Human Trapezius Research Articles

Human trapezius muscle development during the early fetal period.

This study aimed to observe human trapezius muscle (TpzM) development during early fetal period and apply diffusion tensor imaging (DTI) analysis to describe the muscle architecture that leads to physiological functions. Human embryonic and early fetal specimens were selected for this study. TpzM was first detected at Carnegie stage 20. The position of the TpzM changed with the formation of the scapula, clavicle, and vertebrae, which are its insertions and origins. DTI revealed the fiber orientation from each vertebral level to dissect each muscle. Fiber orientation in the ventral view gradually changed from the cervical to thoracic vertebrae, except for the middle part at which the insertions changed, which was almost similar in all early fetal specimens. The TpzM volume increased from C1 to C7 in the upper part, reached local maxima at C6 and C7 in the middle, and then decreased. These muscles can be categorized into three parts according to their insertions and presented with the features of each part. The fiber orientation and distribution of the three parts at the vertebral level were almost constant during the early fetal period. The border between the upper and middle parts was mainly located around the C6 and C7 vertebral levels, whereas the middle and lower parts were between the Th1 and Th2 vertebral levels. A three-dimensional change in the fiber orientation in the upper part of the TpzM according to the vertebral level was noticeable. Our data will help to elucidate the developmental processes of TpzM.

Three-dimensional muscle architecture of the infant and adult trapezius: a cadaveric pilot study

Objectives: The elaborate morphometry of the human trapezius muscle facilitates its involvement in numerous active movements of the shoulder girdle and passive stabilization of the upper extremity. Despite its functional importance throughout the lifespan, little is known about the 3D architecture of trapezius at any post-natal timepoints. Accordingly, the aim of this preliminary cadaveric study was to digitize, quantify, model, and compare the 3D architecture of trapezius at two temporal extremes: infancy and adulthood. Methods: We examined trapezius in two female formalin-embalmed cadavers, aged 6 months and 72 years, respectively. We meticulously dissected each muscle, allowing us to digitize and model the comprehensive muscle architecture in situ at the fiber bundle level. We quantified standard architectural parameters to facilitate comparison between each functional partition of trapezius (i.e., descending, transverse, ascending) and proportionally between the infant and adult specimens. Results: We found markedly different patterns in fiber bundle length range, physiological cross-sectional area, and muscle volume within and between muscles. Notably, the proportional physiological cross-sectional area of the ascending and descending partitions was equal (1:1) in the infant, in contrast to 3:1 in the adult. The transverse partitions were proportionally similar, accounting for over half of the whole muscle physiological cross-sectional area in both specimens. Conclusion: This study provides preliminary insights into infant and adult trapezius architecture at an unparalleled level of detail and precision. The quantifiable architectural differences appear to coincide with functional development-a notion that warrants further investigation in larger samples and with longitudinal approaches.

Fetal development of the human trapezius and sternocleidomastoid muscles

At present, there is no photographic evidence of splitting of the trapezius and sternocleidomastoid muscles (SCMs), which share a common anlage that extends caudally toward the limb bud in the embryo at a length of 9 mm. Therefore, the aim of the present study was to identify which structures divide the caudal end of the common anlage at the first sign of splitting into two muscles. In 11 mm-long specimens, the SCM and trapezius muscles were identified as a single mesenchymal condensation. In 15 and 18 mm-long specimens, the SCM and trapezius muscles were separated and extended posteriorly and lymphatic tissues appeared in a primitive lateral cervical space surrounded by the SCM (anterior). In 21 mm-long specimens, the lymphatic vessels were dilated and the accompanying afferents were forming connections with the subcutaneous tissue through a space between the SCM and trapezius muscles. In 27 mm-long specimens, cutaneous lymphatic vessels were evident and had entered the deep tissue between the SCM and trapezius muscles. Vascular dilation may be viewed as a result of less mechanical stress or pressure after muscle splitting.

Comprehensive 3D Architecture of the Adult Human Trapezius: A Cadaveric Study

Musculoaponeurotic architecture describes the 3D arrangement of the contractile and connective tissue elements within a muscle, or functional partitions thereof. Without a comprehensive appreciation of these features, the certainty with which clinicians may interpret musculoskeletal imaging, assess pathology, and evaluate recovery associated with these muscles is inherently restricted. Current musculoaponeurotic literature is limited by insufficient fibre bundle (FB) sampling, lack of data regarding connective tissue elements, and 2D measurement approaches for 3D parameters. Morphometrically‐complex muscles, like trapezius, are particularly susceptible to overgeneralization using these approaches. A thorough understanding of the contractile and connective tissues of trapezius may help elucidate the etiology and pathophysiology of musculoskeletal disorders associated with this muscle. Accordingly, the primary objective of this study was to capture, quantify, and model the comprehensive 3D musculoaponeurotic architecture of the adult human trapezius muscle from cadaveric data.Ten trapezius muscles from five lightly embalmed cadavers (3F, 2M; ages 64–85 years) were meticulously dissected for this study. Contractile tissue elements were serially digitized in situ at the FB level with a MicroScribe® G digitizer and modelled in Autodesk® Maya®. The surfaces of connective tissue elements (i.e. aponeuroses) were digitized in a grid pattern and integrated into 3D musculoaponeurotic models. Architectural parameters, including FB length, pennation angle, and physiological cross‐sectional area (PCSA), were quantified for the whole muscle and each functional partition (ascending, transverse, and descending).Each trapezius muscle included a minimum of 1000 FBs. Preliminary data analyses reveal consistent patterns of relative mean FB length (ascending = descending > transverse), range of FB lengths (ascending > descending > transverse), and PCSA (transverse > ascending > descending). FBs throughout the muscle volume span between independent medial and lateral aponeuroses, resulting in extensive regions of musculoaponeurotic junction. Aponeuroses are notably substantial in the medial transverse and lateral ascending partitions, while the clavicular attachment of the descending partition had minimal connective tissue present.This study presents a comprehensive 3D model of the adult human trapezius that provides a foundation for improved longitudinal assessment of pathology and continuing clinical education. Future studies will use these data to guide in vivo imaging and functional electromyographic studies investigating musculoskeletal disorders associated with trapezius, such as myofascial pain syndrome.Support or Funding InformationSponsored by a Vanier Canada Graduate Scholarship from CIHR and an Educational Advancement Stipend from the University Health Network.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Investigation of biomarkers alterations after an acute tissue trauma in human trapezius muscle, using microdialysis

Alterations in muscle milieu are suggested as important activity of peripheral drive in patients with chronic musculoskeletal pain (CMP). Microdialysis (MD) has been used in monitoring altered metabolic response pattern in muscles. However, the insertion of MD probe causes a local tissue trauma. Whether and how metabolites in trapezius muscle are affected by acute tissue trauma is unknown. Hence, this study investigated the metabolic response and nociceptive reaction of the tissue following MD probe insertion in patients with CMP and healthy individuals. Fifty-nine patients and forty pain-free volunteers were recruited. Pressure pain thresholds (PPTs) were obtained at the trapezius and tibialis muscles. Pain questionnaires determined the levels of pain related aspects. MD (20 kDa cut-off) was performed in the trapezius and samples were collected within 40 min. Interstitial concentration of the metabolites was analyzed by a two-way-mixed-ANOVA. The metabolic response pattern changed over time and alterations in the level of metabolites could be seen in both CMP and healthy controls. Pain questionnaires and pain intensities manifested clinical aspects of pain closely to what CMP patients describe. Analyzing metabolites due to acute tissue trauma by aid of MD may be a useful model to investigate altered metabolic response effect in CMP.

Crossed responses found in human trapezius muscles are not H‐reflexes

We sought to confirm the presence of crossed short-latency reflexes in trapezius. Mmax and Hmax were measured in the ipsilateral trapezius in 10 subjects by percutaneous electrical stimulation of the accessory nerve and the cervical nerves of C3/4 respectively. Repeated stimulation of the C3/4 cervical nerves was performed during 3 different tasks (relaxation, contraction of ipsilateral side, contraction of contralateral side). Ipsilaterally, responses increased significantly with an increase in the prestimulus electromyogram (EMG) and decreased significantly with a decrease in prestimulus EMG. Contralateral potentials increased significantly with contraction of the trapezius ipsilateral to the stimulus compared with contraction of the muscle in which they were recorded and decreased significantly with increasing distance from the ipsilateral side. We found ipsilateral and contralateral responses consistent with previous findings. However, we conclude that the contralateral response in trapezius is not a crossed reflex.

Comparative metabolomics of muscle interstitium fluid in human trapezius myalgia: an in vivo microdialysis study

PurposeThe mechanisms behind trapezius myalgia are unclear. Many hypotheses have been presented suggesting an altered metabolism in the muscle. Here, muscle microdialysate from healthy and myalgic muscle is analysed using metabolomics. Metabolomics analyse a vast number of metabolites, enabling a comprehensive explorative screening of the cellular processes in the muscle.MethodsMicrodialysate samples were obtained from the shoulder muscle of healthy and myalgic subjects that performed a work and stress test. Samples from the baseline period and from the recovery period were analysed using gas chromatography—mass spectrometry (GC–MS) together with multivariate analysis to detect differences in extracellular content of metabolites between groups. Systematic differences in metabolites between groups were identified using multivariate analysis and orthogonal partial least square discriminate analysis (OPLS-DA). A complementary Mann–Whitney U test of group difference in individual metabolites was also performed.ResultsA large number of metabolites were detected and identified in this screening study. At baseline, no systematic differences between groups were observed according to the OPLS-DA. However, two metabolites, l-leucine and pyroglutamic acid, were significantly more abundant in the myalgic muscle compared to the healthy muscle. In the recovery period, systematic difference in metabolites between the groups was observed according to the OPLS-DA. The groups differed in amino acids, fatty acids and carbohydrates. Myristic acid and putrescine were significantly more abundant and beta-d-glucopyranose was significantly less abundant in the myalgic muscle.ConclusionThis study provides important information regarding the metabolite content, thereby presenting new clues regarding the pathophysiology of the myalgic muscle.

Androgens Affect Myogenesis In Vitro and Increase Local IGF-1 Expression

The mechanism whereby anabolic androgens are associated with hypertrophy of skeletal muscle is incompletely understood but may involve an interaction with locally generated insulin-like growth factor (IGF) 1. The present investigation utilized a cell culture model of human skeletal muscle-derived cell maturation to test the hypothesis that androgens increase differentiation of human muscle precursor cells in vitro and to assess effects of androgen with or without IGF-1 on IGF-1 messenger RNA (mRNA) expression in human muscle precursor cells. Differentiation of muscle-derived cells was induced under standard low-serum conditions. Cultures were then exposed to androgen (testosterone (T)) at 50, 100, and 500 nM or IGF-1 (10-50 ng·mL⁻¹). Immunocytochemistry and real-time polymerase chain reaction (RT-PCR) were used to assess effects of androgens and IGF-1 after 3- (early) or 7-d (late) muscle differentiation, respectively; RT-PCR was used to quantify the effects on androgen receptor expression. Under low-serum conditions, 3-d exposure to androgens or IGF-1 or both resulted in no significant increase in cellular myogenic commitment. After 7-d exposure, however, T and IGF-1 were both found to increase fusion index with no observable synergistic effect. T also increased IGF-1 mRNA generation (P < 0.0001), whereas exogenous IGF-1 (P < 0.001) reduced IGF-1 mRNA transcription relative to control. The T effect was reversible after treatment with flutamide, an androgen receptor antagonist. Both T and IGF-1 increase myogenic commitment after 7-d exposure to a differentiation medium. With T causing a concomitant increase in IGF-1 mRNA underpinning IGF-1 as a central mediator in the cellular pathways associated with muscle hypertrophy, including those affected by androgens. The novel system described has the potential for elucidating the pattern of growth factor effects associated with androgens in skeletal muscle.

Topography of the human trapezius muscle: possibility for imaging of shoulder stiffness

Topography of the human trapezius muscle: possibility for imaging of shoulder stiffness

Protein differences between human trapezius and vastus lateralis muscles determined with a proteomic approach

BackgroundThe trapezius muscle is a neck muscle that is susceptible to chronic pain conditions associated with repetitive tasks, commonly referred to as chronic work-related myalgia, hence making the trapezius a muscle of clinical interest. To provide a basis for further investigations of the proteomic traits of the trapezius muscle in disease, two-dimensional difference gel electrophoresis (2D-DIGE) was performed on the healthy trapezius using vastus lateralis as a reference. To obtain as much information as possible from the vast proteomic data set, both one-way ANOVA, with and without false discovery rate (FDR) correlation, and partial least square projection to latent structures with discriminant analysis (PLS-DA) were combined to compare the outcome of the analysis.ResultsThe trapezius and vastus lateralis showed significant differences in metabolic, contractile and regulatory proteins, with different results depending on choice of statistical approach and pre-processing technique. Using the standard method, FDR correlated one-way ANOVA, 42 protein spots differed significantly in abundance between the two muscles. Complementary analysis using immunohistochemistry and western blot confirmed the results from the 2D-DIGE analysis.ConclusionsThe proteomic approach used in the present study combining 2D-DIGE and multivariate modelling provided a more comprehensive comparison of the protein profiles of the human trapezius and vastus lateralis muscle, than previously possible to obtain with immunohistochemistry or SDS-PAGE alone. Although 2D-DIGE has inherent limitations it is particularly useful to comprehensively screen for important structural and metabolic proteins, and appears to be a promising tool for future studies of patients suffering from chronic work related myalgia or other muscle diseases.

Histological Description of the Interaction Between Muscle Fibers and Connective Tissue of the Fascia of the Human Trapezius Muscle

La fascia del musculo esqueletico corresponde a una condensacion de tejido conectivo. Las fascias estan inervadas y son sensibles y pueden cubrir estructuras no distensibles, asi como las fibras musculares esqueleticas. Tienen un rol importante en la regulacion de la postura, la coordinacion motora periferica y la propiocepcion. Ademas, se ha descrito la presencia de inflamacion y microcalcificaciones en la fascia de los pacientes con dolor muscular localizado, lo que sugiere un rol patogenico en el dolor. El objetivo del trabajo fue describir la estructura histologica de la fascia externa profunda del musculo trapecio, con enfasis en el contenido y la disposicion de las fibras musculares, colageno tipo I y el tejido adiposo. Material y metodos: El material de la muestra fue obtenido de un cadaver de sexo masculino (60-70 anos), por la diseccion de la region cervical posterior de la fascia superficial del musculo trapecio e inmediatamente fijado en formalina tamponada (pH 7,2) durante 48 horas. La muestra fue procesada por tecnicas histologicas e impregnada en parafina (punto de fusion 56-58 C). Secciones de 5 µm de espesor fueron montadas en laminas silanizada para el desarrollo del protocolo de la tecnica de van Gieson. Resultados y discusion: Se observa que la fascia del trapecio esta compuesta por tejido conectivo denso irregular con abundante colageno tipo I, organizado en paquetes grandes como verdaderos haces de colageno de alta densidad orientada en diferentes direcciones; y por adipocitos dispuestos en grupos longitudinales en el eje principal de la fascia. Las fibras musculares estriadas estan organizadas en paquetes que se insertan lateralmente en el espesor de la fascia. Es posible que la transmision lateral de la tension entre las fibras este presente.

Interstitial concentration of serotonin is increased in myalgic human trapezius muscle during rest, repetitive work and mental stress – an in vivo microdialysis study

Objectives. The pathophysiology of trapezius myalgia is not fully elucidated. Serotonin (5-HT) is involved in modulation of nociception and hyperalgesia. Our aim was to compare the interstitial 5-HT levels of the trapezius muscle in women with chronic trapezius myalgia and in pain-free controls. Materials and methods. Microdialysate of the trapezius muscle collected every 20 minutes during rest, work (100 min) and stress (20 min) was used to study the dynamics of 5-HT in women with chronic trapezius myalgia (MYA; n=18) and in pain-free controls (CON; n=30). Results. MYA had higher levels of 5-HT than CON at baseline, during repetitive work, during mental stress and during recovery. There were no significant time effects on 5-HT levels. Conclusion. 5-HT has the potential of a biomarker of chronic myalgia. Elevated levels of 5-HT may be involved in maintenance of habitual chronic pain and might contribute to increased pain during exercise by facilitating the effect of released algesic substances linked to such muscle demands.

Responses of algesic and metabolic substances to 8 h of repetitive manual work in myalgic human trapezius muscle

The trapezius muscle often develops pain as the result of repetitive and stressful work tasks although it is unclear to what extent this pain is due to alterations in muscle concentrations of algesic/nociceptive substances. Twenty women with chronic neck- and shoulder pain (TM) whose work required highly repetitive work tasks and 20 pain-free female colleagues (CON) were studied during and after a full 8-hour workday. We collected microdialysates from their dominant/most painful trapezius muscle; concentrations of serotonin, glutamate, lactate, pyruvate, potassium, bradykinin, and cytokines and blood flow were determined. In addition, we measured surface electromyogram, task exposure level, pain intensity, perceived mental stress, and urine-cortisol. In connection to the clinical neck and shoulder examination, we determined pressure pain thresholds (PPTs) over the trapezius and tibialis muscles. TM had higher concentrations of glutamate (71 ± 42 vs. 36 ± 15 μmol l −1) and pyruvate (187 ± 89 vs. 125 ± 63 μmol l −1) than CON. Interstitial serotonin was higher in TM (before work: 10.6 ± 10.8 vs. 2.2 ± 1.2 nM; after work: 9.2 ± 8.3 vs. 1.5 ± 2.9 nM). The trapezius blood flow during the working day was higher in TM than in CON. TM had lower PPT and higher pain intensity throughout the working day. No differences in EMG, task exposure level, mental stress, or urine–cortisol in the groups were found. These findings support the idea that peripheral nociceptive processes are activated in occupationally active subjects, who are diagnosed with trapezius myalgia. In contrast, no sign of low blood flow or increased stress or muscle activity markers were found in TM.

Selective activation of neuromuscular compartments within the human trapezius muscle

Task-dependent differences in relative activity between “functional” subdivisions within human muscles are well documented. Contrary, independent voluntary control of anatomical subdivisions, termed neuromuscular compartments is not observed in human muscles. Therefore, the main aim of this study was to investigate whether subdivisions within the human trapezius can be independently activated by voluntary command using biofeedback guidance. Bipolar electromyographical electrodes were situated on four subdivisions of the trapezius muscle. The threshold for “active” and “rest” for each subdivision was set to >12% and <1.5% of the maximal electromyographical amplitude recorded during a maximal voluntary contraction. After 1 h with biofeedback from each of the four trapezius subdivisions, 11 of 15 subjects learned selective activation of at least one of the four anatomical subdivisions of the trapezius muscle. All subjects managed to voluntarily activate the lower subdivisions independently from the upper subdivisions. Half of the subjects succeeded to voluntarily activate both upper subdivisions independently from the two lower subdivisions. These findings show that anatomical subdivisions of the human trapezius muscle can be independently activated by voluntary command, indicating neuromuscular compartmentalization of the trapezius muscle. The independent activation of the upper and lower subdivisions of the trapezius is in accordance with the selective innervation by the fine cranial and main branch of the accessory nerve to the upper and lower subdivisions. These findings provide new insight into motor control characteristics, learning possibilities, and function of the clinically relevant human trapezius muscle.

Respiratory and stress-induced activation of low-threshold motor units in the human trapezius muscle

The study aimed to characterize trapezius motor unit firing pattern in low-amplitude contractions, with emphasis on respiratory modulated activity. Constant-amplitude contractions with shoulder elevation, controlled by feedback of the root mean square detected surface electromyographic (SEMG) signal, typing with arm movement and tasks with mental stress were performed. Single motor unit activity was recorded by a quadrifilar fine-wire electrode. A surface electrode simultaneously recorded SEMG activity. Contraction amplitudes ranged from 1 to 10% of the SEMG signal at maximum voluntary contraction (1-10% EMG(max)). The majority ( approximately 80%) of motor units recorded during constant-amplitude contractions showed firing rate modulation at the respiratory frequency. Respiratory firing rate modulation was clear for low amplitude contractions (< 3% EMG(max)), but was reduced at higher amplitudes (3-5.9% EMG(max)). Most motor units had peak firing rate at the transition from inspiration to expiration, but peak firing rate at the transition from expiration to inspiration or at the first harmonic frequency was also observed. The SEMG signal showed little or no respiratory modulation, possibly because respiratory phase varied between motor units. Respiratory modulation of firing rates was significantly reduced in experiments with mental stress and was rarely observed in typing experiments. Both central respiratory drive and peripheral afferent input may contribute to respiratory modulation of firing rates; however, animal studies indicate a central source of the respiratory modulated input. We speculate that the reduction in respiratory modulation of motor activity with mental stress is due to activation of alternative pathways providing excitatory input to trapezius motoneurons.

Gender differences in pain modulation evoked by repeated injections of glutamate into the human trapezius muscle

Gender differences in pain habituation, temporal summation, and pressure hyperalgesia evoked by repeated injections of glutamate into the dominant trapezius muscle were investigated. The glutamate-evoked muscle pain intensity and pressure pain threshold (PPT) were assessed. The PPTs were measured bilaterally in the trapezius muscles (local pain area) and posterolateral neck muscles (referred pain area) after glutamate injection in healthy and age-matched males and females (each n=14). Two glutamate injections (0.4 ml, 2 M each) were injected with an interval of 5 min. One injection of glutamate (0.4 ml, 2 M) served as a control. Males, but not females, rated the second injection (maximal pain intensity) significantly less painful than the first injection. The area under the visual analogue scale pain curve of the second injection was significantly larger than the first injection in females. Repeated glutamate injections, but not one-glutamate injection, significantly decreased PPTs in the local pain area, with no significant gender differences. No PPTs changes were observed either in the contralateral trapezius muscle or bilaterally in the referred pain areas in either sex. These results suggest that a less efficient pain habituation and a greater susceptibility to the development of temporal summation of muscle pain in females, but not in males, might be one of the contributing factors to the higher incidence of neck shoulder pain in females. In addition, the reduction of PPTs in the local pain area evoked by intramuscular glutamate injection may represent an early process of peripheral pressure hyperalgesia, which is most likely gender independent.

Motor unit recruitment and derecruitment induced by brief increase in contraction amplitude of the human trapezius muscle

The activity pattern of low-threshold human trapezius motor units was examined in response to brief, voluntary increases in contraction amplitude (‘EMG pulse’) superimposed on a constant contraction at 4–7% of the surface electromyographic (EMG) response at maximal voluntary contraction (4–7% EMGmax). EMG pulses at 15–20% EMGmax were superimposed every minute on contractions of 5, 10, or 30 min duration. A quadrifilar fine-wire electrode recorded single motor unit activity and a surface electrode recorded simultaneously the surface EMG signal. Low-threshold motor units recruited at the start of the contraction were observed to stop firing while motor units of higher recruitment threshold stayed active. Derecruitment of a motor unit coincided with the end of an EMG pulse. The lowest-threshold motor units showed only brief silent periods. Some motor units with recruitment threshold up to 5% EMGmax higher than the constant contraction level were recruited during an EMG pulse and kept firing throughout the contraction. Following an EMG pulse, there was a marked reduction in motor unit firing rates upon return of the surface EMG signal to the constant contraction level, outlasting the EMG pulse by 4 s on average. The reduction in firing rates may serve as a trigger to induce derecruitment. We speculate that the silent periods following derecruitment may be due to deactivation of non-inactivating inward current (‘plateau potentials’). The firing behaviour of trapezius motor units in these experiments may thus illustrate a mechanism and a control strategy to reduce fatigue of motor units with sustained activity patterns.

Motor unit recruitment and derecruitment induced by brief increase in contraction amplitude of the human trapezius muscle

The activity pattern of low-threshold human trapezius motor units was examined in response to brief, voluntary increases in contraction amplitude ('EMG pulse') superimposed on a constant contraction at 4-7 % of the surface electromyographic (EMG) response at maximal voluntary contraction (4-7 % EMGmax). EMG pulses at 15-20 % EMGmax were superimposed every minute on contractions of 5, 10, or 30 min duration. A quadrifilar fine-wire electrode recorded single motor unit activity and a surface electrode recorded simultaneously the surface EMG signal. Low-threshold motor units recruited at the start of the contraction were observed to stop firing while motor units of higher recruitment threshold stayed active. Derecruitment of a motor unit coincided with the end of an EMG pulse. The lowest-threshold motor units showed only brief silent periods. Some motor units with recruitment threshold up to 5 % EMGmax higher than the constant contraction level were recruited during an EMG pulse and kept firing throughout the contraction. Following an EMG pulse, there was a marked reduction in motor unit firing rates upon return of the surface EMG signal to the constant contraction level, outlasting the EMG pulse by 4 s on average. The reduction in firing rates may serve as a trigger to induce derecruitment. We speculate that the silent periods following derecruitment may be due to deactivation of non-inactivating inward current ('plateau potentials'). The firing behaviour of trapezius motor units in these experiments may thus illustrate a mechanism and a control strategy to reduce fatigue of motor units with sustained activity patterns.

INTERSTITIAL LACTATE AND POTASSIUM DYNAMICS IN TRAPEZIUS MUSCLE DURING REPETITIVE LOW-FORCE CONTRACTIONS, MEASURED WITH MICRODIALYSIS

Muscle metabolic responses to low-force contractions may not be reflected in systemic blood concentrations. Therefore, local measurements performed with microdialysis might offer an insight into muscle metabolism during e.g. repetitive monotonous work that has been proposed to cause muscle disorders. PURPOSE To test if microdialysis can quantify metabolic changes in the human trapezius muscle during low-force contractions typical for repetitive monotonous work. METHODS A microdialysis catheter was inserted in the upper trapezius muscle in 6 healthy male subjects (28–33 yrs). Microdialysis, electromyography (EMG) and rating of perceived exertion (RPE, 0–9) were recorded from the trapezius muscle and venous blood samples were taken during: 20 min of repetitive low-force contractions (RLC), 60 min of rest (R1), 10 min of static shoulder flexion (SSF) followed by 60 min of rest (R2). RESULTS The average muscle activity was 8 ±2% (%EMGmax) during RLC and 22 ±5% during SSF. RPE increased from 0 to 3 ±1 during RLC and from 0 to 8.5 ±1 during SSF. Plasma lactate was constant at 0.8 ±0.1 mM during RLC and R1, increased to 1.9 ±0.3 during SSF (p < 0.05) and dropped to 0.9 ±0.1 mM during R2. Plasma K+ was unchanged during RLC, R1, SSF and R2. In contrast, interstitial muscle lactate increased during RLC from 2.1 ±0.2 to 2.9 ±0.2 mM (p < 0.05) and returned to 2.2 ±0.2 mM during R1 and K+ increased from 6.3 ±0.8 to 10.4 ±1.4 mM (p < 0.05) and returned to 6.6 ±0.8 mM during R1. During SSF interstitial muscle lactate increased from 2.3 ±0.2 to 6.5 ±1.3 mM (p < 0.05) and decreased to 2.6 ±0.3 mM during R2 while K+ increased from 6.7 ±0.8 to 9.4 ±1.1 mM (p < 0.05) and returned to 6.1 ±1.1 mM during R2. CONCLUSION The microdialysis technique was effective in revealing muscle metabolic events that were not seen systemically. Furthermore, the trapezius muscle showed an anaerobic metabolism during RLC, which could indicate an inhomogeneous muscle activation.

Motor unit identification in two neighboring recording positions of the human trapezius muscle during prolonged computer work.

Work-related shoulder-neck pain is a major health risk in computer operators. To understand the physiological mechanisms behind the development of these disorders, EMG recordings of some minutes up to several hours must be accurately decomposed. For this reason we developed EMG-LODEC, an automatic decomposition software program, especially designed for multi-channel long-term recordings of signals detected during slight muscle movements. The subjects executed a 30-min computer task to simulate real work conditions while working at an ergonomically designed workstation. Six-channel intramuscular EMG signals were recorded from two positions of the upper trapezius muscle. The EMG signals were decomposed into individual motor unit action potential trains using EMG-LODEC. The study design enabled us first to study the dependence of intramuscular analysis on the insertion points and second to test the accuracy of the decomposition technique under laboratory conditions during a real experiment. The two positions yielded 887 motor units--452 located in position 1 and 435 in position 2. Although the numbers of detected action potentials were strongly correlated between the two insertion positions, different motor units were mostly recorded. In particular, the detection of continuously active motor units is specific for the selected insertion points and may not be representative of a muscle, not even for parts with common functions. The approach for the quantitative evaluation of the decomposition technique was to independently decompose two signals that were simultaneously detected by separate sets of wire electrodes placed close to each other in the muscle. Common trains discovered in each signal were compared for consistency. A cross-correlation analysis was performed to find corresponding motor unit pairs that were concurrently active. Concurrently active motor units were found in six subjects. For these motor units the extent of simultaneous occurrence of motor unit action potentials between the two positions ranged from 23% to 78% depending on the distinction of the single motor units and the number of superimposed motor unit action potentials. High concordance was seen in 3 out of the 15 motor unit pairs. Based on the results, EMG-LODEC is capable of providing reliable decompositions with satisfying accuracy and reasonable processing time. EMG-LODEC is suitable for the study of motor unit discharge patterns and recruitment order in subjects with and without musculoskeletal pain during long-term measurements to study work-related musculoskeletal disorders.