Methodologies for evaluating electromyographic field data in ergonomics.

Surface electromyography (SEMG) is an important tool for work load assessment in ergonomics. Several different approaches using amplitude as well as frequency parameters give fruitful information depending on question at issue in the laboratory as well as in field studies. One basic factor determining the choice of analysis method is whether the SEMG is interpreted as an indicator of forces/torques or pure muscular activation. Two methods for occupational SEMG data reduction representing two different approaches to SEMG applications in ergonomics, Exposure Variation Analysis (EVA), and Joint Analysis of EMG Spectrum and Amplitude (JASA), applied on the same SEMG recording from three muscles during urology surgeon work, have been compared. The EVA method categorised the three muscle recordings as too static with no EMG gaps while the JASA method identifies fatigue in two of the three recordings. The practical relevance of these findings is discussed.     

Assessment of muscle fatigue in low level monotonous task performance during car driving.

Knowledge of the muscle activation and the development of muscle fatigue may provide more inside in the effects of long-term driving in the occurrence of health problems in the neck/shoulder/back area. The basic assumption behind fatigue detection with electromyography (EMG) is an increase in the EMG amplitude and a decrease of the mean frequency (MF). This study aimed at checking this assumption in monotonous task performance with low level activity during car driving. Surface electromyography was captured from left and right trapezius and deltoid muscles, during a repetitive, non-continuous, driving task (gearing and steering) and the active parts were separated from the non-active parts. Muscle stiffness was reported by more than half of the subjects after a 1 h drive. Only for the active parts a significant decrease of the MF was seen. But also the EMG amplitude decreased significantly. Two possible mechanisms are posted in literature for this finding: no extra recruitment of motor units (MU) and potentiation of muscle fibers. Literature also hypothesizes that low-force occupational work engages only a fraction of the MU available for recruitment and that these units are selectively type I muscle fibers (Cinderella fibers). Initiators of this phenomenon are probably the time lag between activations and the stress from driving and vibration exposure.     

Muscle fatigue and calibration of EMG measurements

Amplitude electromyography (EMG) is often used as an estimator of muscular load. Such measurements can, however, be biased by other factors, for example muscular fatigue. The aim of this study was to examine the influence of fatigue on amplitude parameters of the EMG. The test subjects raised the arm to 90⁹ of abduction in the plane of the scapula. The hand was loaded with 0, 1 and 2 kg during 5, 3 and 2 min respectively. EMG was recorded from the trapezius muscle, and spectral and amplitude parameters were calculated. There was a significant rise of the EMG amplitude as a sign of fatigue at all load levels: 7% min⁻¹ at 0 kg, 15% min⁻¹ at 1 kg, and 19% min⁻¹ at 2 kg. At 0 kg hand load there was no change of the spectral parameters but at higher load levels there was a significant decline of mean power frequency: 3% min⁻¹ at 1 kg and 11% at 2 kg. The amplitude rise due to muscle fatigue may seriously jeopardize calibration measurements unless the duration of the load is kept limited.     

Comparison of an EMG-based and a stress-based method to predict shoulder muscle forces

The estimation of muscle forces in musculoskeletal shoulder models is still controversial. Two different methods are widely used to solve the indeterminacy of the system: electromyography (EMG)-based methods and stress-based methods. The goal of this work was to evaluate the influence of these two methods on the prediction of muscle forces, glenohumeral load and joint stability after total shoulder arthroplasty. An EMG-based and a stress-based method were implemented into the same musculoskeletal shoulder model. The model replicated the glenohumeral joint after total shoulder arthroplasty. It contained the scapula, the humerus, the joint prosthesis, the rotator cuff muscles supraspinatus, subscapularis and infraspinatus and the middle, anterior and posterior deltoid muscles. A movement of abduction was simulated in the plane of the scapula. The EMG-based method replicated muscular activity of experimentally measured EMG. The stress-based method minimised a cost function based on muscle stresses. We compared muscle forces, joint reaction force, articular contact pressure and translation of the humeral head. The stress-based method predicted a lower force of the rotator cuff muscles. This was partly counter-balanced by a higher force of the middle part of the deltoid muscle. As a consequence, the stress-based method predicted a lower joint load (16% reduced) and a higher superior–inferior translation of the humeral head (increased by 1.2 mm). The EMG-based method has the advantage of replicating the observed cocontraction of stabilising muscles of the rotator cuff. This method is, however, limited to available EMG measurements. The stress-based method has thus an advantage of flexibility, but may overestimate glenohumeral subluxation.     

Shoulder muscle co-ordination during chronic and acute experimental neck-shoulder pain. An occupational pain study

Little is known about the mechanisms leading to chronic neck-shoulder musculoskeletal disorders (MSD). The aim of the present study was to investigate and compare motor function during controlled, low load, repetitive work together with chronic or acute experimental neck-shoulder pain. The clinical study was performed on workers with (n = 12) and without (n = 6) chronic neck-shoulder pain. In the experimental study, experimental muscle pain was induced in healthy subjects by intra-muscular injection of hypertonic saline into the trapezius muscle (n = 10). The assessed parameters related to motor performance were: work task event duration, cutting forces, surface electromyogram (EMG) activity in four shoulder muscles, displacement of the centre of pressure, and arm and trunk 3D movements. For controlled cutting force levels, chronic and acute experimental pain provoked a series of changes: a decreased working rhythm and a protective reorganisation of muscle synergy (experimental study), higher EMG frequency contents which may indicate altered motor unit recruitment, and greater postural activity and a tendency towards increased arm and trunk movements. These pain-related changes can play a role in the development of MSD. The present clinical and experimental study demonstrated similar interactions between motor co-ordination and neck-shoulder pain in occupational settings. We therefore suggest that this experimental model can be used to study mechanisms related to MSD. Information on such modulatory processes may help in the design of new strategies aimed at reducing the development of MSD.     

Sensitivity of trapezius electromyography to differences between work tasks - influence of gap definition and normalisation methods.

Surface electromyography (EMG) has been used extensively to estimate muscular load in studies of work related musculoskeletal disorders, especially for the trapezius muscle. The occurrences of periods of EMG silence (gaps), the time below a predetermined threshold level (muscular rest) and various percentiles of the amplitude distribution (APDF) are commonly used summary measures. However, the effects of the criteria used to calculate these measures (e.g., gap duration, threshold level, normalisation method) on the sensitivity of these measures to accurately differentiate work loads is not well known.Bilateral trapezius EMG was recorded, for a full workday, for 58 subjects following both maximal (MVE) and submaximal (RVE) reference contractions. Gap frequency, muscular rest, and percentiles were derived for eight fundamental work tasks. The calculations were performed using different gap duration criteria, threshold levels and normalisation methods.A gap duration of less than 1/2 s, and threshold level approximately 0.3% MVE for gap frequency, and approximately 0.5% MVE for muscular rest, were the criteria that optimised sensitivity to task differences. Minimal sensitivity to tasks and a high sensitivity to individuals was obtained using gap frequency with a threshold level of approximately 1% MVE. Normalisation to RVE, rather than MVE, improved sensitivity to differences between tasks, and reduced undesirable variability. Muscular rest was more sensitive to task differences than APDF percentiles.     

Electromyographic response is altered during robotic surgical training with augmented feedback

There is a growing prevalence of robotic systems for surgical laparoscopy. We previously developed quantitative measures to assess robotic surgical proficiency, and used augmented feedback to enhance training to reduce applied grip force and increase speed. However, there is also a need to understand the physiological demands of the surgeon during robotic surgery, and if training can reduce these demands. Therefore, the goal of this study was to use clinical biomechanical techniques via electromyography (EMG) to investigate the effects of real-time augmented visual feedback during short-term training on muscular activation and fatigue. Twenty novices were trained in three inanimate surgical tasks with the da Vinci Surgical System. Subjects were divided into five feedback groups (speed, relative phase, grip force, video, and control). Time- and frequency-domain EMG measures were obtained before and after training. Surgical training decreased muscle work as found from mean EMG and EMG envelopes. Grip force feedback further reduced average and total muscle work, while speed feedback increased average muscle work and decreased total muscle work. Training also increased the median frequency response as a result of increased speed and/or reduced fatigue during each task. More diverse motor units were recruited as revealed by increases in the frequency bandwidth post-training. We demonstrated that clinical biomechanics using EMG analysis can help to better understand the effects of training for robotic surgery. Real-time augmented feedback during training can further reduce physiological demands. Future studies will investigate other means of feedback such as biofeedback of EMG during robotic surgery training.     

Hypotheses of peripheral and central mechanisms underlying occupational muscle pain and injury

In an overview of the problem of occupational muscle pain the evidence indicates that injury is more common the greater the load and the worse the posture in which the work is performed. The commonest are backstrains or ligament or joint damage due to overuse. Fatigue is associated with alterations in energy metabolites in muscle while pain is often due to microscopical damage to the cellular architecture. The progress of pathological changes in muscle following occupational injury may be similar to those seen in primary fibromyalgia (fibrositis) because of a final common pathway involving calcium-induced secondary damage. Occupational muscle pain frequently occurs in the muscles supporting the upper limb girdle and head in workers engaged in repetitively performing skilled manipulations or activities requiring high or sustained mental concentration. It is suggested that both occupational myalgia of this kind may be due to an imbalance in the use of muscles for postural activity (holding or supporting fine movements) compared to phasic use in dynamic work. While there are undoubtedly muscular indications of damage these may be secondary to alterations in (unconscious) central motor control mechanisms.     

A non-MVC EMG normalization technique for the trunk musculature: Part 2. Validation and use to predict spinal loads.

Estimates of the amount of force exerted by a muscle using electromyography (EMG) rely partially upon the accuracy of the reference point used in the normalization technique. Accurate representations of muscle activities are essential for use in EMG-driven spinal loading models. The expected maximum contraction (EMC) normalization method was evaluated to explore whether it could be used to assess individuals who are not capable of performing a maximum exertion such as a person with a low back injury. Hence, this study evaluated the utility of an EMG normalization method (Marras and Davis, A non-MVC EMG normalization technique, Part 1, method development. Journal of Electromyography and Kinesiology 2000) that draws upon sub-maximal exertions to determine the reference points needed for normalization of the muscle activities. The EMC normalization technique was compared to traditional MVC-based EMG normalization by evaluating the spinal loads for 20 subjects (10 males and 10 females) performing dynamic lifts. The spinal loads (estimated via an EMG-assisted model) for the two normalization techniques were very similar with differences being <8%. The model performance variables indicated that both normalization techniques performed well (r(2)>0.9 and average error below 6%) with only the muscle gain being affected by normalization method as a result in different reference points. Based on these results, the proposed normalization technique was considered to be a viable method for EMG normalization and for use in EMG-assisted models. This technique should permit the quantitative evaluation of muscle activity for subjects unable to produce maximum exertions.     

Testing of a biomechanical model of the lumbar muscle force distribution using quasi-static loading exercises.

The study of lumbar muscle force distribution in response to externally applied loads is based on the introduction of biomechanical models of the lumbar region. The evaluation of such models requires the execution of loading exercises while monitoring the EMG activity of certain lumbar muscles. This work uses muscle activity maps as the major design tool of such exercises, provided that the subject is constrained to an upright erect posture. The maps describe the predicted muscle force for a given combination of externally applied bending moments. A series of shoulder adduction exercises were designed and the EMG signals of eight lumbar muscles were measured while subjects performed the exercises. The results show good agreement between the model predictions and the EMG measurements, especially when the load and the muscle were contralateral to one another.     

The elixir of muscle activity and kinesiology in a health perspective: Evidence of worksite tailored exercise training alleviating muscle disorders

Physical activity is known to benefit health while muscle activation and movements performed during occupational work in contrast may result in work-related musculoskeletal disorders. Therefore, we posed the research question: which mode of muscle activation may result in a reversal of work-related disorders? To address this, we performed electromyographic (EMG) and kinematic assessments of workers with diverse exposure categories: sedentary monotonous work, prolonged walking/standing, and physically heavy work. The various job-specific exposure variables could be categorized in terms of duration, intensity, repetition, static component, peak force etc. that were subsequently identified as risk factors. Based on sports science principles we developed tailored exercise programs to counteract job exposure. EMG activity during exercise training was monitored to identify principal differences between exercise training and job patterns. Evidence from more than 20 RCT studies including >4000 workers showed positive effects such as decreased muscle pain and increased workability. Finally, we identified plausible underlying mechanisms in muscle tissue – human and animal - that confirmed metabolic, morphological, and hormonal changes with e.g. repetitive work that were reversal to adaptations reported with exercise training. Progress has been made in developing intelligent physical exercise training, IPET, as the best complementary activity to job exposure and includes muscle activations and movements that limit work-related inactivity atrophy as well as overload injury.     

Influence of memory demand and contra lateral activity on muscle activity.

Computer mouse work often includes memory demands and contra lateral activity. This study simulated video display unit (VDU) mouse-work and the focus was on forearm muscle activity as a result of standardised postural loads, memory demands and contra lateral activity. Surface and intramuscular electromyography (EMG) were recorded from the right forearm muscles during finger elevation and rest with and without memory demands and with and without contra lateral activity i.e. activity of the left hand. In most situations, memory demand increased activity in the m. extensor carpi radialis brevis and m. flexor digitalis superficialis. Also contra lateral activity increased activity in situations with and without memory demands. While surface EMG level of the m. extensor digitorum communis did not increase during memory demands, intramuscular EMG level increased when memory demands and contra lateral activity was combined. Influence of memory demands and contra lateral activity were most pronounced, in situations where activity levels were small.We presume that it is not only prolonged time of active computer mouse use that is a risk for development of musculoskeletal disorders, but also the time when people interact with the computer mentally or with the 'non-mouse hand', while resting their 'mouse-hand' on the mouse.     

表面肌电信号的肌肉疲劳判别研究进展

肌肉在周期的收缩或静态的拉伸过程中,会渐渐进入疲劳状态,肌肉疲劳特性的研究在康复医学、运动医学领域具有重要作用。表面肌电信号是从肌肉表面通过电极记录下来的反映神经肌肉系统活动的一维时间序列非平稳生物电信号,是评价局部肌肉疲劳的有效工具。本研究从时域和频域、时频域线性方法下的测量指标和非线性方法下的指标来综述表面肌电信号的疲劳研究进展,同时比较各种方法的优缺点,并对使用表面肌电信号来判别疲劳研究做了进一步的展望。     

Changes in muscle activity in response to different impact forces affect soft tissue compartment mechanical properties

Electromyographic (EMG) activity is associated with several tasks prior to landing in walking and running including positioning the leg, developing joint stiffness and possibly control of soft tissue compartment vibrations. The concept of muscle tuning suggests one reason for changes in muscle activity pattern in response to small changes in impact conditions, if the frequency content of the impact is close to the natural frequency of the soft tissue compartments, is to minimize the magnitude of soft tissue compartment vibrations. The mechanical properties of the soft tissue compartments depend in part on muscle activations and thus it was hypothesized that changes in the muscle activation pattern associated with different impact conditions would result in a change in the acceleration transmissibility to the soft tissue compartments. A pendulum apparatus was used to systematically administer impacts to the heel of shod male participants. Wall reaction forces, EMG of selected leg muscles, soft tissue compartment and shoe heel cup accelerations were quantified for two different impact conditions. The transmissibility of the impact acceleration to the soft tissue compartments was determined for each subject/soft tissue compartment/shoe combination. For this controlled impact situation it was shown that changes in the damping properties of the soft tissue compartments were related to changes in the EMG intensity and/or mean frequency of related muscles in response to a change in the impact interface conditions. These results provide support for the muscle tuning idea--that one reason for the changes in muscle activity in response to small changes in the impact conditions may be to minimize vibrations of the soft tissue compartments that are initiated at heel-strike.     

Shoulder muscle load and muscle fatigue among industrial sewing-machine operators

Physiological responses to physical work were assessed for 29 female industrial sewing-machine operators during an 8-h working day under ordinary working conditions. During sewing-machine work, the average (left and right) static load in the trapezius muscle was 9% of the maximal electromyogram (EMG) amplitude (% EMG_max), while the average mean load was 15% EMG_max, and the average peak load was 23% EMG_max. The static load level was unrelated to the muscle strength of the sewing-machine operators, which for the group as a whole was within the normal range. The load levels remained unchanged during the working day, while changes in the EMG mean power frequency and zero crossing frequency rate occurred, both indicating the development of muscle fatigue in left and right trapezius muscle during the working day. In line with this, the rating of perceived exertion in the shoulder and neck region increased during. the working day. Dividing the group of sewing-machine operators into two groups, those with the highest frequency and those with the lowest frequency of shoulder/neck troubles showed that the former group had significantly lower muscle strength, despite the fact that no differences in the surface EMG during sewing were found between the two groups. It was concluded that industrial sewing-machine work involves a pattern of shoulder muscle activity which induces fatiguing processes in the shoulder and neck regions. Furthermore, since the static shoulder muscle load was independent of muscle strength, factors other than working posture may be of significance for the static shoulder muscle load.     

Neuromuscular fatigue during dynamic maximal strength and hypertrophic resistance loadings

The purpose of this study was to compare the acute neuromuscular fatigue during dynamic maximal strength and hypertrophic loadings, known to cause different adaptations underlying strength gain during training. Thirteen healthy, untrained males performed two leg press loadings, one week apart, consisting of 15 sets of 1 repetition maximum (MAX) and 5 sets of 10 repetition maximums (HYP). Concentric load and muscle activity, electromyography (EMG) amplitude and median frequency, was assessed throughout each set. Additionally, maximal bilateral isometric force and muscle activity was assessed pre-, mid-, and up to 30 min post-loading. Concentric load during MAX was decreased after set 10 (P<0.05), while the load was maintained throughout HYP. Both loadings caused large reductions in maximal isometric force (MAX=-30±6.4% vs. HYP=-48±9.7%, P<0.001). The decreased concentric and isometric strength during MAX loading was accompanied by reduced EMG amplitude (P<0.05). Conversely, hypertrophic loading caused decreased median frequency only during isometric contractions (P<0.01). During concentric contractions, EMG amplitude increased and median frequency decreased in HYP (P<0.01). Our results indicate reduced neural drive during MAX loading and more complex changes in muscle activity during HYP loading.     

Normalizing upper trapezius EMG amplitude: Comparison of different procedures

Different procedures have been used for normalization of upper trapezius electromyographic (EMG) amplitudes. This complicates comparisons between studies. The present study aimed at investigating the influence of some commonly used trapezius EMG normalization procedures on the results of ergonomic analyses, as well as the test-retest repeatability of these procedures. EMG activity from the upper trapezius was recorded during an occupational task. The EMG activity was then normalized by seven different normalization procedures. It was shown that at the group level, a unilateral shoulder elevation maximal voluntary electrical (MVE) activation procedure gave 1.2 times higher occupational load estimates than a corresponding bilateral MVE. At the group level, the median load during the occupational task was 1.6 times higher when expressed as %MVC (maximal voluntary contraction) obtained from a power regression of relative force on EMG amplitude than when expressed as %MVE determined from a single maximal shoulder elevation. Normalizations in terms of a submaximal reference voluntary electrical (RVE) activation had similar test-retest repeatability in terms of the coefficient of variation (CV: 11–13%) as normalizations in terms of an MVE (CV: 11–15%), but the power regression procedures had considerably larger CVs (21–36%). The paper provides a basis for comparing previous studies using different normalization methods, as well as a qualitative evaluation of normalization methods for future use.     

Backpack load affects lower limb muscle activity patterns of female hikers during prolonged load carriage

This study investigated the effect of prolonged load carriage on lower limb muscle activity displayed by female recreational hikers. Electromyography (EMG) signals from vastus lateralis (VL), biceps femoris (BF), semitendinosus (ST), tibialis anterior (TA) and gastrocnemius (GM) were recorded for fifteen female hikers carrying four loads (0%, 20%, 30% and 40% body weight (BW)) over 8 km. Muscle burst duration, muscle burst onset relative to initial contact and integrated EMG signals (iEMG) were calculated to evaluate muscle activity, whereas the shift in mean power frequency (MPF) was used to evaluate muscle fatigue. Increased walking distance significantly decreased the MPF of TA; decreased the iEMG for VL, ST and GM; and shortened VL muscle burst duration. Furthermore, carrying 20–40% BW loads significantly increased VL and GM iEMG and increased BF muscle burst duration, whereas a 40% BW load caused a later VL muscle burst onset. The differences observed in muscle activity with increased load mass seem to be adjustments aimed at maintaining balance and attenuating the increased loads placed on the lower limbs during gait. Based on the changes in muscle activity, a backpack load limit of 30% BW may reduce the risk of lower limb injury for female hikers during prolonged walking.     

A method to quantify frequency and duration of sustained low-level muscle activity as a risk factor for musculoskeletal discomfort

AimThe purpose of this paper was to describe and evaluate different aspects of muscle activity patterns associated with musculoskeletal discomfort/pain.MethodSurface electromyography (sEMG) of the right upper trapezius and the right extensor digitorum muscles was conducted continuously during one working day in 19 male forest machine operators driving harvesters, 20 driving forwarders and 20 researchers at the Forest Research Institute.Perceived discomfort/pain in the right side of the neck and the right forearm was rated morning, noon and afternoon with Borg’s CR-10 scale. Static, median and peak levels of muscle activity were analyzed and the number and total duration of EMG gaps (muscular rest) were calculated. Sustained low-level muscle activity (SULMA) was defined as continuous muscle activity above 0.5% of the maximal EMG activity quantified into 10 periods of predetermined duration intervals from 1.6 to 5 s up to above 20 min. The number of SULMA periods is presented within each interval and as cumulative periods above the already determined levels. The operators handled control levers seated in a fixed position while the researchers performed mainly PC work and other varied tasks.ResultsA positive correlation was found between discomfort/pain in the right upper trapezius muscle region in the afternoon and cumulative SULMA periods above 10 min duration, and a negative correlation to cumulative SULMA periods also including the short durations. No specified patterns were found for discomfort/pain in the right extensor digitorum or for the other EMG measurements. All EMG measurements distinguished to some extent between the occupational groups, especially between machine operators driving harvesters and researchers.ConclusionsNumber of SULMA periods longer than 10 min per hour was positively correlated, and predominantly short periods were negatively correlated, to complaints in the neck region. This seems promising in order to find duration limits for sustained low-level muscle activity as a risk factor for musculoskeletal disorders.     

EMG and MMG activities of agonist and antagonist muscles in Parkinson’s disease patients during absolute submaximal load holding

The purpose of the study was (1) to assess changes in electromyographical (EMG) and mechanomyographical (MMG) signals of the biceps and triceps brachii muscles during absolute submaximal load holding in Parkinson’s disease patients tested during their medication “ON-phase” and in age-matched controls, and (2) to check whether mechanomyography can be useful in evaluation of neuromuscular system activity in Parkinson’s disease patients.The data analysis was performed on nine females with Parkinson’s disease and six healthy, age-matched females. The EMG and MMG signals were recorded from the short head of the biceps brachii (BB) and the lateral head of the triceps brachii (TB) muscles.It was concluded that compared to the controls, the Parkinson’s disease patients exhibited higher amplitude in the biceps brachii muscle and lower median frequency of the MMG signal in the both tested muscles. However, no differences in the EMG amplitude and an increase of the EMG median frequency in the triceps brachii muscle of the Parkinson’s disease group were observed. The MMG was not affected by physiological postural tremor and can depict differences between parkinsonians and controls, which may suggest that it is valuable tool for neuromuscular assessment for this condition.