not recruitable for other activity. Sensory input meeting an intermediately activated pool of potentially epileptic neurons is adequate to produce a seizure. In another condition, the same stimulus can depolarize a neuron pool in the same area sufficiently to block the further propagation of nearby epileptic activity. Understanding these interactions facilitates the development of successful nonpharmaceutical therapeutic interventions for epilepsy.

Muscle Activity Determined by Cosine Tuning With a Nontrivial Preferred Direction During Isometric Force Exertion by Lower Limb

We investigated how the CNS selects a unique muscle activation pattern under a redundant situation resulting from the existence of bi-articular muscles. Surface electromyographic (EMG) activity was recorded from eight lower limb muscles while 11 subjects were exerting isometric knee and hip joint torque simultaneously ( Tk and Th, respectively. Extension torque was defined as positive). The knee joint was kept at either 90 or 60°. Various combinations of torque were imposed on both joints by pulling a cable attached to an ankle brace with approximately three levels of isometric force in 16 directions. The distribution of the data in the three-dimensional plot (muscle activation level quantified by the root mean squared value of EMG vs. Tk and Th) demonstrates that the muscle activation level M can be approximated by a single model as M = ⌊ aTk + bTh⌋ where ⌊ x⌋ = max ( x,0) and a and b are constants. The percentage of variance explained by this model averaged over all muscles was 82.3 ± 14.0% (mean ± SD), indicating that the degree of fit of the data to the plane was high. This model suggests that the CNS uses a cosine tuning function on the torque plane ( Tk, Th) to recruit muscles. Interestingly, the muscle's preferred direction (PD) defined as the direction where it is maximally active on the torque plane deviated from its own mechanical pulling direction (MD). This deviation was apparent in the mono-articular knee extensor (MD = 0°, whereas PD = 14.1 ± 3.7° for vastus lateralis) and in the mono-articular hip extensor (MD = 90°, whereas PD = 53.4 ± 6.4° for gluteus maximus). Such misalignment between MD and PD indicates that the mono-articular muscle's activation level depends on the torque of the joint that it does not span. Practical implications of this observation for the motor control studies were discussed. We also demonstrated that the observed shift from the MD to the PD is plausible in the configuration of our musculo-skeletal system and that the experimental results are likely to be explained by the CNS process to minimize the variability of the endpoint force vector under the existence of signal-dependent noise.

Hindlimb suspension inhibits air-righting due to altered recruitment of neck and back muscles in rats

Effects of 9-week hindlimb suspension and 8-week recovery on air-righting reaction in response to drop from a supine position were studied in adult rats. The righting time in rats at the end of suspension (approximately 220 ms) was longer than the age-matched controls (approximately 120 ms, p <0.05). The unloading-related change in righting time was accompanied by lowered activities of electromyogram (EMG) and altered recruitment of both neck and back muscles at a specific stage of drop. After 8 weeks of reambulation, righting time recovered toward the control level (approximately 153 ms, p <0.05), but the EMG activity of back muscle was still less than controls. In contrast, the EMG of neck muscle during fall was even increased. The differences in the characteristics of the muscle fibers between two groups were minor. It is suggested that inhibition of recruitment, rather than the changes in the fiber characteristics, of neck and back muscles is one of the major causes of the slow air-righting.

Effect of Occupational Keyboard Typing on Magnetic Resonance Imaging of the Median Nerve in Subjects with and without Symptoms of Carpal Tunnel Syndrome

OBJECTIVE

To examine the effects of occupational keyboard typing on median nerve shape and T2 relaxation and on forearm muscle T2 in professional typists with and without symptoms of carpal tunnel syndrome.

DESIGN

Based on the Levine Carpal Tunnel Syndrome Symptom Severity scale (LCTSS), 12 female professional typist volunteers were divided into asymptomatic (LCTSS < 1.3, n = 5) and symptomatic (LCTSS > 1.3, n = 7) groups. Magnetic resonance images were acquired from wrist and forearms of all subjects before, immediately after, and 8 hrs after 3 hrs of typing. Forearm muscle T2 and median nerve T2 cross-sectional area and long/short axis ratio were evaluated by blinded observers.

RESULTS

There was no difference between groups in any measured variable before typing. Median nerve T2 increased and long/short axis ratio decreased in asymptomatic subjects after typing, but there were no significant changes in symptomatic subjects. T2 increased in finger flexor muscles after typing, but there was no difference in the pattern of muscle T2 changes between groups.

CONCLUSION

In magnetic resonance images of the median nerve at the carpal tunnel, swelling and T2 increases from baseline are a normal response to typing and may be less likely to occur in subjects with symptoms of carpal tunnel syndrome.

Inhomogeneities in muscle activation reveal motor unit recruitment

This paper presents a novel method to quantify spatial changes in muscle activation pattern by multi-channel surface electromyography (MCSEMG) in order to investigate motor unit recruitment variation. The method is based on non-uniform distributions of motor units that cause spatial inhomogeneous muscle activation. To evaluate the method, 15 subjects performed three isometric elbow flexion contractions consisting of slow sinusoidal changes in force ranging from 0% to 80% of the maximal voluntary contraction. MCSEMG electrodes were placed in a 10 x 13 grid over the biceps brachii muscle. From all channels, root mean square (RMS) values of bipolar leadings were computed over 0.5 s epochs over the whole recording. Thereafter, correlation coefficients were calculated between the RMS values at one epoch, with the RMS values at another epoch. Results showed consistent spatial changes in the distribution of RMS at different contraction levels up to 80% of maximal voluntary contraction and when comparing increasing and decreasing contractions at the same force level. These findings are reproducible within and between subjects, and in agreement with physiological phenomena and therefore indicate that the spatial inhomogeneities of motor unit properties in the biceps brachii muscle can be used to study changes in motor unit recruitment.

Competition for representation is mediated by relative attentional salience

The biased competition model of attentional selection proposes that objects compete with one another for neural representation, with the competition rooted in stimulus and attentionally-based salience. Two experiments explore how the salience of a target item relative to flanking items impacts the speed of target identification. The results of two experiments suggest that spatially proximal items compete for shared, spatially dependent processing resources. In both experiments, subjects identified target elements embedded in multi-element displays. In Experiment 1, attentional salience was manipulated by using abrupt onsets (high-salience) and non-onsets (low-salience). Target identifications were slowest when the target was flanked by two high-salience stimuli (abrupt onsets) and fastest when the target was flanked by two low-salience items, (non-onsets). In Experiment 2, the attentional salience of display items was set through a probability manipulation involving the color of the target. The results mirrored those of Experiment 1, consistent with predictions of the biased competition model.

Regulation of pacing strategies during successive 4-km time trials

PURPOSE

Athletes adopt a pacing strategy to delay fatigue and optimize athletic performance. However, many current theories of the regulation of muscle function during exercise do not adequately explain all observed features of such pacing strategies. We studied power output, oxygen consumption, and muscle recruitment strategies during successive 4-km cycling time trials to determine whether alterations in muscle recruitment by the central nervous system could explain the observed pacing strategies.

METHODS

Seven highly trained cyclists performed three consecutive 4-km time trial intervals, each separated by 17 min. Subjects were instructed to perform each trial in the fastest time possible but were given no feedback other than distance covered. Integrated electromyographic (iEMG) readings were measured at peak power output anti from 90 s before the end of each trial.

RESULTS

Subjects attained VO2 values similar to their VO2(peak) in each interval. Time taken to complete the first and third intervals was similar. Peak power output was highest in the first interval, but average power output, oxygen consumption, heart rate, and postexercise plasma lactate concentrations were not different between intervals. Power output and iEMG activity rose similarly during the final 60 s in all intervals but was not different between trials.

CONCLUSION

The increase in power output and the parallel upward trend in iEMG at the end of each interval indicate that the iEMG changes "tracked" the power output changes dynamically and that therefore the observed pacing strategies were not regulated by peripheral mechanisms. Rather, these findings are compatible with the action of a centrally regulated mechanism that alters the number of motor units that are recruited and de-recruited during exercise based upon peripheral feedback or anticipatory feed-forward.

Neuronal microcircuits: frequency-dependent flow of inhibition

New work suggests that feedback inhibition of neurons in the hippocampus is mediated by two distinct microcircuits. Interneurons targeting a neuron's soma are triggered by onset of activity, while those targeting distal dendrites are recruited by sustained activity. These circuits may thus convey information about the timing and rate of activity, respectively.

Effect of an electric stimulation facilitation program on quadriceps motor unit recruitment after stroke

OBJECTIVE

To compare maximum voluntary isometric torque (MVIT) and motor unit recruitment of the quadriceps after an electric stimulation facilitation program in persons affected by cerebrovascular accident (CVA).

DESIGN

Three-week, randomized controlled trial with an electric stimulation facilitation program added to standard care.

SETTING

Inpatient rehabilitation center.

PARTICIPANTS

Twenty patients receiving rehabilitation for first-time CVA (51.8+/-15.2 y; days post-CVA, 38.4+/-40.0 d). Patients were randomly assigned to study and control groups.

INTERVENTIONS

All patients received standard physical therapy (PT) care. In addition, the study group received an electric stimulation facilitation program during weight-bearing and ambulatory activities of the PT program.

MAIN OUTCOME MEASURES

MVIT and motor unit recruitment measured by interpolated twitch testing. A 2 x 4 repeated-measures analysis of variance was performed on measurements at 4 intervals: pretest, 1 week, 2 weeks, and 3 weeks.

RESULTS

MVIT increased by 77% in patients receiving electric stimulation, compared with a 31% increase for the control group. There was a significant effect for assessment time only. Motor unit recruitment increased from 35% to 53% for the study group, whereas the control group recorded no change in recruitment ability. A significant interaction was recorded, indicating improved motor unit recruitment for the study group.

CONCLUSIONS

A brief and dynamic electric stimulation facilitation program significantly improved motor unit recruitment in persons after CVA.

Brain sensorimotor hand area functionality in acute stroke: insights from magnetoencephalography

An understanding of the functional readjustments that the brain undergoes during the early days after a stroke would give us a major insight into how and how much neurons are capable to react to an insult. Thirty-two patients affected by an acute monohemispheric ischemic stroke were enrolled in the study. Magnetoencephalography was used to record the somatosensory-evoked fields (SEF) generated in response to median nerve stimulation. Latency, strength, and position of the related early cortical components (M20 and M30) were studied both separately within each hemisphere, and in terms of interhemispheric differences. Interhemispheric cross-correlations among SEF waveshapes in the two hemispheres were also investigated. Overall, except for some source displacement possibly induced by the perilesional edema, results did not demonstrate any unusual neural recruitment. The severity of the clinical picture was found related to the sources' strengths (both as absolute values and as interhemispheric differences), to excessive interhemispheric differences in SEF waveshapes and in the M30 latencies. Signs of an enhanced excitability were present in the affected hemisphere (AH) following a cortical lesion, usually in combination with preserved hand functionality. An enhanced excitability of the unaffected hemisphere (UH) was paired with larger lesions with cortical involvement; signs compatible with an abnormal transcallosal transmission and intracortical function of inhibitory GABAergic interneurons in the AH were found subtending UH enhancement. Spared responsiveness from Brodmann's area (BA) 2 and posterior parietal areas despite an altered response from BA 3b was found in six patients, combined to high hand functionality. Present results in acute phase increase the knowledge of the mechanisms governing brain adaptation/reaction capabilities, for future efforts to establish therapeutic and rehabilitative procedures.

Discharge rate variability influences the variation in force fluctuations across the working range of a hand muscle

The goal of this study was to improve the ability of a motor unit model to predict experimentally measured force variability across a wide range of forces. Motor unit discharge characteristics were obtained from 38 motor units of the first dorsal interosseus muscle. Motor unit discharges were recorded in separate isometric contractions that ranged from 4 to 85% of the maximal voluntary contraction (MVC) force above recruitment threshold. High-threshold motor units exhibited both greater minimal and peak discharge rates compared with low-threshold units (P < 0.01). Minimal discharge rate increased from 7 to 23 pps, and peak discharge rate increased from 14 to 38 pps with an increase in recruitment threshold. Relative discharge rate variability (CV) decreased exponentially for each motor unit from an average of 30 to 13% as index finger force increased above recruitment threshold. In separate experiments, force variability was assessed at eight force levels from 2 to 95% MVC. The CV for force decreased from 4.9 to 1.4% as force increased from 2 to 15% MVC (P < 0.01) and remained constant at higher forces (1.2-1.9%; P = 0.14). When the motor unit model was revised using these experimental findings, discharge rate variability was the critical factor that resulted in no significant difference between simulated and experimental force variability (P = 0.22) at all force levels. These results support the hypothesis that discharge rate variability is a major determinant of the trends in isometric force variability across the working range of a muscle.

Selection requirements during verb generation: differential recruitment in older and younger adults

Age-related differences in cognitive performance are well documented. These differences are most pronounced during tasks with high demands on cognitive control, and it has been proposed that selective alteration of prefrontal activity is associated with cognitive changes in old age. Here, differences in the neural systems underlying selection requirements for older and younger adults were investigated using functional magnetic resonance imaging (fMRI). A verb generation task was used, and selection requirements were varied with regard to whether each noun could be associated with either few (scissors-cut) or many (ball-bounce, kick, throw...) competing alternatives. The two age groups showed statistically equivalent behavioral performance across the task conditions but marked differences in activation. Across both age groups, high selection demands activated several regions including bilateral frontal, left anterior frontal, left inferior temporal regions, and the dorsal anterior cingulate cortex (ACC). Between-group comparisons using region-of-interest analyses revealed less activation for senior adults in left inferior frontal gyrus (IFG), left inferior temporal gyrus, and the anterior cingulate and higher activation in right inferior frontal gyrus compared to young adults. These findings indicate age-related changes in multiple regions contributing to aspects of selection requirements during verb generation.

Human medial intraparietal cortex subserves visuomotor coordinate transformation

In the macaque, the posterior parietal cortex (PPC) integrates multimodal sensory information for planning and coordinating complex movements. In particular, the areas around the intraparietal sulcus (IPS) serve as an interface between the sensory and motor systems to allow for coordinated movements in space. Because recent imaging studies suggest a comparable functional and anatomical organization of human and monkey IPS, we hypothesized that in humans, as in macaques, the medial intraparietal cortex (area MIP) subserves visuomotor transformations. To test this hypothesis, changes of neural activity were measured using functional magnetic resonance imaging (fMRI) while healthy subjects performed a joystick paradigm similar to the ones previously employed in macaques for studying area MIP. As hypothesized, visuomotor coordinate transformation subserving goal-directed hand movements activated superior parietal cortex with the local maximum of increased neural activity lying in the medial wall of IPS. Compared to the respective visuomotor control conditions, goal-directed hand movements under predominantly proprioceptive control activated a more anterior part of medial IPS, whereas posterior medial IPS was more responsive to visually guided hand movements. Contrasting the two coordinate transformation conditions, changing the modality of movement guidance (visual/proprioceptive) did not significantly alter the BOLD signal within IPS but demonstrated differential recruitment of modality specific areas such as V5/MT and sensorimotor cortex/area 5, respectively. The data suggest that the human medial intraparietal cortex subserves visuomotor transformation processes to control goal-directed hand movements independently from the modality-specific processing of visual or proprioceptive information.

Muscle control associated with isometric contraction in different joint positions

The purpose of this study was to investigate the dependence of the surface EMG and varied internal effort due to different joint positions, and its muscle control strategies. Ten healthy subjects performed a fixed level (40% MVC measured in the neutral position) of isometric dorsiflexion and plantarflexion contraction in the ankle neutral, dorsiflexion, and plantarflexion positions. Quantitative EMG analyses with feature extraction in the time, frequency domain, as well as time-varying spectrum were employed to estimate recruitment pattern, code rating, and recruitment stationarity of activated motor units. Both RMS and median frequency of the tibial anterior (TA) and gastrocnemius (GS) were strongly dependent on foot position (p < .001). Dominant firing rate of the TA muscle during the effort related isometric contraction was significant different with respect to ankle position (p < .05). Irrespective of ankle position, the regression slope of median frequency across time was not different from zero for both TA and GS muscles (p > .05). Identical torque exertion in different ankle positions called for considerable changes of control strategies of motor units. Possible mechanisms could be augmented excitatory central afferents and release of inhibition from Golgi tendon organs in compensation for biomechanical disadvantage in shorter muscle length.

Electromyographic and mechanomyographic estimation of motor unit activation strategy in voluntary force production

Electromyographic and mechanomyographic estimation of motor unit activation strategy in voluntary force production. In order to determine whether electromyogram (EMG) and mechanomyogram (MMG) are suitable for the noninvasive estimation of the motor unit (MU) activation strategy, the EMG/force and MMG/force relationships were examined simultaneously during isometric ramp contractions in biceps brachii muscle. The highest mean power frequency (MPF) of the EMG, which reflects the full MU recruitment, was determined at 51% MVC. Two obvious inflection points were identified on the MMG-amplitude/force relationship that showed an initial slow increase followed by a rapid increase and a progressive decrease at higher force levels. Our results suggest that the MMG amplitude allows the estimation of the beginning of recruitment of MUs that innervate the first-twitch fibers in addition to identification of the full MU recruitment. The rate coding strategy was qualitatively reflected by the MMG-MPF/force relationship. We conclude that the MU activation strategy is estimated in more detail by the MMG than by the EMG.

Observation of action: grasping with the mind's hand

Engagement of the primary motor cortex (MI) during the observation of actions has been debated for a long time. In the present study, we used the quantitative 14C-deoxyglucose method in monkeys that either grasped 3-D objects or observed the same movements executed by humans. We found that the forelimb regions of the MI and the primary somatosensory (SI) cortex were significantly activated in both cases. Our study resolves a debate in the literature, providing strong evidence for use of MI representations during the observation of actions. It demonstrates that the observation of an action is represented in the primary motor and somatosensory cortices as is its execution. It indicates that in terms of neural correlates, recognizing a motor behavior is like executing the same behavior, requiring the involvement of a distributed system encompassing not only the premotor but also the primary motor cortex. We suggest that movements and their proprioceptive components are stored as motor and somatosensory representations in motor and somatosensory cortices, respectively, and that these representations are recalled during observation of an action.

Recruitment of attentional resources during visuomotor tracking: effects of Parkinson's disease and age

The ability to recruit attentional resources during distracted tracking was studied in 19 moderate PD patients, 21 healthy elderly subjects and 20 young controls. All subjects tracked a 1-cm circle that moved across a computer screen along a sinusoidal path (training) and along a circular path (testing). Tracking consisted of maintaining a dot cursor within the target by moving an unseen manipulandum across a digitizing tablet. Distraction consisted of adding one or three, colored 12-mm circles that moved around and intersected with the target circle, and one or three dots that moved around and intersected with the subject-controlled cursor. The performance of tasks with a low level of distraction (one dot and one circle distractor) and of tasks with a high level of distraction (three dot and three circle distractors) was compared to performance with no distraction. The elderly and young controls did not differ in the baseline task. Both groups surpassed the patients, who failed to keep pace with the target, despite preserved ability to attain the necessary movement speed. Under a low level of distraction, the ability to adjust the direction of hand movement diminished in both control groups, but task management was unaltered. In the patients adjustment of hand movement direction lowered, as in the controls, but task management reduced significantly. Under a high level of distraction, all groups showed further decrement in both aspects of task performance. We conclude that PD, but not age, significantly reduces the attentional resources required for administrative control of tracking, which are associated with prefrontal function.

Extracellular voltage profile for reversing the recruitment order of peripheral nerve stimulation: a simulation study

Electrical stimulation of peripheral nerve activates large-diameter fibers before small ones. A physiological recruitment order, from small to large-diameter axons, is desirable in many applications. Previous studies using computer simulations showed that selective activation of small fibers could be achieved by reshaping the extracellular voltage profile along the nerve using an array of nine electrodes. In this study, several electrode-array configurations were tested in order to minimize the number of contacts. Electrode arrays of 5, 7, 9, and 11 contacts with 0.75 mm contact separation were performed in computer simulations of dog sacral root (S2). Electrode arrays of 5 and 7 contacts recruited 40% of small axons (<10 microm) when recruiting only 10% of larger axons. Effectiveness of 9- and 11-contact arrays decreased with the presence of epineurium and perineurium. The effectiveness of electrode arrays was independent of stimulation pulsewidth. The biphasic-pulse stimulation with the amplitude of the second phase set as low as possible should be used to prevent the excitation of large axons during the second phase and to minimize the electrode corrosion. Arrays of 5 and 7 contacts also decreased the recruitment curve slope to 26% and 51% of the tripolar electrode, respectively. This modeling study predicts that reversing the recruitment order of peripheral nerve stimulation could be achieved by reshaping the extracellular voltage using electrode arrays of 5 or 7 contacts.

Changes in recruitment of the abdominal muscles in people with low back pain: ultrasound measurement of muscle activity

STUDY DESIGN

Ultrasound and electromyographic (EMG) measures of trunk muscle activity were compared between low back pain (LBP) and control subjects in a cross-sectional study.

OBJECTIVES

To compare the recruitment of the abdominal muscles (measured as a change in thickness with ultrasound imaging) between people with and without low back pain and to compare these measurements with EMG recordings made with intramuscular electrodes.

SUMMARY OF BACKGROUND DATA

Although ultrasonography has been advocated as a noninvasive measure of abdominal muscle activity, it is not known whether it can provide a valid measure of changes in motor control of the abdominal muscles in LBP.

METHODS

Ten subjects with recurrent LBP and 10 matched controls were tested during isometric low load tasks with their limbs suspended. Changes in thickness from resting baseline values were obtained for transversus abdominis (TrA), obliquus internus (OI), and obliquus externus (OE) using ultrasonography. Fine wire EMG was measured concurrently.

RESULTS

Study participants with LBP had a significantly smaller increase in TrA thickness with isometric leg tasks compared with controls. No difference was found between groups for OI or OE. Similar results were found for EMG. People with LBP had less TrA EMG activity with leg tasks, and there was no difference between groups for EMG activity for OI or OE.

CONCLUSIONS

This study reinforces evidence for changes in automatic control of TrA in people with LBP. Furthermore, the data establish a new test of recruitment of the abdominal muscles in people with LBP. This test presents a feasible noninvasive test of automatic recruitment of the abdominal muscles.

Recruitment of motor neuronal persistent inward currents shapes withdrawal reflexes in the frog

The details of behaviour are determined by the interplay of synaptic connectivity within neuronal circuitry and the intrinsic membrane properties of individual neurones. One particularly dramatic intrinsic property displayed by neurones in many regions of the nervous system is membrane potential bistability, in which transient excitation of a neurone results in a persistent depolarization outlasting the initial excitation. Here we characterize the contribution of such intrinsic bistability, also referred to as plateau properties and mediated by persistent inward currents (PICs), in spinal motor neurones to the production of withdrawal behaviours in the frog. We performed experiments on the isolated frog spinal cord with attached hindlimb. This preparation allowed the simultaneous monitoring of muscle activations during motor behaviour and intracellular neuronal recordings. We found that PICs, following their potentiation by serotonin (5-HT), are recruited and contribute to the production of withdrawal behaviours. These properties conferred a voltage-dependent prolongation to the duration of motor neuronal activity. Consistent with this potentiation of motor neuronal PICs, 5-HT also increased the duration of evoked muscle activations. This behavioural potentiation, as well as the expression of PICs in individual neurones, was reduced following antagonism of L-type Ca(2+) channels. These results demonstrate that PICs in motor neurones can be recruited during the production of behaviour and play a role in specifying the temporal details of motor output.

Neural basis for the cognitive continuum in episodic memory from health to Alzheimer disease

OBJECTIVE

The authors hypothesize that, behaviorally, episodic memory in health and disease reflects a continuum.

METHODS

Subjects (N=12) with very mild Alzheimer disease (AD) and normal subjects (N=24) were investigated with functional magnetic resonance imaging (fMRI) during an episodic memory task.

RESULTS

Recruitment of a posterior medio-temporal network was correlated with memory performance across the spectrum from high- and low-performing normal subjects to patients with early AD.

CONCLUSIONS

The behavioral spectrum from health to disease in episodic memory function is mirrored neurobiologically with graded recruitment of neuronal activation in medio-temporal regions. The results call for longitudinal assessment of behavioral decline and neuronal recruitment in future studies.

The influence of normal human ageing on automatic movements

There is evidence that aged normal subjects have more difficulty in achieving automaticity than young subjects. The underlying central neural mechanism for this phenomenon is unclear. In the present study, functional magnetic resonance imaging (fMRI) was used to investigate the effect of normal ageing on automaticity. Aged healthy subjects were asked to practice self-initiated, self-paced, memorized sequential finger movements with different complexity until they could perform the tasks automatically. Automaticity was evaluated by having subjects perform a secondary task simultaneously with the sequential movements. Although it took more time, most aged subjects eventually performed the tasks automatically at the same level as the young subjects. Functional MRI results showed that, for both groups, sequential movements activated similar brain regions before and after automaticity was achieved. No additional activity was observed in the automatic condition. While performing automatic movements, aged subjects had greater activity in the bilateral anterior lobe of cerebellum, premotor area, parietal cortex, left prefrontal cortex, anterior cingulate, caudate nucleus and thalamus, and recruited more areas, including the pre-supplementary motor area and the bilateral posterior lobe of cerebellum, compared to young subjects. These results indicate that most healthy aged subjects can perform some complex motor tasks automatically. However, aged subjects appear to require more brain activity to perform automatically at the same level as young subjects. This appears to be the main reason why aged subjects have more difficulty in achieving automaticity.