Vibration-induced extra torque during electrically-evoked contractions of the human calf muscles

Background: High-frequency trains of electrical stimulation applied over the lower limb muscles can generate forces higher than would be expected from a peripheral mechanism (i.e. by direct activation of motor axons). This phenomenon is presumably originated within the central nervous system by synaptic input from Ia afferents to motoneurons and is consistent with the development of plateau potentials. The first objective of this work was to investigate if vibration (sinusoidal or random) applied to the Achilles tendon is also able to generate large magnitude extra torques in the triceps surae muscle group. The second objective was to verify if the extra torques that were found were accompanied by increases in motoneuron excitability. Methods: Subjects (n = 6) were seated on a chair and the right foot was strapped to a pedal attached to a torque meter. The isometric ankle torque was measured in response to different patterns of coupled electrical (20-Hz, rectangular 1-ms pulses) and mechanical stimuli (either 100-Hz sinusoid or gaussian white noise) applied to the triceps surae muscle group. In an additional investigation, M(max) and F-waves were elicited at different times before or after the vibratory stimulation. Results: The vibratory bursts could generate substantial self-sustained extra torques, either with or without the background 20-Hz electrical stimulation applied simultaneously with the vibration. The extra torque generation was accompanied by increased motoneuron excitability, since an increase in the peak-to-peak amplitude of soleus F waves was observed. The delivery of electrical stimulation following the vibration was essential to keep the maintained extra torques and increased F-waves. Conclusions: These results show that vibratory stimuli applied with a background electrical stimulation generate considerable force levels (up to about 50% MVC) due to the spinal recruitment of motoneurons. The association of vibration and electrical stimulation could be beneficial for many therapeutic interventions and vibration-based exercise programs. The command for the vibration-induced extra torques presumably activates spinal motoneurons following the size principle, which is a desirable feature for stimulation paradigms.

Quantitative motor unit action potential analysis in 2 paraspinal neck muscles in adult Royal Dutch Sport horses

BACKGROUND: Reference values for quantitative electromyography (QEMG) in neck muscles of Royal Dutch Sport horses are lacking. OBJECTIVE: Determine normative data on quantitative motor unit action potential (QMUP) analysis of serratus ventralis cervicis (SV) and brachiocephalicus (BC) muscle. ANIMALS: Seven adult normal horses (mean age 9.5 standard deviation [SD] +/- 2.3 years, mean height 1.64 SD +/- 4.5 cm, and mean rectal temperature 37.6 SD +/- 0.3 degrees C). METHODS: An observational study on QMUP analysis in 6 segments of each muscle was performed with commercial electromyography equipment. Measurements were made according to formerly published methods. Natural logarithm transformed data were tested with ANOVA and posthoc testing according to Bonferroni. RESULTS: Mean duration, amplitude, phases, turns, area, and size index (SI) did not differ significantly among the 6 segments in each muscle. Mean amplitude, number of phases, and SI were significantly (P < .002) higher in SV than BC, 520 versus 448 muV, 3.0 versus 2.8 muV, and 0.48 versus 0.30 muV, respectively. In SV 95% confidence intervals (CI) for amplitude, duration, number of phases, turns, polyphasia area, and SI were 488-551 muV, 4.3-4.6 ms, 2.9-3.0, 2.4-2.6, 7-12%, 382-448, and 0.26-0.70, respectively; in BC this was 412-483 muV, 4.3-4.7 ms, 2.7-2.8, 2.4-2.6, 4-7%, 393-469, and 0.27-0.34, respectively. Maximal voluntary activity expressed by turns/second did not differ significantly between SV and BC with a 95% CI of 132-173 and 137-198, respectively. CONCLUSIONS AND CLINICAL IMPORTANCE: The establishment of normative data makes objective QEMG of paraspinal muscles in horses suspected of cervical neurogenic disorders possible. Differences between muscles should be taken into account.

Distribution of intramyocellular lipids in human calf muscles as determined by MR spectroscopic imaging

In this study the distribution of intramyocellular lipids (IMCL) in human calf muscles was determined by 1H-MR spectroscopic imaging (MRSI) measurements. An obstacle for MRSI measurements in the calf, including different muscles, is the inevitable inclusion of regions with high concentrations of extramyocellular lipids (EMCL). This can lead to signal bleeding and consequently to unpredictable overlaps of IMCL resonances with EMCL in voxels of interest. The results of this study show that signal bleeding from EMCL can be substantially reduced in voxels from calf muscles by the application of a lipid extrapolation (LE) procedure (Haupt et al., Magn Reson Med 1996;35:678). The spectra of all voxels located within muscle tissue were fitted, and the metabolite values were assigned to one of 10 different muscles based on image segmentation. Significant IMCL differences between some muscles were obtained, with high values in m. soleus and two to three times lower values in the tibialis anterior, tibialis posterior, and gastrocnemius muscles. In addition to gross differences between muscles, significant intersubject differences were observed in both IMCL content and distribution over different muscles. A significant correlation between fiber orientation (obtained from orientation-dependent dipolar coupling of creatine and taurine resonances) and IMCL content was found, indicating that IMCL content is directly correlated to biomechanical properties.

A new method of isometric dynamometry for the craniocervical flexor muscles

Background and Purpose. A new method of dynamometry has been developed to measure the performance of the craniocervical (CC) flexor muscles by recording the torque that these muscles exert on the cranium around the CC junction. This report describes the method, the specifications of the instrument, and the preliminary reliability data. Subjects and Methods. For the reliability study, 20 subjects (12 subjects with a history of neck pain, 8 subjects without a history of neck pain) performed, on 2 occasions, maximal voluntary isometric contraction (MVIC) tests of CC flexion in 3 positions within the range of CC flexion and submaximal sustained tests (20% and 50% of MVIC) in the middle range of CC flexion (craniocervical neutral position). Reliability coefficients were calculated to establish the test-retest reliability of the measurements. Results. The method demonstrated good reliability over 2 sessions in the measurement of MVIC (intraclass correlation coefficient [ICC] =.79-.93, SEM=0.6-1.4 N-m) and in the measurement of steadiness (standard deviation of torque amplitude) of a sustained contraction at 20% of NMC (ICC=.74-.80, SEM=0.01 N-m), but not at 50% of MVIC (ICC=.07-.76, SEM=0.04-0.13 N-m). Discussion and Conclusion. The new dynamometry method appears to have potential clinical application in the measurement of craniocervical flexor muscle performance.

Further evaluation of an EMG technique for assessment of the deep cervical flexor muscles

A novel surface electromyographic (EMG) technique was recently described for the detection of deep cervical flexor muscle activity. Further investigation of this technique is warranted to ensure EMG activity from neighbouring muscles is not markedly influencing the signals recorded. This study compared deep cervical flexor (DCF) muscle activity with the activity of surrounding neck and jaw muscles during various anatomical movements of the neck and jaw in 10 volunteer subjects. DCF EMG activity was recorded with custom electrodes inserted via the nose and fixed by suction to the posterior mucosa of the oropharynx. Surface electrodes were placed over the sternocleidomastoid, anterior scalene, masseter and suprahyoid muscles. Positioned in supine, subjects performed isometric cranio-cervical flexion, cervical flexion, right and left cervical rotation,jaw clench and resisted jaw opening. Across all movements examined, EMG amplitude of the DCF muscles was greatest during neck movements that would require activity of the DCF muscles, particularly during cranio-cervical flexion, their primary anatomical action. The actions of jaw clench and resisted jaw opening demonstrated significantly less DCF EMG activity than the cranio-cervical flexion action (p < 0.05). Across all other movements, the neighbouring neck and jaw muscles demonstrated greatest EMG amplitude during their respective primary anatomical actions, which occurred in the absence of increased EMG amplitude recorded from the DCF muscles. The finding of substantial EMG activity of the DCF muscles only during neck actions that would require their activity, particularly cranio-cervical flexion, and not during actions involving the jaw, provide further assurance that the majority of myoelectric signals detected from the nasopharyngeal electrode are from the DCF muscles. (C) 2005 Elsevier Ltd. All rights reserved.

Patients with neck pain demonstrate reduced electromyographic activity of the deep cervical flexor muscles during performance of the craniocervical flexion test

Study Design. Cross-sectional study. Objective. The present study compared activity of deep and superficial cervical flexor muscles and craniocervical flexion range of motion during a test of craniocervical flexion between 10 patients with chronic neck pain and 10 controls. Summary of Background Data. Individuals with chronic neck pain exhibit reduced performance on a test of craniocervical flexion, and training of this maneuver is effective in management of neck complaints. Although this test is hypothesized to reflect dysfunction of the deep cervical flexor muscles, this has not been tested. Methods. Deep cervical flexor electromyographic activity was recorded with custom electrodes inserted via the nose and fixed by suction to the posterior mucosa of the oropharynx. Surface electrodes were placed over the superficial neck muscles ( sternocleidomastoid and anterior scalene). Root mean square electromyographic amplitude and craniocervical flexion range of motion was measured during five incremental levels of craniocervical flexion in supine. Results. There was a strong linear relation between the electromyographic amplitude of the deep cervical flexor muscles and the incremental stages of the craniocervical flexion test for control and individuals with neck pain ( P = 0.002). However, the amplitude of deep cervical flexor electromyographic activity was less for the group with neck pain than controls, and this difference was significant for the higher increments of the task ( P < 0.05). Although not significant, there was a strong trend for greater sternocleidomastoid and anterior scalene electromyographic activity for the group with neck pain. Conclusions. These data confirm that reduced performance of the craniocervical flexion test is associated with dysfunction of the deep cervical flexor muscles and support the validity of this test for patients with neck pain.

High frequency tendon reflexes in the human soleus muscle

Tendon reflexes have been often used in studies of the human nervous system in health and disease. They have been investigated either in response to single tendon taps or to long duration vibrations. Tendon reflexes are described here in response to a high frequency vibration burst (3 cycles of a 100 Hz sine wave) applied to the Achilles tendon of standing subjects, either in quiet stance or during a forward leaning posture. The electromyogram from the soleus muscle usually showed three components separated by 10 ms which were interpreted as being three reflexes, each reflex induced by each of the three cycles in a burst. This result indicates that soleus tendon reflexes can respond in fast succession in a phasic manner when a brief high frequency vibration is applied to the Achilles tendon. This occurs in spite of possible depression of the la to motoneuron synapses and the long after hyperpolarization of the motoneurons. An interpretation of the results is that motoneurons from different subsets of the motoneuron pool respond to different cycles of the sinusoidal vibratory burst. (c) 2008 Elsevier Ireland Ltd. All rights reserved.

Visual Display Height

We examined the influence of backrest inclination and vergence demand on the posture and gaze angle that-workers adopt to view visual targets placed in different vertical locations. In the study 12 participants viewed a small video monitor placed in 7 locations around a 0.65-m radius arc (from 650 below to 300 above horizontal eye height). Trunk posture was manipulated by changing the backrest inclination of an adjustable chair. Vergence demand was manipulated by using ophthalmic lenses and prisms to mimic the visual consequences of varying target distance. Changes in vertical target location caused large changes in atlantooccipital posture and gaze angle. Cervical posture was altered to a lesser extent by changes in vertical target location. Participants compensated for changes in backrest inclination by changing cervical posture, though they did not significantly alter atlanto-occipital posture and gaze angle. The posture adopted to view any target represents a compromise between visual and musculoskeletal demands. These results provide support for the argument that the optimal location of visual targets is at least 15 below horizontal eye level. Actual or potential applications of this work include the layout of computer workstations and the viewing of displays from a seated posture.

Repeatability of surface EMG variables in the sternocleidomastoid and anterior scalene muscles

In this study we examined the repeatability and reliability of the surface electromyographic (sEMG) signal mean frequency (MNF), average rectified value (ARV) and conduction velocity (CV) measured for the sternocleidomastoid (SCM) and the anterior scalene (AS) muscles in nine healthy volunteers during 15-s isometric cervical flexion contractions at 50% of the maximal voluntary contraction level over 3 non-consecutive days. Repeatability and reliability estimates were obtained for the initial values and rates of change of each sEMG variable by using both the Intraclass Correlation Coefficient (ICC) and the normalised standard error of the mean (nSEM). Results from SCM indicated good levels of repeatability for the initial value and slope of ARV (ICC > 65%). For the AS, high levels of repeatability were identified for the initial value of MNF (ICC > 70%) and the slope of ARV (ICC > 75%). Values of nSEM in the range 2.8-7.2% were obtained for the initial values of MNF and CV for both SCM and AS, indicating clinically acceptable measurement precision. The low value obtained for the nSEM of the initial value of MNF for the AS, in combination with the high ICC, indicates that of all of the variables examined, this variable could offer the best normative index to distinguish between subjects with and without neck pain, and represents the sEMG variable of choice for future evaluation purposes.

An electromyographic analysis of the deep cervical flexor muscles in performance of craniocervical flexion

Background and Purpose. This study evaluated an electromyographic technique for the measurement of muscle activity of the deep cervical flexor (DCF) muscles. Electromyographic signals were detected from the DCF, sternocleidomastoid (SCM), and anterior scalene (AS) muscles during performance of the craniocervical flexion (CCF) test, which involves performing 5 stages of increasing craniocervical flexion range of motion-the anatomical action of the DCF muscles. Subjects. Ten volunteers without known pathology or impairment participated in this study. Methods. Root-mean-square (RMS) values were calculated for the DCF, SCM, and AS muscles during performance of the CCF test. Myoelectric signals were recorded from the DCF muscles using bipolar electrodes placed over the posterior oropharyngeal wall. Reliability estimates of normalized RMS values were obtained by evaluating intraclass correlation coefficients and the normalized standard error of the mean (SEM). Results. A linear relationship was evident between the amplitude of DCF muscle activity and the incremental stages of the CCF test (F=239.04, df=36, P<.0001). Normalized SEMs in the range 6.7% to 10.3% were obtained for the normalized RMS values for the DCF muscles, providing evidence of reliability for these variables. Discussion and Conclusion. This approach for obtaining a direct measure of the DCF muscles, which differs from those previously used, may be useful for the examination of these muscles in future electromyographic applications.

Neck Pain in Adolescent Headache Sufferers. A Cohort Study of Schoolchildren

Although neck pain (NP) and headache (HA) are often concomitant in adolescents, few data exist on the association of NP with HA in this age group. The aim of the study was to examine the association of concomitant NP with adolescent HA and with the outcome of HA. The associations of self-reported NP, physical findings of the neck and disc degeneration of the cervical spine with adolescent HA were studied. This study is part of a population-based follow-up study of 12-year-old children (N 1135/1409) with and without HA. A sample of adolescents (N = 304) was followed to the age of 16 years. At the age of 17 years, 69 of them participated in a magnetic resonance imaging (MRI) study of the cervical spine. During the follow-up from 13 to 16 years of age, changes in both HA type and frequency were common. A poor outcome of HA was associated with NP interfering with daily activities at the age of 13 years. The changes in HA type were not predictable by NP. At the age of 16 years, local and referred palpation pain of the neck muscles, self-reported NP and NP intensity were associated with HA, and especially with disturbing HA unresponsive to analgesics. The association of NP with HA was not determined by HA type. Mild degenerative changes of the cervical spine were common but did not contribute to headache. HA in adolescence is often episodic, and prevention and treatment of NP could be important in the prevention of future chronic adult HA.

ß2-adrenergic agonists actions on the human skeletal muscle

Les ß2-agonistes sont des bronchodilatateurs qui sont prescrits pour traiter l'asthme et l'asthme induite par l'exercice (AIE). Il est relevant de comprendre s'il y a une utilisation adéquate de ces médicaments pour traiter l'AIE chez les athlètes de haut niveau, ou s'ils sont utilisés pour leur potentiel effet ergogénique sur la performance physique. Ce travail examine les actions centrales et périphériques sur la fonction contractile du muscle squelettique humain in vivo induits par l'ingestion d'une dose thérapeutique de ß2- agonistes. Le premier but était d'évaluer si les ß2-agonistes exerçaient une potentialisation de la contractilité du muscle humain et/ou un effet "anti¬fatigue" comme observé dans le modèle animal. Les résultats n'ont fournit aucune évidence d'une potentialisation sur le muscle squelettique humain in vivo non-fatigué et fatigué induit par l'administration orale de ß2-agonistes. Tout effet excitateur exercé par ce traitement sur le système nerveux central a été aussi exclu. Le deuxième but était de déterminer si les ß2-agonistes affaiblissaient la contractilité du muscle squelettique humain à contraction lente, et d'évaluer si ce changement pouvait interférer avec le contrôle moteur au muscle. Les résultats ont montré que les ß2-agonistes affaiblissent la contractilité des fibres lentes, comme conséquence de l'effet lusitrope positif se produisant dans ces fibres. La capacité de développer une force maximale n'est pas réduite par le traitement, même si une augmentation de la commande centrale au muscle est requise pour produire la même force lors de contractions sous-maximales. Le but final était d'examiner si une adaptation du contrôle moteur était re¬quis pour compenser l'affaiblissement des fibres lentes exercée par les ß2- agonistes pendant un exercice volontaire, et de déterminer si cette adaptation centrale pouvait accroître la fatigue musculaire. Malgré le fait que les résultats confirment l'effet affaiblissant induit par les ß2-agonistes, ce changement contractile n'influence pas le contrôle moteur au muscle pendant les contractions sous-maximales de l'exercice fatiguant, et n'accroît pas le degré de fatigue. Ce travail éclaircit les actions spécifiques des ß2-agonistes sur la fonction contractile du muscle squelettique humain in vivo et leurs influence sur le contrôle moteur. Les mécanismes sous-jacents de l'action ergogénique sur la performance physique produit par les ß2-agonistes sont aussi élucidés. -- ß2-Agonists are bronchodilators that are widely prescribed for the treatment of asthma and exercise-induced asthma (EIA). The extensive use of ß2-agonists by competitive athletes has raised the question as to whether there is a valid need for this class of drugs because of EIA or a misuse because of their potential ergogenic effect on exercise performance. This work investigated the central and peripheral actions that were elicited by the ingestion of a therapeutic dose of ß2-agonists on the contractility of human skeletal muscle in vivo. The first objective was to investigate whether ß2-agonists would potentiate muscle contractility and/or exert the "anti-fatigue" effect observed in animal models. The findings did not provide any evidence for the ß2-agonist-induced potentiation of in vivo human non-fatigued and fatigued skeletal muscle. Moreover, the findings exclude any excitatory action of this treatment on the central nervous system. The second objective was to explore whether the weakening action on the contractile function would occur after ß2-agonist intake in human slow-twitch skeletal muscle and to ascertain whether this contractile change may interfere with muscle motor control. The results showed that ß2-agonists weaken the contractility of slow-twitch muscle fibres as a result of the lusitropic effect occurring in these fibres. The maximal force-generating capacity of the skeletal muscle is not reduced by ß2-agonists, even though an augmented neural drive to muscle is required to develop the same force during submaximal contractions. The final objective was to examine whether a motor control adjustment is needed to compensate for the ß2-agonist-induced weakening effect on slow- twitch fibres during a voluntary exercise and to also assess whether this central adaptation could exaggerate muscle fatigue. Despite the findings confirming the occurrence of the weakening action that is exerted by ß2- agonists, this contractile change did not interfere with muscle motor control during the submaximal contractions of the fatiguing exercise and did not augment the degree of the muscle fatigue. This work contributes to a better understanding of the specific actions of ß2-agonists on the contractile function of in vivo human skeletal muscles and their influence on motor control. In addition, the findings elucidate mechanisms that could underlie the ergogenic effect that is exerted by ß2- agonists on physical performance.

Neck Muscle Function and Adolescent Headache

Muscular function of the neck region may be of importance for the etiology of headache, especially of tension-type headache. However, very few data exist on the association of neck muscle function with different types of headache in adolescents. The main aim of the study was to examine the association of neck muscle function with adolescent headache. The associations between leisure time activities, endurance strength of the upper extremities (UE endurance) and mobility of the neck-shoulder region and adolescent headache were studied. In addition, the associations of force production, EMG/force ratio, co-activation and fatigue characteristics, and cross-sectional area (CSA) of neck muscles with adolescent headache were studied. The study is part of a population-based cohort study of 12-year-old children with and without headache. The study had five phases (years 1998-2003). At the age of 13 years, a sample of 183 adolescents (183/311) participated in endurance strength and mobility measurements of the neck-shoulder region. In addition, the type and level of physical and other leisure activity were elicited with open and structured questions. At the age of 17 years, a random sample of 89 adolescents (89/202) participated in force and EMG measurements of the neck-shoulder muscles. In addition, at the age of 17 years, a sample of 65 adolescents (65/89) participated in CSA measurements of the neck muscles. At the age of 13 years, intensive participation in overall sports activity was associated with migraine. Frequent computer use was associated both with migraine and tension-type headache. The type of sports or other leisure activity classified them on the basis of body loading was not associated with headache type. In girls, low UE endurance of both sides, and low cervical rotation of the dominant side, were associated with tension-type headache, and low UE endurance of non-dominant side with migraine. In boys, no associations occurred between UE endurance and mobility variables and headache types. At the age of 17 years, in girls, high EMG/force ratios between the EMG of the left agonist sternocleidomastoid muscle (SCM) and maximal neck flexion and neck rotation force to the right side as well as high co-activation of right antagonist cervical erector spinae (CES) muscles during maximal neck flexion force were associated with migraine-type headache. In girls, neck force production was not associated with headache types but low left shoulder flexion force was associated with tension-type headache. In boys, no associations were found between EMG and force variables and headache. Increased SCM muscles fatigue of both sides was associated with tension-type headache. In boys, the small CSA of the right SCM muscle and, in girls, of combined right SCM and scalenus muscles was associated with tension-type headache. Similarly, in boys, the large CSA of the right SCM muscle, of the combined right SCM and scalenus muscles, of the left semispinalis capitis muscle, of the combined left semispinalis and splenius muscles was associated with migraine. No other differences in the CSA of neck flexion or extension muscles were found. Differences in the neuromucular function of the neck-shoulder muscles were associated with adolescent headache, especially in girls. Differences in the cross-sectional area of unilateral neck muscles were associated with headache, especially in boys. Differences in the neuromuscular function and in the cross-sectional area of the neck muscles also occurred between different types of headache. It remains to be established whether the findings are primary or secondary to adolescent migraine and tension headache. Keywords: adolescent, cross-sectional area, electromyography, endurance strength, fatigue, force, headache, leisure time activity, migraine, mobility, neck muscles, tension-type headache

The occipital lateral plate mesoderm - a novel source for vertebrate neck musculature

In vertebrates, body musculature originates from somites, whereas head muscles originate from the cranial mesoderm. Neck muscles are located in the transition between these regions. We show that the chick occipital lateral plate mesoderm has myogenic capacity and gives rise to large muscles located in the neck and thorax. We present molecular and genetic evidence to show that these muscles not only have a unique origin, but additionally display a distinct temporal development, forming later than any other muscle group described to date. We further report that these muscles, found in the body of the animal, develop like head musculature rather than deploying the programme used by the trunk muscles. Using mouse genetics we reveal that these muscles are formed in trunk muscle mutants but are absent in head muscle mutants. In concordance with this conclusion, their connective tissue is neural crest in origin. Finally, we provide evidence that the mechanism by which these neck muscles develop is conserved in vertebrates.

The occipital lateral plate mesoderm is a novel source for vertebrate neck musculature

In vertebrates, body musculature originates from somites, whereas head muscles originate from the cranial mesoderm. Neck muscles are located in the transition between these regions. We show that the chick occipital lateral plate mesoderm has myogenic capacity and gives rise to large muscles located in the neck and thorax. We present molecular and genetic evidence to show that these muscles not only have a unique origin, but additionally display a distinct temporal development, forming later than any other muscle group described to date. We further report that these muscles, found in the body of the animal, develop like head musculature rather than deploying the programme used by the trunk muscles. Using mouse genetics we reveal that these muscles are formed in trunk muscle mutants but are absent in head muscle mutants. In concordance with this conclusion, their connective tissue is neural crest in origin. Finally, we provide evidence that the mechanism by which these neck muscles develop is conserved in vertebrates.