Surface EMG amplitude and frequency dependence on exerted force for the upper trapezius muscle: a comparison between right and left sides

Why cumulative loading calculated using non-weighted integration may not be suitable for assessing physical stress of the lower back: an empirical investigation of strain during lifting and lowering tasks

When work-related physical stress is assessed using non-weighted integration, it is assumed that different loading conditions have a sufficiently comparable effect on the human body as long as the area under the loading curve is the same. Growing evidence cast doubt on whether this simple calculation can adequately estimate physical work-related strain. This study investigates in vivo, focussing on the lower back, whether the non-weighted method adequately reflects work-related physical strain of the lower back. Strain data resulting from lifting/lowering tasks performed in a laboratory study with an identical area under the loading curve but different load intensities were compared. Results showed that the non-weighted method does not sufficiently reflect the resulting muscular, cardiovascular and perceived strain but underestimates the influence of higher load intensity even in the range of medium physical exposure. Further research is needed regarding the determination of weighting factors and limit values. Practitioner Summary Given the dynamic nature of most physical work activities, the assessment of time-varying loading of the lower back is of particular interest in practice. Results show that the widely used non-weighted calculation method does not accurately reflect the resulting physical strain but underestimates the influence of higher load intensity.Abbreviations: MSD: musculoskeletal disorders; WMSD: work-related musculoskeletal disorders; KIM-LHC: Key Indicator Method Lifting, Holding, Carrying; RES: right erector spinae longissimus; LES: left erector spinae longissimus; HR: heart rate; RPE: rating of perceived exertion; EMG: surface electromyography; ECG: electrocardiography; SENIAM: Surface ElectroMyoGraphy for the Non-Invasive Assessment of Muscles; MVC: maximum voluntary contraction; ANOVA: analysis of variance; Std. error: standard error HIGHLIGHTSResults of this empirical investigation suggest that the widely used non-weighted calculation method is not fully suitable for calculating cumulative loading of the lower back.Even in the range of medium physical exposure the non-weighted calculation method does not accurately reflect the resulting strain on the human body but tends to underestimate the influence of higher load intensity due to higher external weight.Despite the same cumulative loading value obtained when using the non-weighted method, the resulting physical strain values are generally about 20-25% higher.The results may be used to further develop ergonomic assessment methods in order to avoid a misclassification of loading conditions and to prevent the risk of overexertion.

High-density surface electromyography maps after computer-aided training in individual with congenital transverse deficiency: a case study

BACKGROUND

The aim of this study was to determine whether computer-aided training (CAT) of motor tasks would increase muscle activity and change its spatial distribution in a patient with a bilateral upper-limb congenital transverse deficiency. We believe that our study makes a significant contribution to the literature because it demonstrates the usefulness of CAT in promoting the neuromuscular adaptation in people with congenital limb deficiencies and altered body image.

CASE PRESENTATION

The patient with bilateral upper-limb congenital transverse deficiency and the healthy control subject performed 12 weeks of the CAT. The subject's task was to imagine reaching and grasping a book with the hand. Subjects were provided a visual animation of that movement and sensory feedback to facilitate the mental engagement to accomplish the task. High-density electromyography (HD-EMG; 64-electrode) were collected from the trapezius muscle during a shrug isometric contraction before and after 4, 8, 12 weeks of the training. After training, we observed in our patient changes in the spatial distribution of the activation, and the increased average intensity of the EMG maps and maximal force.

CONCLUSIONS

These results, although from only one patient, suggest that mental training supported by computer-generated visual and sensory stimuli leads to beneficial changes in muscle strength and activity. The increased muscle activation and changed spatial distribution of the EMG activity after mental training may indicate the training-induced functional plasticity of the motor activation strategy within the trapezius muscle in individual with bilateral upper-limb congenital transverse deficiency. Marked changes in spatial distribution during the submaximal contraction in the patient after training could be associated with changes of the neural drive to the muscle, which corresponds with specific (unfamiliar for patient) motor task. These findings are relevant to neuromuscular functional rehabilitation in patients with a bilateral upper-limb congenital transverse deficiency especially before and after upper limb transplantation and to development of the EMG based prostheses.

Surface Electromyography Assessments of the Vastus medialis and Rectus femoris Muscles and Creatine Kinase after Eccentric Contraction Following Glutamine Supplementation

Purpose

L-glutamine is the most abundant amino acid found in human muscle and plays an important role in protein synthesis and can reduce the levels of inflammation biomarkers and creatine kinase (CK) after training sessions. Delayed onset muscle soreness (DOMS) develops after intense exercise and is associated with an inflammatory response. The purpose of this study was to investigate the effect of glutamine supplementation on surface electromyography activity of the vastus medialis muscle (VMM) and rectus femoris muscle (RFM) and levels of creatine kinase after an eccentric contraction.

Methods

Seventeen healthy men (age: 22.35±2.27yr; body mass: 69.91± 9.78kg; height: 177.08±4.32cm) were randomly assigned to experimental (n=9) and control groups (n=8) in a double-blind manner. In both groups, subjects were given L-glutamine supplementation (0.1g.kg^-1) or placebo three times a week for 4 weeks. Median frequency (MDF) and mean power frequency (MPF) for VMM and RFM muscles and also CK measurements were performed before, 24h and 48 h after a resistance training session. The resistance training included 6 sets of eccentric leg extensions to exhaustion with 75% of 1RM.

Results

There was no significant difference between groups for MDF or MPF in VMM and RFM. The difference of CK level between the groups was also not significant.

Conclusion

The results of this study indicate that glutamine supplementation has no positive effect on muscle injury markers after a resistance training session.

Effects of exposure to excessive drafts on myoelectric activity in shoulder muscles

Surface electromyography was recorded from the lateral and the cervical portion of the descending trapezius muscle and the levator scapulae muscle in 12 word processor operators. All operators were exposed to air flows with velocities ranging from 0.96 to 1.96 m/s during three continuous 6-min work periods. The skin temperature measured on the descending trapezius muscle dropped from 31.8 to 30.2°C. The air temperature remained constant at 22°C. For the lateral part of the descending trapezius muscle, a significant increase in both mean power frequency (MPF) and root-mean-square (RMS) amplitudes were observed. It was suggested that this finding was a reflex recruitment of motor units. Similar but not as distinct changes were observed for the levator scapulae muscle. For the cervical part of the descending trapezius and the levator scapulae muscles, the main finding was a significant increase in RMS values with a significant decrease in MPF values. This may be regarded as temperature effects, since it was unlikely that muscular fatigue developed.

The effect of handedness on electromyographic activity of human shoulder muscles during movement

The aim of the study was to investigate whether there was a difference in the electromyographic (EMG) activity of human shoulder muscles between the dominant and nondominant side during movement and to explore whether a possible side-difference depends on the specific task. We compared the EMG activity with surface and intramuscular electrodes in eight muscles of both shoulders in 20 healthy subjects whose hand preference was evaluated using a standard questionnaire. EMG signals were recorded during abduction and external rotation. During abduction, the normalized EMG activity was significantly smaller on the dominant side compared to the nondominant side for all the muscles except for infraspinatus and lower trapezius (P <or= 0.002). In contrast, during external rotation, higher EMG activity was seen in the supraspinatus, infraspinatus, lower and upper trapezius and latissimus muscles of the dominant side (P <or= 0.01). We demonstrated a side-difference in shoulder muscle activity, which was dependent on the type of motion carried out, suggesting a qualitative difference in the activation of muscles during the two types of movement. Dynamic abduction has the characteristics of a dominant arm task (i.e., task performed almost exclusively by the dominant arm) and reduced muscle activity for the dominant side during abduction indicates a dominance-related advantage in arm dynamics.

Behaviour of a surface EMG based measure for motor control: motor unit action potential rate in relation to force and muscle fatigue

Surface electromyography parameters such as root-mean-square value (RMS) and median power frequency (FMED) are commonly used to assess the input of the central nervous system (CNS) to a muscle. However, RMS and FMED are influenced not only by CNS input, but also by peripheral muscle properties. The number of motor unit action potentials (MUAPs) per second, or MUAP Rate (MR), being the sum of the firing rates of the active motor units, would reflect CNS input solely. This study explored MR behaviour in relation to force and during a fatiguing contraction in comparison to RMS and FMED. In the first experiment (n=10) a step contraction of shoulder elevation force (20-100 N) was performed while multi-channel array EMG was recorded from the upper trapezius muscle. The sensitivity of MR for changes in force (1.8%/N) was almost twice as high as that of RMS (0.97%/N), indicating that MR may be more suitable for monitoring muscle force. The second experiment (n=6) consisted of a 15-min isometric contraction of the biceps brachii. MR increased considerably less than RMS (0.9% vs. 4.1%), suggesting that MR selectively reflects central motor control whereas RMS also reflects peripheral changes. These results support that, at relatively low force levels, MR is a suitable parameter for non-invasive assessment of the input of the CNS to the muscle.

Relationship between perceived exertion and mean power frequency of the EMG signal from the upper trapezius muscle during isometric shoulder elevation

The aim of the study was to investigate the relationship between a fatigue-induced increase of perceived exertion in the neck with a decrease of mean power frequency (MPF) in the surface electromyography (sEMG) signal during repeated shoulder elevation endurance tasks. About Thirty-two healthy women (age range 20-62) performed two maximum 6-min shoulder elevation endurance tasks at 30% of their maximal voluntary contraction (MVC) level, separated by a rest of 6 min. During these exercises, perceived exertion was estimated using the Borg scale (range 0-10), whereas the MPF of the sEMG signal from the upper trapezius was simultaneously detected. Linear regression analysis was applied over time for each trial and subject for both MPF and Borg scale rating values. The MPF was normalized by the intercept of the linear regression analysis. The resulting slopes of normalized mean power frequency (nMPF) and Borg scale rating were correlated with each other by linear regression for both trials. In order to investigate the individual behavior of fatigue effects between trials, Delta (trial 2-trial 1) slopes of nMPF and Borg scale ratings were calculated for each subject. These slopes of nMPF and Borg scale ratings were correlated with each other as well by linear regression. The increase of Borg scale ratings, as well as the decrease of nMPF, were significantly higher in trial 2 than trial 1 (P<0.01). The results show a linear correlation between slopes of nMPF and Borg scale ratings for both trials 1 and 2 (r=0.76, P<0.01). Trial-to-trial slopes (Delta (trial 2-trial 1)) of nMPF and Borg scale rating, were also significantly correlated (r=0.68, P<0.05). Thus, the individually sensed increase of perceived exertion in the neck during trial 2 was accompanied by a simultaneously higher detected decrease of nMPF. These findings indicate a close relationship between subjective perception of exertion in the neck and objectively assessed muscle fatigue of the upper trapezius.

Surface EMG and mechanomyogram disclose isokinetic training effects on quadriceps muscle in elderly people

Maximum voluntary contraction (MVC) and cross-sectional area (CSA) of fast and slow twitch fibers are reduced in the lower limb muscles of elderly subjects. Isokinetic training at medium and high velocities has been widely used to improve muscle performance and force in young as well as elderly subjects. EMG and mechanomyogram (MMG) are compound signals in which the electrical and mechanical activities of recruited motor units (MUs) are summated. The aim of the present study was to verify the hypothesis that isokinetic training in the elderly induces changes in EMG and MMG parameters, compatible with a functional retrieval of fast twitch fiber MUs. In ten sedentary males (62-78 years), the surface EMG and MMG were recorded from the vastus lateralis muscle during isometric contractions at 20, 40, 60, 80 and 100% of the MVC, before and after 12 weeks of isokinetic training (six series of ten repetitions, each at an angular velocity of 2.09 rad s(-1) and 4.19 rad s(-1), two times a week). With training: (a) MVC and CSA increased by about 35+/-5% and 8+/-1%, respectively (P<0.05); (b) the ratio MVC/CSA increased significantly in all subjects by 25+/-5%; (c) the EMG root mean square and MMG spectral mean frequency increased significantly at the highest workloads. In conclusion, our data indicate that isokinetic training in the elderly improved muscle size and performance significantly. The EMG and MMG changes suggest that these results may be due to a retrieval of the fast twitch fiber MUs, contributing to muscle action.

Comparison of the electromyographic activity in the upper trapezius and biceps brachii muscle in subjects with muscular disorders: a pilot study

The aim of the present study was to investigate the extent to which work-related muscular disorders of the upper trapezius affect the activity of other pain-free muscles, in particular in the biceps brachii. Two groups of female subjects (age >43 years) participated in the study: seven affected subjects with self-reported disorders in the shoulder/neck region (cases) and nine healthy subjects (control group). Multi-channel electromyography (EMG) and force were recorded during maximum voluntary contractions (MVC) and during 6 min sustained contractions (at 30% MVC) of the upper trapezius and biceps brachii on the dominant side. From the EMG signals, the root mean square (RMS), median frequency (MDF) and single motor unit (MU) conduction velocity (CV) were estimated. From the force signal, the coefficient of variation was calculated. All data are presented as mean values and standard deviation. Differences between the cases and controls were found in the MVC force of the upper trapezius, which was lower in cases [253 (70) N] than in controls [357 (75) N], while the coefficient of variation of force during the sustained contraction was increased [cases 5.5 (2.2); controls 4.1 (1.9)]. The RMS (normalized to the RMS at MVC) during the 6 min sustained contractions was significantly lower in the cases than in the controls for both the upper trapezius and the biceps brachii. A tendency towards a smaller increase in the RMS with fatigue was only found in the trapezius muscle [slope: cases 6.5 (14.1) %/min, controls 10.2 (12.9) %/min]. No differences were found between the two subject groups with respect to the MDF and single MU CV in both muscles. While the lower RMS for the trapezius muscles of the cases may reflect changes at the local level, as well as in motor control, the lower biceps activity indicates a change in the central control strategies of the primarily unaffected muscle. Indications for a changed fatigability of the muscle were only found in the trapezius.

Single motor unit and spectral surface EMG analysis during low-force, sustained contractions of the upper trapezius muscle

Intramuscular and surface electromyographic (EMG) activities were recorded from the left and right upper trapezius muscle of eight healthy male subjects during 5-min long static contractions at 2% and 5% of the maximal voluntary contraction (MVC) force. Intramuscular signals were detected by wire electrodes while surface EMG signals were recorded with linear adhesive electrode arrays. The surface EMG signals were averaged using the potentials extracted from the intramuscular EMG decomposition as triggers. The conduction velocity of single motor units (MUs) was estimated over time from the averaged surface potentials while average rectified value and mean power spectral frequency were computed over time from 0.5 s epochs of surface EMG signal. It was found that (1) MUs were progressively recruited after the beginning of sustained contractions of the upper trapezius muscle at 2% and 5% MVC, (2) the conduction velocity of the MUs active since the beginning of the contraction significantly decreased over time, and (3) although the CV of single MUs significantly decreased, the mean power spectral frequency of the surface EMG did not show a consistent trend over time. It was concluded that spectral surface EMG analysis, being affected by many physiological mechanisms, may show limitations for the objective assessment of localized muscle fatigue during low force, sustained contractions. On the contrary, single motor unit conduction velocity may provide an early indication of changes in muscle fiber membrane properties with sustained activity.

Upper trapezius muscle conduction velocity during fatigue in subjects with and without work-related muscular disorders: a non-invasive high spatial resolution approach

The development of fatigue at the muscle fibre level can be assessed in terms of a decrease in conduction velocity (CV). The present study aimed to investigate if work-related muscular disorders in occupations characterised by static loads of long duration affect fatigue resistance in the painful muscle. A group of eight secretaries suffering from bilateral chronic muscle pain in the shoulder/neck region was compared to a group of healthy subjects. The upper trapezius muscle was studied under isometric contractions, holding the arm in the horizontal plane up to the endurance point. Changes in CV estimated at the motor unit level were investigated using a non-invasive high spatial resolution electromyographic (EMG) approach. In addition, the number of motor unit potentials per second (PPS), and the root mean square (RMS) of bipolar signals were assessed, and the results reported as the mean and standard error for each value. Subjects with work-related disorders showed less pronounced changes in CV with respect to healthy subjects. No differences between subjects with and without work-related disorders were encountered for the PPS and RMS. The present findings on CV indicate an increased fatigue-related recruitment of MUs in the painful muscle with respect to a healthy muscle. The fact that this recruitment is not reflected in the PPS and RMS estimates might be due to a fatigue-induced decrease in the firing rate and/or the de-recruitment of fatigued MUs. Furthermore, methodological limitations of the adopted method in the estimation of 'global' parameters such as the PPS and RMS have to be considered.

EMG power spectra of cervical muscles in lateral flexion and comparison with sagittal and oblique plane activities

The purpose of this study was to calculate electromyographic (EMG) power spectra of the sternocleidomastoid (SCM), the splenius capitis (SPL) and the trapezius (TRP) muscles from both sides of the body in bilateral lateral flexion and compare them with the EMG spectral characteristics of the same muscles in the sagittal and oblique planes recorded from the same subjects in the same experimental session but reported elsewhere in the literature. Forty normal and healthy young adults (21 males, 19 females) volunteered for the study. The subjects were prepared and positioned appropriately to exert linearly ramping isometric exertion aided by visual feedback against a rigid and load cell instrumental device. Exertions were carried out in a random order. The maximal voluntary contraction was reached within a 5-s test period. The torque and EMG from the SCM, SPL and TRP muscles were sampled bilaterally at a rate of 1 kHz. The EMG data were subjected to fast Fourier transform analysis. During lateral flexion, the torque generated by females was significantly lower than males ( P<0.01) and represented at 75% of male torque. Contrary to flexion/extension and oblique plane activities, the ipsilateral SCM was most active with highest power and up to four dominant frequencies demonstrating four distinct peaks. The bandwidth of this muscle progressively increased with the grade of contraction up to 400 Hz, similar to flexion/extension with median frequency (MF) spread between 21 and 109 Hz for males and 78 and 99 Hz for females. The ipsilateral SPL was the second most active muscle, which maintained a narrow bandwidth (200 Hz) and one dominant frequency, rising in power with increasing grade of contraction. Flexion and extension caused symmetrical activity bilaterally. The results of MANOVA revealed a significant main effect of gender, activity direction, grade of contraction and individual muscles ( P<0.001). There was a significant interaction between muscle and activity direction ( P<0.001) implying significantly different MF and mean power frequency if the direction of exertion was changed. Male and female subjects responded to direction differently ( P<0.001).

Intrapericardial algogenic chemicals evoke cardiac-somatic motor reflexes in rats

Many patients suffer from secondary muscle hyperalgesia after experiencing angina pectoris. Electrophysiological studies have also demonstrated electromyography (EMG) activities evoked during brief occlusion of the coronary artery in cats. Our objectives are: (1) to develop an animal model to study muscle changes resulting from cardiac dysfunction and (2) to investigate possible links between muscle hyperalgesia and EMG activities observed with ischemic heart diseases. Intrapericardial infusion of algogenic chemicals produced EMG activities in paraspinal muscles. Among these paraspinal muscles, the spinotrapezius produced the most consistent responses (38/53 animals). The evoked responses, in most cases, persisted longer than 90 s (29/53 muscles). The EMG activities consisted of three patterns of motor-unit discharge, namely single-unit, complex, and ventilation-related. The duration of the patterns, as well as the shape and amplitude of the motor-unit waveforms, are similar to the characteristics of muscle spasms. Since sensitization of muscle afferent fibers by noxious stimuli could produce angina-like pain, the spasm-like EMG activities might sensitize muscle afferent fibers that contribute to acute angina pain and secondary muscle hyperalgesia of cardiac patients.

The effects of four time-varying factors on the mean frequency of a myoelectric signal

Daily activities involve dynamic muscle contractions that yield nonstationary myoelectric signals (MESs). The purpose of this work was to determine the individual effects of four time-varying factors (the number and firing rate of active motor units, muscle force and joint angle) on the mean frequency of a MES. Previous theoretical and experimental work revealed that although changes in the number and firing rate of active motor units contribute to the nonstationarities of the signal, they do not significantly affect the mean frequency. In the experimental work, 12 subjects performed 25 static contractions, one for each force (20, 30, 40, 50 and 60% of maximum voluntary contraction) and elbow joint angle (50, 70, 90, 110 and 130 degrees extension) combination. A MES was recorded from the surface of the biceps brachii during each contraction. The results indicated that muscle force only weakly affects the mean frequency. Also shown was that alteration in muscle geometry resulting from changes in elbow joint angle does significantly affect the mean frequency. Knowing this is important for the assessment of muscle fatigue during dynamic contractions.

Mean frequency and signal amplitude of the surface EMG of the quadriceps muscles increase with increasing torque--a study using the continuous wavelet transform

The continuous wavelet transform (CWT), a time-frequency method, was used when calculating mean frequency of the power spectrum (MNF) and signal amplitude (RMS) of the surface EMG to investigate their relationships to force during a gradually increasing knee extension (ramp). Based upon the CWT, MNF was redefined to include time dependence on the EMG signal frequency contents, the short-time MNF (STMNF). Surface EMG was recorded from vastus lateralis, rectus femoris and vastus medialis in 21 clinically healthy subjects during a brief, gradually increasing contraction up to 100% of a maximum voluntary contraction (MVC), with a duration of approximately 10 s. The relationships between the EMG variables and force using linear regression were determined for each subject. For vastus lateralis, we also investigated if certain aspects of the muscle morphology (i.e., proportions and areas of different fibre types) influenced the EMG-force relationship. For the majority of subjects (17-18 out of 21 subjects) there were significant positive correlations between STMNF and force in the three muscles. No sex differences were found in intercepts or regression coefficients of STMNF. The muscle morphology had a significant influence on the STMNF-force intercept and the regression coefficient. Positive and highly significant linear correlations between RMS and force were found for all subjects and all three muscles.In conclusion, time frequency methods can be applied when investigating EMG during brief contractions associated with non-stationarity. In a great majority of the subjects, and in the three muscles, significant linear force dependencies were found for STMNF. Thus, when evaluating muscle fatigue, e.g., in ergonomic situations, it is important to consider the force level as one factor that can influence the results. Morphological variables (fibre proportions and fibre areas) influenced the STMNF-force relationship in vastus lateralis.

Self-perceived exertion level and objective evaluation of neuromuscular fatigue in a training session of orchestral violin players

The aim of the study was to investigate the self-perceived exertion level and an objective measurement of muscle fatigue on violin players before and after a training session. Fourteen professional violin players volunteered in this study. Surveillance study was used to investigate the demographic characteristics, instrument playing background, playing habits variables and factors associated with playing-related musculoskeletal complaints (PRMCs). The subjective rating of the training-induced exertion was evaluated by the Borg scale ratings of perceived exertion (RPE). Surface electromyography (EMG) was used to record the fatigue level of the upper trapezius muscles before and after a training session. Medium frequency (MF) of the EMG signals was used to document the fatigue rate of this muscle. Descriptive statistics revealed a 79% prevalence rate of PRMCs with neck and shoulder region accounting for 57.1% of the areas reported. On the self-perceived exertion level associated with the training session, results indicated a significant increase in fatigue level (p = 0.003) after the training session. Regression analysis and paired samples t-tests revealed no significant difference in the slopes of MF on both sides of trapezius muscle, before and after the training sessions. The disparity in the subjective perception with the objective findings indicated that the violinists' self-perceived exertion arises from multiple sources. The high prevalence of PRMCs in this profession warrants further ergonomic investigation of possible work-related risk factors.

Relation between differences in electromyographic adaptations during static contractions and the muscle function

This study determines whether changes in the EMG values of two important muscles of the shoulder and neck region, the anterior deltoid and the upper trapezius, are due to changes in torque production or due to fatigue processes during sustained activity. Contractions at 20, 40, 60, 80 and 100% MVC were performed during a flexion of the arm in the sagittal plane at 90 degrees, to examine the relation between torque and EMG. A sustained contraction at 20% MVC was performed to endurance point in the same position. RMS, a new parameter called activity, (ACT), and MPF of the deltoid anterior and the upper trapezius were analysed. The amplitude values correlated highly with increasing torque production, both for the deltoid muscle (range r = 0.95-0.96), and the trapezius muscle (range r = 0.83-0.87), whereas no significant difference was found for MPF. For the endurance task, the decrease in MPF was far more pronounced for the deltoid than for the trapezius, whereas the opposite occurred with RMS (P < or = 0.01). Furthermore, there was no significant difference over time for the ACT values of the deltoid, whereas there were significant increases in ACT for the trapezius (P < or = 0.01). The RMS/ACT ratio correlated highly (r = 0.81) with the MPF. Regression coefficients of these parameters differed significantly for the trapezius muscle but not for the deltoid muscle. Therefore, the RMS/ACT ratio may be extremely important in analysing the fatigue effects during sustained efforts, independent of torque variations, which can influence indicators of fatigue.

Relationship between reaction time of eye movement and activity of the neck extensors

This study investigated the relationship between the reaction time of eye movement (RTEM) and activity of the superficial neck extensor muscles when the shoulder girdle elevator muscles contracted isometrically. The results were compared with those of a previous study in which the subjects's head was fixed and loaded with the neck flexed. When the shoulder girdle elevator muscles contracted isometrically, RTEM decreased significantly in comparison to RTEM at rest. This demonstrated that the reaction time significantly decreased not only as a result of the neck in flexion, which activated the deep and superficial neck extensor muscles, but also from contraction of the shoulder girdle elevator muscles which mainly activated the superficial extensor muscles. The relationship between RTEM and relative muscle load of the shoulder girdle elevator muscles showed that RTEM decreased up to 30% loading, and with loading above 40% the RTEM was longer than with 30%. The relative muscle load for the shortest RTEM demonstrated a subject-to-subject variance ranging from 24.7% to 49.6%. The difference between RTEM at rest and at their shortest reaction time was approximately 20 ms, which was consistent with the data for the neck in flexion. However, the relative muscle load for the shortest RTEM differed between the current and previous studies. The parameters obtained in this study were higher than for those in the previous study.

The relationship between the electromyogram-amplitude and isometric extension torques of neck muscles at different positions of the cervical spine

A group of 12 healthy men volunteered for the experiment. Electromyograms (EMG) were obtained from semispinalis capitis, splenius capitis, levator scapulae, and trapezius muscles. The flexion angle of the cervical spine was precisely adjusted to 0 degrees, 10 degrees, 20 degrees, and 30 degrees relative to the horizontal, with a constant angle of the atlanto-occipital joint. The subjects made eight short (about 2 s) vertical extension forces (6%, 12%, 18%, 24%, 30%, 36%, 42%, and 48% of maximal voluntary peak contraction force). For each position, the centre of pressure under the head was determined as the basis for the calculation of the external lever arm. The presence of motor endplate regions was ascertained by multiple surface electrodes. The slopes of individual linear regression lines for the root mean square (rms)-values were dependent on the existence of endplates in the area of the electrodes - endplates caused smaller rms values per Newton metres of external torque. Significant intersubject differences between regression equations could not be eliminated by the normalization of EMG-parameters and/or torques. The elimination of gravity, the continuous monitoring of positions, and the consideration of localization of motor endplate regions were essential prerequisites for the acquisition of reliable relationships between EMG of different neck muscles and external torques. Two important conclusions were derived for the prediction of torques from EMG measurements: firstly, individual regression equations which take into account the position of the head and neck should be used; secondly, normalization procedures do not justify the application of average regressions to a group of subjects.

The influence of electrode position on bipolar surface electromyogram recordings of the upper trapezius muscle

The effect of electrode position on the upper trapezius muscle on the myo-electric signal amplitude was investigated with special reference to arm position and estimate of force output. Previously, a depression of the electromyogram (EMG) signal has been reported midway between the seventh cervical vertebrae (C7) and acromion (Veiersted 1991, Eur J Appl Physiol 62:91-98) although this electrode position has been recommended (Zipp 1982, Eur J Appl Physiol 50: 41-54). Ten healthy subjects performed maximal shoulder elevations with the arm in vertical, abducted and flexed positions and they performed a dynamic movement test. The myo-electric signal was recorded along the length of the right upper trapezius muscle by a 16-channel bipolar array electrode and was integrated with a 0.2-s time resolution. A region just lateral to the midpoint between C7 and the lateral edge of acromion was found with high and stable amplitudes (% coefficient of variation equalled 5.6). At the midpoint a dip in the amplitude profile appeared which was slightly displaced by arm abduction or flexion probably due to sliding of the skin relative to the muscle. A linear EMG-force relationship was found in the region with high signal amplitudes, whereas the more lateral and the dip region showed highly variable EMG-force relationships. Thus, it was found that when using bipolar surface electrodes with an interelectrode distance of 2 cm a centre position 2 cm lateral to the midpoint between C7 and acromion provided good repeatability and high signal yield.

Electromyographic changes in work-related myalgia of the trapezius muscle

In 11 patients, all women, 21-55 years of age, with unilateral work-related myalgia of the trapezius muscle, the right and left trapezius muscles were examined simultaneously for electromyogram (EMG) signs of localized muscle fatigue. All patients were tested with 0-kg hand load for 5 min, holding the arms straight at 90 degrees of elevation in the scapular plane. Only 4 of the patients tolerated exposure to higher load levels. They were tested with 1 kg hand load for 3 min and 2 kg hand load for 2 min, with a period of rest of 30 min between the trials. The EMG mean power frequency (MPF) and root mean square (rms) were calculated. Data were normalized with the initial value as a reference and regression analyses were performed. On both sides a decrease of MPF and an increase of rms were found with increasing time and load, i.e. classical EMG signs of localized muscle fatigue. Compared with the nonaffected side smaller changes were found on the affected side, possibly due to pain inhibition, impaired microcirculation and biochemical changes along the muscle fibres. At 0-kg hand load we found no change of MPF on either side despite subjective feelings of fatigue and pain. We interpreted these findings as an indication of reduced capacity of the affected trapezius muscle to sustain static load with early development of pain-associated local fatigue.