Electromyographic analysis of the serratus anterior and trapezius muscles during push-ups on stable and unstable bases in subjects with scapular dyskinesis

The present study was performed to assess the electromyographic activity of the scapular muscles during push-ups on a stable and unstable surface, in subjects with scapular dyskinesis. Muscle activation (upper trapezius [UT]; lower trapezius [LT]; upper serratus anterior [SA_5th]; lower serratus anterior [SA_7th]) and ratios (UT/LT; UT/SA_5th; UT/ SA_7th) levels were determined by surface EMG in 30 asymptomatic men with scapular dyskinesis, during push-up performed on a stable and unstable surface. Multivariate analysis of variance with repeated measures was used for statistical analyses. The unstable surface caused a decrease in the EMG activity of the serratus anterior and an increase in EMG activity of the trapezius (p = 0.001). UT/SA_5th and UT/ SA_7th ratios were higher during unstable push-ups (p = 0.001). The results suggest that, in individuals with scapular dyskinesis, there is increased EMG activity of the trapezius and decreased EMG activity of the serratus anterior in response to an unstable surface. These results suggest that the performance of the push up exercise on an unstable surface may be more favorable to produce higher levels of trapezius activation and lower levels of serratus anterior activation. However, if the goal of the exercise program is the strengthening of the SA muscle, it is suggested to perform the push up on a stable surface.     

Rotator cuff muscle function and its relation to scapular morphology in apes

It is widely held that many differences among primate species in scapular morphology can be functionally related to differing demands on the shoulder associated with particular locomotor habits. This perspective is largely based on broad scale studies, while more narrow comparisons of scapular form often fail to follow predictions based on inferred differences in shoulder function. For example, the ratio of supraspinous fossa/infraspinous fossa size in apes is commonly viewed as an indicator of the importance of overhead use of the forelimb, yet paradoxically, the African apes, the most terrestrial of the great apes, have higher scapular fossa ratios than the more suspensory orangutan. The recent discovery of several nearly complete early hominin scapular specimens, and their apparent morphological affinities to scapulae of orangutans and gorillas rather than chimpanzees, has led to renewed interest in the comparative analysis of human and extant ape scapular form. To facilitate the functional interpretation of differences in ape scapulae, particularly in regard to relative scapular fossa size, we used electromyography (EMG) to document the activity patterns in all four rotator cuff muscles in orangutans and gibbons, comparing the results with previously published data for chimpanzees.     

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.     

Assessing the validity of surface electromyography for recording muscle activation patterns from serratus anterior

PurposeNo direct evidence exists to support the validity of using surface electrodes to record muscle activity from serratus anterior, an important and commonly investigated shoulder muscle. The aims of this study were to determine the validity of examining muscle activation patterns in serratus anterior using surface electromyography and to determine whether intramuscular electromyography is representative of serratus anterior muscle activity.MethodsSeven asymptomatic subjects performed dynamic and isometric shoulder flexion, extension, abduction, adduction and dynamic bench press plus tests. Surface electrodes were placed over serratus anterior and around intramuscular electrodes in serratus anterior. Load was ramped during isometric tests from 0% to 100% maximum load and dynamic tests were performed at 70% maximum load. EMG signals were normalised using five standard maximum voluntary contraction tests.ResultsSurface electrodes significantly underestimated serratus anterior muscle activity compared with the intramuscular electrodes during dynamic flexion, dynamic abduction, isometric flexion, isometric abduction and bench press plus tests. All other test conditions showed no significant differences including the flexion normalisation test where maximum activation was recorded from both electrode types. Low correlation between signals was recorded using surface and intramuscular electrodes during concentric phases of dynamic abduction and flexion.ConclusionsIt is not valid to use surface electromyography to assess muscle activation levels in serratus anterior during isometric exercises where the electrodes are not placed at the angle of testing and dynamic exercises. Intramuscular electrodes are as representative of the serratus anterior muscle activity as surface electrodes.     

Normalization procedures using maximum voluntary isometric contractions for the serratus anterior and trapezius muscles during surface EMG analysis.

The serratus anterior and trapezius muscles are considered to be the only upward rotators of the scapula and are very important for normal shoulder function. A variety of methods have been used to produce a maximum voluntary isometric contraction (MVIC) of these muscles for normalization of EMG data. The purpose of this study was to quantify the surface EMG activity of the serratus anterior muscle and the upper, middle, and lower parts of the trapezius during 9 manual muscle tests performed with maximum effort in 30 subjects. It was found that no one muscle test produced a MVIC for all individuals. Therefore, to perform normalization within each subject, it is suggested that the 2 or 3 tests identified in this study that produce high levels of EMG activity for each muscle be performed. The scapular protraction muscle test that is often used to normalize data for the serratus anterior muscle produced relatively low levels of EMG activity and was not found to be an optimal test. Muscle tests in which an attempt was made to de-rotate the scapula from an upwardly rotated position produced much higher levels of EMG activity in the serratus anterior muscle.     

Variability of the EMG mean power frequency: A study on the trapezius muscle

Calculation of the EMG mean power frequency (MPF) is a common procedure applied in evaluation of the frequency shift associated with local muscle fatigue. Variations of the MPF that are unrelated to muscle fatigue may jeopardize the estimation of the frequency shift. Different kinds of variation include random variation and systematic variation due to changes in posture or load. In a previous article we have evaluated the systematic linear variation of the MPF. The aim of the present study was to examine the random variation. Data sequences of 10 s, each obtained from nonfatigued trapezius muscle of 19 healthy subjects, were examined over a functional range of load and joint angles with multiple regression analysis. The random variation was evaluated with residual analysis. The residual standard deviation within the whole group was 10% for surface recordings and 13% for intramuscular recordings. If only within-subject variation was considered, the corresponding values were 5 and 8%. Based on this, confidence and prediction intervals for the regression models were calculated. Ninety-five percent confidence intervals were ±1–3% around the regression surfaces, whereas 95% prediction intervals for single measurements were as large as ±20–26% for the whole group, and ±11–20% if only within-subject variations were considered. Assessment of localized muscle fatigue using single MPF estimates should therefore be avoided. Multiple measurements and regression analysis are discussed as methods to minimize the effects of random variations.     

Red and white muscle activity and kinematics of the escape response of the bluegill sunfish during swimming

Summary We quantified midline kinematics with synchronized electromyograms (emgs) from the red and white muscles on both sides of bluegill sunfish (Lepomis macrochirus) during escape behaviors which were elicited from fish both at a standstill and during steady speed swimming. Analyses of variance determined whether or not kinematic and emg variables differed significantly between muscle fiber types, among longitudinal positions, and between swimming versus standstill trials.At a given longitudinal location, both the red and white muscle were usually activated synchronously during both stages of the escape behavior. Stage 1 emg onsets were synchronous; however, the mean durations of stage 1 emgs showed a significant increase posteriorly from about 11 to 15 ms. Stage 2 emgs had significant posterior propagation, but the duration of the stage 2 emgs was constant (17 ms). Posterior emgs from both stages occurred during lengthening of the contractile tissue (as indicated by lateral bending). Steady swimming activity was confined to red muscle bursts which were propagated posteriorly and had significant posterior decrease in duration from about 50% to 37% of a cycle. Fish performed escape responses during all phases of the steady swimming motor pattern. All kinematic events were propagated posteriorly. Furthermore, no distinct kinematic event corresponded to the time intervals of the stage 1 and 2 emgs. The rate of propagation of kinematic events was always slower than that of the muscle activity. The phase relationship between lateral displacement and lateral bending also changed along the length of the fish. Escape responses performed during swimming averaged smaller amplitudes of stage 2 posterior lateral displacement; however, most other kinematic and emg variables did not vary significantly between these two treatments.Abbreviations A angle of lateral flexion (bending) of midline at a single point in time - A1, A2 change in A from T₀ to T₁ and from T₁ to T₂ - AMX maximal lateral flexion concave towards the side of the stage 1 emg - AMXR equals AMX minus A at T₀ - AT1, AT2 lateral flexion at T₁ and T₂ - DUR1, DUR2 durations of stage 1 and stage 2 emgs - emg electromyogram - ON2 onset time of stage 2 emg - RELDUR relative duration of steady swimming emg - T₀, T₁, T₂ times of stage 1 emg onset, latest stage 1 emg offset and latest stage 2 emg offset standardized such that T₀ = 0 - TAMX, TAMN, TYMX times of maximal lateral flexion, no lateral flexion and maximum lateral displacement - Y1, Y2 amounts of lateral displacement from T₀ to T₁ and from T₁ to T₂ - YMXR relative amount of lateral displacement from T₀ to TYMX     

How swimming fish use slow and fast muscle fibers: implications for models of vertebrate muscle recruitment

We quantified the intensity and duration of electromyograms (emgs) from the red and white axial muscles in five bluegill sunfish (Lepomis macrochirus) which performed three categories of behavior including steady swimming and burst and glide swimming at moderate and rapid speeds. Steady swimming (at 2 lengths/s) involved exclusively red muscle activity (mean posterior emg duration = 95 ms), whereas unsteady swimming utilized red and white fibers with two features of fiber type recruitment previously undescribed for any ectothermic vertebrate locomotor muscle. First, for moderate speed swimming, the timing of red and white activity differed significantly with the average onset time of white lagging behind that of red by approximately 40 ms. The durations of these white emgs were shorter than those of the red emgs (posterior mean = 82 ms) because offset times were effectively synchronous. Second, compared to steady and moderate speed unsteady swimming, the intensity of red activity during rapid unsteady swimming decreased while the intensity of white muscle activity (mean white emg duration = 33 ms) increased. Decreased red activity associated with increased white activity differs from the general pattern of vertebrate muscle recruitment in which faster fiber types are recruited in addition to, but not to the exclusion of, slower fiber types.     

Normalizing upper trapezius EMG amplitude: Comparison of ramp and constant force procedures

The present study compared three procedures for normalization of upper trapezius surface electromyographic (EMG) amplitudes: (a) a ramp procedure (providing data in per cent of maximal voluntary contraction, MVC); (b) a constant force procedure based on two reference contractions (two-force procedure) (%MVC) and (c) a procedure expressing muscle activation in per cent of a reference voluntary electrical activity (%RVE). The study also evaluated the repeatability of the ramp and the RVE procedures and estimated the force exertion (%MVC) corresponding to the RVE. To illustrate the ergonomic effect of different normalization procedures, trapezius EMG during two work tasks was compared after normalization by the two-force and the RVE procedures. Fifteen subjects participated in the whole study. We found that force estimates obtained by the ramp procedure equation could be translated to force estimates obtained by the two-force procedure by the equation: %MVC_2force = − 0.6 + 0.9*%MVC_ramp, although with a considerable imprecision due to large inter-individual differences. In the ramp procedure, the intra-individual test-retest coefficient of variation (CV) depended on the force level; it was 45% at 5% MVC and 10% at 30% MVC. The CV of the RVE was 15%. The reference contraction used in the RVE procedure corresponded from 13–79% MVC (median 33%MVC). The load reducing effect of an ergonomic intervention was less obvious with the RVE procedure than with the two-force procedure due to a larger inter-individual variation. The advantages and disadvantages of the different procedures are discussed.     

Predicting muscle forces in gait from EMG signals and musculotendon kinematics

An EMG-driven muscle model for determining muscle force-time histories during gait is presented. The model, based on Hill's equation (1938), incorporates morphological data and accounts for changes in musculotendon length, velocity, and the level of muscle excitation for both concentric and eccentric contractions. Musculotendon kinematics were calculated using three-dimensional cinematography with a model of the musculoskeletal system. Muscle force-length-EMG relations were established from slow isokinetic calibrations. Walking muscle force-time histories were determined for two subjects. Joint moments calculated from the predicted muscle forces were compared with moments calculated using a linked segment, inverse dynamics approach. Moment curve correlations ranged from r = 0.72 to R = 0.97 and the root mean square (RMS) differences were from 10 to 20 Nm. Expressed as a relative RMS, the moment differences ranged from a low of 23% at the ankle to a high of 72% at the hip. No single reason for the differences between the two moment curves could be identified. Possible explanations discussed include the linear EMG-to-force assumption and how well the EMG-to-force calibration represented excitation for the whole muscle during gait, assumptions incorporated in the muscle modeling procedure, and errors inherent in validating joint moments predicted from the model to moments calculated using linked segment, inverse dynamics. The closeness with which the joint moment curves matched in the present study supports using the modeling approach proposed to determine muscle forces in gait.     

Dynamic muscle force predictions from EMG: an artificial neural network approach

EMG signals of dynamically contracting muscle have never been used to predict experimentally known muscle forces across subjects. Here, we use an artificial neural network (ANN) approach to first derive an EMG–force relationship from a subset of experimentally determined EMGs and muscle forces; second, we use this relationship to predict individual muscle forces for different contractile conditions and in subjects whose EMG and force data were not used in the derivation of the EMG–force relationship; and third, we validate the predicted muscle forces against the known forces recorded in vivo. EMG and muscle forces were recorded from the cat soleus for a variety of locomotor conditions giving a data base from three subjects, four locomotor conditions, and 8–16 steps per subject and condition. Considering the conceptual differences in the tasks investigated (e.g. slow walking vs. trotting), the intra-subject results obtained here are superior to those published previously, even though the approach did not require a muscle model or the instantaneous contractile conditions as input for the force predictions. The inter-subject results are the first of this kind to be presented in the literature and they typically gave cross-correlation coefficients between actual and predicted forces of >0.90 and root mean square errors of <15%, thus they were considered excellent.

From the results of this study, it was concluded that ANNs represent a powerful tool to capture the essential features of EMG–force relationships of dynamically contracting muscle, and that ANNs might be used widely to predict muscle forces based on EMG signals.     

Effect of isometric horizontal abduction on pectoralis major and serratus anterior EMG activity during three exercises in subjects with scapular winging

The aim of this study was to determine the effect of isometric horizontal abduction using Thera-Band during three exercises (forward flexion, scaption, and wall push-up plus) in subjects with scapular winging by investigating the electromyographic (EMG) amplitude of the pectoralis major, serratus anterior and the pectoralis major/serratus anterior activity ratio. Twenty-four males with scapular winging participated in this study. The subjects performed the forward flexion, scaption, and wall push-up plus with and without isometric horizontal abduction using Thera-Band. Surface EMG was used to collect the EMG data of the pectoralis major and serratus anterior during the three exercises. Two-way repeated analyses of variance with two within-subject factors (isometric horizontal abduction condition and exercise type) were used to determine the statistical significance of pectoralis major and serratus anterior EMG activity and the pectoralis major/serratus anterior EMG activity ratio. Pectoralis major EMG activity was significantly lower during forward flexion and wall push-up plus with isometric horizontal abduction, and serratus anterior EMG activity was significantly greater with isometric horizontal abduction. Additionally, the pectoralis major/serratus anterior activity ratio was significantly lower during the forward flexion and wall push-up plus with isometric horizontal abduction. The results of this study suggest that isometric horizontal abduction using Thera-Band can be used as an effective method to facilitate the serratus anterior activity and to reduce excessive pectoralis major activity during exercises for activating serratus anterior.     

EMG spectral characteristics of masticatory muscles and upper trapezius during maximum voluntary teeth clenching

To assess the surface electromyographic spectral characteristics of masticatory and neck muscles during the performance of maximum voluntary clench (MVC) tasks, 29 healthy young adults (15 men, 14 women, mean age 22 years) were examined. Electromyography of masseter, temporalis and upper trapezius muscles was performed during 5-s MVCs either on cotton rolls or in intercuspal position. Using a fast Fourier transform, the median power frequency (MPF) was obtained for the first and last seconds of clench, and compared between sexes, muscles, sides, tests and time intervals using ANOVAs.On average, the MPFs did not differ between sexes or sides (p > 0.05), but significant effects of muscle (MPF temporalis larger than masseter, larger than trapezius muscles), test (larger MPFs when clenching in intercuspal position than when clenching on cotton rolls) and time (larger MPFs in the first than in the fifth second of clench) were found.In conclusion, a set of data to characterize the sEMG spectral characteristics of jaw and neck muscles in young adult subjects performing MVC tasks currently in use within the dental field was obtained. Reference values may assist in the assessment of patients with alterations in the cranio-cervical-mandibular system.     

Reliability of surface EMG matrix in locating the innervation zone of upper trapezius muscle

The identification of the motor unit (MU) innervation zone (IZ) using surface electromyographic (sEMG) signals detected on the skin with a linear array or a matrix of electrodes has been recently proposed in the literature. However, an analysis of the reliability of this procedure and, therefore, of the suitability of the sEMG signals for this purpose has not been reported.The purpose of this work is to describe the intra and inter-rater reliability and the suitability of surface EMG in locating the innervation zone of the upper trapezius muscle.Two operators were trained on electrode matrix positioning and sEMG signal analysis. Ten healthy subjects, instructed to perform a series of isometric contractions of the upper trapezius muscle participated in the study. The two operators collected sEMG signals and then independently estimated the IZ location through visual analysis.Results showed an almost perfect agreement for intra-rater and inter-rater reliability. The constancy of IZ location could be affected by the factors reflecting the population of active MUs and their IZs, including: the contraction intensity, the acquisition period analyzed, the contraction repetition. In almost all cases the IZ location shift due to these factors did not exceed 4 mm. Results generalization to other muscles should be made with caution.     

An electromyographic study of serratus anterior in atelines and Alouatta: Implications for hominoid evolution

The serratus anterior pars caudalis muscle of nonhuman primates displays anatomical differences among genera that can be attributed to differences in the mechanical demands placed on these genera by their diverse locomotor: behaviors. In primates that engage extensively in climbing and suspensory behaviors, the caudal digitations of this fan-shaped muscle are aligned more nearly parallel to the long axis of the trunk. In order to clarify the selective factors promoting such a morphological change, we have conducted a telemetered electromyographic study of the caudal and middle digitations of the serratus anterior pars caudalis. During voluntary elevation of the forelimb, only the middle, more obliquely disposed digitations are powerfully recruited. The caudal digitations are either inactive or function just to initiate scapular rotation. During locomotion, the middle digitations act in the swing (recovery) phase, whereas the caudal digitations are predominatly active in the support (propulsive)These Pashe findings suggest that the caudal digitations are important in propelling the trunk past the scapula during locomotion. Evolution of a fiber orientation more parallel to the long axis of the trunk is suggested to have occurred in broad chested primates for the purpose of facilitating locomotor behaviors requiring caudal scapular retraction for propulsion, but which would be deleteriously affected if such retraction were linked to simultaneous ventral displacement of the shoulder girdle. In its current state, the human serratus anterior seems clearly adapted for arm-raising functions and indicates descent from a small ape with a thoracic shape similar to atelines.     

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

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 (microV) 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.     

Influence of locomotion speed on biomechanical subtask and muscle synergy

This paper investigates the relationship of biomechanical subtasks, and muscle synergies with various locomotion speeds. Ground reaction force (GRF) of eight healthy subjects is measured synchronously by force plates of treadmill at five different speeds ranging from 0.5 m/s to 1.5 m/s. Four basic biomechanical subtasks, body support, propulsion, swing, and heel strike preparation, are identified according to GRF. Meanwhile, electromyography (EMG) data, used to extract muscle synergies, are collected from lower limb muscles. EMG signals are segmented periodically based on GRF with the heel strike as the split points. Variability accounted for (VAF) is applied to determine the number of muscle synergies. We find that four muscle synergies can be extracted in all five situations by non-negative matrix factorization (NMF). Furthermore, the four muscle synergies and biomechanical subtasks keep invariant as the walking speed changes.     

The recruitment order of scapular muscles depends on the characteristics of the postural task

Previous studies show that the scapular muscle recruitment order could possibly change according to the characteristics of the postural task. We aimed to compare the activation latencies of serratus anterior (SA), upper, middle, and lower trapezius (UT, MT and LT, respectively) between an unpredictable perturbation (sudden arm destabilization) and a predictable task (voluntary arm raise) and, to determine the differences in the muscle recruitment order in each task. The electromyographic signals of 23 participants were recorded while the tasks were performed. All scapular muscles showed earlier onset latency in the voluntary arm raise than in the sudden arm destabilization. No significant differences were observed in the muscle recruitment order for the sudden arm destabilization (p > 0.05). Conversely, for voluntary arm raise the MT, LT SA and anterior deltoid (AD) were activated significantly earlier than the UT (p < 0.001). Scapular muscles present a specific recruitment order during a predictable task: SA was activated prior to the AD and the UT after the AD, in a recruitment order of SA, AD, UT, MT, and LT. While in an unpredictable motor task, all muscles were activated after the destabilization without a specific recruitment order, but rather a simultaneous activation.     

The use of EMG biofeedback for learning of selective activation of intra-muscular parts within the serratus anterior muscle: A novel approach for rehabilitation of scapular muscle imbalance

Motor control and learning possibilities of scapular muscles are of clinical interest for restoring scapular muscle balance in patients with neck and shoulder disorders. The aim of the study was to investigate whether selective voluntary activation of intra-muscular parts within the serratus anterior can be learned with electromyographical (EMG) biofeedback, and whether the lower serratus anterior and the lower trapezius muscle comprise the lower scapula rotation force couple by synergistic activation. Nine healthy males practiced selective activation of intra-muscular parts within the serratus anterior with visual EMG biofeedback, while the activity of four parts of the serratus anterior and four parts of the trapezius muscle was recorded. One subject was able to selectively activate both the upper and the lower serratus anterior respectively. Moreover, three subjects managed to selectively activate the lower serratus anterior, and two subjects learned to selectively activate the upper serratus anterior. During selective activation of the lower serratus anterior, the activity of this muscle part was 14.4 ± 10.3 times higher than the upper serratus anterior activity (P < 0.05). The corresponding ratio for selective upper serratus vs. lower serratus anterior activity was 6.4 ± 1.7 (P < 0.05). Moreover, selective activation of the lower parts of the serratus anterior evoked 7.7 ± 8.5 times higher synergistic activity of the lower trapezius compared with the upper trapezius (P < 0.05). The learning of complete selective activation of both the lower and the upper serratus anterior of one subject, and selective activation of either the upper or lower serratus anterior by five subjects designates the promising clinical application of EMG biofeedback for restoring scapular muscle balance. The synergistic activation between the lower serratus anterior and the lower trapezius muscle was observed in only a few subjects, and future studies including more subjects are required before conclusions of a lower scapula rotation couple can be drawn.     

Normalization of surface EMG amplitude from the upper trapezius muscle in ergonomic studies — A review

Surface electromyographic (EMG) amplitude from the upper trapezius muscle is widely used as a measure of shoulder-neck load in ergonomic studies. A variety of methods for normalizing EMG amplitude from the upper trapezius (EMGamp_ut) have been presented in the literature. This impedes meta-analyses of, for instance, upper trapezius load in relation to development of shoulder-neck disorders. The review offers a thorough discussion of different normalization procedures for EMGamp_ut. The following main issues are focused: output variable, location of electrodes, posture and attempted movement during normalization, load and duration of reference contractions, signal processing and test-retest repeatability. It is concluded that translations of EMGamp_ut into biomechanical variables, for example relative force development in the shoulder or in the upper trapezius itself, suffer from low validity, especially if used in work tasks involving large and/ or fast arm movements. The review proposes a standard terminology relating to normalization of EMGamp_ut and concludes in a concrete suggestion for a normalization procedure generating bioelectrical variables which reflect upper trapezius activation.