Agonist Muscle Activity Research Articles

Modulation of corticospinal output in agonist and antagonist proximal arm muscles during motor preparation.

Previous studies have shown modulation of corticospinal output of the agonist muscle when a known-movement is prepared but withheld until a response signal appearance, reflecting motor preparation processes. However, modulation in the antagonist muscles has not been described, despite the fact that reaching movements require precise coordination between the activation of agonist and antagonist muscles. In this study, participants performed an instructed-delay reaction time (RT) task, with randomized elbow flexion and extension movements. The aim was to assess the time course modulation of corticospinal output in two antagonist muscles, by simultaneously quantified the amplitude of motor evoked potentials (MEPs) in biceps brachii and triceps brachii, and the amplitude and direction of elbow movements evoked by transcranial magnetic stimulation (TMS). Depending on the prepared movement direction, a specific modulation of corticospinal output was observed, MEPs and TMS-evoked movements amplitude being relatively greater for extension compared to flexion. At the end of motor preparation, a decrease in MEPs amplitude was observed for both biceps brachii and triceps brachii, regardless of the prepared movement direction. In contrast, the probability of evoking movement in the flexion direction and the amplitude of TMS-evoked movement decreased at the end of preparation for flexion, but not for extension. Together, these results confirm the existence of inhibitory processes at the end of the motor preparation, probably to avoid a premature motor response. Moreover, they provide evidence of differences in the corticospinal control of elbow flexor and extensor muscles with patterns of modulation that are not necessarily reciprocal during motor preparation.

Repeated magnetic stimulation improves the effectiveness of exercise therapy in relieving ankle spasticity after stroke

Objective To study the effect of repetitive peripheral magnetic stimulation combined with exercise on spasticity in the ankle plantar flexors among stroke survivors. Methods Sixty stroke survivors with spasticity in their ankle plantar flexors were randomly divided into a treatment group and a control group, each of 30. Both groups were given conventional kinesitherapy, while the treatment group were additionally provided with repetitive peripheral magnetic stimulation for 4 weeks. The myoelectric activity of the agonist and antagonist muscles was recorded using surface electromyography during maximum isometric voluntary contractions of the ankle dorsiflexors and co-contraction ratios (CRs) were calculated. The motor function, walking ability and ankle plantar flexor spasticity were evaluated using the Fugl-Meyer lower extremity assessment (FMA-LE), functional ambulation categories (FACs) and a composite spasticity scale (CSS). Results Before the intervention there was no signification difference between the two groups in terms of any of the assessments. After 4 weeks of treatment, the average integrated EMG of the anterior tibialis in the treatment group was significantly better than in the control group. That group′s average FMA-LE and FAC scores were also significantly better. The experimental group′s average spasticity score and co-contraction ratio during maximum isometric voluntary contractions of the ankle dorsiflexors had both decreased significantly. All of the improvements in the treatment group were significantly better than those in the control group. Conclusion Repetitive peripheral magnetic stimulation combined with the exercise therapy can effectively reduce ankle plantar flexior spasticity while improving motor function and walking ability. It is more effective than exercise alone. Key words: Magnetic stimulation; Exercise therapy; Stroke; Spasticity; Surface electromyography

Botulinum Toxin in Management of Limb Tremor.

Essential tremor is characterized by persistent, usually bilateral and symmetric, postural or kinetic activation of agonist and antagonist muscles involving either the distal or proximal upper extremity. Quality of life is often affected and one’s ability to perform daily tasks becomes impaired. Oral therapies, including propranolol and primidone, can be effective in the management of essential tremor, although adverse effects can limit their use and about 50% of individuals lack response to oral pharmacotherapy. Locally administered botulinum toxin injection has become increasingly useful in the management of essential tremor. Targeting of select muscles with botulinum toxin is an area of active research, and muscle selection has important implications for toxin dosing and functional outcomes. The use of anatomical landmarks with palpation, EMG guidance, electrical stimulation, and ultrasound has been studied as a technique for muscle localization in toxin injection. Earlier studies implemented a standard protocol for the injection of (predominantly) wrist flexors and extensors using palpation and EMG guidance. Targeting of muscles by selection of specific activators of tremor (tailored to each patient) using kinematic analysis might allow for improvement in efficacy, including functional outcomes. It is this individualized muscle selection and toxin dosing (requiring injection within various sites of a single muscle) that has allowed for success in the management of tremors.

Muscular responses appear to be associated with existence of kinesthetic perception during combination of tendon co-vibration and motor imagery.

The afferent inputs from peripheral sensory receptors and efferent signals from the central nervous system that underlie intentional movement can contribute to kinesthetic perception. Previous studies have revealed that tendon vibration to wrist muscles elicits an excitatory response-known as the antagonist vibratory response-in muscles antagonistic to the vibrated muscles. Therefore, the present study aimed to further investigate the effect of tendon vibration combined with motor imagery on kinesthetic perception and muscular activation. Two vibrators were applied to the tendons of the left flexor carpi radialis and extensor carpi radialis. When the vibration frequency was the same between flexors and extensors, no participant perceived movement and no muscle activity was induced. When participants imagined flexing their wrists during tendon vibration, the velocity of perceptual flexion movement increased. Furthermore, muscle activity of the flexor increased only during motor imagery. These results demonstrate that kinesthetic perception can be induced during the combination of motor imagery and co-vibration, even with no experience of kinesthetic perception from an afferent input with co-vibration at the same frequency. Although motor responses were observed during combined co-vibration and motor imagery, no such motor responses were recorded during either co-vibration alone or motor imagery alone, suggesting that muscular responses during the combined condition are associated with kinesthetic perception. Thus, the present findings indicate that kinesthetic perception is influenced by the interaction between afferent input from muscle spindles and the efferent signals that underlie intentional movement. We propose that the physiological behavior resulting from kinesthetic perception affects the process of modifying agonist muscle activity, which will be investigated in a future study.

Gluteus Medius and Minimus Muscle Structure, Strength, and Function in Healthy Adults: Brief Report.

Purpose: This article describes gluteus medius and minimus muscle thickness and hip abductor strength and function in healthy adults and explores the relationships between muscle thickness and function. Methods: Gluteus medius and minimus muscle thickness (B-mode ultrasound), isometric hip abductor strength (Biodex dynamometer), and lower extremity function (timed Trendelenburg test, Five-Times-Sit-to-Stand Test [FTSST], and lateral step-down test) were measured in healthy adults using a cross-sectional study design. Results: A total of 22 subjects were included: 10 men and 12 women, mean age 25.2 (SD 3.1) years, mean BMI 22.9 (SD 3.5) kilograms per metre squared. Muscle thickness of the gluteals was a mean 3.88 (SD 0.13) centimetres, and mean hip abductor peak torque was 111 (SD 43) newton-metres. FTSST mean time was 5.3 (SD 0.2) seconds, and median scores were 2.0 points for lateral step-down and 90 seconds for timed Trendelenburg. No significant relationships were found between gluteal muscle thickness and functional tests (rs=-0.28 to 0.37, ps=0.09-0.80) or strength (r=-0.24, p=0.28). Conclusion: Although hip abductors are key pelvic stabilizers for functional movements, gluteal muscle thickness was not associated with strength or function. This may be a result of agonist muscle activity, leading to an inability to isolate the gluteals, and to the ceiling effects of the functional tests.

Chronic pain alters spatiotemporal activation patterns of forearm muscle synergies during the development of grip force.

It is largely unknown how the CNS regulates multiple muscle systems in the presence of pain. This study used muscle synergy analysis to investigate multiple forearm muscles in individuals with chronic elbow pain during the development of grip force. Eleven individuals with chronic elbow pain and 11 healthy age-matched control subjects developed grip force to 15% and 30% of maximum voluntary contraction (MVC). Surface electromyography was obtained from six forearm muscles during force development before nonnegative matrix factorization was performed. The relationship between muscle synergies and standard clinical tests of elbow pain were examined by linear regression. During grip force development to 15% MVC the pain group had a lower number of forearm muscle synergies, increased similarity in spatial activation patterns, increased cocontraction of forearm flexors, and a greater magnitude of muscle weightings across the forearm when performing the task. During the 30% MVC grip the numbers of muscle synergies were the same for both groups; however, the pain group had lower activation and reduced variability in the timing of peak activation. The timing of peak activation was delayed in the pain group regardless of the task, and performing the grip in different wrist postures did not affect muscle synergy characteristics in either group. Although localized pain causes direct dysfunction of an affected muscle, this study provides evidence that the timing and amplitude of agonist and antagonist muscle activity are also affected with chronic pain.NEW & NOTEWORTHY Muscle activation patterns of individuals with chronic elbow pain are simplified compared with healthy individuals. This is apparent as individuals with pain exhibit fewer forearm muscle synergies, and increased similarity of activation patterns between forearm muscles, when performing pain-free isometric gripping. As such, even during pain-free tasks it is possible to observe changes in motor control in people with chronic pain.

Compensating for intersegmental dynamics across the shoulder, elbow, and wrist joints during feedforward and feedback control

Moving the arm is complicated by mechanical interactions that arise between limb segments. Such intersegmental dynamics cause torques applied at one joint to produce movement at multiple joints, and in turn, the only way to create single joint movement is by applying torques at multiple joints. We investigated whether the nervous system accounts for intersegmental limb dynamics across the shoulder, elbow, and wrist joints during self-initiated planar reaching and when countering external mechanical perturbations. Our first experiment tested whether the timing and amplitude of shoulder muscle activity account for interaction torques produced during single-joint elbow movements from different elbow initial orientations and over a range of movement speeds. We found that shoulder muscle activity reliably preceded movement onset and elbow agonist activity, and was scaled to compensate for the magnitude of interaction torques arising because of forearm rotation. Our second experiment tested whether elbow muscles compensate for interaction torques introduced by single-joint wrist movements. We found that elbow muscle activity preceded movement onset and wrist agonist muscle activity, and thus the nervous system predicted interaction torques arising because of hand rotation. Our third and fourth experiments tested whether shoulder muscles compensate for interaction torques introduced by different hand orientations during self-initiated elbow movements and to counter mechanical perturbations that caused pure elbow motion. We found that the nervous system predicted the amplitude and direction of interaction torques, appropriately scaling the amplitude of shoulder muscle activity during self-initiated elbow movements and rapid feedback control. Taken together, our results demonstrate that the nervous system robustly accounts for intersegmental dynamics and that the process is similar across the proximal to distal musculature of the arm as well as between feedforward (i.e., self-initiated) and feedback (i.e., reflexive) control.NEW & NOTEWORTHY Intersegmental dynamics complicate the mapping between applied joint torques and the resulting joint motions. We provide evidence that the nervous system robustly predicts these intersegmental limb dynamics across the shoulder, elbow, and wrist joints during reaching and when countering external perturbations.

A new EMG-based index towards the assessment of elbow spasticity for post-stroke patients.

The commonly used method for grading spasticity in clinical applications is Modified Ashworth Scale (MAS). The MAS-based method depends on the subjective evaluations and the experience of physicians, which may lead to imprecise and inconsistent evaluations. In this study, we propose a novel index (A-ApA, which was calculated with the root mean square (RMS) of agonist muscle activity by the mean between the RMS of agonistic and antagonistic muscle activations extracted from surface electromyography (sEMG) signals to quantitatively assess elbow spasticity. 39 post-stroke patients with elbow spasticity caused by hemiplegia participated in the experiments, and their elbow spasticity was assessed with MAS by one experienced physiotherapist. Patients were thereafter divided into four groups according to the MAS scales. The sEMG signals were recorded simultaneously on the patients' biceps and triceps when they extended or bended their elbows passively. The correlations between MAS and RMS of sEMG signals as well as the newly proposed index were calculated. The results demonstrated that the correlation between the MAS and the sEMG-based index in the assessment of elbow spasticity was significant. This suggests that the EMG-based index would be helpful for the assessment of spasticity..

Factors contributing to lower metabolic demand of eccentric compared with concentric cycling.

This study compared muscle-tendon behavior, muscle oxygenation, and muscle activity between eccentric and concentric cycling exercise at the same work output to investigate why metabolic demand is lower during eccentric cycling than with concentric cycling. Eleven untrained men (27.1 ± 7.0 y) performed concentric cycling (CONC) and eccentric cycling (ECC) for 10 min (60 rpm) at 65% of the maximal concentric cycling power output (191 ± 45 W) 4 wk apart. During cycling, oxygen consumption (V̇o2), heart rate (HR), vastus lateralis (VL) tissue total hemoglobin (tHb), and oxygenation index (TOI) were recorded, and muscle-tendon behavior was assessed using ultrasonography. The surface electromyogram (EMG) was recorded from VL, vastus medialis (VM), rectus femoris (RF), and biceps femoris (BF) muscles, and cycling torque and knee joint angle during each revolution were also recorded. Average V̇o2 (-65 ± 7%) and HR (-35 ± 9%) were lower and average TOI was greater (16 ± 1%) during ECC than CONC, but tHb was similar between bouts. Positive and negative cycling peak crank torques were greater (32 ± 21 and 48 ± 24%, respectively) during ECC than CONC, but muscle-tendon unit and fascicle and tendinous tissue length changes during pedal revolutions were similar between CONC and ECC. VL, VM, RF, and BF peak EMG amplitudes were smaller (24 ± 15, 22 ± 18, 16 ± 17, and 18 ± 9%, respectively) during ECC than CONC. These results suggest that the lower metabolic cost of eccentric compared with concentric cycling was due mainly to a lower level of muscle activation per torque output.NEW & NOTEWORTHY This study shows that lower oxygen consumption of eccentric compared with concentric cycling at the same workload is explained by lower muscle activity of agonist and antagonist muscles during eccentric compared with during concentric cycling.

Sex differences in spatial accuracy relate to the neural activation of antagonistic muscles in young adults.

Sex is an important physiological variable of behavior, but its effect on motor control remains poorly understood. Some evidence suggests that women exhibit greater variability during constant contractions and poorer accuracy during goal-directed tasks. However, it remains unclear whether motor output variability or altered muscle activation impairs accuracy in women. Here, we examine sex differences in endpoint accuracy during ankle goal-directed movements and the activity of the antagonistic muscles. Ten women (23.1±5.1years) and 10 men (23±3.7years) aimed to match a target (9° in 180ms) with ankle dorsiflexion. Participants performed 50 trials and we recorded the endpoint accuracy and the electromyographic (EMG) activity of the primary agonist (Tibialis Anterior; TA) and antagonist (Soleus; SOL) muscles. Women exhibited greater spatial inaccuracy (Position error: t=-2.65, P=0.016) but not temporal inaccuracy relative to men. The motor output variability was similar for the two sexes (P>0.2). The spatial inaccuracy in women was related to greater variability in the coordination of the antagonistic muscles (R 2 0.19, P=0.03). These findings suggest that women are spatially less accurate than men during fast goal-directed movements likely due to an altered activation of the antagonistic muscles.

Abstract P25: A Caprine Model of a Novel Amputation Paradigm for Bi-directional Neural Control of a Bionic Limb

PURPOSE: No significant changes have been made to limb amputation surgical paradigms since the Civil War era. Traditional approaches are plagued by poor nerve treatment, which leads to neuromas and phantom pain, and complicates neural communication with advanced limb prostheses. In this study we present a caprine model of the Linked Residual Musculature Peripheral Nerve Interface (LRM-PNI), a surgical approach designed to reduce phantom pain and improve bi-directional neural control of a bionic limb. The key advancement in this architecture is the surgical coaptation of natively-innervated agonist-antagonist muscle pairs within the residuum. The benefits of this approach are two-fold. First, preservation of the mechanical coupling between agonist contraction and antagonist stretch allows physiologically-relevant muscle state feedback from a prosthesis through mechanical activation of native mechanoreceptors within these linked muscles. Second, by preserving functionality of these native mechanoreceptors, we limit the presence of non-meaningful phantom sensation, including phantom pain. We hypothesize that these advancements will allow for afferent proprioceptive sensation of a limb prosthesis through native neural pathways, which is crucial in both reflexive and volitional lower extremity control during gait. METHODS: A transtibial amputation was performed on the hindlimb of a goat, during which the lateral tarsal tunnel from the amputated ankle was grafted transversely to the medial surface of the tibia, serving as a “synovial pulley”. Distal ends of the lateral gastrocnemius and tibialis cranialis were then coapted to either end of this synovial pulley, allowing coupled motion of the agonist-antagonist muscle pair. The residual musculature was sensorized with implantable epimysial and intramuscular electrodes, as well as sonomicrometry crystals. Over a two-month period, electromyography and muscle-state measurements were recorded from the residuum while the animal ambulated without a prosthesis, and fluoroscopic evaluations of coupled motion in the presence of artificial muscle stimulation were carried out under anesthesia. The animal was then sacrificed, and the residuum was examined. RESULTS: Electromyography from residual musculature during gait indicate that the animal continued to send neural commands to the coapted agonist-antagonist muscle pair in the absence of a prosthetic limb device. Muscle-state measurements and fluoroscopic evaluations show clear coupled motion of the agonist-antagonist pair in the presence of both natural neural commands and artificial muscle stimulation. Integrated EMG from the agonist muscle was cross-correlated against muscle state recordings from the antagonist; it was determined that the average time delay from agonist muscle activation to antagonist muscle stretch was 93 ms, with a correlation coefficient of .73. Post-mortem examination of the residual tissues showed no signs of necrosis and no abundance of scarring. CONCLUSIONS: These findings indicate that the LRM-PNI architecture has the potential to provide coupled motion of agonist-antagonist muscle pairs, as in the pre-amputation physiological milieu. Guided by these findings, it is our expectation that further development of the LRM-PNI architecture will improve bi-directional neural control of advanced limb prostheses.

Motor plan differs for young and older adults during similar movements.

Older adults exhibit altered activation of the agonist and antagonist muscles during goal-directed movements compared with young adults. However, it remains unclear whether the differential activation of the antagonistic muscles in older adults results from an impaired motor plan or an altered ability of the muscle to contract. The purpose of this study, therefore, was to determine whether the motor plan differs for young and older adults. Ten young (26.1 ± 4.3 yr, 4 women) and 16 older adults (71.9 ± 6.9 yr, 9 women) participated in the study. Participants performed 100 trials of fast goal directed movements with ankle dorsiflexion while we recorded the electromyographic activity of the primary agonist (tibialis anterior; TA) and antagonist (soleus; SOL) muscles. From those 100 trials we selected 5 trials in each of 3 movement end-point categories (fast, accurate, and slow). We investigated age-associated differences in the motor plan by quantifying the individual activity and coordination of the agonist and antagonist muscles. During similar movement end points, older adults exhibited similar activation of the agonist (TA) and antagonist (SOL) muscles compared with young adults. In addition, the coordination of the agonist and antagonist muscles (TA and SOL) was different between the two age groups. Specifically, older adults exhibited lower TA-SOL overlap (F1,23 = 41.2, P < 0.001) and greater TA-SOL peak EMG delay (F1,25 = 35.5, P < 0.001). This finding suggests that although subjects in both age groups displayed similar movement end points, they exhibited a different motor plan, as demonstrated by altered coordination between the agonist and antagonist muscles.NEW & NOTEWORTHY We aimed to determine whether the altered activation of muscles in older adults compared with young adults during fast goal-directed movements is related to an altered motor plan. For matched movements, there were differences in the coordination of antagonistic muscles but no differences in the individual activation of muscles. We provide novel evidence that the differential activation of muscles in older adults is related to an altered motor plan.

Isometric Contraction of Scapular Muscles Activities during Horizontal Abduction and Adduction of the Shoulder

Background: Scapular stability is important for shoulder movement; however, the mechanism of scapular stability remains unknown. In addition, activities of scapular muscles have been only evaluated during shoulder elevation, but not extensively observed those of other shoulder movements. We investigated muscle activity, including scapular muscle activity, during horizontal abduction and adduction of the shoulder joint. Methods: Eighteen healthy men without orthopedic or neurological abnormalities participated in this study. Muscle activity was measured during horizontal isometric abduction and adduction of the shoulder joint in the sitting position. Measurements were conducted in three positions horizontal adduction at 0°, 45° and 90°, and external loads were set at 3, 5 and 10% of each subject's body weight. External load was added to the direction of horizontal abduction on horizontal adduction task and the direction of horizontal adduction on horizontal abduction task. Surface electromyography was used to collect raw data during the task. Based on scapular movements during shoulder motion, the measured muscles were classified as agonist muscles of the glenohumeral joint, and agonist and antagonist muscles of the scapulohumeral joint. Results: The same patterns of muscles activities were identified during horizontal abduction and adduction in all positions. Electromyographic activities of agonist muscles of both the glenohumeral and scapulothoracic joints became higher with greater load; however, activities of antagonist muscles of the scapulohumeral joint did not show statistical significance. Conclusion: Most scapular muscles engage during isometric contraction, and the high activating muscles were changed by the direction of shoulder movement. We have to evaluate the scapular muscle activities individually in consideration of this finding.

Linking Theoretical Decision-making Mechanisms in the Simon Task with Electrophysiological Data: A Model-based Neuroscience Study in Humans.

A current challenge for decision-making research is in extending models of simple decisions to more complex and ecological choice situations. Conflict tasks (e.g., Simon, Stroop, Eriksen flanker) have been the focus of much interest, because they provide a decision-making context representative of everyday life experiences. Modeling efforts have led to an elaborated drift diffusion model for conflict tasks (DMC), which implements a superimposition of automatic and controlled decision activations. The DMC has proven to capture the diversity of behavioral conflict effects across various task contexts. This study combined DMC predictions with EEG and EMG measurements to test a set of linking propositions that specify the relationship between theoretical decision-making mechanisms involved in the Simon task and brain activity. Our results are consistent with a representation of the superimposed decision variable in the primary motor cortices. The decision variable was also observed in the EMG activity of response agonist muscles. These findings provide new insight into the neurophysiology of human decision-making. In return, they provide support for the DMC model framework.

Progressive resistance exercise restores some properties of the triphasic EMG pattern and improves bradykinesia: the PRET-PD randomized clinical trial.

In Parkinson's disease (PD), the characteristic triphasic agonist and antagonist muscle activation pattern during ballistic movement is impaired: the number of agonist muscle bursts is increased, and the amplitudes of the agonist and antagonist bursts are reduced. The breakdown of the triphasic electromyographic (EMG) pattern has been hypothesized to underlie bradykinesia in PD. Progressive resistance exercise has been shown to improve clinical measures of bradykinesia, but it is not clear whether the benefits for bradykinesia are accompanied by changes in agonist and antagonist muscle activity. This study examined the spatiotemporal changes in agonist and antagonist muscle activity following 24 mo of progressive resistance exercise and the combined relationship between spatiotemporal muscle activity and strength measures and upper limb bradykinesia. We compared the effects of progressive resistance exercise training (PRET) with a nonprogressive exercise intervention, modified Fitness Counts (mFC), in patients with PD. We randomized 48 participants with mild-to-moderate PD to mFC or PRET. At the study endpoint of 24 mo, participants randomized to PRET compared with mFC had significantly faster movement velocity, accompanied by significant increases in the duration, magnitude, and magnitude normalized to duration of the 1st agonist burst and fewer number of agonist bursts before peak velocity. The antagonist muscle activity was increased relative to baseline but did not differ between groups. Spatiotemporal EMG muscle activity and muscle strength were significantly associated with upper limb bradykinesia. These findings demonstrate that progressive resistance exercise improves upper limb movement velocity and restores some aspects of the triphasic EMG pattern.

Reduced Maximal Force during Acute Anterior Knee Pain Is Associated with Deficits in Voluntary Muscle Activation.

Although maximal voluntary contraction (MVC) force is reduced during pain, studies using interpolated twitch show no consistent reduction of voluntary muscle drive. The present study aimed to test if the reduction in MVC force during acute experimental pain could be explained by increased activation of antagonist muscles, weak voluntary activation at baseline, or changes in force direction. Twenty-two healthy volunteers performed maximal voluntary isometric knee extensions before, during, and after the effects of hypertonic (pain) and isotonic (control) saline injections into the infrapatellar fat pad. The MVC force, voluntary activation, electromyographic (EMG) activity of agonist, antagonist, and auxiliary (hip) muscles, and pain cognition and anxiety scores were recorded. MVC force was 9.3% lower during pain than baseline (p < 0.001), but there was no systematic change in voluntary activation. Reduced MVC force during pain was variable between participants (SD: 14%), and was correlated with reduced voluntary activation (r = 0.90), baseline voluntary activation (r = − 0.62), and reduced EMG amplitude of agonist and antagonist muscles (all r > 0.52), but not with changes in force direction, pain or anxiety scores. Hence, reduced MVC force during acute pain was mainly explained by deficits in maximal voluntary drive.

Psychological and behavioral manifestations in alzheimer’ disease and mild cognitive impairment. Artemisa province

The synchronous activation of the muscles involved in force production is crucial for the neuromuscular performance, but the underlying mechanisms remain to be fully elucidated. Our aim was thus to contribute to understand the mechanisms involved in the synergistic activation of agonist muscles. Through wavelet-based time–frequency analysis, this study investigated the modulation of ‘beta’ intermuscular interactions (IM) during maximum isometric knee extensions performed before and after repetitive submaximal fatiguing contractions. Three groups of participants were included: 9 untrained subjects (control group, CO), 10 elite rugby league players (strength-trained group, ST) and 7 trail runners (endurance-trained group, ED), engaged for 5+ years in intense strength and endurance training, respectively. Before fatigue, CO showed higher IM when compared to ED, and a trend to higher IM when compared to ST. Following fatiguing contractions, all groups showed a decline in neuromuscular performance concomitant with a change (decline) in IM values for CO only. No differences were found between ST and ED regarding to IM either before or after fatiguing contractions. These findings suggested both a form of optimization of intermuscular coupling in trained individuals and the functional importance of intermuscular coupling as a mechanism responsible for the maintenance of the neuromuscular performance.

Physiological mechanisms for stabilizing the limb when acting against physical constraints.

Smooth physical interaction with our environment, such as when working with tools, requires adaptability to unpredictable perturbations that can be achieved through impedance control of multi-joint limbs. Modulation of arm stiffness can be achieved either increasing co-contraction of antagonistic muscles or by increasing the gain of spinal reflex loops. According to the "automatic gain scaling" principle, the spinal reflex gain, as measured via the H-reflex, scales with muscle activation. A previous experiment from our labs suggested, however, that reflex gains might instead be scaled to the force exerted by the limb, perhaps as a means to counteract destabilizing external forces. The goal of our experiment was to test whether force output, rather than the muscular activity per se, could be the critical factor determining reflex gain. Five subjects generated different levels of force at the wrist with or without assistance to dissociate applied force from agonist muscular activity. We recorded contact force, EMG and H-reflex response from a wrist flexor. We did not find a strict relationship between reflex gain and contact force but nor did we observe consistent modulation of reflex gain simply as a function of agonist muscle activity. These results are discussed in relation to the stability of the task constraints.

Strength performance parameters and muscle activation adopting two antagonist stretching methods before and between sets

Summary Objectives The purpose of this study was to investigate the acute effects of passive static stretching and proprioceptive neuromuscular facilitation applied in antagonist muscles (shoulder horizontal adductors) in inter-set rest period on repetition performance and muscle activation. Methods Fifteen ( n = 15) resistance trained men completed three different protocols on wide-grip cable row exercise: traditional set – one set repetition to failure on cable row; antagonist static stretching – 40 s of static stretching on horizontal adductor muscles before each set of cable row; antagonist proprioceptive neuromuscular facilitation – 40 s of contract-relax stretching was applied on horizontal adductor muscles before each set of cable row. Electromyographic signal of latissimus dorsi, pectoralis major, triceps brachii and biceps brachii were recorded. Results Higher numbers of repetitions were found in static stretching protocol compared to traditional set and proprioceptive neuromuscular facilitation protocols for all sets ( P Conclusion The antagonist static stretching applied in inter-set rest period may induce a potential effect on repetition performance and agonist muscle activity when compared to traditional set condition.

Voluntary motor commands reveal awareness and control of involuntary movement

The capacity to inhibit actions is central to voluntary motor control. However, the control mechanisms and subjective experience involved in voluntarily stopping an involuntary movement remain poorly understood. Here we examined, in humans, the voluntary inhibition of the Kohnstamm phenomenon, in which sustained voluntary contraction of shoulder abductors is followed by involuntary arm raising. Participants were instructed to stop the involuntary movement, hold the arm in a constant position, and ‘release’ the inhibition after ∼2s. Participants achieved this by modulating agonist muscle activity, rather than by antagonist contraction. Specifically, agonist muscle activity plateaued during this voluntary inhibition, and resumed its previous increase thereafter. There was no discernible antagonist activation. Thus, some central signal appeared to temporarily counter the involuntary motor drive, without directly affecting the Kohnstamm generator itself. We hypothesise a form of “negative motor command” to account for this novel finding. We next tested the specificity of the negative motor command, by inducing bilateral Kohnstamm movements, and instructing voluntary inhibition for one arm only. The results suggested negative motor commands responsible for inhibition are initially broad, affecting both arms, and then become focused. Finally, a psychophysical investigation found that the perceived force of the aftercontraction was significantly overestimated, relative to voluntary contractions with similar EMG levels. This finding is consistent with the hypothesis that the Kohnstamm generator does not provide an efference copy signal. Our results shed new light on this interesting class of involuntary movement, and provide new information about voluntary inhibition of action.