Specific modulation of corticomuscular coherence during submaximal voluntary isometric, shortening and lengthening contractions.

During voluntary contractions, corticomuscular coherence (CMC) is thought to reflect a mutual interaction between cortical and muscle oscillatory activities, respectively measured by electroencephalography (EEG) and electromyography (EMG). However, it remains unclear whether CMC modulation would depend on the contribution of neural mechanisms acting at the spinal level. To this purpose, modulations of CMC were compared during submaximal isometric, shortening and lengthening contractions of the soleus (SOL) and the medial gastrocnemius (MG) with a concurrent analysis of changes in spinal excitability that may be reduced during lengthening contractions. Submaximal contractions intensity was set at 50% of the maximal SOL EMG activity. CMC was computed in the time-frequency domain between the Cz EEG electrode signal and the unrectified SOL or MG EMG signal. Spinal excitability was quantified through normalized Hoffmann (H) reflex amplitude. The results indicate that beta-band CMC and normalized H-reflex were significantly lower in SOL during lengthening compared with isometric contractions, but were similar in MG for all three muscle contraction types. Collectively, these results highlight an effect of contraction type on beta-band CMC, although it may differ between agonist synergist muscles. These novel findings also provide new evidence that beta-band CMC modulation may involve spinal regulatory mechanisms.

Acute effects of single dose transcranial direct current stimulation on muscle strength: A systematic review and meta-analysis.

Previous studies investigating the effects of transcranial direct current stimulation (tDCS) on muscle strength showed no consensus. Therefore, the purpose of this article was to systematically review the literature on the effects of single dose tDCS to improve muscle strength. A systematic literature search was conducted on PubMeb, ISI Web of Science, SciELO, and Scopus using search terms regarding tDCS and muscle strength. Studies were included in accordance with Population, Intervention, Comparison, Outcomes, and Setting (PICOS) including criteria. Healthy men and women, strength training practitioners or sedentary were selected. The acute effects of single dose anode stimulus of tDCS (a-tDCS) and the placebo stimulus of tDCS (sham) or no interventions were considered as an intervention and comparators, respectively. Measures related to muscle strength were analyzed. To conduct the analyses a weighted mean difference (WMD) and the standardized mean difference (SMD) were applied as appropriate. A total of 15 studies were included in this systematic review and 14 in meta-analysis. Regarding the maximal isometric voluntary contraction (MIVC), a small effect was seen between tDCS and Sham with significant difference between the conditions (SMD = 0.29; CI95% = 0.05 to 0.54; Z = 2.36; p = 0.02). The muscular endurance measured by the seconds sustaining a percentage of MIVC demonstrated a large effect between tDCS and Sham (WMD = 43.66; CI95% = 29.76 to 57.55; Z = 6.16; p < 0.001), showing an improvement in muscular endurance after exposure to tDCS. However, muscular endurance based on total work showed a trivial effect between tDCS and Sham with no significant difference (SMD = 0.22; CI95% = -0.11 to 0.54; Z = 1.32, p = 0.19). This study suggests that the use of tDCS may promote increase in maximal voluntary contraction and muscular endurance through isometric contractions.

Dose-response effect of photobiomodulation therapy on neuromuscular economy during submaximal running.

The purpose of this study was to verify the photobiomodulation therapy (PBMT) effects with different doses on neuromuscular economy during submaximal running tests. Eighteen male recreational runners participate in a randomized, double-blind, and placebo-controlled trial, which each participant was submitted to the same testing protocol in five conditions: control, placebo, and PBMT with doses of 15, 30, and 60 J per site (14 sites in each lower limb). The submaximal running was performed at 8 and 9 km h-1 during 5 min for each velocity. Muscle activation of the vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF), biceps femoris (BF), and gastrocnemius lateralis (GL) was collected during the last minute of each running test. The root mean square (RMS) was normalized by maximal isometric voluntary contraction (MIVC) performed a priori in an isokinetic dynamometer. The RMS sum of all muscles (RMSLEG) was considered as main neuromuscular economy parameter. PBMT with doses of 15, 30, and 60 J per site [33 diodes = 5 lasers (850 nm), 12 LEDs (670 nm), 8 LEDs (880 nm), and 8 LEDs (950 nm)] or placebo applications occurred before running tests. For the statistical analysis, the effect size was calculated. Moreover, a qualitative inference was used to determine the magnitude of differences between groups. Peak torque and RMS during MIVCs showed small effect sizes. According to magnitude-based inference, PBMT with dose of 15 J per site showed possibly and likely beneficial effects on neuromuscular economy during running at 8 and 9 km h-1, respectively. On other hand, PBMT with doses of 30 and 60 J per site showed possible beneficial effects only during running at 9 km h-1. We concluded that PBMT improve neuromuscular economy and the best PBMT dose was 15 J per site (total dose of 420 J).

Laser therapy and needling in myofascial trigger point deactivation.

The aim of this study was to evaluate different approaches to deactivating myofascial trigger points (MTPs). Twenty-one women with bilateral MTPs in the masseter muscle were randomly divided into three groups: laser therapy, needle treatment and control. Treatment effectiveness was evaluated after four sessions with intervals ranging between 48 and 72 h. Quantitative and qualitative methods were used to measure pain perception/sensation. The Wilcoxon test based on results expressed on a visual analog scale (VAS) demonstrated a significant (P < 0.05) decrease in pain only in the laser and needle treatments groups, although a significant increase in the pressure pain threshold was evident only for needling with anesthetic injection (P = 0.0469), and laser therapy at a dose of 4 J/cm² (P = 0.0156). Based on these results, it was concluded that four sessions of needling with 2% lidocaine injection with intervals between 48 and 72 h without a vasoconstrictor, or laser therapy at a dose of 4 J/cm², are effective for deactivation of MTPs.

Efectos Inmediatos de la Técnica de Manipulación de Ashmore C5/C6 en la Actividad Muscular, en pacientes con Cervicalgia Mecánica / Immediate Effects of the Ashmore Manipulation Technique C5/C6, in Muscle Activity in patients with Mechanical Neck Pain

Introducción: La manipulación vertebral se relaciona con el aumento de la fuerza muscular, aunque no existen suficientes evidencias relativas a su aplicación en el raquis cervical. Objetivo: Determinar los efectos inmediatos de la técnica de manipulación de C5/C6 (Ashmore) en la actividad electromiográfica en reposo y en las contracciones del músculo deltoides medio bilateralmente. Material y Métodos: Estudio experimental, aleatorizado, cegado y controlado. Treinta (n=30) pacientes con Cervicalgia Mecánica (CM) se distribuyeron aleatoriamente en dos grupos, experimental (GE; n=15) y control (GC; n=15). Los pacientes fueron evaluados mediante el cuestionario Neck Disability Index (NDI), el test de la arteria vertebral y electromiografía (EMG) antes de la intervención. Después de las intervenciones en los grupos de estudio, realizamos otra vez la prueba EMG. Resultados: Los análisis comparativos intergrupos post-intervención presentaron diferencias estadísticamente significativas para las variables Root Mean Square (RMS) en isometría de 30 segundos bilateral, para el músculo deltoides medio. Conclusiones: La manipulación vertebral C5-C6 disminuye la actividad electromiográfica en la contracción isométrica, pero no produce cambios electromiográficos en reposo ni en contracción isotónica (AU)

Introduction: The effects of spinal manipulation are not yet entirely clear. Previous studies have found both increased and decreased electromyographic (EMG) activity of muscles related to the level being manipulated, although few of them have considered the cervical region or symptomatic individuals. Objetives: To determine the immediate effects of the C5/C6 (Ashmore) manipulation technique on bilateral EMG activity of the middle deltoid muscle at rest and in contractions. Patients, Materials and Methods: A randomized, controlled, single blind, experimental study was conducted. A total of 30 individuals presenting with mechanical neck pain were assigned randomly to two groups: 15 formed the experimental group (EG), and 15 the control group (CG). All participants completed a data questionnaire and the NDI (Neck Disability Index), and underwent a vertebral artery and EMG evaluation before their participation. After C5/C6 manipulation in the intervention group and no manipulation in the control group, the EMG evaluation was repeated. Results: All the variables were normally distributed, indicative of the total sample's initial homogeneity. Comparative post-intervention inter-group analyses showed statistically significant differences in the root mean square (RMS) values of the 30-s isometric bilateral EMG measurements of the middle deltoid muscle's activity. Conclusions: C5-C6 spinal manipulation reduced EMG activity in the longer isometric contractions, but no changes were observed neither in the resting EMG values nor in the isotonic contractions performed(AU)

Low-level laser therapy improves skeletal muscle performance, decreases skeletal muscle damage and modulates mRNA expression of COX-1 and COX-2 in a dose-dependent manner.

We tested if modulation in mRNA expression of cyclooxygenase isoforms (COX-1 and COX-2) can be related to protective effects of phototherapy in skeletal muscle. Thirty male Wistar rats were divided into five groups receiving either one of four laser doses (0.1, 0.3, 1.0 and 3.0 J) or a no-treatment control group. Laser irradiation (904 nm, 15 mW average power) was performed immediately before the first contraction for treated groups. Electrical stimulation was used to induce six tetanic tibial anterior muscle contractions. Immediately after sixth contraction, blood samples were collected to evaluate creatine kinase activity and muscles were dissected and frozen in liquid nitrogen to evaluate mRNA expression of COX-1 and COX-2. The 1.0 and 3.0 J groups showed significant enhancement (P < 0.01) in total work performed in six tetanic contractions compared with control group. All laser groups, except the 3.0 J group, presented significantly lower post-exercise CK activity than control group. Additionally, 1.0 J group showed increased COX-1 and decreased COX-2 mRNA expression compared with control group and 0.1, 0.3 and 3.0 J laser groups (P < 0.01). We conclude that pre-exercise infrared laser irradiation with dose of 1.0 J enhances skeletal muscle performance and decreases post-exercise skeletal muscle damage and inflammation.

Muscle injury after repeated bouts of voluntary and electrically stimulated exercise.

UNLABELLED: Repeated bouts of eccentric exercise reduce the amount of exercise-induced muscle injury. PURPOSE: This study sought to evaluate the importance of neural adaptations by comparing the repeated bout effect on muscle injury caused by voluntary and electrically stimulated eccentric exercise. METHODS: Sixteen subjects (nine men, seven women) were assigned into two groups; electrical stimulation (STIM) and voluntary (VOL). Each group performed 2 identical bouts of 80 eccentric contractions of the quadriceps femoris (QF) through a 90 degree arc at approximately 45 degrees x s(-1), separated by 7 wk. T2-weighted magnetic resonance images of the QF were obtained before and 3 d after each exercise bout. Injury was assessed by determining changes in T2 relaxation time and muscle volume 3 d after exercise, and changes in isometric force and ratings of soreness for 28 d after exercise. RESULTS: The initial bout of exercise caused significant changes in T2 relaxation time, isometric force, and ratings of soreness in both STIM and VOL groups (P < 0.05). After the repeated bout, significantly smaller changes were noted in soreness ratings (P < 0.05), mean change in T2 (P<0.05), and percentage of the QF demonstrating an increase in T2 (P < 0.05) compared with the initial bout in both exercise groups. CONCLUSIONS: A repeated-bout effect was observed after electrically stimulated exercise, and the magnitude of the effect was similar to that observed with voluntary exercise. This suggests that the primary mechanism for the reduction in muscle injury after repeated exercise bouts is not related to changes in muscle recruitment and is potentially related to structural changes within the muscles.

Force per active area and muscle injury during electrically stimulated contractions.

UNLABELLED: Multiple mechanical factors have been implicated in the initiation of exercise-induced muscle injury. Although high absolute force levels are associated with greater injury, the importance of high force per active area independent of absolute force remains to be determined, especially in humans. PURPOSE: This study sought to examine the role of specific force in muscle injury when peak eccentric force and total eccentric force were matched between two exercise bouts. METHODS: Ten subjects (six men, four women) performed 80 electrically stimulated (EMS) eccentric contractions of the right and left quadriceps femoris (QF) through a 90 degree arc at approximately 45 degrees x s(-1). Specific force was manipulated by applying 25-Hz EMS to one thigh and 100-Hz EMS to the contralateral thigh, whereas force production was matched by lowering the EMS amplitude during the 100-Hz bout. T2 magnetic resonance images of the QF were collected before and 3 d after the eccentric exercise bouts. Injury was assessed via changes in isometric force and ratings of soreness in the QF over the course of 28 d after exercise and by determining changes in T2 relaxation time and muscle volume 3 d after exercise. RESULTS: The 100-Hz EMS induced a greater force loss (P < 0. 05), soreness (P < 0.05), change in muscle volume (P = 0.03), and volume of muscle demonstrating an increase in T2 (P = 0.005) compared with 25-Hz EMS. CONCLUSIONS: Our findings suggest that in humans, high specific force potentially plays an important role in the initiation of exercise-induced muscle injury.

Rhythmic arm cycling suppresses hyperactive soleus H-reflex amplitude after stroke.

OBJECTIVE: Rhythmic arm cycling movement suppresses the amplitude of soleus H-reflexes in neurologically intact participants. This suppression is greater when the movement frequency is increased. If rhythmic arm movement can still suppress the amplitude of H-reflexes in the legs after stroke, it could potentially be used as a rehabilitation technique to reduce exaggerated reflexes such as those occurring in spasticity. The purpose of this study was to test for maintenance of this suppressive effect after stroke. Since a portion of the effect of arm cycling had previously been ascribed to subcortical and spinal mechanisms, we hypothesized that the suppressive effect of arm cycling would be partially maintained after stroke. METHODS: Participants with history of single chronic (> 6 months) stroke performed rhythmic arm cycling at approximately 1 Hz and also at the highest frequency possible ( approximately 1.5 Hz). Soleus H-reflexes were evoked in the more and less affected legs simultaneously and full recruitment curves obtained. RESULTS: H-reflex amplitudes in both the more and less affected legs of stroke participants were significantly suppressed during arm cycling. However, the extent of the suppression is weaker compared to neurologically intact and age-matched subjects. CONCLUSIONS: Neural activity related to arm cycling can still access interlimb pathways after stroke and activate spinal control mechanisms leading to suppression of H-reflex amplitudes. SIGNIFICANCE: The suppressive effect of arm cycling could be exploited in the modification of exaggerated muscle afferent reflexes in leg muscles after stroke. Whether this has a significant effect on modulation of spasticity requires further substantiation.

Cortical and spinal modulation of antagonist coactivation during a submaximal fatiguing contraction in humans.

This study investigates the control mechanisms at the cortical and spinal levels of antagonist coactivation during a submaximal fatiguing contraction of the elbow flexors at 50% of maximal voluntary contraction (MVC). We recorded motor-evoked potentials in the biceps brachii and triceps brachii muscles in response to magnetic stimulation of the motor cortex (MEP) and corticospinal tract (cervicomedullary motor-evoked potentials--CMEPs), as well as the Hoffmann reflex (H-reflex) and maximal M-wave (Mmax) elicited by electrical stimulation of the brachial plexus, before, during, and after the fatigue task. The results showed that although the coactivation ratio did not change at task failure, the MVC torque produced by the elbow flexors declined by 48% (P < 0.01) with no change in MVC torque for the elbow extensors. While the MEP and CMEP areas (normalized to Mmax) of the biceps brachii increased ( approximately 50%) over the first 40% of the time to task failure and then plateaued, both responses in the triceps brachii increased ( approximately 150-180%) gradually throughout the fatigue task. In contrast to the monotonic increase in the MEP and CMEP of the antagonist muscles, the H-reflex of the triceps brachii exhibited a biphasic modulation, increasing during the first part of the contraction before declining subsequently to 65% of its initial value. Collectively, these results suggest that the level of coactivation during a fatiguing contraction is mediated by supraspinal rather than spinal mechanisms and involves differential control of agonist and antagonist muscles.

A comparison of monopolar and bipolar recording techniques for examining the patterns of responses for electromyographic amplitude and mean power frequency versus isometric torque for the vastus lateralis muscle.

The purpose of the present study was to compare monopolar and bipolar recording techniques for the patterns of responses and mean values for absolute and normalized electromyographic (EMG) amplitude and mean power frequency (MPF) versus isometric torque for the vastus lateralis muscle. Ten healthy men (mean+/-S.D. age=23.6+/-3.0 years; body weight=80.9+/-15.6 kg) volunteered to perform submaximal to maximal isometric muscle actions of the dominant leg extensors. Monopolar and bipolar surface EMG signals were detected simultaneously from the vastus lateralis with an eight-channel linear electrode array. The results indicated that in 70-80% of the cases, monopolar and bipolar recording techniques resulted in the same patterns of responses for absolute and normalized EMG amplitude and MPF versus isometric torque. There were, however, differences between the two techniques for mean absolute EMG amplitude and MPF values, but not for the normalized values. Thus, these results supported the practice of normalization, and suggested that comparisons can be made between monopolar and bipolar recording methods for the patterns of responses and mean values for normalized (but not absolute) EMG amplitude and MPF versus isometric torque.

Mathematical model that predicts the force-intensity and force-frequency relationships after spinal cord injuries.

We have previously developed and tested a muscle model that predicts the effect of stimulation frequency on muscle force responses. The aim of this study was to enhance our isometric mathematical model to predict muscle forces in response to stimulation trains with a wide range of frequencies and intensities for the quadriceps femoris muscles of individuals with spinal cord injuries. Isometric forces were obtained experimentally from 10 individuals with spinal cord injuries (time after injury, 1.5-8 years) and then compared to forces predicted by the model. Our model predicted accurately the force-time integrals (FTI) and peak forces (PF) for stimulation trains of a wide range of frequencies (12.5-80 HZ) and intensities (150-600-mus pulse duration), and two different stimulation patterns (constant-frequency trains and doublet-frequency trains). The accurate predictions of our model indicate that our model, which now incorporates the effects of stimulation frequency, intensity, and pattern on muscle forces, can be used to design optimal customized stimulation strategies for spinal cord-injured patients.

The effects of stimulation frequency and fatigue on the force-intensity relationship for human skeletal muscle.

OBJECTIVE: Functional electrical stimulation (FES) has not gained widespread application for a number of factors; two of which are rapid muscle fatigue and imprecise control in force. Stimulation intensity is adjusted during FES to overcome the decline in muscle force due to fatigue and precisely control muscle force output. The purpose of this study was to understand the relationship between muscle force output and stimulation intensity and to see how this relationship changes with fatigue. METHODS: Quadriceps femoris muscles of 10 able-bodied adults were tested isometrically. Pre- and post-fatigue muscle force responses to stimulation trains with different intensities and frequencies were recorded and analyzed. In addition, a case study using a subject with spinal cord injury was presented to illustrate the use of the force-intensity relationship to reduce muscle fatigue and improve the control of muscle force during repetitive electrical stimulation. RESULTS: An exponential relationship between muscle force and stimulation intensity was observed; interestingly, the normalized force-intensity relationship did not change with stimulation frequency or fatigue. CONCLUSIONS: The observed consistencies in the force-intensity relationship should assist scientists and clinicians to more accurately predict the forces produced by a muscle with changes in pulse duration during repetitive electrical stimulation. SIGNIFICANCE: The findings of this study provide guidelines for clinicians and researchers to adjust the stimulation intensity to achieve precise control of force repetitively during the application of FES.

Muscle contractile properties in children with spastic diplegia.

The aim of the present study was to evaluate contractile properties of the plantarflexor muscles in children with spastic diplegia (SD) in comparison of age-matched healthy children. Twelve prepubertal children with SD aged 11-12 years (6 girls and 6 boys) and 12 age- and gender-matched healthy control children (6 girls and 6 boys) participated in this study. Subjects were seated in a custom-made dynamometric chair with the dominant leg flexed 90 degrees at the knee and ankle joints. Twitch contraction characteristics of the plantarflexor muscles were measured by supramaximal electrical stimulation of posterior tibial nerve in popliteal fossa using square-wave pulses of 1 ms duration at rest and after a brief (5 s) isometric maximal voluntary contraction (MVC), i.e., during post-activation potentiation (PAP). Children with SD had significantly lower (p<0.05) MVC force, twitch contraction peak force (PF), PAP of twitch force, and twitch maximal rates of force development and relaxation compared to control group. Twitch contraction PF:MVC force ratio was higher (p<0.05) in children with SD than in the control group. However, no significant differences in twitch contraction and half-relaxation times were observed between the measured groups. It was concluded that prepubertal children with SD in comparison of normal children are characterized by markedly reduced isometric voluntary and electrically evoked twitch contraction maximal force, capacity for twitch PAP, and rates of twitch force production and relaxation of the plantarflexor muscles. The time-course characteristics of isometric twitch contraction were similar in children with SD and normal children.

The amplitude of force variability is correlated in the knee extensor and elbow flexor muscles.

The purpose of this study was to determine motor output variability for different muscles in the same individuals. Ten young (21.7 +/- 3.4 years) and ten elderly (72.1 +/- 3.9 years) men underwent assessment of maximal isometric (MVC) and dynamic (1-RM) strength, and performed constant-force (2-50% MVC), constant-load (5-50% 1-RM load), and unloaded postural contractions as steadily as possible with the first dorsal interosseus (FDI), elbow flexors (EF), and knee extensors (KE). The coefficient of variation (CV) of force for isometric contractions and the standard deviation (SD) of acceleration for concentric, eccentric, and postural contractions were calculated. The 1-RM load, the CV of force for four of five isometric target forces, and the SD of acceleration during postural contractions were correlated between the EF and KE muscles. MVC force, 1-RM load, and SD of acceleration during postural contractions were not correlated between the FDI/EF or FDI/KE. The CV of force was correlated between the FDI/EF and FDI/KE for two of five isometric target forces. The SD of acceleration during concentric and eccentric contractions was not correlated between muscles. The normalized fluctuations during isometric contractions were greater for the FDI compared with the EF and KE. Elderly adults displayed greater fluctuations only for the FDI during low-force isometric and postural contractions. The dominant frequency of fluctuations was similar for the EF and KE muscles. The correlated fluctuations for the EF and KE muscles, within subjects, suggests that the two motor neuron pools transform the various neural inputs similarly.

Doublet discharges in motoneurons of young and older adults.

The purpose of this study was to investigate the occurrence of motor unit doublet discharges in young and older individuals at different rates of increasing force. Participants included eight young (21.9 +/- 3.56 yr) and eight older (74.1 +/- 8.79 yr) individuals, with equal numbers of males and females in each group. Motor unit activity was recorded from the tibialis anterior during isometric dorsiflexion using a four-wire needle electrode. Subjects performed three ramp contractions from zero to 50% maximal voluntary contraction (MVC) force at each of three rates: 10, 30, and 50% MVC/s. Overall, the occurrence of doublets was significantly higher in the young than in the older individuals. However, neither group showed differences in the occurrence of doublets across the three rates of force production. Doublet firings were observed in 45.6 (young) and 35.1% (old) of motor units at 10% MVC/s; 48.6 (young) and 22.5% (old) of motor units at 30% MVC/s; and 48.4 (young) and 31.4% (old) at 50% MVC/s. The maximal firing rate was significantly higher and the force at which the motor units were recruited was significantly lower for those units that fired doublets than those that did not. The force at which doublets occurred ranged from 3.42 to 50% MVC in the young subjects and from 0 (force onset) to 50% MVC in the older subjects. The results of this study suggest that the occurrence of doublets is dependent on both motor unit firing rate and force level. The lower incidence of doublets in older individuals may be attributable to changes in the intrinsic properties of the motoneurons with aging, which appear to play a role in doublet discharges.

Effects of nicotine on rat sternohyoid muscle contractile properties.

Obstructive sleep apnoea (OSA) is a major clinical disorder characterised by recurring episodes of pharyngeal collapse during sleep. At present, there remains no satisfactory treatment for OSA. Pharmacological therapies as a potential treatment for the disorder are an attractive option and include agents that increase the contractility of the pharyngeal muscles. The aim of the present study was to examine the effects of nicotine on upper airway muscle contractile properties. In vitro isometric contractile properties were determined using strips of rat sternohyoid muscle in physiological salt solution containing nicotine (0-100 microg/ml) at 25 degrees C. Isometric twitch and tetanic tension, contraction time, half-relaxation time and tension-frequency relationship were determined by electrical field stimulation with platinum electrodes. Fatigue was induced by stimulation at 40 Hz with 300 ms trains at a frequency of 0.5 Hz for 5 min. Nicotine at a concentration of 1 microg/ml was associated with a significant increase in sternohyoid muscle specific tension compared to control data. Dose-dependent increases in contractile tension were not observed. Nicotine had effects on tension-frequency relationship and endurance properties of the sternohyoid muscle at some but not all doses. A leftward shift in the tension-frequency relationship was observed at low stimulus frequencies (20-30 Hz) for nicotine at a concentration of 1 and 5 microg/ml and a significant increase in fatigue resistance was observed with nicotine at a concentration of 10 microg/ml. As fatigue of the upper airway muscles has been implicated in obstructive airway conditions, a pharmacological agent that improves muscle endurance may prove useful as a potential treatment for such disorders. Therefore, further studies of the effects of nicotinic agonists on upper airway function are warranted.

Periodontal anaesthesia reduces common 8 Hz input to masseters during isometric biting.

During isometric contractions of the jaw muscles, oscillations in the rectified masseteric EMG record that are coherent with the mandibular force output are evident at ~8 Hz. We have investigated the load dependence of these oscillations under both force and EMG feedback conditions and the extent to which these oscillations are coupled bilaterally in the jaw muscles. We further investigated the extent to which afferent information arising from the periodontium during biting influenced the extent of ~8 Hz EMG tremor and the bilateral coupling between masseters at this frequency. Using coherence analysis we have shown that a significant load-independent coupling of EMG between the closing muscles of the jaw occurs at ~8 Hz as a result of common ~8 Hz input to the masseters. This common input is significantly reduced when afferent information from the periodontium is blocked. These results suggest that afferent information arising from the periodontium enhances the expression of peripheral tremulous activity, which may be important for optimising the response of the jaw to changes in forces occurring between the teeth.

Common input across motor nuclei mediating precision grip in humans.

Short-term synchrony was measured for pairs of motor units located within and across muscles activated during a task that mimicked precision grip in the dominant and nondominant hands of human subjects. Surprisingly, synchrony for pairs of motor units residing in separate muscles (flexor pollicis longus, a thumb muscle, and flexor digitorum profundus, an index-finger muscle) was just as large as that for pairs of units both within the thumb muscle. Furthermore, the high level of synchrony seen across muscles in the dominant hand was absent in the nondominant hand. These results suggest that descending pathways diverge to provide extensive common input across motor nuclei involved in the precision grip and that such divergence might contribute to the preferred use of one hand over the other.

Pain-related factors contributing to muscle inhibition in patients with chronic low back pain: an experimental investigation based on superimposed electrical stimulation.

OBJECTIVE: To evaluate muscle strength, as a component of physical deconditioning, and central activation ratio, representing the performance level during testing, in patients with chronic low back pain as compared to healthy controls, and to evaluate the contribution of cognitive-behavioral and pain-related factors to the central activation ration of patients with chronic low back pain. METHODS: Twenty-five patients with chronic low back pain and 25 age and gender-matched controls participated. Muscle strength, that is, peak torque of the quadriceps muscle, was measured on a Cybex dynamometer. During peak torque, the quadriceps muscle was percutaneously stimulated using superimposed electrical stimulation, generating an additional twitch torque in case of submaximal performance. The central activation ratio was calculated as peak torque/(peak torque + additional twitch torque). To evaluate cognitive-behavioral and pain-related factors influencing the central activation ratio, measures of fear of injury, pain catastrophizing, psychologic distress, and pain intensity were used. Differences between groups were tested using either T tests or Mann-Whitney U tests. Associations were tested by partial correlation coefficients controlling for gender. RESULTS: The male:female ratio was 15:10. Mean age and chronic low back pain duration were 42.7 (+/-9.5) and 9.9 (+/-8.3) years, respectively. Mean muscle torque (per kg lean body mass) in patients (1.95 Nm/kg +/-0.8) was less than in controls (3.16 Nm/kg +/-0.7) (P < 0.01). Median central activation ratio was lower in patients (P < 0.05). Patients experiencing increased psychologic distress and patients with a higher current pain level showed a lower central activation ratio (P < 0.05). CONCLUSIONS: When interpreting decreased muscle strength in terms of physical deconditioning in patients with chronic low back pain, submaximal performance has to be taken into account. The results suggest that patients with chronic low back pain who report increased psychologic distress and a higher level of current pain tend to show increased inhibition of muscle activity, leading to submaximal performance.