Surface EMG recordings during maximum static shoulder forward flexion in different positions

Summary This study investigated how position in the range of motion influences the power spectral density function during static shoulder forward flexion. 23 healthy females (20–30 years) volunteered as subjects. They performed maximum static shoulder forward flexions in three positions: 45, 65 and 90° of shoulder flexion. An isokinetic dynamometer was used and the subjects were seated in a specially constructed chair to enable adequate fixation. The elbow was extended and the hand pronated. Electromyographic (EMG) signals (using surface electrodes) were obtained from the descending part of the right trapezius, the anterior portion of the right deltoid, the right infraspinatus and the common belly of the right biceps brachii. The four EMG-signals and the torque and shoulder angle were analyzed by computer. For each 256 ms, mean power frequency, root mean square value and mean torque were calculated. At each of the three positions four 256 ms periods were analyzed and the data are presented as their means. In the trapezius and the biceps brachii the mean power frequency did not change between the three positions. Deltoid and infraspinatus had significantly higher mean power frequencies at 90° than at 45° of flexion. Different factors behind the change in mean power frequency are discussed. The need to standardize the range of motion when studying dynamic fatiguing contractions is emphasised.     

Reproducibility of surface EMG during dynamic shoulder forward flexions: a study of clinically healthy subjects.

There is a shortage of studies concerning the reproducibility of surface electromyography (EMG) during dynamic contractions. The movement per se introduces factors that could have the potential to affect the characteristics of the electromyogram. The aim of this study was to investigate the during-the-day reproducibility (using intra-class coefficient, ICC) of the peak torque (PT) and the EMG variables (without removing electrodes) of dynamic shoulder forward flexions. Eleven healthy women performed three sets of 10 dynamic maximum right-shoulder flexions at 1 h intervals using an isokinetic dynamometer. The PT of each flexion was determined. EMG signals were recorded from four muscles (trapezius, deltoid, infraspinatus and biceps brachii) using surface electrodes, and the mean frequency of the power spectrum (MNF [Hz]) and the signal amplitude (RMS [microV]), were computed. The ability to relax between maximum flexions was calculated as a ratio between the RMS during the passive extension phase and the RMS during the active flexion phase of each contraction cycle. This ratio is the signal amplitude ratio (SAR). The present study showed good reproducibility for PT, MNF and RMS, while the reproducibility of SAR was generally acceptable (fair) according to the criteria used.     

Changes in the surface electromyogram during increasing isometric shoulder forward flexions

Summary When using electromyographic techniques in the evaluation of muscular load it is necessary to determine the mathematical relationship between the torque and the amplitude of the electromyographic signal. Isometric gradually increasing contractions up to 100% MVC can then be used. Often more than linear increases for the amplitude (RMS) — force regression have been reported. The present study was designed to test whether changes in power spectral density function take place during a gradually increasing isometric contraction (duration 10 s). Twenty-two clinically healthy females performed an increasing isometric shoulder forward flexion for 10 s using an isokinetic dynamometer. Electromyographic activity was measured in trapezius, deltoid, infraspinatus and biceps brachii using surface electrodes. Mean torque values were determined together with mean power frequency (MPF) and root mean square values (RMS) from the EMG signals for each 256 ms period. The RMS-torque regressions showed higher regression coefficients during the 6th to 9th sec than during the first 5 s. No significant correlation existed between MPF for the four muscles and the torque. A gradual decrease in MPF was generally found from the 6th s. It is concluded that this decrease in power spectral density function might have contributed to the significantly higher regression coefficient for the RMS torque regression at the high output part of the gradually increasing isometric contraction.     

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.     

Effect of arm-shoulder fatigue on carpenters at work

Summary The purpose of the study was to analyse the effect of arm-shoulder fatigue on manual performance. Ten experienced carpenters performed three standardized tasks (nailing, sawing and screwing). Electromyographic activity was recorded from six arm-shoulder muscles and the performances were video-filmed. After 45 min of standardized arm-cranking (arm-shoulder-fatiguing exercise of approximately 70%–80% maximal oxygen consumption), the tasks were repeated. The number of work movements and the time taken for each task were recorded and the quality of the work performed was compared. After the fatiguing exercise, only nailing was perceived as being harder and more mistakes were made during nailing and sawing. Movement performance was not influenced during nailing but was slightly slower during sawing and faster during screwing. However, there were increased mean EMG amplitudes in the upper trapezius and biceps muscles during nailing, in the upper trapezius, anterior deltoid and infraspinatus muscles during sawing and in the anterior deltoid muscle during screwing. Of the muscles studied the upper trapezius and anterior deltoid muscles increased their activity most after the arm-shoulder-fatiguing exercise.     

冻结肩三维运动表面肌电信号特征分析

目的 通过肩周肌肉手法治疗前、后积分肌电值差异,探讨肩周肌肉功能变化。方法选取冻结肩患者7例,利用AMT-8肌电仪对手法治疗前、后肌肉肌电值进行采集,将数据用MATLAB软件进行分析,求积分肌电值。用Stata11.0软件统计。结果①外展时三角肌、冈下肌治疗前、后积分肌电值差异有统计学意义(P＜0.05),肱二头肌、肱...     

Low-frequency oscillations (<0.3 Hz) in the electromyographic (EMG) activity of the human trapezius muscle during sleep

The surface electromyographic (EMG) signal from right and left trapezius muscles and the heart rate were recorded over 24 h in 27 healthy female subjects. The root-mean-square (RMS) value of the surface EMG signals and the heartbeat interval time series were calculated with a time resolution of 0.2 s. The EMG activity during sleep showed long periods with stable mean amplitude, modulated by rhythmic components in the frequency range 0.05-0.2 Hz. The ratio between the amplitude of the oscillatory components and the mean amplitude of the EMG signal was approximately constant over the range within which the phenomenon was observed, corresponding to a peak-to-peak oscillatory amplitude of approximately 10% of the mean amplitude. The duration of the periods with stable mean amplitude ranged from a few minutes to approximately 1 h, usually interrupted by a sudden change in the activity level or by cessation of the muscle activity. Right and left trapezius muscles presented the same pattern of FM. In supplementary experiments, rhythmic muscle activity pattern was also demonstrated in the upper extremity muscles of deltoid, biceps, and forearm flexor muscles. There was no apparent association between the rhythmic components in the muscle activity pattern and the heart rate variability. To our knowledge, this is the first time that the above-described pattern of EMG activity during sleep is documented. On reanalysis of earlier recorded trapezius motor unit firing pattern in experiments on awake subjects in a situation with mental stress, low-FM of firing with similar frequency content was detected. Possible sources of rhythmic excitation of trapezius motoneurons include slow-wave cortical oscillations represented in descending cortico-spinal pathways, and/or activation by monoaminergic pathways originating in the brain stem reticular formation. The analysis of muscle activity patterns may provide an important new tool to study neural mechanisms in human sleep.     

Bilateral responses of upper limb muscles to transcranial magnetic stimulation in human subjects

Anatomical and behavioural work on primates has shown bilateral innervation of axial and proximal limb muscles, and contralateral control of distal limb muscles. The following study examined if a clear boundary exists between the distal and proximal upper limb muscles that are controlled contralaterally or bilaterally. The right motor cortical area representing the upper limb was stimulated, while surface EMG was recorded bilaterally from various upper limb muscles during rest and phasic voluntary contractions. Peak-to-peak amplitude of motor evoked potential (MEP) was measured for each muscle on both sides. The ratio R = (ipsilateral MEP: contralateral MEP) was calculated for seven pairs of muscles. For each of the seven pairs, R was less than 1.0, implying that for each muscle and subject, the contralateral control is stronger. The boundary where R changed from almost zero to a clearly measurable magnitude depended on the subject. Ipsilateral MEPs from trapezius and pectoralis could be recorded with a small background contraction from almost all subjects; on the other hand, in deltoid and biceps brachii, ipsilateral MEPs were observed only with bimanual phasic contractions. The forearm and hand muscles, in general, did not show any ipsilateral MEPs. Major differences between subjects lay in the presence or the absence of ipsilateral MEPs in biceps brachii and deltoid, without defining a sharp boundary between proximal and distal muscles.     

Muscle activity and fatigue in the shoulder muscles during repetitive work

Summary The amplitude distribution probability function (ADPF) and the power spectrum of the surface electromyogram from m. deltoideus anterior, m. infraspinatus and m. trapezius pars descendens were analyzed from 7 persons working at a pillar drill. Recordings were performed 6 times during a working day. The ADPF was analyzed from 2–3 work-cycles from each recording. The static contraction level was 11.0% MCV in m. deltoideus anterior, 8.5% MVC in m. infraspinatus, and 20.5% MCV in m. trapezius, without any change occuring throughout the day. When compared to previous suggested upper limits of ADPF levels, both the static and the medium contraction levels were too high in the performance of this particular task. The power spectrum of the EMG was analyzed during isometric contractions of the shoulder muscles. The mean power frequency decreased during the day in m. trapezius only, suggesting muscular fatigue in this area.     

Experimental muscle pain changes motor control strategies in dynamic contractions

This study investigated the effect of muscle pain on muscle activation strategies during dynamic exercises. Ten healthy volunteers performed cyclic elbow flexion/extension movements at maximum speed for 2 min after injection of (1) hypertonic (painful) saline in the biceps brachii, (2) hypertonic saline in both biceps brachii and triceps brachii, and (3) isotonic (nonpainful) saline in the biceps brachii muscle. Surface electromyographic (EMG) signals were collected from the upper trapezius, biceps brachii, triceps brachii, and brachioradialis muscles (to estimate EMG amplitude) and with an electrode arrays from biceps brachii (to estimate muscle fiber conduction velocity [CV]). In all conditions, the acceleration of the movement decreased throughout the exercise, and kinematic parameters were not altered by pain. With respect to the control condition, pain induced a decrease of the biceps brachii (mean ± SE, –23±4%) and brachioradialis (–10±0.4%) integrated EMG (IEMG) in the beginning of the exercise, and an increase (45±3.5%) of the upper trapezius IEMG at all time points during the exercise. The biceps brachii IEMG decreased over time during the nonpainful exercises (–11±0.6%) while it remained constant in the painful condition. Biceps brachii CV decreased during painful conditions (–12.8±2.2%) while it remained constant during the nonpainful condition. In conclusion, muscle pain changes the motor control strategy to sustain the required dynamic task both in the relative contribution between synergistic muscles and in the motor unit activation within the painful muscle. Such a changed motor strategy may be highly relevant in models of occupational musculoskeletal pain conditions.     

俯卧撑中前臂旋转对上肢肌肉电活动的影响

目的 分析俯卧撑中前臂旋转对上肢肌肉电活动的影响。方法 采集十个对象在前臂内旋90°（IR90）,中立位（NEU）和外旋90°（ER90）三个支撑位下肱三头肌长头和侧头、三角肌前部和中部、胸大肌和肱二头肌的肌电信号,计算肌电信号的均方根值,并作归一化处理,运用单因素方差分析法比较不同支撑位下肌电信号的差异。同时对一个周期内肌电信号进行包络线分析。结果 从IR90到ER90,肱三头肌长头信号无显著变化,肱三头肌侧头和三角肌中部信号逐渐减小,而三角肌前部、胸大肌和肱二头肌则逐渐增大。另外,包络分析结果显示了三种不同类型的肌电信号曲线。结论 不同前臂转动支撑位对肌肉活性产生影响,而对各肌肉的影响程度和方式并不相同。     

Differences in activation patterns in elbow flexor muscles during isometric,concentric and eccentric contractions

Summary To investigate the relative activation of the synergistic muscles during three different types of muscle contraction, the electromyograms (EMG) of two elbow flexor muscles, the biceps brachii (BB) and the brachioradialis (BR), have been compared. To accomplish this eight healthy human subjects performed the following elbow flexions against the same load — concentric, eccentric and isometric contractions. The isometric contractions were performed at three elbow angles: 10, 45 and 90° (0° equal to full expension). The EMG were recorded by bipolar surface electrodes, and the relative activation between the two muscles was evaluated as the quotient of mean EMG activities (BR/BB). For the isotonic elbow flexions, BR/BB were calculated at three angle divisions: 0–30°, 30–60° and 60–90°. Results indicated that the relative activation of the BR during the concentric contractions was higher than that of the eccentric contraction, particularly at the extended elbow angles, i.e. the BR/BB of the concentric contractions for the elbow joint angles ranging from 0–30° and 30–60° were significantly greater (P<0.05) than those of the eccentric contractions. During the isometric and eccentric contractions, the BR/BB at the flexed joint angles tended to be greater than those at the extended angles. In contrast, there were no angle-dependent BR/BB variations during the concentric elbow flexions. Further, changing patterns in the EMG power spectra due to the type of contraction were different between BB and BR. These results indicated that the activation pattern in the two elbow flexor muscles varied with the muscle contraction pattern.     

Lack of gender differences in the ability to relax between repetitive maximum isokinetic shoulder forward flexions: a population-based study among northern Swedes

The aim of the present study was to analyse the effects of gender and age in a population-based sample of clinically healthy subjects on: (1) strength, endurance and perception of fatigue, (2) mean frequency (MNF) of the electromyogram (EMG), and (3) the ability to relax between active contractions (signal amplitude ratio; SAR) of the surface EMG of an isokinetic shoulder forward flexion test. With this aim we have analysed aspects of the validity of MNF as an indicator of fatigue, using peak torque as a criterion variable. The subjects were 27 men and 28 women (age range: 20–60 years), who were obtained by random sample from the official census lists (participation rate: 66%). The peak torque and surface EMG of two portions of the trapezius, deltoid and infraspinatus muscles were recorded throughout 100 repetitive maximal isokinetic shoulder forward flexions. No significant differences in the perception of fatigue and relative endurance levels of peak torque and work were found between males and females. Males were significantly stronger than females and, on average, females produced approximately 60% of the output of the males; 76% after normalisation for body mass. The men had significantly lower MNF endurance levels for three of the investigated muscles. When controlled for age and body mass, the men had a significantly higher MNF of the deltoid muscle than did the females. These differences were only found for the deltoid muscle. Significant correlations existed between the MNF of the four muscles and biomechanical output, indicating criterion validity for the MNF variable with respect to fatigue. There were no significant effects of gender or age on the ability to relax between repetitive contractions (SAR). The higher prevalence of musculoskeletal complaints of the neck-shoulder region in females cannot be explained by a higher intrinsic muscle tension. Age, gender, body mass index and biomechanical output can have significant effects upon MNF. These effects are important considerations in the interpretation of MNF, for instance in ergonomic situations. Accepted: 17 July 2000     

Influences of handgrip on shoulder muscle activity

Four shoulder muscles (the supraspinatus, the infraspinatus, the middle portion of the deltoid and the descending part of the trapezius muscle) were examined using electromyography (EMG) in abducted and flexed arm positions, in nine subjects who had no history of illness from arm or shoulder. The subjects were asked to supply an intermittent isometric handgrip force of 30% and 50% of maximal voluntary contraction, in a total of eight different arm positions. The EMG activity with and without hand activity was compared in all positions. There was a statistically significant increase in the EMG activity in the supraspinatus muscle in humeral flexion from and above 60°. In the infraspinatus muscle the changes were less; a significant decrease was however noticed in abduction. In the deltoid muscle there was a significant decrease with hand activity in flexion from and above 90°. There was no statistically significant alteration regarding the EMG activity of the trapezius muscle. The result of this study implies that high demands on handgrip force, particularly while using hand tools in elevated arm positions, adds further to the already high load on some shoulder muscles. This factor should be considered in the design of manual work and in the places of work.     

Effects of repetitive dynamic contractions upon electromechanical delay

The effect of repeated maximal effort isotonic contractions on electromechanical delay was studied. Over 4 days, 17 male subjects performed 400 rapid elbow flexion trials. The kinematics and surface electromyographic (EMG) activity of the biceps brachii of these subjects were recorded. The period from the onset of the EMG until the beginning of movement was defined as the electromechanical delay. The period from the beginning of movement until the end of the EMG was defined as the second component of the contraction. Over the 4 day period there was an increase in the speed of limb movement. The mean power frequency and the duration of the EMG during the electromechanical delay did not change, while the root-mean-square amplitude increased. The duration of the EMG during the second component of the contraction remained stable. The mean power frequency and the root-mean-square amplitude of the EMG during the second component of the contraction increased with the speed of limb movement. We conclude that the faster contractions were a result of changes in motor unit recruitment during the second component of the contraction, rather than in the electromechanical delay.     

The trapezius muscle uniquely lacks adaptive process in response to a repeated moderate cognitive stressor

The aim of this study was to examine the adaptive process of muscular responses in healthy subjects over two repeated exposures to the same moderate cognitive stressor. The electromyographic (EMG) activity of the flexor pollicis brevis, biceps brachii, triceps brachii, trapezius, gastrocnemius and soleus muscles was recorded in 35 males during video-recorded Stroop color-word interference tests. The results showed lower EMG activity in all muscles during the second exposure to the stressful task, but not in the trapezius muscle. These findings could help to the understanding of the role of stressful situations in the development of musculoskeletal disorders.     

Worsening of neck and shoulder complaints in humans are correlated with frequency parameters of electromyogram recorded 1-year earlier

The aim was to investigate whether output and electromyogram (EMG) variables obtained from an isokinetic endurance test of the shoulder flexor muscles of 23 women with neck and shoulder problems in a car and truck industry correlated with improvement or worsening of complaints 1 year later. Each subject performed 100 maximal isokinetic shoulder forward flexions at 60°?·?s^?1. Surface EMG of the trapezius, deltoid, biceps brachii and infraspinatus muscles and mechanical output (peak torque) were determined for each contraction. The EMG was used to determine mean frequency f _mean and the ratio between the signal amplitudes of the EMG of the passive relaxation and active flexion parts of each contraction cycle (SAR). The subjects also rated the degree of fatigue they experienced throughout the test. The magnitude of the shift in f _mean was correlated with whether improvement or worsening occurred for complaints in the neck and or shoulders; a significant relationship (r ²?=?0.44; P?=?0.001) existed between the total frequency shift of the four muscles and the variables measuring improvement in complaints. In the multivariate predictions other f _mean variables and perception of fatigue were also of significance. The present study would indicate that a high degree of f _mean shift correlates with improvement in neck and shoulder complaints 1 year later. One possible reason could be that f _mean reflects the muscle morphology and/or a pathological situation for the type-1 muscle fibres.     

Mechanical performance and electromyography during repeated maximal isokinetic shoulder forward flexions in female cleaners with and without myalgia of the trapezius muscle and in healthy controls

This cross-sectional study aimed at investigating the influence of occupational exposure to static and highly repetitive work involving the neck and shoulder muscles, myalgia of and tender point in the trapezius muscle on biomechanical output, and electromyogram (EMG) variables (mean frequency MNF, signal amplitude and ability to relax) during maximal forward flexions of the shoulder muscles. Groups of 25 cleaners suffering from chronic myalgia of the trapezius muscle, 25 cleaners free from myalgia of the trapezius muscle and 21 teachers performed 150 forward flexions using an isokinetic dynamometer. Perception of fatigue was reported and surface EMG was recorded from four muscles during the endurance test. The cleaners were stronger than the teachers. Myalgia was associated with lower levels of endurance and a high degree of perceived fatigue. The ability to relax the trapezius muscle decreased with age and was even lower in cleaners with and without myalgia. Higher MNF of the deltoid muscle but not of the trapezius muscle was found in the group suffering from myalgia compared to the groups free from myalgia. This cross-sectional study indicated that myalgia of the trapezius muscle did not influence the strength but did influence the endurance of the forward flexor muscles of cleaners. The observed decrease in the ability to relax the trapezius muscle in cleaners compared to healthy teachers might be indicative of a future insufficiency in the muscle. Prospective studies are needed to define the significance of the results presented here. Accepted: 26 June 2000     

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

Summary Surface electromyographic (EMG) amplitude and mean power frequency (MPF) were used to study the isometric muscular activity of the right versus the left upper trapezius muscles in 14 healthy right-handed women. The EMG activity was recorded simultaneously with force signals during a 10–15 s gradually increasing exertion of force, up to maximal force. Only one side at a time was tested. On both sides there was a significant increase in EMG amplitude (V) during the gradually increasing force from 0% to 100% maximal voluntary contraction (MVC). The right trapezius muscle showed significantly less steep slopes for regression of EMG amplitude versus force at low force levels (0%–40% MVC) compared intra-individually with high force levels (60%–100% MVC). This was not found for the left trapezius muscle. At 40% MVC a significantly lower MPF value was found for the right trapezius muscle intra-individually compared with the left. An increase in MPF between 5% and 40% MVC was statistically significant when both sides were included in the test. The differences in EMG activity between the two sides at low force levels could be due to more slow-twitch (type I fibres) motor unit activity in the right trapezius muscles. It is suggested that this is related to right-handed activity.     

Lack of gender differences in the ability to relax between repetitive maximum isokinetic shoulder forward flexions: a population-based study among northern Swedes

The aim of the present study was to analyse the effects of gender and age in a population-based sample of clinically healthy subjects on: (1) strength, endurance and perception of fatigue, (2) mean frequency (MNF) of the electromyogram (EMG), and (3) the ability to relax between active contractions (signal amplitude ratio; SAR) of the surface EMG of an isokinetic shoulder forward flexion test. With this aim we have analysed aspects of the validity of MNF as an indicator of fatigue, using peak torque as a criterion variable. The subjects were 27 men and 28 women (age range: 20-60 years), who were obtained by random sample from the official census lists (participation rate: 66%). The peak torque and surface EMG of two portions of the trapezius, deltoid and infraspinatus muscles were recorded throughout 100 repetitive maximal isokinetic shoulder forward flexions. No significant differences in the perception of fatigue and relative endurance levels of peak torque and work were found between males and females. Males were significantly stronger than females and, on average, females produced approximately 60% of the output of the males; 76% after normalisation for body mass. The men had significantly lower MNF endurance levels for three of the investigated muscles. When controlled for age and body mass, the men had a significantly higher MNF of the deltoid muscle than did the females. These differences were only found for the deltoid muscle. Significant correlations existed between the MNF of the four muscles and biomechanical output, indicating criterion validity for the MNF variable with respect to fatigue. There were no significant effects of gender or age on the ability to relax between repetitive contractions (SAR). The higher prevalence of musculoskeletal complaints of the neck-shoulder region in females cannot be explained by a higher intrinsic muscle tension. Age, gender, body mass index and biomechanical output can have significant effects upon MNF. These effects are important considerations in the interpretation of MNF, for instance in ergonomic situations.